Tuesday, June 8, 2010

The unit delay operator

The transfer function of a digital filter

In the last section, we used two different ways of expressing the action of a digital filter: a form giving the output yn directly, and a "symmetrical" form with all the output terms (y's) on one side and all the input terms (x's) on the other.

In this section, we introduce what is called the transfer function of a digital filter. This is obtained from the symmetrical form of the filter expression, and it allows us to describe a filter by means of a convenient, compact expression. Tthe transfer function of a filter can be used to determine many of the characteristics of the filter, such as its frequency response.

The unit delay operator

First of all, we must introduce the unit delay operator, denoted by the symbol

z-1

When applied to a sequence of digital values, this operator gives the previous value in the sequence. It therefore in effect introduces a delay of one sampling interval.

Applying the operator z-1 to an input value (say xn) gives the previous input (xn-1):

z-1 xn = xn-1

Suppose we have an input sequence

x0 = 5
x1 = -2
x2 = 0
x3 = 7
x4 = 10

Then

z-1 x1 = x0 = 5
z-1 x2 = x1 = -2
z-1 x3 = x2 = 0

and so on. Note that z-1 x0 would be x-1 which is unknown (and usually taken to be zero, as we have already seen).

Similarly, applying the z-1 operator to an output gives the previous output:

z-1 yn = yn-1

Applying the delay operator z-1 twice produces a delay of two sampling intervals:

z-1 (z-1 xn) = z-1 xn-1 = xn-2

We adopt the (fairly logical) convention

z-1 z-1 = z-2

i.e. the operator z-2 represents a delay of two sampling intervals:

z-2 xn = xn-2

This notation can be extended to delays of three or more sampling intervals, the appropriate power of z-1 being used.

Let us now use this notation in the description of a recursive digital filter. Consider, for example, a general second-order filter, given in its symmetrical form by the expression

b0yn + b1yn-1 + b2yn-2 = a0xn + a1xn-1 + a2xn-2

We will make use of the following identities:

yn-1 = z-1 yn

yn-2 = z-2 yn

xn-1 = z-1 xn

xn-2 = z-2 xn

Substituting these expressions into the digital filter gives

(b0 + b1z-1 + b2z-2) yn = (a0 + a1z-1 + a2z-2) xn

Rearranging this to give a direct relationship between the output and input for the filter, we get

yn / xn = (a0 + a1z-1 + a2z-2) / (b0 + b1z-1 + b2z-2)

This is the general form of the transfer function for a second-order recursive (IIR) filter.

For a first-order filter, the terms in z-2 are omitted. For filters of order higher than 2, further terms involving higher powers ofz-1 are added to both the numerator and denominator of the transfer function.

A non-recursive (FIR) filter has a simpler transfer function which does not contain any denominator terms. The coefficient b0 is regarded as being equal to 1, and all the other b coefficients are zero. The transfer function of a second-order FIR filter can therefore be expressed in the general form

yn / xn = a0 + a1z-1 + a2z-2

Transfer function examples

  1. The three-term average filter, defined by the expression

    yn = 1/3 (xn + xn-1 + xn-2)

    can be written using the z-1 operator notation as

    yn = 1/3 (xn + z-1xn + z-2xn)

    = 1/3 (1 + z-1 + z-2) xn

    The transfer function for the filter is therefore

    yn / xn = 1/3 (1 + z-1 + z-2)

  2. The general form of the transfer function for a first-order recursive filter can be written

    yn / xn = (a0 + a1z-1) / (b0 + b1z-1)

    Consider, for example, the simple first-order recursive filter

    yn = xn + yn-1

    which we discussed earlier. To derive the transfer function for this filter, we rewrite the filter expression using the z-1 operator:

    (1 - z-1) yn = xn

    Rearranging gives the filter transfer function as

    yn / xn = 1 / (1 - z-1)

  3. As a further example, consider the second-order IIR filter

    yn = xn + 2xn-1 + xn-2 - 2yn-1 + yn-2

    Collecting output terms on the left and input terms on the right to give the "symmetrical" form of the filter expression, we get

    yn + 2yn-1 - yn-2 = xn + 2xn-1 + xn-2

    Expressing this in terms of the z-1 operator gives

    (1 + 2z-1 - z-2) yn = (1 + 2z-1 + z-2) xn

    and so the transfer function is

    yn / xn = (1 + 2z-1 + z-2) / (1 + 2z-1 - z-2)
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The transfer function of a digital filter

In the last section, we used two different ways of expressing the action of a digital filter: a form giving the output yn directly, and a "symmetrical" form with all the output terms (y's) on one side and all the input terms (x's) on the other.

In this section, we introduce what is called the transfer function of a digital filter. This is obtained from the symmetrical form of the filter expression, and it allows us to describe a filter by means of a convenient, compact expression. Tthe transfer function of a filter can be used to determine many of the characteristics of the filter, such as its frequency response.

