Running universal electric motor on 1 phase with cap? - Page 1
Forum section: electronics / beginners

author: jurk0
author_rank: Newbie
timestamp: August 06, 2017, 10:23:21 am
content: Hi,I recently wired an AC electric motor designed to run on 3 phase electricity to 230V (EU) 1 phase using the delta wiring setup and a run capacitor. The motor is a 350W motor and I am running it with a 25uF capacitor. I can use it without the capacitor if I manually turn the motor to start it. I read somewhere that using the capacitor can make the motor overheat and damage it. My question is: Is it better for the motor to run with the cap or without in the delta configuration? Or do you you disconnect the cap after the motor has started running? I do not want to damage the motor. Thanks for your input!Jurk0

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2017, 10:30:13 am
content: It seems like you've confused a universal motor with a capacitor run AC motor. A universal motor is single phase and doesn't need a capacitor. It has brushes and will run off DC, as well as AC.

author: jurk0
author_rank: Newbie
timestamp: August 06, 2017, 10:35:06 am
content: You're right, I fixed it thanks

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2017, 10:42:10 am
content: Yes, the motor will start, without a capacitor, if it's already moving.I haven't heard of any problems with using a capacitor.The motor will only run at a fraction of its rate power from single phase. The correct way to do this is to get an inverter to generate three phase, from single phase. It also enables you to control the speed and soft start.

author: jurk0
author_rank: Newbie
timestamp: August 06, 2017, 11:08:51 am
content: Thanks for the info. The motor was basically bought just to experiment with running it on 1 phase. Maybe later I'll turn it into a bench sander or something. I have looked at the inverters and I am quite tempted to get one))

author: SeanB
author_rank: Super Contributor
timestamp: August 06, 2017, 06:11:22 pm
content: With this motor running you can use it to power smaller 3 phase motors as well, as it will generate the missing phases on the terminals. Lowewr power than the motor itself, but useful to see how 3 phase motors run.

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2017, 03:35:44 pm
content: Quote from: SeanB on August 06, 2017, 06:11:22 pmYes, that can be useful with larger motors but this one is only 350W, so you'd be looking at around a third of the power and there's not much you can do with that.

author: Lupin III.
author_rank: Contributor
timestamp: August 09, 2017, 12:54:12 pm
content: Quote from: jurk0 on August 06, 2017, 10:23:21 amWhere did you hear that? Because it's not true. Sure, you have to pick the right capacity (for some motors it will state the value right on its label), but an uncountable number of motors are running like that.

author: Circlotron
author_rank: Super Contributor
timestamp: August 11, 2017, 08:47:12 am
content: Definitely use a cap.The ideal value cap will vary with load, and also needs much larger cap for starting. Probably the right cap will give equal currents in each motor phase. If you haven't got a clamp type ammeter then I'd say choose a cap that gives the quietest operation. Note how the motor hums a bit with no cap.I did see a circuit many years ago that used both a capacitor and an inductor, one going from the third motor wire to active and the other going from the third motor wire to neutral. That's all I remember of it. No idea of the values of each but you might be able to use a simulator to get the right phase shift. Might actually work better than just a cap over a wide range of loads.

author: EPTech
author_rank: Regular Contributor
timestamp: August 12, 2017, 12:30:17 pm
content: Hi there,If you use to large of a running capacitor on a 1 phase induction motors with an auxiliary winding, you may end up burning the auxiliary winding because this winding is sometimes wound in smaller gage copper than the "work" winding.In 3 phase to 1 phase conversion this is less critical since all windings have the same cage. You can still burn them though if the capacitor is large enough or if the motor gets blocked or has excessive slip.Happy commutating.