It is a very tedious and time consuming process to determine torque and current at locked rotor and at various slips during the acceleration test on a synchronous machine. Induction motors and synchronous machines behave similarly during acceleration except that the synchronous machine acceleration can include a pulsation torque. At locked rotor because of the salient poles, there will be conditions where the synchronous motor will have minimum torque at quadrature axis and maximum torque at direct axis. Since these machines do not have large numbers of rotor bars in the amortisseur winding like the induction motor has in the squirrel cage, their stall times tend to be a little shorter. Due to the shorter stall time and large starting torque, locked rotor torque and current are determined from acceleration tests conducted at reduced voltage rather than by tests at stand-still. The paper presents data showing that saturation effects impact both torque and current. It also shows that the voltage index which has to be applied to the torque and current measured at reduced voltage varies with speed and can be obtained from a series of acceleration tests.
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