The unit delay operator

First of all, we must introduce the unit delay operator, denoted by the symbol

z-1

When applied to a sequence of digital values, this operator gives the previous value in the sequence. It therefore in effect introduces a delay of one sampling interval.

Applying the operator z-1 to an input value (say xn) gives the previous input (xn-1):

z-1 xn = xn-1

Suppose we have an input sequence

x0 = 5
x1 = -2
x2 = 0
x3 = 7
x4 = 10

Then

z-1 x1 = x0 = 5
z-1 x2 = x1 = -2
z-1 x3 = x2 = 0

and so on. Note that z-1 x0 would be x-1 which is unknown (and usually taken to be zero, as we have already seen).

Similarly, applying the z-1 operator to an output gives the previous output:

z-1 yn = yn-1

Applying the delay operator z-1 twice produces a delay of two sampling intervals:

z-1 (z-1 xn) = z-1 xn-1 = xn-2

We adopt the (fairly logical) convention

z-1 z-1 = z-2

i.e. the operator z-2 represents a delay of two sampling intervals:

z-2 xn = xn-2

This notation can be extended to delays of three or more sampling intervals, the appropriate power of z-1 being used.

Let us now use this notation in the description of a recursive digital filter. Consider, for example, a general second-order filter, given in its symmetrical form by the expression

b0yn + b1yn-1 + b2yn-2 = a0xn + a1xn-1 + a2xn-2

We will make use of the following identities:

yn-1 = z-1 yn

yn-2 = z-2 yn

xn-1 = z-1 xn

xn-2 = z-2 xn

Substituting these expressions into the digital filter gives

(b0 + b1z-1 + b2z-2) yn = (a0 + a1z-1 + a2z-2) xn

Rearranging this to give a direct relationship between the output and input for the filter, we get

yn / xn = (a0 + a1z-1 + a2z-2) / (b0 + b1z-1 + b2z-2)

This is the general form of the transfer function for a second-order recursive (IIR) filter.

For a first-order filter, the terms in z-2 are omitted. For filters of order higher than 2, further terms involving higher powers ofz-1 are added to both the numerator and denominator of the transfer function.

A non-recursive (FIR) filter has a simpler transfer function which does not contain any denominator terms. The coefficient b0 is regarded as being equal to 1, and all the other b coefficients are zero. The transfer function of a second-order FIR filter can therefore be expressed in the general form

yn / xn = a0 + a1z-1 + a2z-2

Transfer function examples

  1. The three-term average filter, defined by the expression

    yn = 1/3 (xn + xn-1 + xn-2)

    can be written using the z-1 operator notation as

    yn = 1/3 (xn + z-1xn + z-2xn)

    = 1/3 (1 + z-1 + z-2) xn

    The transfer function for the filter is therefore

    yn / xn = 1/3 (1 + z-1 + z-2)

  2. The general form of the transfer function for a first-order recursive filter can be written

    yn / xn = (a0 + a1z-1) / (b0 + b1z-1)

    Consider, for example, the simple first-order recursive filter

    yn = xn + yn-1

    which we discussed earlier. To derive the transfer function for this filter, we rewrite the filter expression using the z-1 operator:

    (1 - z-1) yn = xn

    Rearranging gives the filter transfer function as

    yn / xn = 1 / (1 - z-1)

  3. As a further example, consider the second-order IIR filter

    yn = xn + 2xn-1 + xn-2 - 2yn-1 + yn-2

    Collecting output terms on the left and input terms on the right to give the "symmetrical" form of the filter expression, we get

    yn + 2yn-1 - yn-2 = xn + 2xn-1 + xn-2

    Expressing this in terms of the z-1 operator gives

    (1 + 2z-1 - z-2) yn = (1 + 2z-1 + z-2) xn

    and so the transfer function is

    yn / xn = (1 + 2z-1 + z-2) / (1 + 2z-1 - z-2)
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DSP - Digital signal processing - Introduction to Digital Signal Processing

What is DSP?

DSP, or Digital Signal Processing, as the term suggests, is the processing of signals by digital means. A signal in this context can mean a number of different things. Historically the origins of signal processing are in electrical engineering, and a signal here means an electrical signal carried by a wire or telephone line, or perhaps by a radio wave. More generally, however, a signal is a stream of information representing anything from stock prices to data from a remote-sensing satellite. The term "digital" comes from "digit", meaning a number (you count with your fingers - your digits), so "digital" literally means numerical; the French word for digital is numerique. A digital signal consists of a stream of numbers, usually (but not necessarily) in binary form. The processing of a digital signal is done by performing numerical calculations.

