AC Induction Gearmotors & Motors

Although commutator and brush assemblies may be used in some types of alternating current (AC) gearmotors and motors (series/universal wound), brushless induction-type designs are by far the most common and most reliable for industrial AC motors and gearmotors that operate from an AC power source or from an AC control (adjustable speed drive).

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AC Motor Action

In an AC induction motor or gearmotor, the stator winding sets up a magnetic field which reacts with the current-carrying conductors of the rotor to produce rotational torques. The rotor currents are induced in the rotor conductors by the stator’s changing magnetic field, rather than by means of a commutator and brushes. This induction action is the central operating principle of AC induction motors.

AC power is commercially supplied in both single-phase and three-phase forms. The essential operating characteristics of AC induction motors and gearmotors will vary according to:

1. winding types (split-phase, shaded-pole, three-phase, etc.), and
2. the number of phases, the frequency, and the voltage of the power source.

Fig. 2-1: Simplified diagram of a two-phase AC motor (left), and a cross section of a two-phase AC motor showing phase 1 and phase 2 windings (right).

Polyphase Motors (Two or Three Phases)

AC power is commercially supplied in both single-phase and three-phase forms. The essential operating characteristics of AC induction motors and gearmotors will vary according to:

1. winding types (split-phase, shaded-pole, three-phase, etc.), and
2. the number of phases, the frequency, and the voltage of the power source.

Fig. 2-1: Simplified diagram of a two-phase AC motor (left), and a cross section of a two-phase AC motor showing phase 1 and phase 2 windings (right).

Let us first consider Coil 1 only. When the phase 1 current is in its positive portion of the cycle (current enters Coil 1 from the right and exits on the left), a magnetic field is set up which points in the positive (+Y) direction. See Fig. 2-3. When the current flows in the opposite direction during the negative portion of its cycle, the magnetic field points in the negative (-Y) direction. See Fig. 2-4. Since the strength of the magnetic field (H) is proportional to the amount of current flowing through the coil, the field strength also oscillates sinusoidally in time.

Similarly, we can illustrate in Figs. 2-5a and 2-5b the magnetic field due to current flowing in Coil 2.

Fig. 2-6a AC motor stator

Similarly, we can illustrate in Figs. 2-5a and 2-5b the magnetic field due to current flowing in Coil 2.

Since BY and BX vary sinusoidally with their corresponding currents we can express them in the following equations:

Fig. 2-6: Progression of the magnetic field in a two-phase stator at eight different instants

Fig. 2-7: The vector sum of BY and BX is resultant field Br

Hence, Θ is increasing at a rate of 2π f radians per unit time. In other words, Br is rotating with the same frequency as the supply current. We can also show that the magnitude of Br remains constant during rotation, since:

B_r² = B_Y² + B_X²

Since B is independent of time, the magnitude of the rotating resultant field (Br) is constant.

We have demonstrated that a rotating magnetic field is generated in a two-phase stator. These basic analyses can be extended to a three-phase stator and show that it also has a rotating field. Therefore, we will not go into detail with three-phase stators.

The rotor of a typical AC induction motor is constructed from a series of steel laminations, each punched with slots or holes along its periphery. When laminations are stacked together and riveted, these holes form channels which are filled with a conductive material (usually aluminum, but also copper in newer high-efficiency designs) and short-circuited to each other by means of conducting end rings. The conductors are typically formed by die-casting.

This one-piece rotor casting can also include integral fan blades which create a built-in cooling device (for open design AC motors). The common term for this type of rotor is “squirrel cage” (because of its resemblance to the runway of an old-fashioned squirrel cage or hamster wheel). It is an inexpensive and common type of AC induction motor rotor design. See Fig. 2-8.

Fig. 2-8: Aluminum conductors in an AC induction rotor. The steel laminations have been removed to illustrate the “squirrel cage” form of the cast aluminum conductor

Fig. 2-8a: Bodine 30R-D gearmotor cutaway view.

As the rotating field sweeps past the bars in the rotor, an induced current is developed. Since the flow of current in a conductor sets up a magnetic field with a corresponding polarity, an attraction will result between the rotating magnetic field of the stator and the induced field in the rotor. Rotation results from the motor’s attempt to keep up with the rotating magnetic field. The rate of change at which the lines of flux cut the rotor determines the voltage induced. When the rotor is stationary, this voltage is at its maximum. As rotor speed increases, the current and corresponding torque decreases. At the point of synchronous speed (speed of the rotating field), the induced current and developed torque both equal zero.

The rotor in a non-synchronous AC induction motor will always operate at some speed less than synchronous unless it is aided by some supplementary driving device. This lag of the rotor behind the rotating magnetic field is called “slip”, and is expressed as a percentage of synchronous speed: (RPM = revolutions per minute)

% Slip =	synchronous speed - actual speed	x 100
	synchronous speed

In designing rotors for induction motors, the shape and dimensions of the slots have a demonstrable effect on the performance characteristics of the motor. This variation is illustrated in Fig. 2-9.

