Will a 240 Volt AC Motor Operate on 120 Volts AC

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. 


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iceng1 year ago

FYI background most 3 phase motors are dual voltage 220/440 or 110/220...

Click the pictures.

vincedis1 year ago

Thank you for all of your help!

Update on my progress. I hooked it up and it works but...

It has poor performance (it isn't pushing as much air as i would have thought) and the motor definatly gets hot. It is a 2.0 amp 1/4HP motor so either the load of pushing the squirrel cage is a bit much at 120v or the current increase is too much for the windings.

Might just have to try to find a different fan.

vincedis1 year ago

Hate to resurect an old post but I have a similar question. When you wired the motor to your new power source did you wire both positive leads from the fan to the hot of your new voltage source? I'm going from four wires down to three and I'm not sure if i should cap one or just put them together.

find a wiring diagram for your motor, vince. Find the mfg info plate, and if you can't find the original manual, Google that diagram.

I'd also suggest talking to an instructor at a technical college. Maybe get a general tech manual for motors. With ac motors, it isn't the "positive leads" as you say. There is "hot" and neutral, but both switch from + to - and back. I get the feeling that you are gonna need more than casual advice.

The motor has 4 wires. Ground, Neutral, Black hot, and red hot, (the two phases to get to 240v) If you are running the fan on 120v you only have ground, neutral, and one of the "hot" wires.

I'm just not sure what happens once the juice gets inside. I would hate to short something stupid before i even get it started.

I'll try to Find a wiring diagram but any advice from a professional is going to basically be don't do it for liability reasons.

I'm not too worried about the increased current. The fan was free so if i melt the windings i'm out zero dollars and its only going to be used when i'm in the garage so if it catches on fire hopefully i'll notice

biochemtronics (author)  vincedis1 year ago

make sure the motor is squirrel cage motor. It should not have a capacitor on the side or contacts inside the end of the motor on the end opposite the shaft. If that is alright just hook the 120 volts to the red and black wires. Ground wire to the frame. Cap the neutral white since you won't need it. Remember I only tested this on a fan and it used less current than it did on 240 volts. On a machine with a varying or heavy load the results may be different. Something like a small saw or lathe should work fine.

biochemtronics (author) 1 year ago

the 240 volt motor has been running.for several.years now with no problems so the answer is yes a 240 volt squirrel cage motor will run on 120 volts -at least in a fan application.

biochemtronics (author) 1 year ago
This is simple. Hook the the 240 volt motor to the 120 volt supply just like you would hook it up to 240 volts. Just don't try to run a120 volt motor on 240 volts. That will turn into a hot and smoky situation pretty quickly.
BenY22 years ago

I encountered a couple of cases that dual voltage motors were configured for 230v and connected to 115volt power, and it seems like they have been running for years.

biochemtronics (author)  BenY22 years ago

A dual voltage motor is designed to run on either voltage so it is not surprising that it runs fine on the lower voltage. This instructable is about running a single voltage 240 volt motor on half of the voltage it was designed for. As an update, my 240 volt motors have been running great on 120 volts for a couple of years now, However, keep in mind that this is a fan application. It may not be the same for motors in high current high torque machine applications.

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