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I have a 3 phase motor and 3 phase power supply (400V, 50Hz)

Currently this motor produces 1.5hp in torque and ~1750rpm. I need it to produce less.

My question is:

Is the best way to go about this, a rheostat or a VFD? The cheaper the better.

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    $\begingroup$ What type of motor is it? You need "less"... what? Lower speed or lower power? A rheostat is a good way to start a fire, I would avoid that solution at all costs. Whatever power you siphon off of the motor with a rheostat gets converted directly to heat. $\endgroup$
    – Chuck
    Sep 19 '15 at 0:23
  • $\begingroup$ VFD, is a better solution, not only avoids heat dissipation issue. Apart from that it also other benefits like controlling the motor speed and improving the motor starting characteristics. Sharing a link which gives good insight on VFD - ecmweb.com/power-quality/basics-variable-frequency-drives $\endgroup$
    – shri
    Sep 23 '15 at 6:02
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1.5hp is 1.12kW, this is no torque, this is power.

You want to decrease the output power of your motor. I assume that, for your load, the torque is a function of the rpm. With a synchronous 3 phase motor, the rpm are set by the power supply frequency, and this would prevent you to adjust output power. All you'll change is the system efficiency.

As your current speed is 1750rpm, however, I assume that your motor is asynchronous. Therefore, you can decrease your effective speed by using a rheostat, as long as your slip is low enough to keep your motor running. Obviously, you'll still decrease your system efficiency.

In my opinion, you should go for VFD: for a 1.12kW motor, this is cheap. Maybe not as cheap as a rheostat (it depends on your needs), but the price difference will probably be paid by the power saving, you'll avoid heat dissipation issues, and you may control your system more precisely than with rheostat.

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