Variable Speed Drives or AC Inverters

Variable Speed Drives or AC Inverters are a common subject that I get questions asked by apprentices and thus thought I might tell others in my words how these are used in an easy and fairly non technical way.

50HZ (60Hz in the US) is the cycles per second of the mains frequency which most apprentices can tell me. Now we look at a 3 phase squirrel cage induction motor which reveals that it will rotate at a given speed. The fastest motor speed found by the apprentice in the UK will be slightly less than 3000 rpm. So the formula is given N (rpm) = f * 60/P where f is frequency in hertz and P is the pair of poles.

So now asking the apprentice what we can change to alter the speed they are most likely to answer either the pair of poles or the frequency and both are correct. Pole change motors are possible but not very practical and are expensive so we are left with changing frequency.

The black box is now brought into play since it is best at this stage to only give the the very basics. Three phases go into the box and another three come out and then we add a speed control potentiometer. Explained is that the mains three phase frequency is now converted to a different frequency. Now many apprentices at this stage begin to ask what is inside the box since it is a little bit simplistic.

So now the black box is opened so to speak and on the drawing additional boxes are drawn. First there is a rectifier a storage capacitor and a box marked chopper. At the stage of asking about speed control most apprentices will understand how the rectification of AC into DC takes place and also how a capacitor can store electricity. The chopper box internals may or not be understood by some. However the simple explanation is that it chops up the DC to produce AC.

It is at this stage that most will accept the explanation of the basic of how inverters or Variable Speed Drives work?