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Motor Fundamentals

Special Motors

Most applications, if studied carefully, will have parameters that will be satisfied more effectively by one type of motor. There are other criteria such as continuous operation at very slow speeds, short duty cycles or high torque requirements within a limited mounting space, to name just a few, that can place very unusual demands on fractional horsepower motors.

To meet these unique design criteria, motor manufacturers have developed a variety of special purpose motors that exceed the specifications of many common motor designs. In this Chapter we will take a brief look at some of these special purpose motors.
For low speed drive applications, electric motor manufacturers have developed compact and efficient integral gearheads. When coupled with common fractional horsepower (fhp) electric motors, these gearheads provide speedreducing/ torque-multiplying units of exceptional reliability. In any application which requires shaft speeds slower than that of a “straight” motor, fhp gearmotors can be a highly desirable alternative to conventional belts, gears and chains.

Gearmotors free the original equipment manufacturer of the burden of designing external reduction devices. They also offer original equipment designers a highlyengineered, field-tested, single-source drive system.

Because gearmotors are rated and selected based on both the motor specifications and the gearhead specifications, they present a
unique situation.
Some applications require high torque combined with rapid stop and start characteristics. Low speed AC synchronous motors are appropriate for applications which require six or more starts per minute. Since the motor has no significant current rise on starting, there is no additional heat rise with repeated starts.

Unlike gearmotors, there is no backlash associated with low speed synchronous motors. As a result, they are used in place of gearmotors in some applications. Most low speed synchronous motors are designed to start typically within 1.5 cycles of the applied frequency. Most low speed synchronous motors will reach full synchronous speed within 5 to 30 milliseconds. See Figure below.
Typical starting characteristics for a low speed AC synchronous motor.
Because of their rapid start characteristics, careful attention must be given to inertial loads especially if the load is to be coupled directly to the motor shaft. As inertia is increased beyond a certain value, the available torque decreases. This inertia is defined by the “knee” in the torque vs. inertia curve shown below.
Typical torque vs. inertia curves for a low speed synchronous motor.
Also, operation with less than minimum inertia can sometimes result in objectionable startup noise or reduced available torque. The
use of gearing can increase the ability of these motors to move inertial loads. Speed change gearing produces reflected load inertia in proportion to the square of the gear ratio. For example, a 2 to 1 reduction from 72 RPM at the motor to 36 RPM at the load reduces reflected inertia 4 to 1, and conversely, an increase of speed at the load to 144 RPM increases reflected inertia 4 to 1.

Resilient couplings can be used in applications with high inertial loads to provide some free shaft rotation so the motor can start the load. A resilient coupling should provide approximately five degrees of rotational freedom before full load is applied. Standard coupling means include rubber components, timing belts and slack chains. On the other hand, adding a resilient coupling in an application, with less than the minimum rated system inertia connected to the motor, may reduce the available torque.

Low speed synchronous motors can usually withstand stalls without overheating since the starting, full load and no-load currents are essentially the same. However, the motor will vibrate in prolonged stalled conditions against a solid stop, which could cause bearing damage over a period of time. The stall torque cannot be measured in the conventional manner, because there is no average torque delivered when the rotor is not in synchronization with the apparent rotation of the stator magnetic field.

Low speed AC synchronous motors decelerate faster than conventional motors, usually stopping within a range of 5° to 15° after turn-off with no external inertia, depending on the RPM rating of the motor. Application of DC to one or both motor windings after removal of AC can produce deceleration times one-tenth to one-twentieth of those attainable with a conventional motor. The motor position remains electrically locked after stopping.

Torque motors are a variation on conventional induction and DC type motors. They are designed for duty in slow speed and tensioning applications. Not only will they deliver maximum torque under stalled or “locked rotor” conditions, but torque
machines can maintain a “stall” for prolonged periods, allowing for the controlled tension essential in such applications
as spooling and tape drives.

Torque motors are especially useful in three general classes of operation:

  1. Motor stalled with no rotation required. Torque motors will operate like a spring in applications which require tension or pressure. They can be easily controlled to change the amount and direction of force applied.
  2. Motor shaft to rotate only a few degrees or a few revolutions to perform its function. Torque motors may be used to open or close a switch, valve or clamping device. In this sense, they are used as “actuators.”
  3. The shaft must rotate a major portion or all of the cycle at a speed much lower than that of a conventional motor. Spooling and reel drives may require torque motor characteristics. Reel drives may also call for slow speeds during the “playback” mode, and higher speeds for short periods in a rewind or “searching” phase.

AC torque motors are normally of the permanent split capacitor (PSC) or polyphase induction type. See Fig.below. Brush-type motors may also be designed to operate as torque motors. This would include shunt and permanent magnet designs which run on DC as well as series wound torque motors capable of running on either AC or DC supplies.

