Synchronous Motor : Concepts - 3

Since the speed of the motor is always constant, above expression also expresses the torque developed by the motor.

The condition for the maximum power is Θ = α

Hence the value of maximum power

Pmax = (EbV)/ Zs – E2b / Zs cos (θ)

When, θ=90o , cos (θ) =0 so that,

Pmax = (Eb . V)/ Zs

'Load angle of Power angle:

It is the space angle between the axis of stator revolving magnetic field and the rotor pole axis, both locked and running at synchronous speed.

Hunting:

Due to sudden change of load, the angle changes causing oscillations of rotor about the mean position due to inertia of the rotor. Thus hunting is usually of low frequency oscillations superimposed on normal synchronous speed.

Synchronous reactance:

A synchronous machine can be represented as an emf source in series with internal impedance of the machine. This internal impedance, Zs is called synchronous impedance. The synchronous reactance takes into account the effect of armature reactions and the flux produced by the armature current. The emf takes into account the flux produced by field excitation.

Effect of change of Excitation in synchronous motor:

The real power is P = VI cosφ .The value of excitation for which back emf, Eb is equal to applied voltage V, is known as 100% excitation. An over excited motor (more than 100% excitation) takes leading power factor current. An under excited motor takes lagging power factor current.

Change in excitation currents does not change the load or power output of the synchronous motor. To change the power output, the mechanical load should change.

SYNCHRONOUS MOTOR V-CURVES:

The power factor of a synchronous motor is controlled by the variation of the filed current. Increasing the field current from small value, the line current decreases until a minimum line current occurs showing that the motor is operating at unity power factor. If we go on increasing the field current, the line current increases again operating the motor at leading power factor.

(a)V curves of a synchronous motor

(b) Power Factor versus field current at different loads

Open circuit characteristics of Synchronous Motor:

The open circuit characteristics of a synchronous machine is a curve of the armature terminal voltage on open circuit as a function of the field excitation, when the machine is running at synchronous speed.

Short circuit characteristics of Synchronous Motor:

If the armature terminals of a synchronous machines (driven as a generator at synchronous speed) are short-circuited through suitable, ammeter, and the field current is gradually increased until the armature current has reached a maximum safe value (about 1.5 times rated current), data can be obtained from which short circuit armature current can be plotted against the field current. Such a characteristic is known as short-circuit characteristic.

SYNCHRONOUS CONDENSERS:

It is a synchronous motor used for power factor correction which is operated at no load but at greatly over excited field. Such conditions cause the motor to take a current that leads the voltage by 90° which is equivalent to a capacitor.

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