Analog and digital signals

In many cases, the signal of interest is initially in the form of an analog electrical voltage or current, produced for example by a microphone or some other type of transducer. In some situations, such as the output from the readout system of a CD (compact disc) player, the data is already in digital form. An analog signal must be converted into digital form before DSP techniques can be applied. An analog electrical voltage signal, for example, can be digitised using an electronic circuit called an analog-to-digital converter or ADC. This generates a digital output as a stream of binary numbers whose values represent the electrical voltage input to the device at each sampling instant.

Signal processing

Signals commonly need to be processed in a variety of ways. For example, the output signal from a transducer may well be contaminated with unwanted electrical "noise". The electrodes attached to a patient's chest when an ECG is taken measure tiny electrical voltage changes due to the activity of the heart and other muscles. The signal is often strongly affected by "mains pickup" due to electrical interference from the mains supply. Processing the signal using a filter circuit can remove or at least reduce the unwanted part of the signal. Increasingly nowadays, the filtering of signals to improve signal quality or to extract important information is done by DSP techniques rather than by analog electronics.

Development of DSP

The development of digital signal processing dates from the 1960's with the use of mainframe digital computers for number-crunching applications such as the Fast Fourier Transform (FFT), which allows the frequency spectrum of a signal to be computed rapidly. These techniques were not widely used at that time, because suitable computing equipment was generally available only in universities and other scientific research institutions.

Digital Signal Processors (DSPs)

The introduction of the microprocessor in the late 1970's and early 1980's made it possible for DSP techniques to be used in a much wider range of applications. However, general-purpose microprocessors such as the Intel x86 family are not ideally suited to the numerically-intensive requirements of DSP, and during the 1980's the increasing importance of DSP led several major electronics manufacturers (such as Texas Instruments,Analog Devices and Motorola) to develop Digital Signal Processor chips - specialised microprocessors with architectures designed specifically for the types of operations required in digital signal processing. (Note that the acronym DSP can variously mean Digital Signal Processing, the term used for a wide range of techniques for processing signals digitally, or Digital Signal Processor, a specialised type of microprocessor chip). Like a general-purpose microprocessor, a DSP is a programmable device, with its own native instruction code. DSP chips are capable of carrying out millions of floating point operations per second, and like their better-known general-purpose cousins, faster and more powerful versions are continually being introduced. DSPs can also be embedded within complex "system-on-chip" devices, often containing both analog and digital circuitry.

Applications of DSP

DSP technology is nowadays commonplace in such devices as mobile phones, multimedia computers, video recorders, CD players, hard disc drive controllers and modems, and will soon replace analog circuitry in TV sets and telephones. An important application of DSP is in signal compression and decompression. Signal compression is used in digital cellular phones to allow a greater number of calls to be handled simultaneously within each local "cell". DSP signal compression technology allows people not only to talk to one another but also to see one another on their computer screens, using small video cameras mounted on the computer monitors, with only a conventional telephone line linking them together. In audio CD systems, DSP technology is used to perform complex error detection and correction on the raw data as it is read from the CD.

Although some of the mathematical theory underlying DSP techniques, such as Fourier and Hilbert Transforms, digital filter design and signal compression, can be fairly complex, the numerical operations required actually toimplement these techniques are very simple, consisting mainly of operations that could be done on a cheap four-function calculator. The architecture of a DSP chip is designed to carry out such operations incredibly fast, processing hundreds of millions of samples every second, to provide real-time performance: that is, the ability to process a signal "live" as it is sampled and then output the processed signal, for example to a loudspeaker or video display. All of the practical examples of DSP applications mentioned earlier, such as hard disc drives and mobile phones, demand real-time operation.

The major electronics manufacturers have invested heavily in DSP technology. Because they now find application in mass-market products, DSP chips account for a substantial proportion of the world market for electronic devices. Sales amount to billions of dollars annually, and seem likely to continue to increase rapidly
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Moving coil meters

Moving Coil Meters

The design of a voltmeter, ammeter or ohmmeter begins with a current-sensitive element. Though most modern meters have solid state digital readouts, the physics is more readily demonstrated with a moving coil current detector called agalvanometer. Since the modifications of the current sensor are compact, it is practical to have all three functions in a single instrument with multiple ranges of sensitivity. Schematically, a single range "multimeter" might be designed as illustrated.


Voltmeter

A voltmeter measures the change in voltage between two points in an electric circuit and therefore must be connected in parallel with the portion of the circuit on which the measurement is made. By contrast, an ammeter must be connected in series. In analogy with a water circuit, a voltmeter is like a meter designed to measure pressure difference. It is necessary for the voltmeter to have a very high resistance so that it does not have an appreciable affect on the current or voltage associated with the measured circuit. Modern solid-state meters have digital readouts, but the principles of operation can be better appreciated by examining the older moving coil meters based on galvanometer sensors.

