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).

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.

Polyphase Motors (Two or Three Phases)

The production of a rotating magnetic field can be simply illustrated by considering a two-phase motor with two embedded stator windings for establishing the magnetic fields. Each coil, for simplicity, shall consist of a single loop of wire connected to one phase of a two-phase AC supply. We shall refer to the coil supplied by phase 1 current as Coil 1, and the coil supplied by phase 2 current as Coil 2. The two coils are placed at a right angle to each other in the stator core, with each coil creating a two-pole field. See Fig. 2-1.

The output waveform of the two-phase AC supply is represented in Fig. 2-2. The voltage in each phase varies sinusoidally in time and one lags the other by π/2 radians or 90° (electrical). Note: one complete cycle = 2π radians or 360° (electrical).

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.

Now we have two perpendicular fields. Each varies sinusoidally in time, and one lags the other by π/2 radians. The combined effect (vector sum) of the two fields is a rotating resultant field. Figure 2-6 illustrates the progression of the rotation at eight different points in time. The letters (A-H) in Fig. 2-6 correspond to the points (A-H) on the waveform diagram in Fig. 2-2.

It can also be shown mathematically that the magnetic field rotates. If we choose the center of the stator as our reference point, we can define B_Y and B_X as the magnitudes of the magnetic flux densities due to the currents flowing through Coil 1 and Coil 2 respectively. Both B_Y and B_X are functions of their respective currents* and are functions of time. (*Note: This assumes a constant permeability in the ferromagnetic structure). Also, due to symmetry, their peak values are the same.

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

B_Y = COS (2π ft)
B_X = SIN (2π ft)
Where:
B = peak value of either B_Y or B_X f = frequency of the supply current (cycles/unit time) t = time

Let Br be the resultant value of B_Y and B_X and let Θ be the angle of B_r with respect to the axis as shown in Fig. 2-7. For example:

or

Θ = (2π ft)

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.

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)

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.

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

Variable Speed, Three-Phase Gearmotors, Motors and matching Speed Controls

Bodine Electric’s Pacesetter™, variable-speed AC three-phase motors and gearmotors are rated from 1/25 - 3/4 HP, 230 VAC or 230/460 VAC, 60 Hz. All feature Bodine’s Quintsulation™ 5-stage insulation system, which meets NEMA MG 1-1993, Section IV, Part 31. This insulation protects the motor from potential spikes or corona damage caused by the inverter. Pacesetter inverter duty gearmotors are UL recognized for construction and cURus or UR/CSA marked. In addition, these products are in compliance with the Low Voltage Directive “CE”, and all products meet the European RoHS Directive. Most Bodine 34R, 42R and 48R frame inverter duty gearmotors and motors are now available with optional 230/460 VAC, 60 Hz windings.

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.

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.

Split-Phase (Non-synchronous)

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

 

 

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.

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.

 

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

 

Design and Operation: Capacitor action described in the Bodine Handbook, chapter one, has been found to provide specific performance improvements when used with single-phase AC motors and gearmotors (Fig. 2-14). The types of capacitors (see Fig. 2-15.) used and the method of operation varies with motor type. The operating characteristics of each type are quite different and will be discussed separately. In general, there are three distinct capacitor motor types:

1. Capacitor Start (CS) — motors use one capacitor in the starting mode only,
2. Permanent Split Capacitor (PSC) — motors may operate with one permanently-connected, continuous-duty AC-type capacitor for both starting and running, and
3. Two Capacitor Start/One Capacitor Run — motors use one continuous-duty capacitor and one capacitor in the start mode only, and then switch out the start capacitor while running.

The addition of a capacitor, in series with the “start” winding, can significantly enhance the starting characteristics by improving the phase relationship between the “run” and the “start” windings. With the proper selection of capacitor value, the starting torque can be increased and/or the starting current decreased. Of course, capacitor values must be carefully selected to produce this effect. Because the CS motor’s capacitor is used only when starting, its duty cycle is very intermittent. Thus, a relatively small AC electrolytic-type capacitor can be used in CS designs.

