60Hz motors @ 50Hz

"We will not be out-gunned"

A favourite saying of Americans. It would be foolish to start slating them as it's probably the very attitude that has made them the success they are (although they need to be careful of South Africans!).

So, it comes as no surprise that their home appliances are bigger than most, but now other countries are catching on and wanting to install these "American style fridge-freezers" etc. As they are already built, all that needs to be done to acquire these desirable beasts is to simply import them. And with it comes a rather unforeseen problem. Ah! Before you think they cannot be imported because the voltage is wrong, think again. There are a lot of 230VAC appliances in the USA. They've been running dual voltage homes for years!

When the compressors of such appliances are switched on the motors are expecting to see a 90° quadrature between the two magnetic fields pulling on the armature. This would be at 60Hz, but at 50Hz this angle is not the intended 90° and the amount of "pull" (i.e. torque) is therefore reduced. This results in the motor taking a longer time to spin up to full speed. A further complication is the motor is designed with 60Hz in mind i.e. the impedances are just that little bit higher at 60Hz than at 50Hz. When the motor is stalled the inrush current is that much higher (because of the lower impedance) meaning the whole start current is about double that of 60Hz operation.

On a slight tnagent for a moment; One needs to be careful using such motors as they are usually squirral cage and therefore spin at the mains frequency divided by the number of poles. This comes as no surprise. However, the pump is being asked to be as efficient with a whole 17% knocked off the speed! Those with experience of using variable speed drives and impeller pumps will know what I am referring to.

In essence start current is not an issue. Where it becomes serious is when there isn't sufficient supply 'solidity' (high feed impedance). Although the secondary issue is likely to be flicker owing to the high start currents, the primary one could be premature failure of the motor.


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© 03.08.02