Ammeter

An ammeter is an instrument for measuring the electric current in amperes in a branch of an electric circuit. It must be placed in series with the measured branch, and must have very low resistance to avoid significant alteration of the current it is to measure. By contrast, an voltmeter must be connected in parallel. The analogy with an in-line flowmeter in a water circuit can help visualize why an ammeter must have a low resistance, and why connecting an ammeter in parallel can damage the meter. Modern solid-state meters have digital readouts, but the principles of operation can be better appreciated by examining the older moving coil metersbased on galvanometer sensors.



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Sunday, June 6, 2010

Power Quality causes

Power quality is depends upon the rated voltage, frequency(pure sinusoidal). The power delivered to the customer is must be in quality. The causes of power quality in electrical system are listed as follows,
  • Sags, dips & swells,
  • Transient over voltage,
  • Harmonics,
  • Flickers,
  • Voltage regulation,
  • Other disturbances.
Power quality is the set of limits of electrical properties that allows electrical systems to function in their intended manner without significant loss of performance or life. The term is used to describeelectric power that drives an electrical load and the load's ability to function properly with that electric power. Without the proper power, an electrical device (or load) may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality and many more causes of such poor quality power.


Voltage sags (dips) and swells

Voltage sags -- or dips which are the same thing -- are brief reductions in voltage, typically lasting from a cycle to a second or so, or tens of milliseconds to hundreds of milliseconds. Voltage swells are brief increases in voltage over the same time range.

(Longer periods of low or high voltage are referred to as "undervoltage" or "overvoltage".)

Voltage sags are caused by abrupt increases in loads such as short circuits or faults, motors starting, or electric heaters turning on, or they are caused by abrupt increases in source impedance, typically caused by a loose connection. Voltage swells are almost always caused by an abrupt reduction in load on a circuit with a poor or damaged voltage regulator, although they can also be caused by a damaged or loose neutral connection.

A typical voltage sag.

Voltage sags are the most common power disturbance. At a typical industrial site, it is not unusual to see several sags per year at the service entrance, and far more at equipment terminals.

Voltage sags can arrive from the utility; however, in most cases, the majority of sags are generated inside a building. For example, in residential wiring, the most common cause of voltage sags is the starting current drawn by refrigerator and air conditioning motors.

Sags do not generally disturb incandescent or fluorescent lighting. motors, or heaters. However, some electronic equipment lacks sufficient internal energy storage and, therefore, cannot ride through sags in the supply voltage. Equipment may be able to ride through very brief, deep sags, or it may be able to ride through longer but shallower sags.

1996 Version of the IT Industry Tolerance Curves (update from original CBEMA curve). The vertical axis is percent of nominal voltage. "Well-designed" equipment should be able to tolerate any power event that lies in the shaded area. Note that the curve includes sags, swells, and transient overvoltages.

The semiconductor industry developed a more recent specification (SEMI F47) for tools used in the semiconductor industry in an effort to achieve better ride through of equipment for commonly occurring voltage dips and therefore improving the overall process performance. It is basically the same as the ITI Curve but specifies an improved ride through requirement down to 50% retained voltage for the first 200 msec. Many short voltage dips are covered by this additional requirement. IEC 61000-4-11 and IEC 61000-4-34 provide similar voltage dip immunity standards.

Many utilities have benchmarked performance of the supply system for voltage dips but it has not been the general practice to specify any required performance levels for the system. Performance is often specified using the SARFI index that provides a count of all events with magnitudes and durations outside of some specifications. For instance, SARFI-70 would provide a count of all voltage dips with a retained voltage less than 70% (regardless of duration). SARFI-ITIC would provide a count of all voltage dips that exceeded the ride through specifications of the ITI Curve.

The table below provides a summary of voltage dip performance levels from a few major benchmarking efforts. Note that these are average performance levels and it would not be reasonable to develop limits based on an average expected performance (although these are the correct values to use when evaluating the economics of investments in ride through solutions).

Example of average voltage dip performance from major benchmarking projects. These values represent voltage dip performance on medium voltage systems.

The voltage dip performance can vary dramatically for different kinds of systems (rural vs urban, overhead vs underground). It may be important to include some of these important factors in the specification of the power quality grades.

It will also be important to specify the performance for momentary interruptions. These events can be a particular problem for customers and are not included in most assessments of reliability.

A previous CEA Technologies report prepared by Electrotek Concepts recommended that the SARFI indices be calculated for the following magnitude and duration categories:

Recommended magnitude and duration categories for calculating voltage dip performance.

