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
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
- Low Cost
- Reliable Operation
- Easily Found Replacements
- Variety of Mounting Styles
- Many Different Environmental Enclosures
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:
- The fixed number of winding sets (known as poles) built into the motor, which determines the motor's base speed.
- The frequency of the AC line voltage. Variable speed drives change this frequency to change the speed of the motor.
- 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:
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:
Disadvantages
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
Advantages
- Easy to understand design
- Easy to control speed
- Easy to control torque
- Simple, cheap drive design
Easy to understand design
- Permanent magnets
- Electro-magnetic windings
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
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
Simple, cheap drive design
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
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