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I am a bit confused with V/f ratio. I know it is used to control the speed, as the output is constant. However, for an induction motor I was doing an experiment on, the rated V/f ratio was 230V/50Hz with a rated speed of 1375rpm, whilst experimentally I got 254V/49.26Hz at a speed of 1480rpm. I have read it is possible to get a motor speed greater than a rated speed by increasing the frequency. Surely though, if you increase the frequency, the V/f ratio will be smaller, hence it is no longer constant as it should be?

Hope that makes sense! Thank you.

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  • \$\begingroup\$ Rated speed is under rated load. Experimental result looks like you left the motor unloaded. Did you? \$\endgroup\$ – Brian Drummond Apr 24 '17 at 16:27
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You can use constant V/f up to rated voltage. For many motors, you can continue to increase frequency above rated frequency, but you should not increase the voltage. Increasing frequency will cause the motor to spin faster, but, if the voltage is held constant, the available torque will decline as frequency increases.

Note that the rotor never quite catches up with the frequency you supply to the motor. Your motor has a synchronous speed of 1500 rpm. If you supply it with 50 Hz, it will try to spin at 1500 rpm, but won't quite get there. This difference, 1500 rpm - actual speed, is called slip. Slip is proportional to torque.

As you increase the mechanical load on the motor, it will slow down a bit, but this will increase slip and increase torque until the torque matches the load torque.

The nameplate speed (1375 rpm) is the speed at which the motor will deliver its rated torque and power when supplied with the nameplate voltage. If you do not apply a mechanical load, the slip and torque will be less than nameplate rating, and the motor will spin somewhere between 1375 and 1500.

If you need to control the speed of the motor, you may need to look at a variable frequency drive (VFD). It may also be called an AC drive.

One other thing to consider is cooling. There are different types of motors. But if your motor has a fan built-in to it, as many do, it probably relies on that fan to keep the motor cool. If you run it at full torque, but low speed, the fan will not be able to keep the motor cool. For example, if the nameplate says TEFC, then the motor is totally enclosed fan cooled, and has an internal fan.

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If you increase the speed and the V/f ratio drops (as a result of f increasing, but V staying constant) then the available torque will drop.

If you continue to increase the frequency with constant voltage, at some point you will either exceed the maximum rated RPM of the motor, or the available torque will drop to the point where speed no longer increases.

Here (from this website) is an inverter-duty motor graph showing the torque-speed behavior (above the rated speed, the voltage is assumed to be constant):

enter image description here

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