The reasons for these categories were explained as follows:

  • The 90% level provides an indication of performance for the most sensitive equipment.
  • The 80% level corresponds to an important break point on the ITI curve and some sensitive equipment may be susceptible to even short sags at this level.
  • The 70% level corresponds to the sensitivity level of a wide group of industrial and commercial equipment and is probably the most important performance level to specify.
  • The 50% level is important, especially for the semiconductor industry, since they have adopted a standard that specifies ride through at this level.
  • Interruptions affect all customers so it is important to specify this level separately. These will usually have longer durations than the voltage sags.
  • The first range of durations is up to 0.2 seconds (12 cycles at 60 Hz). This is the range specified by the semiconductor industry that equipment should be able to ride through sags as long as the minimum voltage is above 50%.
  • The second range is up to 0.5 seconds. This corresponds to the specification in the ITIC standard for equipment ride through as long as the minimum voltage is above 70%. It is also an important break point in the definition of sag durations in IEEE 1159 (instantaneous vs. momentary).
  • The third duration range is up to 3 seconds. This is an important break point in IEEE 1159 and in IEC standards (momentary to temporary).
  • The final duration is up to one minute. Events longer than one minute are characterized as long duration events and are part of the system voltage regulation performance, rather than voltage sags.

As a final note, remember that voltage sags are voltages, and therefore always occur between two conductors - there is no such thing as a "sag on phase A" -- it must be a sag between phase A and phase B, or a sag between phase A and Neutral.



TRANSIENT OVER VOLTAGE

Information about Transient Overvoltages

Transient overvoltages are brief, high-frequency increases in voltage on AC mains.

Broadly speaking, there are two different types of transient overvoltages: low frequency transients with frequency components in the few-hundred-hertz region typically caused by capacitor switching, and high-frequency transients with frequency components in the few-hundred-kilohertz region typically caused by lighting and inductive loads.

Low frequency transients are often called "capacitor switching transients". High frequency transients are often called "impulses", "spikes", or "surges".

Surge suppressors are devices that conduct across the power line when some voltage threshold is exceeded. Typically, they are used to absorb the energy in high frequency transients. However, the resulting high frequency current pulses (often in the hundreds of amps) can still create problems for sensitive electronic systems, especially delicate instrumentation.

Low frequency transients are caused when a discharged power-factor-correction capacitor is switched on across the line. The capacitor then resonates with the inductance of the distribution system, typically at 400 - 600 Hz, and produce and exponentially damped decaying waveform. The peak of this waveform, in theory, cannot exceed twice the peak voltage of the sine wave, and is more typically 120% - 140% of the sine peak. However, in some specific cicumstances, there can be "multiplication" of this transient by resonance with other power factor correction capacitors.

High frequency transients are caused by lightning, and by inductive loads turning off. Typical rise times are on the order of a microsecond; typical decay times are on the order of a tens to hundreds of microseconds. Often, the decay will be an exponential damped ringing waveform, with a frequency of approximately 100 kHz, which corresponds to the frequency of equivalent inductor/capacitor model of low voltage power lines. Typical peak voltages for end-use applications are hundreds of volts to a few thousand volts; several thousand amps of current may be available.

(Extremely fast transients, or EFT's, have rise and fall times in the nanosecond region. They are caused by arcing faults, such as bad brushes in motors, and are rapidly damped out by even a few meters of distribution wiring. Standard line filters, included on almost all electronic equipment, remove EFT's.)

Contact Alex McEachern for more information about:

  • transient standards
  • effects of transients on equipment
  • solutions to transient problems.
Example capacitor switching transient.



Information about Harmonics

The electric power distribution system is designed to operate with sinusoidal voltages and currents.

But not all waveforms are sine waves. Electronic loads, for example, often draw current only at the peak of the voltage waveform, which always means that the current is distorted, and may distort the voltage as well. One convenient way to describe these waveforms is to make a list of sine waves that, when added together, reproduce the distorted waveform. The sine waves in this list are always multiples, or harmonics, of the fundamental frequency (50 Hz or 60 Hz).

A typical input circuit of a single-phase supply.

All of the graphs below are automatically produced by the Industrial Power Corruptor's Power Flow Option.

A typical distorted current waveform, drawn by the supply above. It only draws current at the peak of the voltage waveform, because the diodes in BR1 only conduct when the AC voltage is higher than the voltage on C1.

This is the same waveform, expressed as a frequency spectrum. Note that the frequency content of the waveform consists of odd multiples (3,5,7,9, etc.) of the fundamental. This is typical for electronic loads.

Again, the same waveform, expressed as a frequency spectrum. This time the values are listed. Sometimes, the phase angles of the harmonics can be important, too, but they are not shown here.

THD, or Total Harmonic Distortion, is one measure of the total distortion. It is the RMS sum of the harmonics, divided by one of two values: either the fundamental value, or the RMS value of the total waveform. Both are legitimate definitions of THD. For small values of distortion, they both produce roughly the same number. For the waveform above, using the fundamental as the reference produces a THD value of 93.2%, and using the RMS as the reference produces a THD value of 67.8%. Both values are correct.

