I need help understanding the numbers on a start capacitor

[Zachary Hoyt]

We have a Rockwell 14" bandsaw that has been in storage since 2015 and the start capacitor was bad on it then. I took the blade off and spun the lower wheel by hand and the motor started up, but without the spin it just hums, so I'm pretty sure it must be the start capacitor that's the problem. It has a lot of numbers on it and I have not had much luck googling the ones that look like part numbers, and I don't know which of the other numbers are important to match exactly. This is what it says:

AeroM
MADE IN USA
PSA6R12250N
250-300 MFD
125VAC 60HZ
275464-114
677-9218-03

I have seen some capacitors on eBay that have the same MFD range but are marked 110v, and I don't know if that is the same or different for these purposes. I'll be very grateful for any advice.

[Paul F Franklin]

You want the MFD value to have the same range, if a range is specified, or to be within the range if only a single value is specified (say, 275 MFD).
The voltage needs to be the same or higher; I wouldn't use one marked 110 VAC.
The other numbers are likely manufacturer part no's or the like. AeroM is a manufacturer, now part of cornell Dublier, I believe.
The other considerations are physical size and terminal type. Size is only important if it has to fit somewhere specific, like in a can on the side of the motor. Differing terminal types are usually easy to work around.

Finally, it needs to be an AC capacitor intended for motor use. If the voltage rating indicates VAC (as opposed to VDC) then it is an AC capacitor.

[Maurice Mcmurry]

from Ugly's. I remember the poem for resistors.
475562

[Rod Sheridan]

The capacitor has two values because electrolytic capacitors have a -10/+50% tolerance.

You need a 250uF, 125 VAC non-polarized electrolytic capacitor, in the trade referred to as a start capacitor because it’s intermittent duty.

Any motor repair shop will have one for you, bring the old one with you to make sure it will physically fit….Regards, Rod

[Bill Dufour]

I assume it is wired 120 volt not 240/. If 240 volts use a 240 volt cap. The 120/240 is minimum voltage. no penalty in going higher voltage.
Bill D.

250-300 MFD, or microfarad or UF. The U is really the greek letter with a tail on it

[Zachary Hoyt]

Thank you all very much for your help. It is indeed wire for 120 volts. I just ordered a capacitor that is rated 110-125 VAC and 243-292 MFD on eBay and if it doesn't work I'll take the motor to a shop next time I get to the city. I figured I might as well try the eBay one for under $10 and see if it is close enough. It's physically a bit smaller than the old one so it will fit inside the cover on the side of the motor.

[Ralph Okonieski]

Zachary,

I recently went through a capacitor issue and learned a lot about them. Here is one reference that I found to be informative, although there were several I looked at.

https://inspectapedia.com/electric/Motor-Capacitor-Decoder.php

[Zachary Hoyt]

The new capacitor arrived in the mail today and I put it in tonight. When I first flipped the switch it still just hummed till I spun the lower wheel, but after I turned it off and the motor stopped it started perfectly the next two times, so I guess it just hadn't gotten charged up till the motor had been run, or something. I haven't tried starting it under the load of the blade and upper wheel, as I am still waiting for delivery of a new tire for the lower wheel, but it seems like it is working. Thank you all very much for your help.

[Rod Sheridan]

Hi Zachary, it indicates that the centrifugal starting switch in your motor wasn’t closed when the motor was stationary.

The second time it was and the motor started properly……Regards, Rod.

[Zachary Hoyt]

Thank you Rod, that makes sense. Maybe the capacitor wasn't the problem after all, but a sticky centrifugal switch? I'll have to see what happens as I keep turning the motor off and on from time to time.

[Tom Bender]

I assume it is wired 120 volt not 240/. If 240 volts use a 240 volt cap. The 120/240 is minimum voltage. no penalty in going higher voltage.
Bill D.

So you're saying that we can use a 120 volt or a 240 volt capacitor. Ok not doubting you but the math makes me unsure. From Maurice's post, Capacitance = Coulombs per volt. So if we use a capacitor with the right MFD rated at 250 volts it will only have half the needed capacitance at 120 volts, yes?

[Rod Sheridan]

It has nothing to do with charge Tom, it has to do with impedance.

The reactance of the capacitor is inversely proportional to the frequency and capacitance.

The start winding is highly inductive, causing a high impedance and a poor power factor, this reduces starting torque.

To cancel the inductive reactance of the winding, we put a capacitor in series with the winding, this cancels the inductive reactance, increasing winding current and power factor, resulting in more starting torque.

The voltage rating of the capacitor does not affect the capacitive reactance.

Regards, Rod

[John K Jordan]

So you're saying that we can use a 120 volt or a 240 volt capacitor. Ok not doubting you but the math makes me unsure. From Maurice's post, Capacitance = Coulombs per volt. So if we use a capacitor with the right MFD rated at 250 volts it will only have half the needed capacitance at 120 volts, yes?

