What is the purpose of the capacitor in this circuit?

[punkykatt]

I know this is not amp related, but I want to use this fan/motor unit in my shop to increase ventilation when soldering. The fan motor works without the  capacitor but Im not sure if  the circuit requires the capacitor??  Is this what they call a capacitor run motor?  If so what type of capacitor would I need and what voltage rating.  The guy that gave me this thing said it came out of a huge microwave oven. Thanks for any help you can give me.

[kagliostro]

May be the motor is a 3 phase motor and the cap is used to rephase it because the use is with 2 phase lines

Kagliostro

[John]

I think the cap helps the motor to start running.

If I'm right, you owe me a beer. 

[jjasilli]

See:  http://en.wikipedia.org/wiki/Motor_capacitor

Also, the speed of some AC electric motors, like ceiling fans, can be varied by switching from no cap, to one or more caps of different value.   E.g., hi, med, lo speed.

[kagliostro]

I was thinking to something like this



old washing machine has those kind of capacitor, it is an AC cap and is rated, in voltage, more than the line, about capacitance it differs with motor

Kagliostro

[Willabe]

I think it's either a motor run or motor start cap. I don't think that's a 3 phase motor, electric motors have an extra start winding that can (kinda sometimes) make the schematic look like it's 3 phase when it's not.

I fix my house AC last summer and came accross this;

This is a motor run cap test chart;

http://www.toad.net/~jsmeenen/capacitor.html

Here's the home site;

http://toad.net/~jsmeenen


              Brad                  

[punkykatt]

Its definitely  single phase 120v  60 cyl.  The white wire and the sky blue wire connected to 120 vac is the high speed, the white wire and the violet wire connected to 120 vac is the low speed. I don`t have the original capacitor that came with the unit.  For ha ha`s  I connected a 10uf 100v NP cap (largest NP cap I had) between red and orange wires. The cap lasted about 5 seconds and pushed out smoke.

[RicharD]

It's a start cap.  The motor may start w/o it but that's hard on the windings.  Since torque is linear in electric motors, at 0 RPM, the motor is pulling a lot of current.  This cap is connected parallel through a centrifugal switch and aids the motor up until it hits a certain RPM at which point the switch disconnects it.

[punkykatt]

On the circuit drawing in the first post it shows the capacitor wired direct to the motor with no switching device in the circuit?

[DummyLoad]

cap is part of the motor start circuit. the cap and start winding provides added reactance to overcome high initial torque and ensure the motor starts spinning in the proper direction - it's usually used in split phase motors that have a load on start-up, (unlike smaller fan motors) such as compressors or appliances. once the motor has started, the start winding circuit is disconnected, usually by a centrifugal or magnetic switch.

--DL

[RicharD]

On the circuit drawing in the first post it shows the capacitor wired direct to the motor with no switching device in the circuit?

It's a wiring diagram depicting the leads coming off the motor only.  There's a bit -o- circuitry within the circle M not depicted.

[punkykatt]

It's a wiring diagram depicting the leads coming off the motor only.  There's a bit -o- circuitry within the circle M not depicted.

Ok,  Thats whats confusing me.  Now the next step is how to get a replacement cap since I don`t have the original. All I have for info is on that label (10uf), and it came from a large microwave oven, so I was told..
Here is another pic.

[mresistor]

Maybe call a motor shop, and see it they have or can get a 10uf motor start cap for the Samsung motor in question.

[RicharD]

Any ol motor start cap rated 10ish uF at 250ish volts will work.  The uF's isn't all that important.  You want your voltage rating to be at least 2x your run voltage.

[punkykatt]

Thanks for the great info Guys.  I will start hunting  for a start cap now that I know what Im looking for.  You Guys are the best.
Punky

[punkykatt]

John !!!  Here is the BEER I owe ya.