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V/hz Control


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#1 billstrong

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Posted 28 December 2008 - 03:29 AM

hi,
I am trying to design vfd using v/hz contorl to drive a three phase induction motor(delta connected and 230v/50hz,bus voltage is 400v),as i know:
1.during the motor start up, the frequency should ramp up gradually to 50hz
2.there should be deadtime between the upper switch and lower switch in each inverter leg
3.there should be a brake circuitry to discharge the extra energy when motor is running from high speed to low speed
since this is the first time i design vfd, is there anything that i must also take into consideration for v/hz control?
it would be highly appreciated if you can provide me some reference link or documents!

thanks
bill

#2 marke

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Posted 28 December 2008 - 05:39 PM

Hello billstrong

1. That is correct, you should be able to control the ramp up and the ramp down time independently. As it is ramping up (down) the frequency, it must also ramp up (down) the voltage. In addition, there should be provision to provide voltage boost at very low speed to compensate for the resistance of the windings. At line frequency, the impedance is primarily due to the magnetising current and the load torque. At low frequencies, the winding resistance (in series with the magnetising circuit) reduces the magnetizing current and therefore reduces the flux in the iron and hence the shaft torque available.

2. This is quite critical as it introduces distortion to the output waveform (I assume that you are using some form of SVM to generate the sinewaves).

3. The brake circuitry is usually optional and is only necessary where the load is required to decellerate faster than it would if power was removed while at speed. Decelleration faster than the natural slow down due to load losses, results in an increasing DC bus voltage. It is common to provide a dynamic decelleration such that if the rate is to fast and the DC bus voltage begins to rise, the setting is overridden and the rate decreased, reducing the need for a brake control.

4. You need to have over voltage and undervoltage prtection on the DC bus to shut down the inverter if either too high or too low.

Best regards,
Mark.




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