Capacitors - 10uf vs 16 uf

I see that it’s a “250 W” capacitor. It’s probably sold as “110/220 V” capacitor – but I can’t see the part number or anything apart from the manufacturer.

Note: it’s a bench grinder, so it’s got a run capacitor, not a start capacitor. You can hold the wheel in your fingers and turn it on, and it will burn out rather than spin, because it doesn’t have a start capactor. (Don’t try that - its a way to loose your fingers).

Also, if it was a start capacitor, when you took the grinder apart, you probably would have seen the centrifugal switch that disconnects the capacitor after the motor has spun up.

A run capactitor is a way to get two phase power, to run a two phase moter, off a single phase supply. I don’t know anything about motor design.

It might have said something like “250 WV” before exploding, not 250W (why would it have a wattage rating?) Either way, we know it would be perfectly safe to replace it with a motor-run-rated polymer film capacitor rated to a higher voltage like 440V, but I would be less cavalier about changing from 10uF, especially since those seem to be readily available.

Is this correct? I do not understand what you mean about the capacitance listed on the capacitor not being correct. Are you saying the capacitance changes according to the applied voltage? I thought as long as the capacitance was the same it was ok to replace a capacitor with one that has the same capacitance, as long as the voltage rating was the same or higher.

If the quote is correct, will someone please explain for me?


Specifying just the capacitance is enough to describe an ideal condenser, but real capacitors are made of different types of materials and in different sizes and their capacitance can vary as a function of temperature, voltage (including frequency), and aging. For some applications it does not really matter, while in others it is important to specify the class of capacitor. When in doubt, check the data sheet.

Let’s say you have a capacitor with the following printed on it:

10 μF
250 VAC

This means the manufacturer guarantees it will have a capacitance between 9.5 μF and 10.5 μF within a limited frequency range (typically 50 Hz to 120 Hz for AC capacitors), voltage range, and temperature range. As mentioned by others, capacitance is a function of all of these things, though they’re usually second and third-order effects. The “250 VAC” is the maximum AC voltage you can put across the cap, which means the maximum peak voltage rating is around 350 V.

Run capacitors for motors must carry quite a bit of current on a continuous basis. Therefore the capacitor must have very low ESR, else it will quickly overheat and fail. For this application, a “film” capacitor must be used. These capacitors use a polyester or polypropylene dielectric and have a low dissipation factor. They’re also bulky and relatively expensive.

Putting capacitors in parallel is fine. But you will quickly find out that putting two capacitors in series will not double the voltage rating; it will result in smoke and embarrassment. :wink: This is because the two capacitors will not have the exact same leakage resistance. As a result, one of the caps will have more voltage on it than the other at all times. And the more unequal the capacitors’ leakage resistances are, the more unequal the voltages will be across the caps.

When connecting “identical” capacitors in series in an effort to increase the voltage rating, you must connect an external resistor across each capacitor. The resistors act as a voltage divider, and (assuming proper values) will guarantee each capacitor sees the same voltage. Each resistor should be of the same value, and should be approximately 10X less than the nominal leakage resistance for one of the caps. The disadvantage to all of this, of course, is that it’s inefficient - energy is going to be dissipated by the resistors.

You can always use a higher VAC. Always. The 2 common VAC sizes in my line of work would be 370 VAC and 440 VAC. Because I can always go higher, and to make it easy, I try to carry 440 VAC when that is available.

A 10 uf capacitor is very common in HVAC applications. I probably have 10 of them right now. 10uf @ 370 VAC.

Thanks for the replies. I was reading this thread late last night when I probably should have been sleeping. I realize now he was probably talking about the capacitors reactance.

As mentioned previously, the magnitude of the voltage across a capacitor will affect the capacitance value, but it’s usually a subtle/third-order effect. Same goes for frequency. Most designers don’t worry about it because - for most applications - a small change in capacitance won’t affect the performance of the circuit.

The reactance of a capacitor, OTOH, is very strongly dependent on the frequency.

That’s sort of the point…

Indeed. If the reactance wasn’t dependent on frequency, it wouldn’t be a capacitor… :slight_smile:

And here we see the difference between a physicist and an electrical engineer.

30% -~ 50% is not a subtle effect!.

It’s often an unimportant effect.

The actual shift in capacitance (due to changes in frequency, voltage, or temperature) greatly depends on the construction and dielectric material of the capacitor. But you’re correct… I should not have waved it off as insignificant, as it can be quite important for some capacitors.

Here is a good paper on the subject.

From everything I’ve gleaned, the OP is dealing with a PSC motor. A low amp, low power motor at that.
The capicitor is a run cap on a PSC motor, not a start cap. I AM NOT AN electrical engineer. I know that all the catalogs, stats, manufactures pubs state that … not to exceed -+ 5% of the capacitance and not to go below the rated voltage. BUT, I have routinely exceeded the voltage and capacitance ratings of many motors with no detriment. ON PSC Motors. Whenever I replace a cap I always note the original value on the unit.
You shouldn’t get too worked up about caps on PSC motors. I’ve replaced 40mfd with 50 mfd (temporarily) and 5mfd with 10 mfd and 10 mfd (again temporarily) with 20 mfd with no bad side effects. There are plenty of 5 mfd rated motors out there running on 3, 7.5, 10 and even 15 mfd caps with no problems – because that’s what the tech had on the truck. Dirty secret, but true. Fan motors have small, 5 - 10 mfd values, compressors run 30 to 80 mfd.

For what it’s worth, I went to a class in 85-87 time frame on AC compressors. The Lennox electrical engineer (booger brains in my book) said that any cap will start any compressor. OK but not good. But I’ve never had a PSC motor not start with a good capacitor of any value. As long as you have a phase shift, the motor will start. Might not run long, but it will start.

Bought a new capacitor on ebay for $9. Here is a photo of the new one. Grinder starts right up and spins. Haven’t yet used it to grind anything, but so far all is good.

Thanks for all the replies.