We’ve often substituted capacitors with a higher voltage rating, and for motors, a higher capacitance. Just don’t use one with a lower voltage rating. It’s been decades since I looked into the circuitry but my understanding is the voltage rating is the maximum voltage the capacitor will handle before it may be destroyed. The capacitor stores electrons for a short time, kind of like a very fast rechargeable battery, so using one with a somewhat higher capacitance never caused a problem. (The capacitance value can be critical for some applications such as designing oscillators, wave shaping, certain filters, etc.) And of course, using a non-polarized (AC) capacitor for motor applications is very important - most electrolytic capacitors are polarized with the polarity marked.

The biggest problem I’ve had with replacing start capacitors is finding one that will physically fit in the “wart” on most motor housings. Some I couldn’t even find at a good motor shop or a local industrial supplier. For one lathe I had to call Jet and pay a shameful amount for one that would fit. If the capacitor is physically too big it is possible to cobble up a substitute enclosure but with the appropriate electrical protection to prevent shock or electrocution.

If details are needed I can ask my friend Joe - he knows more about both motors as electronic circuits than almost anyone.

[Ken Fitzgerald]

Using a higher voltage rating is never a problem in my experience. Using one with lower voltage rating could cause a failure that could occur instantly or over a length of time. Capacitance should be similar but it doesn't have to be exact, in my experience.

As John said, the physical size is often the problem.

[Maurice Mcmurry]

Here are the next two pages from Ugly's. Hopefully there is a reference some where that deals with motors less than 10 H.P. . I have to get my Wife, Daughter, or Sons to help me when I try to do this kind of math.

476170

[Rod Sheridan]

Those are tables for power correction capacitors for three phase motors.

KVAR = Kilo Volt Amperes Reactive and is the current in the capacitor multiplied by the voltage and divided by 1,000. For three phase it would be multiplied by the square root of 3.

Those tables aren’t for starting capacitors….Regards, Rod

[Rod Sheridan]

Try blowing it out with compressed air gently… Rod

[Tom Bender]

Did a little reading and my understanding is now better, though probably still not very good. Anyway, writing it down helps so here it is.

A motor will not start on just it's running windings so it has a second winding or set of start windings to get it moving. These are given a current lag by the start capacitor so they provide torque at low and zero rpm.

My comment about the capacitance being wrong if rated at the wrong voltage is still correct I think but given the brief action of the run capacitor it does not usually cause a failure. The motor still starts but with less vigor.

[Michael Schuch]

So you're saying that we can use a 120 volt or a 240 volt capacitor. Ok not doubting you but the math makes me unsure. From Maurice's post, Capacitance = Coulombs per volt. So if we use a capacitor with the right MFD rated at 250 volts it will only have half the needed capacitance at 120 volts, yes?

Wrong. The capacitor is rated to handle voltage up to 240v but you will be running only 120v through it. The higher voltage rating just gives you more head room before the capacitor will short internally and release the magic smoke.

When I built my 10hp rotary phase convertor 20 or so years ago I had problems with the cheap Russian 220v capacitors I purchased popping. I replaced them with more expensive 350v capacitors of the same value and haven't had another problem in 20+ years.

[Maurice Mcmurry]

This table is from the web. Those 4 pages are the entire motor capacitor content from my version of Ugly's.

476413 476419 https://www.uglysbooks.com/products.php?cat=10&gclid=EAIaIQobChMI8JGA1e_e9gIVQic4Ch0m3waIEAAYAiAA EgJcn_D_BwE

[Bill Dufour]

This came up before but I thought caps used to be rated at full voltage not rms. So for 120 volt rms you need 170 volt peak to peak. Maybe those Russian capacitors are rated that way. I suppose frequency comes into play here as well. I assume russia is 50 hz.
Bill D

[Michael Schuch]

This came up before but I thought caps used to be rated at full voltage not rms. So for 120 volt rms you need 170 volt peak to peak. Maybe those Russian capacitors are rated that way. I suppose frequency comes into play here as well. I assume russia is 50 hz.
Bill D

I still have some of them in a box somewhere. They were cheap and my fault for cheaping out. I don't think they were even made to be starting capacitors. When the RPC is coming up to speed the voltages can get pretty high before the starting capacitors pulled out of the circuit. I don't remember how high the voltages got but it was substantially above the capacitors 240v rating.

[Paul F Franklin]

This came up before but I thought caps used to be rated at full voltage not rms. So for 120 volt rms you need 170 volt peak to peak. Maybe those Russian capacitors are rated that way. I suppose frequency comes into play here as well. I assume russia is 50 hz.
Bill D

Since motor caps are rated for rms AC volts, the conversion is already factored in.

[Bill Dufour]

Dc caps are rated peak to peak not rms.
Bill D

[Rod Sheridan]

Tom, the capacitance does not vary with voltage, the current varies directly in proportion to the voltage and frequency…..Regards, Rod

[Maurice Mcmurry]

We are encouraged to avoid the topic, I suspect and hope components from Russia will become scarce if a change for the better is not achieved. Hopefully the few US manufacturers of capacitors, vacuum tubes and the like will have a renaissance. Thinking of Western Electric, RCH labs, hopefully others. I am thinking of divesting many of my tubes? It is is confusing and would be very frustrating.