For this and other reasons, most experts in power system harmonics frown on using THD as a measure of harmonics. Other measures such as TDD (IEEE 519) or volts and amps make more sense. For example, the waveform above consists of 32.4 amps at 60 Hz, plus 25.4 amps at 180 Hz, plus 14.8 amps at 300 Hz, etc.

Many devices on the power system respond poorly to non-sinusoidal waveforms. Transformers, for example, become less efficient. Many revenue meters become less accurate. Protection devices such as circuit breakers may trip too soon, or too late.

Balanced harmonics at multiples-of-3-of-the-fundamental, or triplen harmonics (3rd, 9th, 15th, etc.), fail to rotate on three-phase systems. As a result, neutral conductors may overheat, and transformers and motors become less efficient.


Information about Flicker



Flicker is a very specific problem related to human perception and incandescent light bulbs.
It is not a general term for voltage variations.
Humans can be very sensitive to light flicker that is caused by voltage fluctuations.
Human perception of light flicker is almost always the limiting criteria for controlling small voltage fluctuations. The figure illustrates the level of perception of light flicker from a 60 watt incandescent bulb for rectangular variations. The sensitivity is a function of the frequency of the fluctuations and it is also dependent on the voltage level of the lighting.



Voltage changes that will result in perceptible light flicker with a 60 watt incandescent light bulb.

Limits for flicker levels are not specified in IEEE standards. Curves similar to the one shown above have been used by individual utilities as guidelines for controlling flicker.

Flicker levels in IEC standards are characterized by two parameters:

  • Pst is a value measured over 10 minutes that characterizes the likelihood that the voltage fluctuations would result in perceptible light flicker. A value of 1.0 is designed to represent the level that 50% of people would perceive flicker in a 60 watt incandescent bulb.
  • Plt is derived from 2 hours of Pst values (12 values combined in cubic relationship).

Note that IEEE is also adopting this method of characterizing flicker (IEEE 1453).

IEC 61000-2-2 specifies flicker compatibility levels:

  • Compatibility level for short term flicker (Pst) is 1.0.
  • Compatibility level for long term flicker (Plt) is 0.8.

Recognizing that it is not always possible to maintain flicker levels within these compatibility levels, EN 50160 specifies less restrictive requirements for the supply system performance. The EN 50160 limit is that 95% of the long term flicker values (Plt) should be less than 1.0 in one week measurement period.

Note that individual step changes in the voltage, such as would be caused by motor starting or switching a capacitor bank, are often limited separately from the continuous flicker limits. IEC 61000-2-2 specifies a compatibility level of 3% for the individual voltage variations. EN 50160 specifies a limit of 5% for these variations but mentions that more significant variations (up to 10%) can occur for some switching events. Specific recommendations are not provided in IEEE but individual utilities usually have their own guidelines in the range 4-7%.


Information about Voltage Regulation

The term "voltage regulation" is used to discuss long-term variations in voltage. It does not include short term variations, which are generally called sags, dips, or swells.

The ability of equipment to handle steady state voltage variations varies from equipment to equipment. The steady state voltage variation limits for equipment is usually part of the equipment specifications. The Information Technology Industry Council (ITIC) specifies equipment withstand recommendations for IT equipment according to the ITI Curve (formerly the CBEMA curve). The 1996 ITI Curve specifies that equipment should be able to withstand voltage variations within ± 10% (variations that last longer than 10 seconds).

Voltage regulation standards in North America vary from state to state and utility to utility. The national standard in the U.S.A. is ANSI C84.1. Voltage regulation requirements are defined in two categories:

  • Range A is for normal conditions and the required regulation is ± 5% on a 120 volt base at the service entrance (for services above 600 volts, the required regulation is -2.5% to +5%).
  • Range B is for short durations or unusual conditions. The allowable range for these conditions is -8.3% to +5.8%. A specific definition of these conditions is not provided.
Voltage regulation requirements from ANSI C84.1. This is not a universal standard; it is only used in North America.

Other countries have different standards. For example, IEC 61000-2-2 mentions that the normal operational tolerances are ± 10% of the declared voltage. This is the basis of requirements for voltage regulation in EN 50160 for the European Community. EN 50160 requires that voltage regulation be within ± 10% for 95% of the 10 minute samples in a one week period, and that all 10 minute samples be within -15% to +10%, excluding voltage dips.

Other countries have established different limits, based on the characteristics of their distribution systems: Australia, Japan, etc.


Other Disturbances

The most common disturbances on AC power systems are voltage sags or dips. Other problems, such as transient overvoltages and brief interruptions, occur almost everywhere. Problems with harmonics, voltage regulation, and flicker occur at a wide range of sites.

