Electrical machines miscellaneous Easy Questions and Answers

Electrical machines miscellaneous

A synchronous motor is operated from a bus voltage of 1.0 pu and is drawing 1.0 pu zero power factor leading current. Its synchronous reactance is 0.5 pu. The excitation emf of the motor will be

1. higher than actual as it does not account for magnetic saturation
1. lower than actual as it does not account for magnetic saturation
1. nearly accurate as it accounts for magnetic saturation
1. nearly accurate as the generator is normally operated in the unsaturated regi on of magnetization

A synchronous motor is running from busbars and has a torque angle of δ = 15°. The bus voltage and frequency are reduced by 10% while field current and load torque are held constant (all losses are negligible).The new torque angle will

1. remain the same
1. increase slightly
1. decrease slightly
1. the motor will lose synchronism

A synchronous generator is loaded and the armature current lags behind the excitation emf by angle Ψ . The electrical angle between the field axis and the axis of the armature reaction field will be

The subtransient reactance of a synchronous generator is much less than its synchronous reactance in the initial phase of short circuit from no load as

1. magnetizing ampere-turns appear because of induced field and damper winding current as the air-gap flux cannot change instantaneously from its no load value.
1. demagnetizing ampere-turns appear because of induced filed and damper winding currents as the air -gap flux cannot change instantaneously from its no-load value.
1. magnetizing ampere-turns, appear because of induced field and damper winding currents as the air-gap flux must reduce instantaneously from its no-load value.
1. demagnetizing ampere-turns appear because of induced field and damper winding currents as the air-gap flux must reduce instantaneously from its no-load value.