1. Chapter 4 Three-Phase Motors
Three-Phase Motor Construction • Operating Principles • Motor Load and Torque • Motor Power • Motor Efficiency

2. The stator consists of a core and windings and is enclosed within a housing.
A stator is the fixed, unmoving part of a motor, consisting of a core and windings, or coils, that converts electrical energy to the energy of a magnetic field. The stator of a three-phase motor is enclosed within a housing made of cast iron, rolled steel, or cast aluminum. See Figure 4-1. The windings are coils of wire wrapped through slots in the stator core.

3. Many individual laminated sheets are pressed together into the housing, becoming the stator frame.
The laminated sheets of the core have notches punched around their inner diameter and are stacked in the housing. When the sheets are stacked and pressed into the housing, these notches become the slots for the windings. See Figure 4-2.

4. A 3-phase stator is wound with coils that are connected to produce the three separate phases, A, B, and C.
A 3-phase stator is wound with coils that are con-nected to produce the three separate phases, A, B, and C. See Figure 4-3. Each phase must have the same number of coils, so the total number of coils must be divisible by the number 3. For example, if the total number of coils wound in the stator is 36, then each phase contains 12 coils equally divided among the poles.

5. During motor manufacture, an insulating material called slot paper is first laid in the slot to provide protection and electrical insulation.
During motor manufacture, an insulating material called slot paper is first laid in the slot to provide protection and electrical insulation. When the windings are laid in the slots, a holder made of Glastic® or fiberglass, called a topstick, is driven into the slot over each winding to hold it in place. See Figure 4-4. In some cases, a midstick is also used to hold the windings in place.

6. Varnish is sanded from the bore to allow for a minimal air gap between the stator and rotor.
When the stator is removed from the oven and cooled, the varnish is sanded from the bore of the core to allow for a minimal air gap between the stator and rotor. See Figure 4-5. The size of the air gap must be very small to prevent loss of magnetic flux between the stator and the rotor. The flux created in the stator must cross the air gap to induce current in the rotor windings. Any loss of flux affects torque, increases slip, and decreases the motor’s efficiency.

7. Feet are attached to the housing to provide a method of mounting the motor to a base.
Feet are attached to the housing to provide a method of mounting the motor to a base. See Figure 4-6. The core is pressed into the housing. The end of the housing is machined to allow precise fit of the end bell as it is attached to the housing. Close tolerances of the endbell to the housing are critical, as the rotor must be centered in the core to ensure an even air gap. The air gap is the space between the rotor and stator. The endbells contain the bearings to support the rotor, and in some types of motors, close the ends of the housing as well. Larger motors also may have cooling fins to help remove heat and a lifting hook to facilitate installation.

8. The rotor core consists of many thin iron sheets laminated together.
Like the stator, the rotor core consists of many thin iron sheets laminated together. See Figure 4-7. The rotor sheets are usually thicker than the stator sheets. Since the rotor frequency is lower than line frequency, other than at locked rotor, the induced eddy currents are much smaller than in the stator. These sheets are slightly smaller in outer diameter than the inner bore of the sheets of the stator to allow a small space for the air gap. In the most common design, the notches are punched on the outer diameter of the sheet.

9. The end of the shaft is machined with a keyway to contain a bar-type key.
The motor shaft is a cylindrical bar used to carry the revolving rotor and to transfer power from the motor to the load. The end of the shaft is machined with a keyway to contain a bar-type key or with a circular slot that contains a half-moon key. See Figure 4-8. This key prevents the coupling device from slipping on the shaft. Some smaller motors have a flat spot milled on the shaft against which a setscrew on the coupling device can be set.

10. It takes 720 electrical degrees, or two electrical cycles, to complete one revolution in a 4-pole motor.
During one rotor revolution in a 2-pole motor, the rotor passes from one north pole to a south pole and back to the original north pole, completing 360 mechanical degrees for each 360 electrical degrees. During one revolution in a 4-pole motor, a rotor passes from one north pole, through a south pole, through a north pole, through a south pole, and back to the original north pole. Since there are 180 electrical degrees per pole, it takes 720 electrical degrees, or two electrical cycles, to complete one revolution in a 4-pole motor. See Figure 4-9.

11. Inductive reactance increases with increasing frequency and decreases with decreasing frequency.
At startup when a motor is in locked rotor, the winding resistance and reactance are the only limits to current flow. Inductive reactance increases with increasing frequency and decreases with decreasing frequency. The rotor conductors have very low resistance, but have much higher reactance because of the relatively high frequency. Since the circuit is at least 10 times more reactive than resistive, it is a reactive circuit and the current lags the voltage induced in the rotor by 90 electrical degrees. See Figure 4-10.