Fig. 2-9: Comparison of speed / torque characteristics for single cage (left) and double cage (right) integral hp rotor design.

Another design factor common to most squirrel cage induction rotors is the deliberate “skewing” of the slots (positioning the slots at a slight angle to the shaft) to avoid cogging action and wide variations in starting torque which may result when bars are placed parallel to the stator slots.

Inverter-Duty, Three-Phase Motors / Gearmotors

AC power is commercially supplied in both single-phase and three-phase forms. The essential operating characteristics of AC induction motors and gearmotors will vary according to:

1. winding types (split-phase, shaded-pole, three-phase, etc.), and
2. the number of phases, the frequency, and the voltage of the power source.

Fig. 2.2.1-a Variable Speed AC Motors and Gearmotors (TEFC)

Safe Operating Torque and Speed Area (SOA)

Rated torque is either the value of torque which corresponds to nameplate output power and speed at 60 Hz, or it is the maximum torque at gear strength limits (rated torque can be either motor limited or gear limited). We define SOA Torque as the maximum torque at which the motor still operates within Class F thermal limits, or as the maximum torque of a gearmotor when it is gear-limited. Continuous duty operation must be limited to the area below the SOA or gear-limited torque curves.

The SOA torque for synchronous motors is close to the pull-in torque; that is, the motor will pull out of synchronism if the required torque exceeds the SOA torque. The SOA speed of Bodine’s Pacesetter™ non-synchronous motors is BELOW rated speed. For example, Bodine model 2295, a type 34R6BFPP motor has a rated torque of 148 oz-in. at a rated speed of 1700 rpm (60 Hz), but only a SOA speed of 1572 rpm (60 Hz) at 247 oz-in. SOA torque. The higher SOA torque will be produced at the trade-off of motor speed. Starting currents for standard Pacesetter motors were measured with the motor connected to a three-phase power source. Starting currents may be different when the motors or gearmotors are operated with an inverter drive (VFD/ASD). The SOA Graphs for Bodine Electric inverter duty, three-phrase, AC induction motors and gearmotors were generated by performance-testing all standard models over the full rated speed/frequency range. The SOA graphs provide the data needed to successfully apply these variable speed AC motors and gearmotors.

Fig. 2.2.1-b: Examples of typical “SOA” Graphs.

Benefits:

Inverter duty, three-phase gearmotors offer performance improvements over comparable single-phase units. When operated with an AC speed control (inverter), the motor or gearmotor speed can be easily matched to varying application loads. Pacesetter gearmotors and motors are more efficient than their single-phase counterparts, they are more compact, and provide higher output torques in the same size package. In addition, these variable-speed AC gearmotors and motors don’t require brush replacement or brush maintenance, and the gearheads are lubricated for life.

Features:

- Quintsulation™ 5-stage insulation system designed to meet NEMA MG 1-1993, Section IV, Part 31.

- 230VAC or 230/460VAC, 60 Hz, 3-phase for operation with a wide range of inverter products.

- Inverter-Grade magnet wire and Class “F” insulation system for increased protection against spikes and corona damage caused by the inverter.

- UL recognized for construction, CSA certified, and in compliance with the Low Voltage Directive “CE”. Products comply also with the RoHS Directive.

As a true system-solution provider, Bodine Electric Company also offers several new AC speed controls (Variable Frequency Drives or Adjustable Speed Drives), including chassis type (IP-20) and enclosed (NEMA-1, -4 and NEMA-4X). When purchased as a “matched system”, Bodine customers benefit from an extended two-year warranty for the motor or gearmotor and control.

Typical Applications: Conveyor systems, food processing equipment, medical equipment and factory automation.

AC Motor Speed Controls from Bodine Electric Company

Single-Phase Motors / Gearmotors

We have demonstrated in the previous section that two-phase and three-phase induction motors will create a rotating magnetic field corresponding to excitation of the stator windings.

In the single-phase induction motor, there is only one phase active during normal running. Although it will pulse with intensity, the field established by the single-phase winding will not rotate. If a squirrel cage rotor were introduced into the air gap between the stator poles of a single-phase motor, it might vibrate intensely but would not initiate rotation. However, the rotor shaft will start to rotate in either direction if given a push.

This rotation sets up an elliptical revolving field which turns in the same direction as the rotor. The “double rotating field theory” and the “cross-field theory” explain why a single-phase motor will rotate if it is started by some means. Due to the complexity of the mathematics involved, they will not be discussed here. What is important to remember is that single-phase AC motors require an auxiliary starting scheme.

Single-Phase AC Motor Types

Single-phase motors, without the aid of a starting device, will have no inherent “starting” torque. To produce torque, some means must be employed to create a rotating field to start the rotor moving. A number of different methods are used. The particular method used determines the “motor type.” An explanation of the various types follows.