Typical AC torque motor.
Torque motor characteristics are usually obtained by “deviating” from conventional stator winding, rotor winding (squirrel cage), rotor lamination and air gap designs. The Figure below shows the substantially different speed / torque curves achieved in one basic motor design (frame) by changing one or more of the above-mentioned design parameters.

Curve A is a motor designed for low slip, high output running performance and a high breakdown of torque. By changing one parameter, we can get performance characteristics indicated by curve B. By making additional parameter changes, we can obtain the characteristics shown in curve C, which is very nearly a straight line (curve C approaches the “ideal” for torque motor service).

Because there is a reduction in the power input, giving the motor prolonged stall capability, the locked rotor torque in curve C must be lower than that in the other two curves. It is common practice to operate torque motors at different levels of power input in applications which have wide variations in torque demand. For example, in tape reel drives, high speed is needed for fast rewind while relatively low speeds are required for recording and playback.
Torque motor design vs. high
and low slip motor design
Reduced output is usually obtained by reducing the voltage applied to the motor. This may be accomplished by a variable ratio transformer, saturable reactor, silicon controlled rectifier (SCR) supply, or in the case of small motors, by a series resistor. The output of a torque motor will be affected by voltage change in the same way as conventional motors — by the square of the voltage, as shown in the figure below. While the curves(shown below)are for a voltage reduction across the entire motor winding, it is sometimes advisable to reduce only the voltage across the main winding of a PSC motor. This keeps the full line voltage on the capacitor and capacitor winding combination so that torque stability at extremely low operating speeds can be maintained. When connected in this manner, the torque can be varied approximately in proportion to voltage.
Family of speed / torque curves
for various input voltages

Many torque applications require that the motor be driven against the normal rotation of its rotating field during a portion of each cycle. The reverse rotation (resisting) torque is normally never greater than stalled torque and will decrease slightly as the reverse speed increases from zero.

A typical tape reel application can be used to demonstrate this requirement. When a tape is being wound from one reel to another, resisting torque is necessary on one reel motor to provide tape tension. The voltage is reduced on the motor that resists being pulled against its normal rotation to provide the desired tension on the tape.

There are several specific differences in rating concepts between conventional induction motors and their torque motor counterparts. An understanding of these differences is essential for proper application. In contrast to ordinary induction motors, torque motor input and output are considered at locked rotor rather than operating speed. While output is normally expressed as horsepower or watts, torque motor output is described as torque (ounce-inches, ounce-feet, pound-feet or newton-meters).

The speed rating of a torque motor is either its “no-load” speed or the theoretical synchronous speed if the motor is an induction

Duty cycle ratings of torque motors are also important, and should include two factors:

  1. the percentage of the duty cycle during which the motor may be “stalled” at rated voltage, and
  2. the maximum time duration of the stall.

For example, if a motor has a 40% duty and 30 minute time rating, the motor can be stalled for 40% of the entire duty cycle, and the continuous stalled time cannot exceed 30 minutes out of a 75 minute duty cycle.

During the remaining 45 minutes, the motor must be de-energized to permit the heat generated during the stalled period to

Of course, the duty cycle of this motor could have many other variations. If the stalled time was only three minutes, the total cycle could be as short as 7.5 minutes (the motor will be de-energized for 4.5 minutes). A motor designed with a torque sufficiently low to permit continuous stall, and not exceed the maximum acceptable temperature, would be rated 100% duty and a time rating would be unnecessary.

In general, the best torque-to-watt ratio is obtained in low speed induction motors (six or more poles). The relationship of motor poles to torque and speed is shownbelow. Having no commutator or brushes, induction motors are rugged and require a minimum of service. The permanent split capacitor (PSC) motor is by far the most popular in fractional and subfractional sizes. It operates on single-phase AC and has a torque-output-to-watt input ratio that compares favorably with the polyphase motor under locked rotor conditions.

Speed vs. torque for various
numbers of stator poles.
Another advantage of the PSC motor as a torque motor is that it can be designed with a three-wire reversible winding which will permit it to be stopped, started and reversed by a simple single pole / double throw switch. The shaded pole design may satisfy some torque motor applications, but its torque-to-watt ratio is low, and it cannot be reversed.

While the output of a torque motor is usually taken from the rotor shaft directly, the motor may have a speed reducing gearhead through which the torque is increased by the mechanical advantage of the ratio minus the losses in gearing. When a gearmotor is being considered, the gearing type and ratio are very important and must be chosen with care. This is especially true if part of the motor’s function requires it to be driven by the load, or if the operation requires the motor and load to be brought to rest by bumping a rigid stop. The mechanical parts in a gearhead must be able to withstand the shocks and stresses imposed by the application.

