low voltage high frequency drives

[Harjitsingh]

I am using variable frequency drives for altering the speed of a 3 phase motor. As frequency is increased from 50 to 150 hz, my 2 pole motor attains 9000 rpm. Now i wish to use the same drive for low voltage 3 ph motor. can the output from a VFD be fed to a step down transformer for reducing the voltage from 415 v to 50 v.Can someone provide me with a standard circuit for such an arrangement. my motor runs on 50 volts 3ph ~ suppply of 150 hz, capacity 1.5 KW.

 

[mikekilroy]

yes, you can use an output autoxfmr to reduce your 0-415v 3ph to 0-50v.

you will need to take the extra heating of the vfd's pwm into account when sizing it. If you are in the USA I would be happy to quote you just such a transformer.

 

[ozmosis]

Heating yes, as a secondary effect but importantly, the very high p-p voltages the transformer will see from the VFD. This will have a direct impact on the insulation of the transformer windings, especially if it is likely to be connected immediately after the drive.

 

[waross]

It may be cheaper to oversize the VFD and set the voltage down in the VFD. You may be able to use a 240V single phase VFD. A reactor on the output of the VFD is worth considering.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricuwe]

Omitting the transformer as suggested by waross might be dangerous due to insufficient isolation (phase to GND--> safety, phase to phase --> reliability) of the motors.

What is the application of the low voltage high speed drive ? The low voltage might have been chosen due to especially restrictive safety concerns.

 

[Harjitsingh]

Dear Electricuwe, low voltage is necessary as the motors are physically handled in the yard by unskilled labour in damp conditions. my only concern is that i understand that the vfd changes the voltage with change in frequency. the frequency has to be kept at 150 hz only and is not to be changed. i want to know how the transformer will react to high frequency input and would i have to make any changes in the transformer for this specific purpose.

 

[waross]

Auto-transformers won't be considered safe for safety isolation. You should go with a two winding transformer, preferably with a grounded shield between the windings. Transformers generally perform well at increased frequencies (within reason, 150 Hz should be OK.) If transformers are used at much below rated frequency the voltage should be reduced in the same ratio as the frequency but the VFD does that at reduced frequency so no problem..

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mikekilroy]

most vfd's have output phase to ground protection built in, so I would not be excessively concerned over a phase to ground voltage on 0-50v output..... the autoxfmr will reduce the vfd's output voltage directly by its ratio - you will NOT get the vfd output high voltage spikes into the 50v motor.

We have never used an isolation transformer ("2 winding transformer") on output of a vfd as theoretically it is a bad idea: from 0 to prob around 30+hz its output would be zero, so you would be basically applying 1/2 motor voltage instantly to the motor, which will result in the normal 600% current draw, causing the vfd to simply shut down on ourput overcurrent. But ok by me for someone to try it. If you do try it, please report back how it works.

These reasons are why only autoxfmrs are used on output of vfd drives: dc thru the max freq go thru just fine and don't mess up the v/hz ratio.

 

[electricuwe]

Using fixed frequency higher than line frequency is a common approach to operate handheld tools with high power density, e.g. for grinding, polishing and screwing. For an introduction take a look in the catalog you will find here:

http://www.boschproductiontools.com...ice/high-frequency-electric-tools-24245.shtml

You will also find the safety/low voltage issue beeing discussed there.

Beside the rotating equipment described in the catalog there are also inverters available specifically for this purpose. Of course it is tempting to apply a standard drives inverter for this purpose because of cost, but going with speciallized equipment looks more appropriate to me unless you are a specialist on this application yourself. The low voltage approach for safety is only viable if you provide isolation from the mains.

 

[LionelHutz]

I've run a motor using a VFD through a full isolation transformer before. The setup worked just fine for accelerating the motor starting at 0 speed.