Some other disturbances that occur at specific locations include:

  • Frequency variations. On utility grids, these are rare events, usually associated with catastrophic collapses on the grid. However, at sites with back-up diesel generators, they are common.
  • High frequency noise. This can be caused by anything from arcing brushes on a motor, to local radio transmitters.
  • Mains signalling Some utilities intentionally place small signals on the mains voltage to act as control signals (for example, they may control a capacitor switch, or they may instruct revenue meters to go to a different rate structure).
  • EFT Extremely Fast Transients are nano-second range transient overvoltages. Due to their high frequency content, they do not travel well over the mains circuits, getting damped out within a few meters. However, they can be caused by nearby contact arcing.
  • Unbalance On three-phase systems, the voltages and currents on each phase should, in theory, match the voltages and currents on the other phases. Sometimes they don't.


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Types of motors

This is the structure of electrical motors
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Types of motors

Motor Types

Industrial motors come in a variety of basic types. These variations are suitable for many different applications. Naturally, some types of motors are more suited for certain applications than other motor types are. This document will hopefully give some guidance in selecting these motors.

  • AC Motors
  • DC Motors
  • Brushless DC Motors
  • Servo Motors
  • Brushed DC Servo Motors
  • Brushless AC Servo Motors
  • Stepper Motors
  • Linear Motors

    • AC Motors

    The most common and simple industrial motor is the three phase AC induction motor, sometimes known as the "squirrel cage" motor. Substantial information can be found about any motor by checking its (nameplate).

    Advantages

    Simple Design

    The simple design of the AC motor -- simply a series of three windings in the exterior (stator) section with a simple rotating section (rotor). The changing field caused by the 50 or 60 Hertz AC line voltage causes the rotor to rotate around the axis of the motor.

    The speed of the AC motor depends only on three variables:

    1. The fixed number of winding sets (known as poles) built into the motor, which determines the motor's base speed.
    2. The frequency of the AC line voltage. Variable speed drives change this frequency to change the speed of the motor.
    3. The amount of torque loading on the motor, which causes slip.

    Low Cost

    The AC motor has the advantage of being the lowest cost motor for applications requiring more than about 1/2 hp (325 watts) of power. This is due to the simple design of the motor. For this reason, AC motors are overwhelmingly preferred for fixed speed applications in industrial applications and for commercial and domestic applications where AC line power can be easily attached. Over 90% of all motors are AC induction motors. They are found in air conditioners, washers, dryers, industrial machinery, fans, blowers, vacuum cleaners, and many, many other applications.

    Reliable Operation

    The simple design of the AC motor results in extremely reliable, low maintenance operation. Unlike the DC motor, there are no brushes to replace. If run in the appropriate environment for its enclosure, the AC motor can expect to need new bearings after several years of operation. If the application is well designed, an AC motor may not need new bearings for more than a decade.

    Easily Found Replacements

    The wide use of the AC motor has resulted in easily found replacements. Many manufacturers adhere to either European (metric) or American (NEMA) standards. (For Replacement Motors)

    Variety of Mounting Styles

    AC Motors are available in many different mounting styles such as:

  • Foot Mount
  • C-Face
  • Large Flange
  • Vertical
  • Specialty

    • Many Different Environmental Enclosures

    Because of the wide range of environments in which people want to use motors, the AC motor has been adapted by providing a wide range of enclosures:

  • ODP - Open Drip Proof
  • TEFC - Totally Enclosed Fan Cooled
  • TEAO - Totally Enclosed Air Over
  • TEBC - Totally Enclosed Blower Cooled
  • TENV - Totally Enclosed Non-Ventilated
  • TEWC - Totally Enclosed Water Cooled

    • Disadvantages

  • Expensive speed control
  • Inability to operate at low speeds
  • Poor positioning control

    • Expensive speed control

    Speed control is expensive. The electronics required to handle an AC inverter drive are considerably more expensive than those required to handle a DC motor. However, if performance requirements can be met -- meaning that the required speed range is over 1/3rd of base speed -- AC inverters and AC motors are usually more cost-effective than DC motors and DC drives for applications larger than about 10 horsepower, because of cost savings in the AC motor.

    Inability to operate at low speeds

    Standard AC motors should not be operated at speeds less than about 1/3rd of base speed. This is due to thermal considerations. A DC motor should be considered for these applications.

    Poor positioning control

    Positioning control is expensive and crude. Even a vector drive is very crude when controlling a standard AC motor. Servo motors are more appropriate for these applications.

    DC Motors

    The brushed DC motor is one of the earliest motor designs. Today, it is the motor of choice in the majority of variable speed and torque control applications.

    Advantages

    • Easy to understand design
    • Easy to control speed
    • Easy to control torque
    • Simple, cheap drive design

    Easy to understand design

    The design of the brushed DC motor is quite simple. A permanent magnetic field is created in the stator by either of two means:
    • Permanent magnets
    • Electro-magnetic windings
    If the field is created by permanent magnets, the motor is said to be a "permanent magnet DC motor" (PMDC). If created by electromagnetic windings, the motor is often said to be a "shunt wound DC motor" (SWDC). Today, because of cost-effectiveness and reliability, the PMDC motor is the motor of choice for applications involving fractional horsepower DC motors, as well as most applications up to about three horsepower.