Since the torque motor operates either under a stalled condition or at speeds too low to provide self-ventilation, it is important that a motor with a maximum torqueto- watt ratio be used that will also satisfy all of the other requirements of the application. If the operating temperature of the torque motor chosen for an application exceeds safe limits, and there is no available space to accommodate a larger motor, the problem may be overcome by providing additional cooling with a low cost, motor-blower unit. The use of the smaller
torque motor (with the blower addition) may even result in a cost savings over the use of a larger motor.

A “fail-safe” brake may also be used to reduce temperature in torque motor applications. This would be applicable in cases where the motor must lift a load to a specific location and hold it for an extended period. The brake, connected in parallel with the motor, would be applied by spring pressure when power is removed from the motor. This action will keep the load in position without any heat being generated.


From the above discussion it is apparent that most torque motor applications require the use of a sample motor for tests in the machine before determining final specifications for the optimum motor. Answers to the nine questions below should give the motor manufacturer enough information to supply a sample that is close to “ideal.” The customer could then adjust the voltage to the sample to obtain the desired performance with minimum input power. Temperature tests should also be performed in the equipment
under actual or simulated duty conditions. Consultation with the motor manufacturer should determine whether modifications or
resizing will be necessary.

Criteria for determining torque motor applications are:

  1. What is the available power (voltage, AC or DC, phase and frequency)?
  2. What is the torque requirement and duty cycle?
  3. What are the minimum and maximum speeds and how long will the motor operate at the various speeds?
  4. Will the motor be driven by the load at any time in the cycle?
  5. Is a brake or clutch to be used in the drive mechanism?
  6. Will the motor and load be brought to rest by bumping a rigid stop?
  7. What mounting space is available?
  8. Is surrounding air free of dust and contaminants or should the motor be enclosed to protect against pollutants?
  9. What is the ambient temperature?
The switched reluctance motor is a type of synchronous reluctance motor. The stator and rotor resemble that of a variable
reluctance step motors. See Figure below. The stator of a switched reluctance motor may have three or four phases as does the
step motor. There are no coils on the rotor which eliminates the need for slip rings, commutators and brushes. Both the stator
and the rotor of a switched reluctance motor have salient poles.
Typical switched reluctance
motor design.
The rotor is aligned when the diametrically opposed stator poles are energized. Two of the rotor poles will align to the stator poles. The other rotor poles will be out of alignment with the remaining stator poles. When the next stator pole pair is energized in sequence, they attract the two rotor poles that are out of alignment. By sequentially switching the current from one stator winding to the next, the rotor continually rotates in a kind of “catch-up” mode trying to align itself with the appropriate minimum reluctance position of the energized stator windings — thus the term, “switched reluctance.”

The switched reluctance motor provides inherent characteristics and control functions that are directly equivalent to DC servo motors. The torque of the switched reluctance motor is equal to the square of the current giving it excellent starting torque. Motor direction can be reversed by changing the current switching sequence in the stator windings. Like their DC counterparts, the brushless design of switched reluctance motors simplifies maintenance.

Switched reluctance motors cannot be operated directly from a three-phase AC supply or a DC source. They require a controller which adds to their cost. They are also capable of four quadrant operation, that is, both speed and torque are controllable in both negative and positive directions.

Switched reluctance motors can achieve very high speeds which may be limited only by the type of bearings used. This makes
them ideal for high speed applications. Ironically, their high speed operation causes considerable noise which is one of their
Conventional rotary motors require some type of rotary-to-linear mechanical converter (lead screw, rack and pinion, etc.) if they are used in applications where the final stage results in linear motion. The most obvious advantage of linear induction motors (LIMs) is that they produce linear motion directly without the need of a transmission or conversion stage.

The operation of linear induction motors can be more easily understood if we start with a conventional rotary squirrel cage motor, cut the stator and rotor along a radial plane and roll them out flat. See Figure below. The rotor equivalent of the linear motor
is called the secondary and the stator equivalent is called the primary.
Basic linear motor construction
The figure below shows that the primary consists of a core and windings (multiple phases) which carry current and produce a
sweeping magnetic field along the length of the motor. The secondary can be a sheet, plate or other metallic substance. Linear
motors can have single or dual primaries. The sweeping action induces currents in the secondary and thus creates thrust in a given
direction depending on the direction of current flow.
Thrust developed by single (left) and dual (right) primary linear motors.
In contrast to a rotary motor, either element can be the moving element in a linear motor. LIMs can have a fixed primary and moving secondary or vice versa. This adds to their flexibility in a wide range of applications. The secondary and primary are separated by a small air gap, typically 0.015 to 0.045 inches. This gap is maintained by using bearings, wheels or magnetic levitation.

