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Please guide me how a 3 kW 4 pole induction motor can be modified to run at higher frequency in the range of 0 to 200 hz. If the rotor motor is die cast copper. What should be done with present winding to suit 200 hz.

I want to use this motor for a utility vehicle running on battery.

Can I get a ready vfd to run this motor ?

With warm regards

C. B. Patil

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For a motor designed to run at 50 Hz or 60 Hz to produce rated torque at 200 Hz, the voltage would need to be increased in proportion to the frequency. A 230-volt, 50 Hz motor would need to have 920 volts. The motor could be operated at full require torque up to 133 Hz at full torque and constant power above that. That would require 612 volts at 133 Hz and above. A VFD is very unlikely to be available for 12 kW and 920 volts and not very likely available for 8 kW and 610 volts either. Note that the kW requirement increases with the voltage. In addition, the DC input voltage would need to be about 860 volts for a 610-volt VFD and 1300 volts for a 920-volt VFD.

The motor could possibly be rewound for a lower voltage. For full torque at 200 Hz, the voltage would need to be 25% of the present voltage assuming a 50 Hz motor. That would require 25% of the number of turns in each slot and 4 times the cross sectional area of the wire. It would probably be better to operate with full torque up to 2/3 of rated speed and constant power above that.

If the vehicle is adequately powered by a 3 kW engine an a mechanical transmission, it probably does not need 8 kW at 2/3 speed and above. The motor could possibly be rewound for less power more easily than rewinding for 8 kW.

At 200 Hz, a 4-pole motor would be operating at nearly 6000 RPM. For that speed, the rotor would likely need to re-balanced. Higher-speed bearings may be required. If the rotor has cooling fins, they may need to be trimmed to reduce the aerodynamic drag. The same is true for any external self-cooling fan. If the motor is not properly balanced etc. it will vibrate excessively and possibly break loose from its mounting. An external self-cooling fan may also break loose if operated above its design speed. Bearing failure can result in the rotor jamming against the stator.

There are a lot of VFDs available on the market than can be configured by the user to operate an induction motor up to 200 Hz with a constant torque range and a constant power range. Most industrial VFDs on the market will be designed for 3-phase AC input at voltage levels from 200 to 600 volts. Some will accept DC input without a lot of difficulty.

As mentioned above, VFD output voltage is proportional to frequency (speed) for constant torque operation. For a constant power range the voltage is held constant as the frequency is increased above the constant torque speed range as shown below by the "Torque 1, Power 1 and Voltage 1" curves. In the constant power range the torque curve is defined by Torque = Power / Speed.

It is also possible to have declining power operation above the constant torque range. In that case, the voltage continues to increase at a slower rate while both power and torque decline as shown by the "Torque 2, Power 2 and Voltage 2" curves. The curves are idealized and specific motor designs will have variations in actual performance. There are probably VFDs on the market that offer a declining torque and power mode of operation, but they may be more difficult to find and configure.

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  • \$\begingroup\$ A motor designed for 3000 RPM may be destroyed when rotating with 6000 RPM. Humans standing near the motor may be injured or even killed. \$\endgroup\$ – Uwe Apr 2 '18 at 15:47
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You'll have to be certain that the motor itself is capable of run at 200Hz. NEMA MG1 has values for different size motors and their maximum allowable speeds. 3kW (5hp) TEFC 4-pole has a safe maximum operating speed of 3600RPM (120Hz) for direct coupled motors per table 30-1.

If you have a general purpose induction motor, more than likely you won't be able to simply modify it to speed it up to 200Hz. You may want to reach out to the manufacturer first to see what options are available.

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