I am looking at the datasheet for a motor. This is an AC induction motor designed for use in electric vehicles. It includes the statement [my emphasis]:
The motors are designed for installation in electrically driven
vehicles. and are characterized by their high power density, ruggedness, long lifetime, and overall reliability. The variable-speed three-phase motors are operated from a frequency converter. The direct connection to the three-phase line supply is not permissible.
The AC specifications are never listed (I assume the inverter uses some frequency other than 50/60Hz, and you'd have to calculate the appropriate AC voltage/current requirements based on the DC requirements) so is that the only reason this wouldn't be allowed? Is this statement based on physical limitations, or is this just business?
Best Answer
A frequency converter for motors also known as a variable frequency drive (VFD) is capable of a softer start than any other means of motor starting. A motor that is intended only for VFD use could draw significantly higher starting current if started by the methods usually used for motors connected to a 3-phase power source.
The easiest way to design a motor for high power density is to design it for a frequency higher than 50 or 60 Hz. For a vehicle, the voltage would likely be no more than the usual 3-phase industrial supply voltage range, 200 to 600 volts. If a motor is designed for 400 volts and 200 hz, it could operate at other frequencies if the V/Hz ratio, 2 V/Hz is used. For 50 or 60 Hz, that would mean that anything higher than 100 or 120 Hz would be excess voltage. Connecting to a normal 3-phase supply would likely mean connecting to excess voltage.
Either of the above two situations represent trouble if the motor is connected directly to a 3-phase supply.