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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?

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  • \$\begingroup\$ Inrush current? \$\endgroup\$
    – winny
    Jan 17, 2019 at 22:46
  • \$\begingroup\$ That linked document is titled Operating Instructions; there should be a separate data sheet with electrical characteristics \$\endgroup\$
    – user28910
    Jan 17, 2019 at 22:53

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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.

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Induction motors that are designed to be run off of the AC line have specific tradeoffs between size, efficiency, and the ability to start with line-frequency and full voltage excitation without burning up.

If you know you're never going to run off of a fixed-frequency, fixed-voltage line, then you can toss that start-up requirement out the window, and instead put the onus of not burning up the motor onto the drive electronics. Then the motor gets smaller, more efficient -- oh, and if you hook it up straight to the AC line it'll burn up.

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  • \$\begingroup\$ Can you explain what physical characteristics cause it burn up? Is it solely a startup problem? \$\endgroup\$
    – ericksonla
    Jan 17, 2019 at 23:00
  • \$\begingroup\$ I can't remember them all, in large part because the bulk of my knowledge comes from 35 year old training on "how to design a motor that won't burn up" (because the notion of a VFD was just madness). I know that the rotor resistance is part of it -- the higher the rotor resistance, the more slip the motor can stand and the more torque it develops at high slip. I'm pretty sure that the gap between rotor and stator is an issue, too -- certainly the 1980 design rules had you making it bigger than a similar synchronous machine. On this issue, Google is probably a better friend than me! \$\endgroup\$
    – TimWescott
    Jan 17, 2019 at 23:36

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