Will a 240 Volt AC Motor Operate on 120 Volts AC Answered
I need to know if a 240 volt ac squirrel cage fan motor can be safely run on 120 volts ac. An quick internet search reveals several sources that all say no, but I am concerned that the people answering the question don't know what they are talking about - as is often the case on the internet. I know there are a lot of want-to-be engineers out there ready to answer any question posed even when they have no experience or knowledge of the issue. Please do not guess at this or offer some unsubstantiated opinion. I need a real answer to this question.
Here is what I do know about this issue from researching a couple of electrical engineering sites. Lowering the motor input voltage by 50 % does several things.
1) The motor will run slower.
2) The torque developed will drop to about 25% of its full voltage rating.
3) This decrease in torque can make the motor stall when trying to start under a heavy load.
4) There may be drastic speed fluctuations due to load variations.
5) The motor current will double if the load remains constant.
I can understand how these issues could cause severe problems in some applications, but perhaps not in the application I am looking at. I want to use a squirrel cage motor rated at 1/20 HP, 240 volts, 0.5 amp in a direct drive fan application. Here is how I am looking at this issue:
- I need to reduce the fan speed so the speed reduction is a good thing in my application.
- I have test run the motor on 120 volts for about an hour and there is plenty of torque to operate the fan. I figure at the lower speed there is less air load on the fan blades so less torque is required. Again a good thing for this application.
The is no heavy load at start up and the motor seems to have plenty of starting torque for the fan. The fan does not reach full speed as fast as it does on 240 volts, but it still does so in about 4 to 5 seconds so this shouldn't be an issue either.
Since it is a fan moving air and not some machine moving gears and levers the load is constant so the speed does not fluctuate at all. Again this should not be a problem.
The possibility of the motor current increasing is my only concern so far and apparently it is not an issue since a test run showed the current decreased from 0.5 amps at 240 volts to 0.4 amps at 120 volts.That is a significant current drop instead of the predicted increase. Since this is a fan, I think the current decreases because the load decreases with the lower speed.
For these reasons and based on my apparently successful test run I think the motor will operate fine on the decreased voltage. However, I am not sure how to factor the motor slip into this. Slip is the difference between the synchronous and asynchronous speed of an induction motor. From what I have read the slip will increase dramatically when the voltage is cut in half, but I am not sure what physical affect that will have on the motor. If you know what effect that will have or know of some other factor I am not catching please let me know.