12. The sine curves at 0 degrees show –5 A for phase A, +10 A for phase B, and –5 A for phase C. The + and – signs indicate the direction of the current and the numbers represent the magnitude of the current.
Looking at the 3-phase sine curves, an arbitrary point in time is chosen at the beginning and 0 electrical degrees is assigned to the alternating current at that point. See Figure At this beginning point, the sine waves show –5 A for phase A, +10 A for phase B, and –5 A for phase C. The + and – signs indicate the direction of the current and the numbers represent the magnitude of the current.

13. The motor nameplate typically has a wiring diagram depicting the proper wiring connec-tions for the desired operation.
Several types of motor connections must be evaluated. The stator can be wired in a wye or delta configuration by the manufacturer. In addition, many motors are dual voltage and can be wired to operate at a low voltage or a high voltage. For single-voltage motors, the stator coils may consist of two coils per pole, but the coils are internally connected by the manufacturer so that the motor can only operate at one voltage. The motor nameplate typically has a wiring diagram depicting the proper wiring connections for the desired operation. See Figure 4-12.

14. In a wye-connected, 3-phase motor, one end of each of the three phase windings is internally connected to the other phase windings. The remaining end of each phase is then brought out externally to form T1, T2, and T3.
In a wye-connected, 3-phase motor, one end of each of the three phase windings is internally connected to the other phase windings. The remaining end of each phase winding is then brought out externally to form T1, T2, and T3. When connecting to 3-phase power lines, the power lines and motor terminals are connected L1 to T1, L2 to T2, and L3 to T3. See Figure A 2-pole motor was chosen for this illus-tration for simplicity. For any other number of poles, the principles are the same, but the illustration would be more complex because there would be more coils.

15. As the current changes, the stator poles move to follow the strongest current.
The current starts to change as the sine wave rotates through the cycle and the amount of current flowing through each coil changes. As the current changes, the stator poles move to follow the strongest current. See Figure The magnitude of the current in phase A increases from –5 A toward –10 A, the current in phase B decreases from +10 A toward 0 A, and the current in phase C decreases from –5 A toward 0 A.

16. In a delta-connected, 3-phase motor, each phase is wired end-to-end to form a completely closed circuit. At each point where the phases are connected, leads are brought out externally to form T1, T2, and T3.
In a delta-connected, 3-phase motor, each phase is wired end-to-end to form a completely closed circuit. At each point where the phases are connected, leads are brought out externally to form T1, T2, and T3. See Figure T1, T2, and T3 are connected to the three power lines, with L1 connected to T1, L2 con-nected to T2, and L3 connected to T3. The 3-phase line supplying power to the motor must have the same voltage and frequency rating as the motor.

17. As the current changes, the stator poles move to follow the strongest current.
The current starts to change as the sine wave rotates through the cycle and the amount of current flowing through each coil changes. As the current changes, the stator poles move to follow the strongest current. See Figure The magnitude of the current in phase A increases from –5 A toward –10 A, the current in phase B decreases from +10 A toward 0 A, and the current in phase C decreases from –5 A toward 0 A.

18. Each phase coil (A, B, and C) is divided into two equal parts and the coils are connected in a standard wye connection.
In a dual-voltage, wye-connected, 3-phase motor, each phase coil (A, B, and C) is divided into two equal parts and the coils are connected in a standard wye connection. By dividing the phase coils in two, nine terminal leads are available. These motor leads are marked terminals one through nine (T1 to T9). See Figure 4-17.

19. Each phase coil (A, B, and C) is divided into two equal parts and the coils are connected in a standard delta connection.
In a dual-voltage, delta-connected, 3-phase motor, each phase coil (A, B, and C) is divided into two equal parts and the coils are connected in a standard delta connection. By dividing the phase coils in two, nine terminal leads are available. These motor leads are marked terminals one through nine (T1–T9). The nine terminal leads can be connected for high or low voltage. See Figure 4-18.

20. Manufacturers of dual-voltage, 3-phase motors sometimes do not make the internal connections. The internally uncon-nected motors have 12 leads coming out of the motor box labeled T10, T11, and T12. The connections are made externally by the installer.
Typically, dual-voltage, 3-phase motors have nine leads coming out of the motor box. A 9-lead, wye-connected motor and a 9-lead, delta-connected motor have internal connections made by the manufacturer. However, manufacturers of dual-voltage, 3-phase, wye-connected and delta-connected motors some-times do not make the internal connections. The internally unconnected motors have 12 leads coming out of the motor box. The three additional leads are labeled T10, T11, and T12. The connections are made externally by the installer. See Figure 4-19.

21. The direction of rotation of 3-phase motors can be reversed by interchanging any two of the 3-phase power lines to the motor.
The direction of rotation of 3-phase motors can be reversed by interchanging any two of the 3-phase power lines to the motor. See Figure Although any two lines can be interchanged, the industrial standard is to interchange T1 and T3. This standard holds true for all 3-phase motors. For example, to reverse the direction of rotation of a delta-connected, 3-phase motor, T1 and T3 are interchanged.