Fig. 2-10: Split-phase (non-synchronous) motor.

Single-Phase AC Motor Types

Features:

- Continuous duty
- AC power supply
- Reversibility normally at rest
- Relatively constant speed
- Starting torque 175% and up (of rated torque)
- High starting current (5 to 10 times rated current)
- No run capacitor required

Fig. 2-12: Speed-torque curve for a typical split-phase AC motor

Fig. 2-13: Example of a centrifugal cut-out mechanism used in split-phase motors.

Design and Operation: Split-phase motors are perhaps the most widely used relatively constant speed AC motors (of appreciable output) employed for driving domestic appliances. Also used for a variety of industrial applications, motors of this type are relatively simple in construction and lower in cost than most other types. Low cost, plus good efficiency, starting torque and relatively good output for a given frame size have made the split-phase AC induction motor today’s general purpose drive. See Fig. 2-10.

Split-phase motors are single-phase motors equipped with main and auxiliary windings connected in parallel (during the start cycle). The auxiliary winding shares the same slots as the main winding, but is displaced in space.

To give the design its unique starting characteristic, the auxiliary winding is wound with finer wire and fewer turns (for high resistance and low reactance) than the main winding, and the current flowing through it is substantially in phase with the line voltage. The current flowing through the main windings, because of their lower resistance and higher reactance, will tend to lag behind the line voltage in time. This lagging effect will act to “split” the single-phase of the AC power supply by causing a phase (time) displacement between the currents in the two windings.

The space and phase displacement of the main and auxiliary windings produce a rotating magnetic field which interacts with the rotor to cause it to start (begin rotating). After the split-phase motor has attained approximately 70% of rated speed, the auxiliary winding is automatically disconnected from the circuit by means of a centrifugal switch or current sensitive relay. The motor will then continue to run on the single oscillating field established by the main winding. See Figs. 2-12 and 2-13.

Advantages: Split-phase motors will operate at relatively constant speed, typically from about 1790 RPM at no load to 1725 or 1700 RPM at full load for a four-pole, 60 Hz motor.

A standard four-wire split-phase motor can be reversed at standstill or while operating at a speed low enough to ensure that the auxiliary winding is in the circuit. Split-phase motors could also be reversed at full speed if a special (external) switching device is used to connect the starting winding in the reverse direction sufficiently long to reverse the motor. This is normally not done because of the risk to burn out the starting winding during a long reversal period. Gearmotors should never be reversed at full speed. To prevent gearing damage, the gearmotor (gear train) should come to a full stop before reversing the direction of rotation!

Perhaps the most important feature associated with split-phase motors is their relatively low initial cost. The high starting torque combined with simple, reliable construction make split-phase AC motors ideal for many general purpose applications. Since the rate at which the motor can be accelerated is often a primary concern to the applications engineer, split-phase designs are often specified because of their ability to come up to speed rapidly (reaching running speeds with normal loads in a fraction of a second). Another benefit of the split-phase motor is that it does not require a run capacitor.

Application Considerations: Because of the high resistance of the starting winding, repeated starting and stopping will heat the windings (in particular, the starting winding) and result in loss of torque and possible winding damage. This is one of the reasons why it is not practical to apply split-phase motors when very frequent starts are required, or where high inertial loads must be accelerated.

Split-phase motors have a high starting current which can range from 5 to 10 times the current drawn while running. If the starting load is heavy, the wiring between the motor and the power source must be of adequate size to prevent excessive voltage drop. The low voltage conditions resulting from inadequate wire size will result in decreased motor starting torque. Frequent starts, coupled with inherent high starting current, can also adversely affect starting switch or relay life.

Cautions: The auxiliary starting winding in a split-phase motor is designed for very short duty. If it stays in the circuit for more than a few seconds, the relatively high starting current, which it draws, can cause overheating of the winding. Should this happen, a more powerful motor or a motor having different electrical characteristics should be considered.

Caution should be used when driving high inertial loads with split-phase motors. This type of load can prolong the acceleration and “hang” too long on the starting winding.

Fig. 2-14: Capacitor (non-synchronous) gearmotor.

Capacitor Motors and Gearmotors (Non-synchronous)

Features:

- Continuous duty
- AC power supply
- Reversibility (3-wire or 4-wire reversible)
- Relatively constant speed
- Starting torque 75% to 150% of rated torque
- Normal starting current (3 to 7 times rated current)
- Requires a capacitor

Fig. 2-15: Examples of AC motor run and start capacitors – a) electrolytic type (used as start capacitors for larger, >1HP, motors, and when high capacitance values are required) b) plastic type (will often fit into a motor terminal box); c) large metal can type capacitor (typically used with higher voltage windings; often mounted to the motor frame);

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