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I'm studying from Wildi's textbook on electrical machines and I came across the following on starting a synchronous motor:

"To limit the voltage induced in the rotor windings during the starting of the machine and to improve the starting torque , we either short-circut the slip-rings or connect them to an auxiliary resistor during the starting period".

I get the part about limiting the voltage but since the current will be less how is the torque greater than before?

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This information is not related to synchronous motor but rather to an induction motor with wound rotor and slip rings.

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Adding a resistance in rotor circuit affects the torque vs. speed characteristics, so you can get higher starting torque. Note that this doesn't increase the available torque, it only shifts the max. torque point to the zero speed.

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  • \$\begingroup\$ I believe that the material cited by the OP refers to synchronous motors started by induction motor operation and not wound rotor slip-ring motors. See my answer. \$\endgroup\$ – Charles Cowie Apr 27 '17 at 2:47
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Synchronous motors are often started as induction motors. To implement induction motor starting, the rotor has windings or bars called damper windings that are separate from and additional to the field excitation windings. The damper windings are shorted just as the rotor bars in an induction motor are shorted. With this starting method, the rotor is accelerated by induction motor operation to nearly synchronous speed. At that point the rotor excitation is applied and the motor pulls into synchronism. Once the motor is operating at synchronous speed, induction motor operation ceases naturally because no current is induced in the damper windings once there is no slip.

While the motor is starting, the field winding is shorted by connecting a small resistance across the field terminals. That protects the field windings and slip-ring assembly from excessive voltage that would be induced during starting. The induced current also adds to the induction motor accelerating torque.

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