Page 147 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

2 Running torque
As its name indicates, it is the torque developed by the motor under running conditions. It is determined by the
horse-power and speed of the driven machine. The peak horsepower determines the maximum torque that would
be required by the driven machine. The motor must have a breakdown or a maximum running torque greater than
this value in order to avoid stalling.
3 Pull-in torque
A synchronous motor is started as induction motor till it runs 2 to 5% below the synchronous speed. Afterwards,
excitation is switched on and the rotor pulls into step with the synchronously rotating stator field. The amount of
torque at which the motor will pull into step is called the pull-in torque. Torque motors are designed to provide
maximum torque at locked rotor or near stalled conditions
4 Pull-out torque
The maximum torque which the motor can develop without pulling out of step or synchronism is called the pull-
out torque. Normally, when load on the motor is increased, its rotor progressively tends to fall back in phase by
some angle (called load angle) behind the synchronously-revolving stator magnetic field though it keeps running
synchronously. Motor develops maximum torque when its rotor is retarded by an angle of 90º (or in other words,
it has shifted backward by a distance equal to half the distance between adjacent poles). Any further increase in
load will cause the motor to pull out of step (or synchronism) and stop.
Reluctance torque
If we gradually reduce the excitation of the synchronous motor when it is running at no-load, it can find that the
motor continuous to run at synchronous speed even when the excitation current is zero. The reason is that the
flux produced by the stator prefers to cross the short gap between the salient poles and the stator rather than
the much longer air gap between the poles. In other words, because the reluctance of the magnetic circuit is less
in the axis of the salient poles, the flux is concentrated in that area. On account of this phenomenon, the motor
develops a reluctance torque.
If a mechanical load is applied to the shaft, the rotor poles will fall behind the stator poles and the stator flux
change its shape accordingly. Thus a considerable reluctance torque can be developed without any DC excitation
at all.
The reluctance torque becomes zero when the rotor poles are midway between the stator poles. The reason is
that the N and S poles on the stator attract the salient poles in opposite direction. Consequently, the reluctance
torque is zero precisely at that angle where the regular torque attains its maximum value, namely at 900. The
reluctance torque reaches its maximum at 45°.
The main points regarding the above three cases can be summarized as under:
1 As load on the motor increases, Ia increases regardless of excitation.
2 For under-and over-excited motors, power factor tends to approach unity with increase in load.

3 Both with under-and over-excitation, change in power factor is greater than in Ia with increase in load.
4 With normal excitation, when load is increased change in Ia is greater tends to become increasingly lagging.
Effect of excitation on armature current and power factor
The value of excitation for which back e.m.f. (Eb) is equal (in magnitude) to applied voltage V is known as 100%
excitation. We will now discuss what happens when motor is either over-excited or under-excited.
Consider a synchronous motor in which the mechanical load is constant (and hence output is also constant if
losses are neglected).

Fig 6(a) shows the case for 100% excitation i.e., when Eb = V. The armature current I lags behind V by a small
angle. Its angle with ER is fixed by stator constants i.e. tan = XS / Ra.
In Fig 6(b) excitation is less than 100% i.e., Eb< V. Here, ER is advanced is armature current (because it lags
behind E clockwise and so increased but its power factor is decreased (ɸ has increased). Because input as
well as V are constant, hence the power component of I i.e. I cos ɸ remains the same as before, but wattles
component I sin ɸ is increased. Hence as excitation is decreased, I will increase but power factor will decrease so
that power component of I will remain constant. Incidentally, it may be noted that when field current is reduced,
the motor pull-out torque is also reduced in proportion.

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CITS : Power - Electrician & Wireman - Lesson 76-85