    At five horsepower and greater, various forms of the shunt wound DC motor are most commonly used. This is because the electromagnetic windings are more cost effective than permanent magnets in this power range.

    Caution: If a DC motor suffers a loss of field (if for example, the field power connections are broken), the DC motor will immediately begin to accelerate to the top speed which the loading will allow. This can result in the motor flying apart if the motor is lightly loaded. The possible loss of field must be accounted for, particularly with shunt wound DC motors.

    Opposing the stator field is the armature field, which is generated by a changing electromagnetic flux coming from windings located on the rotor. The magnetic poles of the armature field will attempt to line up with the opposite magnetic poles generated by the stator field. If we stopped the design at this point, the motor would spin until the poles were opposite one another, settle into place, and then stop -- which would make a pretty useless motor!

    However, we are smarter than that. The section of the rotor where the electricity enters the rotor windings is called the commutator. The electricity is carried between the rotor and the stator by conductive graphite-copper brushes (mounted on the rotor) which contact rings on stator. Imagine power is supplied:

    The motor rotates toward the pole alignment point. Just as the motor would get to this point, the brushes jump across a gap in the stator rings. Momentum carries the motor forward over this gap. When the brushes get to the other side of the gap, they contact the stator rings again and -- the polarity of the voltage is reversed in this set of rings! The motor begins accelerating again, this time trying to get to the opposite set of poles. (The momentum has carried the motor past the original pole alignment point.) This continues as the motor rotates.

    In most DC motors, several sets of windings or permanent magnets are present to smooth out the motion.

    Easy to control speed

    Controlling the speed of a brushed DC motor is simple. The higher the armature voltage, the faster the rotation. This relationship is linear to the motor's maximum speed.

    The maximum armature voltage which corresponds to a motor's rated speed (these motors are usually given a rated speed and a maximum speed, such as 1750/2000 rpm) are available in certain standard voltages, which roughly increase in conjuntion with horsepower. Thus, the smallest industrial motors are rated 90 VDC and 180 VDC. Larger units are rated at 250 VDC and sometimes higher.

    Specialty motors for use in mobile applications are rated 12, 24, or 48 VDC. Other tiny motors may be rated 5 VDC.

    Most industrial DC motors will operate reliably over a speed range of about 20:1 -- down to about 5-7% of base speed. This is much better performance than the comparible AC motor. This is partly due to the simplicity of control, but is also partly due to the fact that most industrial DC motors are designed with variable speed operation in mind, and have added heat dissipation features which allow lower operating speeds.

    Easy to control torque

    In a brushed DC motor, torque control is also simple, since output torque is proportional to current. If you limit the current, you have just limited the torque which the motor can achieve. This makes this motor ideal for delicate applications such as textile manufacturing.

    Simple, cheap drive design

    The result of this design is that variable speed or variable torque electronics are easy to design and manufacture. Varying the speed of a brushed DC motor requires little more than a large enough potentiometer. In practice, these have been replaced for all but sub-fractional horsepower applications by the SCR and PWM drives, which offer relatively precisely control voltage and current. Common DC drives are available at the low end (up to 2 horsepower) for under US$100 -- and sometimes under US$50 if precision is not important.

    Large DC drives are available up to hundreds of horsepower. However, over about 10 horsepower careful consideration should be given to the price/performance tradeoffs with AC inverter systems, since the AC systems show a price advantage in the larger systems. (But they may not be capable of the application's performance requirments).

    Disadvantages

    • Expensive to produce
    • Can't reliably control at lowest speeds
    • Physically larger
    • High maintenance
    • Dust

    Special type motors

    • Brushless DC Motors
    • Servo Motors
    • Brushed DC Servo Motors
    • Brushless AC Servo Motors
    • Stepper Motors
    • Linear Motors
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    Electric motor basic

    Inside an Electric Motor

    An el ectric motor is all about magnets and magnetism: A motor uses magnets to create motion. If you have ever played with magnets you know about the fundamental law of all magnets: Opposites attract and likes repel. So if you have two bar magnets with their ends marked "north" and "south," then the north end of one magnet will attract the south end of the other. On the other hand, the north end of one magnet will repel the north end of the other (and similarly, south will repel south). Inside an electric motor, these attracting and repelling forces create rotational motion.

    Parts of an electric motor.


    In the above diagram, you can see two magnets in the motor: The armature (or rotor) is an electromagnet, while the field magnet is a permanent magnet (the field magnet could be an electromagnet as well, but in most small motors it isn't in order to save power).
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