22. Motor power is rated in horsepower or watts.
Work in foot-pounds (ft-lb) is a scalar measurement of the linear movement and weight of an object. The power of a motor represents the rate at which the motor does work. See Figure Motor power is rated in watts or horsepower. Larger motors are rated in kilowatts (kW). A horsepower (HP) is a unit of power equal to 746 watts, 33,000 foot-pounds per minute, or 550 foot-pounds per second. If a motor raises a 550 lb weight to a height of 1¢ in 1 second (550 ft-lb per second), the motor uses 1 horsepower or 746 watts.

23. The four most common types of torque related to motors are locked-rotor torque, full-load torque, pull-up torque, and breakdown torque.
The torque in pound-feet (lb-ft) is not the same as the work in foot-pounds (ft-lb). Torque is a vector mea-sured from the axis of a pivot point. It is used to describe a rotation. The four most common types of torque related to motors are locked-rotor torque, full-load torque, pull-up torque, and break down torque. See Figure 4-22.

24. The torque-speed characteristic of a motor must match the load the motor is to drive.
The torque-speed characteristic of a motor must match the load the motor is to drive. A load may have a definite torque-speed characteristic, such as a pump or fan that has a fixed load. Or the load may have a variable torque-speed characteristic, such as a hoist or conveyor belt used to move loads of varying weights. See Figure 4-23.

25. Constant-horsepower motors are used to drive loads that require the same horsepower output at different speeds.
Motors for constant-horsepower loads are used to drive loads that require the same horsepower output at different speeds. Typical applications include most machine-tool machines, such as boring machines, drilling machines, wheel-driven grinders, lathes, and milling machines. The number of poles in this type of motor is effectively changed by changing the direction of current through the motor windings. See Figure 4-24.

26. Constant-torque motors are used to drive loads that require a constant torque output at different speeds.
Motors for constant-torque loads are used to drive loads that require a constant torque output at different speeds. Typical applications include rotary and reciprocating compressors, conveyors, displace-ment fans, and printing presses. The number of poles in the motor is effectively changed by changing the direction of current through the motor windings. See Figure 4-25.

27. Variable-torque, multiple-speed motors are used to drive fans, pumps, and blowers that require an increase in both torque and horsepower when speed is increased.
Multiple-speed motors for variable-torque loads are used to drive loads that require an increase in both torque and horsepower when speed is increased. Typical applications include fans, pumps, and blowers. The number of poles in the motor is effectively changed by changing the direction of current through the motor windings. See Figure 4-26.

28. True power can be pro-duced only when current and voltage are both are positive or both negative.
True power is the power, in W or kW, drawn by a motor that produces useful work. True power is used by the resistive part of a circuit that performs the work. True power can be produced only when current and voltage are both positive or both are negative. See Figure A resistive load consumes true power when the voltage and current are in the same direction (both positive or both negative).

29. For circuits with mixed inductive and resistive components, the current lags the voltage by a value between 0° and 90°.
Reactive power is the power, in VAR or kVAR, stored and released by the magnetic field around inductors and the electric field across capacitors. Reactive power is measured in volts-amps-reactive (VAR). In a circuit with reactive components, the voltage and current are out of phase. For purely inductive circuits, the current lags the voltage by 90 electrical degrees. For circuits with mixed inductive and resistive components, the current lags the voltage by a value between 0 electrical degrees and 90 electrical degrees. See Figure Reactive power flows through the inductor or capacitor when the voltage and current are not in the same direction (one positive and one negative).

30. Power factor correction capacitors can be placed ahead of an electric motor drive in the AC supply lines but not between the drive and motor.
A power factor correction capacitor is a capacitor used to improve a facility’s power factor by improving voltage levels, increasing system capacity, and reducing line losses. The capacitor should have the same amount of reactance as the inductor to cancel out the reactive power of the inductor. Power factor correction capacitors can be placed ahead of an electric motor drive in the AC supply lines but not between the drive and motor. Power factor correction capacitor units with automatic switching must not be used unless specifically recommended by the manufacturer. See Figure 4-29.

31. The five major components of motor energy losses are resistance losses, core losses, bearing losses, windage losses, and sound losses. These losses add up to the total loss of a motor.
There are always motor energy losses that reduce the efficiency of a motor. Any energy losses reduce the efficiency of a motor because the energy is wasted as heat and does not contribute to driving the load. Older motors typically operate at less than 80% efficiency. Newer, high-efficiency motors typically operate at more than 90% efficiency. The five major components of motor energy losses are resistance loss, core loss, bearing loss, windage loss, and sound loss. These losses add up to the total loss of a motor. See Figure 4-30.