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Induction motor and syncronous motor

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Davehol

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An industrial plant is supplied by the electric utility with 3 phase, 4160 V(line to line), 60 HZ. The total plant load is 5000 kW (3 phase). Each major process in the plant now runs with an induction motor that has a power factor of 0.85 lagging. It has been suggested that one of these major process can be converted over to a synchronous motor, which would also improve the overall power factor. The proposed machine has a series reactance per phase, Xs=6 ohms. It will serve a process that requires 2000kW after installation.
a) Find the current delivered to the plant by the utility before replacement of the induction motor by the synchronous motor.
b) The synchronous motor (Y-connected) is installed and set up to draw 2000 kW (out of the same 5000 kW) and S=2750 kVA with a leading power factor. What is the current delivered to the plant now? What is the plant's overall power factor?
c) With the motor in place, drawing P=2000 kW and S=2750 kVA (leading), find the torque angle and the internal voltage magnitude Ea (line to neutral) in the machine.
 
A synchronous motor is a synchronous machine used for a motor. A synchronous motor
cannot start without being driven. They need a separate starting means.
There are several types of synchronous motors. These include direct current excited
synchronous motor (field poles are excited by direct current), a permanent magnet
synchronous motor (field excitation is provided by permanent magnets) and a reluctance
synchronous motor (starts as an induction motor, is normally provided with a squirrelcage winding, but operates at synchronous speed).
Synchronous motors have fixed stator windings electrically connected to the AC supply with a separate source of excitation connected to a field winding on the rotating shaft. A three-phase stator is similar to that of an induction motor. The rotating field has the same number of poles as the stator, and is supplied by an external source of DC. Magnetic flux links the rotor and stator windings causing the motor to operate at synchronous speed. A synchronous motor starts as an induction motor, until the rotor speed is near synchronous speed where it is locked in step with the stator by application of a field excitation. When the synchronous motor is operating at synchronous speed, it is possible to alter the power factor by varying the excitation supplied to the motor field.

Induction motors are simple and rugged and relatively cheap to construct. They consist of a wound stator and a rotor assembly. They have fixed stator windings that are electrically connected to an AC power source. Current is induced in the rotor circuit. The resulting magnetic field interacts with the stator field for the “induction” to occur. No separate power source is required to provide the rotor field. An induction motor can be started and accelerated to steady state running conditions simply by applying AC power to the fixed stator windings of the motor. They do not rely on brushes like a DC motor does. Induction motors have a longer life than synchronous motors and are common for applications above 1 kW.

**broken link removed**
 
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