Swapping Capacitors with different markings

[ftaylor74]

Hi all. I hope someone can save my weekend. The start capacitor on my pool pump blew out this morning. The pump is wired for 230V and the capacitor is rated as 64-77MF at 250 VAC. It is a 2 1/2 HP pump.

Since it is Saturday, I can't pick up a capacitor until Monday at the earliest, but I need to keep my pool running. I have a second booster pump (125/250V 3/4 HP) that has a capacitor rated as follows 128-148MF at 125VAC.

Both of the capacitors were installed by the factory so they are appropriate for their applications.

My question: CAn I swap the 128-148MF capacitor for the 64-77MF capacitor? I believe they are equivalent. Same dimensions, same connection posts.

Please advise. Thanks

[Elysarian]

Re: Swapping Capacitors with different markings

being as the voltage ratings are different I'd say no - a cap that says 125v on it is likely to fail in the worst way if you apply twice that voltage to it!

[budm]

Re: Swapping Capacitors with different markings

The value is also twice as much and that will cause more current draw on either the start up winding or for running depends on how the cap is used in this motor.

[JonathanAnon]

Re: Swapping Capacitors with different markings

A lot of switching power supplies that come with laptops and computer networking devices have a main filter capacitor that is 400V ~100uF..

I had a similar issue with my washing machine in another thread a while back and I found a capacitor on an old playstation that did the job...

https://www.badcaps.net/forum/showthread.php?t=28094

It shouldnt be too difficult to find one from an old power supply that will get it back running.

[Longbow]

Re: Swapping Capacitors with different markings

Let's back up a little. The capacitors that are strapped to AC motors are NOT power supply filter capacitors, so put away your ESR meters. Their purpose is to provide phase lag in one winding of a single phase AC motor. The phase lag produces rotational torque to start or run the motor. The value of the capacitor is chosen for a specific motor type, speed and torque. Values are not interchangeable. Voltage ratings are important for things like electrical fire hazards, blown circuit breakers and damaged motors. I hope you find the right part.

[Uniballer]

Re: Swapping Capacitors with different markings

You also probably do not want to install a polarized electrolytic capacitor in place of the appropriate unpolarized motor start or run cap.

[ftaylor74]

Re: Swapping Capacitors with different markings

Thanks to all who have replied. I did some math that nearly convinced me that I would be okay to use the the lower voltage, higher capacitance capacitor. It seems that both motors (3/4 HP and 2-1/2 HP require the same starting charge.
125VAC X 138uF ~ 250VAC X 70uF

But since capacitance is fixed, sending twice the voltage to the capacitor will double the starting charge. I'm thinking that may well damage the windings upstream of the capacitor.

However, and this is a big if... The 3/4 HP pump is dual voltage, and in my case it is wired for 230VAC, via a selector switch on the back of the pump motor. So it seems possible to me that 125VAC capacitor can be used on 230 VAC motor.

Does anyone know if the capacitor receives 230 VAC when that voltage is selected?

[Elysarian]

Re: Swapping Capacitors with different markings

A lot of switching power supplies that come with laptops and computer networking devices have a main filter capacitor that is 400V ~100uF..

I had a similar issue with my washing machine in another thread a while back and I found a capacitor on an old playstation that did the job...

https://www.badcaps.net/forum/showthread.php?t=28094

It shouldnt be too difficult to find one from an old power supply that will get it back running.

There's a damn good reason that those mains filter caps are rated for 400v...

the 240v value for the nominal mains voltage in the UK is the RMS value - the peak value is actually around 340v (240*1.414), the equivalent peak values for the mains in other countries can be arrived at the same way - by multiplying the listed RMS value by the square root of 2.

Of course, this is assuming you're getting exactly the listed voltage at your outlets - the standards over here in the UK allow for +/- 10% on that so you could be getting as much as 264v RMS which is almost 374v peak.

[ftaylor74]

Re: Swapping Capacitors with different markings

I'm fairly convinced that it is a bad idea, and with all the expense and aggravation I've already gone through it's probably foolish to try.

I have another 1-1/2 HP pump motor with a 35uF capacitor rated at 330VAC. Does anyone see a problem trying it just to get the motor started and until I can get my hands on the right capacitor? By my calculations, it will provide about 1/2 the starting charge than the original. Not enough to harm the motor, but maybe enough to get it going?

[budm]

Re: Swapping Capacitors with different markings

The capacitive reactance (Xc) of the original cap 64-77MFis twice as much as the sub 128-148MF you are trying to use, so more current will flow through the motor winding. When motor operates in 220V, the windings are connected in series, in 120V in parallel which will maintain the same power.
The size of the cap is based on the motor design, an also if it is used for starting or for running.
Get the right cap or you may taking a chance on more damages.

[ftaylor74]

Re: Swapping Capacitors with different markings

Again, thanks for responses.

Discretion being the better part of valor, I heeded the advice from this forum and opted
not to install the underrated 140uF 125VAC capacitor from my 3/4 HP pump in place of a blown 70uF 250VAC capacitor from my 2-1/2 HP pump.

I thought I would follow-up with what has happened since then.

I went to the pump repair shop this morning to purchase a new capacitor that matched the one that had blown. As luck would have it, they didn't have a direct match, but offered me -hold your breath - a capacitor marked 128-148MF at 125VAC. I explained why I thought it would cause me problems, but he said that it would operate just fine. He could tell that I was reluctant so he offered me a 108-129 MF capacitor rated at 165VAC. He said it would deliver less starting charge than the other one, but would still get it going.

Since this was the same guy that sold me the pump, and who had successfully rebuilt my pool pumps and irrigation pumps over the years, I figured... what the heck.

I took it, installed it, and voila... the motor started. I little slower than I am used to to be sure, but it started nonetheless.

Anyway, we have gone from the theoretical to the practical in this thread, and as is often the case, practical application and sometimes transcends theory.

That said, I really do appreciate the willingness of experts to take the time to answer questions like these from grunts like me.

Thanks again

[budm]

Re: Swapping Capacitors with different markings

One way to check is to monitor the starting/running current with the original value cap and the new cap, higher value capacitance cap will cause more current draw.
If it is the starting cap, once the motor starts spinning the centrifuge switch will open up and disconnect the cap from the circuit. So far it sounds like the motor you have only has starting cap, may be give us the make and model of this motor.
You can also look at the manufacturer of the motor website to get the info about the requirement for the motor to run properly.
http://dnr.louisiana.gov/assets/TAD/...P/hvac/d/d.htm
http://homeenergypros.lbl.gov/profil...ng-a-capacitor

[JonathanAnon]

Re: Swapping Capacitors with different markings

Let's back up a little. The capacitors that are strapped to AC motors are NOT power supply filter capacitors, so put away your ESR meters.

Sorry about that, thought it was the filter cap.. Funnily enough I actually replaced one of these myself a few years back... I had an outdoor water pump, and I powered it back on (after making some plumbing changes) the large capacitor blew.. As far as I remember it was connected from live to ground, so I presumed it was just for getting rid of any non-50Hz frequencies..

I just took out the capacitor, brought it to the local pump store and he gave me the replacement... Wasnt cheap either, cost about 30 euros...

[Elysarian]

Re: Swapping Capacitors with different markings

>.<

Of course a 125v cap will work... I wasn't taking into account the inductive reactance of the motor windings...

I feel a little stupid now