The flat primary can be rolled in the transverse direction creating a cylinder into which a tube or rod-type secondary can be inserted. See Figure below. This is referred to as a tubular or round rod linear motor. An advantage of this type of linear motor is that it has no end connections and can be operated either horizontally or vertically.
Tubular or round rod LIM.
One of the factors that determines LIM performance is the pitch-to-gap ratio of the primary coils. It affects the input power delivered to the secondary and the harmonic content of the sweeping magnetic flux. In general, a larger ratio translates into better performance since it means less harmonics. Flat LIMs are usually more efficient than tubular LIMs.

The maximum speed of a LIM is directly proportional to the operating frequency and the pitch-to-gap ratio. Speed is varied by using a variable frequency controller.

LIMs are ideal for applications such as computer plotters and read head positioning units, drapery openers, X-ray camera positioning and a wide variety of conveyor applications.
Servo motors are available in both DC and AC types. Servo motors are an integral part of a closed-loop feedback control system consisting of the motor, an amplifier which drives the motor, an actuator and a feedback device.

A block diagram for a closed-loop system using a servo motor is shown below. Any change in a system’s load, amplifier gain or other variable will cause the output of the system to deviate from the expected response. In the closed-loop system, these variations in output are monitored, fed back and compared to a reference or desired input. The difference between the reference and the measured output signal is a deviation. The deviation is amplified and used to correct the error. Therefore, the closed-loop system is selfcorrecting.
Block diagram of a closed-loop control system.

Although servo motors show the basic performance characteristics of the motor classes to which they belong (AC induction, PM DC, etc.), they incorporate special design features which make them uniquely suited to applications involving feedback control. Because servo motors must be sensitive to a relatively small control signal, their designs stress reaction to small voltage variations, especially at starting.

Both DC and AC servo motors possess two fundamental characteristics:

  1. the output torque of the motor is roughly proportional to the applied control voltage (which the drive amplifier develops in response to an error signal), and
  2. the instantaneous polarity of the control voltage determines the torque direction.

AC servo motors are used in the 1/ 1500 to 1/8 hp ranges. Beyond this range AC motors become very inefficient and difficult to cool. DC servo motors are usually used in higher hp ranges.

Direct-Drive Servo Motors:
In applications where precise positioning and speed control is required, a directdrive servo motor is often employed. Direct- drive servo motors allow the load to be directly coupled to the motor which eliminates backlash and wear associated with other coupling arrangements. Directdrive servo motors are capable of achieving fast acceleration and have excellent response times.

Direct-drive servo motors are usually brushless and provide all of the advantages of brushless technology. They may also have built-in resolvers which provide precise position monitoring and feedback control. Position accuracy in the range of 30 arc seconds is typical.
While hardly a new idea (patents were granted for shell-type armature designs near the turn of the century), shell-type armature motors have benefited tremendously from advances in polymer resin technology. While early armatures were bonded with metal strapping (which contributed to large eddy current losses), more recent shell-type designs make use of a variety of bonding methods which do not contribute significantly to motor inertia. These innovations have combined to produce motors with extremely low inertia and high acceleration —characteristics which are useful in many servo applications.

Shell-type armature motors operate in much the same way as conventional permanent magnet motors, with an oriented PM field and commutation by spring-loaded brushes. The feature that makes shell armature motors unique is the hollow cylindrical armature composed of a series of aluminum or copper coils (“skeins”) bonded together in polymer resin and fiberglass to form a rigid, “ironless,” shell. See Figure below. Because the armature has no iron core, it has very low inertia and rotates in an air gap with very high flux density.
Basic construction of a shell-type armature motor.
The unusual design characteristics of the shell-type armature motor contribute to low inductance and low electrical time constant
(less than 0.1 millisecond). The absence of rotating iron in the shell-type armature motor results in a very high torqueto- inertia ratio, producing high acceleration and quick response required in many positioning servo and incremental motion applications. The Figure below shows the typical speed / torque curves for a shell-type armature motor.
Typical speed / torque curves
for a shell-type armature motor.
The principal disadvantage to shell-type armature designs is their thermal time constant (typically 20-30 seconds for armature, and 30-60 minutes for housing). Without proper cooling and/or sophisticated control circuitry, the armature could be heated without warning to destructive temperatures in a matter of seconds during an overload condition.

Another difficulty is the tendency for shell-type motors to exhibit audio noise and output shaft “whip” at high speeds. Like printed circuit motors, shell-type armature motors are of somewhat fragile construction and should be operated in a more or less controlled environment. Furthermore, due to the manufacturing techniques and degree of application engineering required for this type of motor, they are relatively expensive and tend to be employed only where their unique performance characteristics are required.
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