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Overview of Motor Types Tutorial

The information on this page is to assist you in selecting motor systems for common industrial applications. Please be sure to use good engineering practices in your designs.

Copyright 1996, 1997 by Brian L. Boley. All Rights Reserved.


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

    The design of the brushed DC motor is quite simple. A permanent magnetic field is created in the stator by either of two means: 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

    Sorry, this portion of the document is still being developed.

    Brushless DC Motors

    Sorry, this portion of the document is still being developed.

    Servo Motors

    Sorry, this portion of the document is still being developed.

    Brushed DC Servo Motors

    Sorry, this portion of the document is still being developed.

    Brushless AC Servo Motors

    Sorry, this portion of the document is still being developed.

    Stepper Motors

    Sorry, this portion of the document is still being developed.

    Linear Motors

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