induction machines above 500 hp?
There is no universal rule about using reduced-voltage starting methods above a specific power level. The decision is mostly determined by the capacity of the power source. If the motor can be started without causing a problematic voltage sag, it can be started at full voltage. Some power suppliers may prohibit their customers from full-voltage starting of motors above a specific power level. The local power distribution in some industrial facilities may experience a voltage sag if a large motor is started. Another limitation is the construction of the motor. Some manufacturers may not construct larger motors to withstand the stress of full-voltage starting. The important things for a student is to understand the possible limitations and the reduced-voltage starting methods that are available. It may be a good idea to check any proposed motor installation above 500 Hp to make sure the motor can be started at full voltage. It would not be a good idea to assume that every motor larger than 500 Hp needs reduced voltage starting or that smaller motors never need reduced-voltage starting. It is also wrong to believe a motor with any type of load can be started with a reduced-voltage starter.