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Chapter 6: Synchronous Motor
In general, a synchronous motor is very similar to a synchronous generator with a
difference of function only.
Steady State Operations
A synchronous motor are usually applied to instances where the load would require a
constant speed. Hence for a synchronous motor, its torque speed characteristic is constant
speed as the induced torque increases. Hence SR = 0%.
Since,
sin
3s
i
n
ind
R
net
A
ind
ms
kB B
VE
X
φ
τ
δ
ω
=
=
Maximum torque (pullout torque) is achieved when sin
δ
= 1. If load exceeds the pullout
torque, the rotor will slow down. Due to the interaction between the stator and rotor
magnetic field, there would be a torque surge produced as such there would be a loss of
synchronism which is known as
slipping poles
.
Also based upon the above equation, maximum induced torque can be achieved by
increasing E
a
hence increasing the field current.
Effect of load changes
Assumption:
A synchronous generator operating with a load connected to it. The field current
setting are unchanged.
Varying load would in fact slow the machine down a bit hence increasing the torque
angle. Due to an increase to the torque angle, more torque is induced hence spinning the
synchronous machine to synchronous speed again.
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 Spring '10
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