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Chapter 4. Three-Phase Motors. Three-Phase Motor Construction • Operating Principles • Motor Load and Torque • Motor Power • Motor Efficiency. The stator consists of a core and windings and is enclosed within a housing.
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Chapter 4 Three-Phase Motors Three-Phase Motor Construction • Operating Principles • Motor Load and Torque • Motor Power • Motor Efficiency
The stator consists of a core and windings and is enclosed within a housing.
Many individual laminated sheets are pressed together into the housing, becoming the stator frame.
A 3-phase stator is wound with coils that are connected to produce the three separate phases, A, B, and C.
During motor manufacture, an insulating material called slot paper is first laid in the slot to provide protection and electrical insulation.
Varnish is sanded from the bore to allow for a minimal air gap between the stator and rotor.
Feet are attached to the housing to provide a method of mounting the motor to a base.
The rotor core consists of many thin iron sheets laminated together.
The end of the shaft is machined with a keyway to contain a bar-type key.
It takes 720 electrical degrees, or two electrical cycles, to complete one revolution in a 4-pole motor.
Inductive reactance increases with increasing frequency and decreases with decreasing frequency.
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.
The motor nameplate typically has a wiring diagram depicting the proper wiring connec-tions for the desired operation.
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.
As the current changes, the stator poles move to follow the strongest current.
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.
As the current changes, the stator poles move to follow the strongest current.
Each phase coil (A, B, and C) is divided into two equal parts and the coils are connected in a standard wye connection.
Each phase coil (A, B, and C) is divided into two equal parts and the coils are connected in a standard delta connection.
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.
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 four most common types of torque related to motors are locked-rotor torque, full-load torque, pull-up torque, and breakdown torque.
The torque-speed characteristic of a motor must match the load the motor is to drive.
Constant-horsepower motors are used to drive loads that require the same horsepower output at different speeds.
Constant-torque motors are used to drive loads that require a constant torque output at different speeds.
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.
True power can be pro-duced only when current and voltage are both are positive or both negative.
For circuits with mixed inductive and resistive components, the current lags the voltage by a value between 0° and 90°.
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.
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.
