Wednesday, April 28, 2010

Some basics on Variable Frequency Drives

Let's take a very basic look at how a VFC works:

Three phase input power enters into the drive and the first thing it arrives at are the Diode Bridge Rectifiers.  At this point, we are converting the input AC power to DC power where it then goes to a Capacitor Bank for smoothing and storage until needed.

The voltage as stored on the bus, which since rectified has gone from 480 VAC now to about 690 VDC.  The reason for this is that when you rectify AC power, you need to multiply the number by 1.414 to get the new DC value.

The Positive and Negitive legs of the Capacitor Bank are connected to Output Transistors, in simple terms "Switches" that are in-turn connected to the Output Terminals.  There is a Positive Transistor and a Negitive Transistor connect to each of the Output Terminals.

When we start the drive, it fires two positive transistors and one negitive transistor on, multiple times and for different periods of time.  Starting out very breifly and getting longer in time until the Voltage reaches desired output.  Then, more and more breifly again until reaching zero.  This is done firing one side of one transistor per phase resulting in one half of a sinewave being delivered from the DC bus to the output terminal.  Then the output changes.  One of the positives goes negitive, the one that was negitive going positive and the other staying on positive thus completing one cycle or hert. 

The above happens very quickly and can only be seen with the use of an o-scope.  The frequency of what I just explained happens at the speed of the desired output frequency.  The rate of the firing of the transistors happens at the speed of the Carrier Frequency which is thousands of times per second.

Now, one question I am always asked is "why can't I change the rotation of the motor with the input terminals?".  The answer is above.  We rectify the power as it comes in therefore we don't care what the phase rotation of the incoming power is, we just rectify it and dump it on the bus.

The above is explained in a very simplified manner but may be useful to know in the future for you.  Questions?  You can always call me and we can discuss this further.


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