Permanent Split Capacitor (PSC): When split-phase or capacitor start (CS) motors are applied in applications that require long or frequent starts, the motor may tend to overheat and adversely affect the system reliability. In this type of application, PSC motors and gearmotors should be considered.

The PSC capacitor winding is permanently connected in series with a continuous-duty “run” capacitor. In contrast to the split-phase or capacitor start motor, the “second” winding is energized at all times.

Permanent split capacitor motors operate in much the same way as two-phase AC motors. The capacitor in the PSC design causes the current in the capacitor winding to be out of phase (with respect to time) with the current in the main winding, thus a rotating magnetic field is created. This action gives the PSC motor greater efficiency and quieter, generally smoother operation than the split-phase or the split-phase capacitor start designs. See Fig. 2-16.

 

Shaded Pole Motors (Non-synchronous)

Features:

 

• Continuous duty
• Low efficiency
• AC power supply
• Relatively constant speed
• Low starting torque (only 50% to 80% of rated torque)
• Low starting current
• Unidirectional (non-reversible)

 

 

Advantages: The shaded pole motor is simple in design and construction, making it readily adaptable to high-volume, low-cost production. Because there are no internal switches, brushes or special parts, motor of this type can be extremely dependable. Depending upon construction, shaded pole motors are relatively quiet and free from vibration. Shaded pole designs are normally available in sized from subfractional to approximately 1/4 hp (186 W).

The shaded pole motor is classified as a relatively constant speed machine, and running efficiency will increase with load. Variation in applied load will not significantly affect motor speed, providing that the motor is not overloaded. See Fig. 2-19.

Normal shaded pole designs also offer the "fail-safe" feature of staring in only one direction. With split-phase and capacitor start motors, there is always the remote possibility that they may start in reverse in some failure modes (cutout switch doesn't operate, open winding, etc.).

 

The phase relationship between the poles of the rotating field and the rotor is known as the coupling angle, expressed in mechanical degrees. This coupling angle is not rigid, but will "increase" with an increase in load. At no load, the rotor poles will line up with the field poles and the coupling angle is considered to be zero.

When a load is applied to reluctance synchronous motors, the magnetic lines of force coupling the rotor to the stator field are stretched, increasing the coupling angle. If the load is increased beyond the motor’s capability, the magnetic coupling between the rotor poles and stator field will break, and the rotor will “pull out” of synchronism. “Pull-out” torque is defined as the maximum torque the motor can deliver at synchronous speed.

Reluctance synchronous motors may be designed for polyphase operation, as well as single-phase versions in splitphase, CS and PSC configurations. These motors have characteristics comparable to their non-synchronous counterparts using the same types of stator windings. For comparable output in a given frame size, the poly-phase or PSC reluctance synchronous motor will provide quieter operation and more nearly uniform angular velocity than the split-phase or CS reluctance synchronous motor. As shown in Fig. 2-20, the reluctance rotor can be skewed to improve smoothness of operation.

 

Hysteresis Synchronous: Although the stator in a hysteresis synchronous design is wound much like that of the conventional squirrel cage motor, its rotor is made of a heat-treated cast permanent magnet alloy cylinder (with a nonmagnetic support) securely mounted to the shaft. The motor’s special performance characteristics are associated with its rotor design. The rotor starts on the hysteresis principle and accelerates at a fairly constant rate until it reaches the synchronous speed of the rotating field.

Instead of the permanently fixed poles found in the rotor of the reluctance synchronous design, hysteresis rotor poles are “induced” by the rotating magnetic field. During the acceleration period, the stator field will rotate at a speed faster than the rotor, and the poles which it induces in the rotor will shift around its periphery. When the rotor speed reaches that of the rotating stator field, the rotor poles will take up a fixed position.

Like the reluctance synchronous motor, the coupling angle in hysteresis motors is not rigid, and if the load is increased beyond the capacity of the motor, the poles on the periphery of the rotor core will shift. If the load is then reduced to the “pullin” capacity of the motor, the poles will take up fixed positions until the motor is again overloaded or stopped and restarted.

The hysteresis rotor will “lock-in” at any position, in contrast to the reluctance rotor which has only the “lock-in” points corresponding to the salient poles on the rotor.