Capacitors - 10uf vs 16 uf

I need to replace a capacitor on a bench grinder. The one I need is a 10uf 250 volt.
The part is discontinued, and can’t fine one online.

Will there be any problem using a 16uf, 250 volt??
Thanks for any help.

That’s probably close enough that it will work.

10uF 250V capacitors aren’t that hard to find. Is there something specific about this one that makes it hard to replace (physical size/location, etc)?

Real easy to replace. 1 screw, 2 wires and I took it out. Just couldn’t fine a 10uF 250V.

Thanks for the reply. I’ll probably go ahead and order the 16uF.

It’s also always safe to replace it with the same capacitance but a higher voltage rating. So if you can find like a 10uF 400V that fits, that will work.

What if the device in question is a magic-smoke emitter? The higher-voltage capacitor might not work there. :slight_smile:

Is it a start capacitor or a run capacitor? If it’s a start capacitor, you’re probably OK, but if it’s a run capacitor, I’d find one that is rated the same as the original.

10uf run capacitors from Amazon.

I’ve been trying to learn electronics for 6 months now. Is there any easy way for you explain why it’s ok for a start capacitor but not a run capacitor?

Are you trying to get me into trouble? Note that I said “probably” in my response. :slight_smile:

Here was my thinking:

The start capacitor is just there to get the motor started. It’s only on for a brief time until the motor reaches a certain speed. I imagine it acts as some sort of “charge reservoir” in case the motor has an erratic draw as it ramps up. Or, it acts like a low pass filter. A larger capacitor is going to be OK in either case.

The run capacitor is on the whole time the motor is operating, and I assume it’s there to regulate the frequency or the phase of the power going to the coil, or something like that. I’m thinking that the continuous operation means it needs to be more accurately tuned to the circuit. My understanding here is pretty sketchy, so I’m open to being corrected if I’m wrong.

And not all motors have run capacitors, so it might not even be applicable, but I think start capacitors are usually larger than what the OP is describing.

Having said that, you should keep in mind that capacitor ratings are +/- “something”, probably around 10%, so a 10uf capacitor might be an 8 or 12uf capacitor. But a 16uf capacitor is pretty far outside the error range I would expect. And since 10uf capacitors seem to be readily available, why risk it?

“Safe” yes, it’s almost certainly safe to do almost anything in our society- there is a massive margin of safety built into all our activities.

And “functional” for the same reason – in order to get good manufacturing outcomes, most stuff mostly works even with components that are only mostly alright.

But technically no. A typical capacitor doesn’t actually have the capacitance value given on the label - the actual value depends on the voltage, and what the voltage has been. So a capacitor with a different voltage rating is a different capacitor, and may, for example, make the circuit unstable, and just not work. (But see point two).

I would expect the capacitance of a capacitor with a higher breakdown voltage (400V versus 250V) to vary less when subjected to a similar voltage, all other things being equal.

What does the original capacitor look like? Is it labelled?

The answer may make it easier to figure out its exact role in the circuit, and whether or not a 60% deviation will kill your motor.

Here is a picture of the capacitor.

The low capacitance value (10 μF) makes me think it’s a run capacitor and not a start capacitor. If it is a run capacitor, you want to replace it with a 10 μF capacitor of equal or greater voltage rating.

OK, electronics nerds, would something like this capacitor be appropriate for the OP?

Points for effort, John Mace, but I think this is pretty far off. What the capacitors are there to do is to create a second phase of electrical power to drive motor coils that are offset at a 90 degree angle from the coils that are just connected to the line.

The run capacitor, if it is strictly a run capacitor, helps the motor run much more efficiently, although it might be able to run without the capacitor. Without that run capacitor it would trade much more current back and forth with the line, though out of phase with the voltage. It wouldn’t draw way more power, but the extra power it does draw would get converted to heat by resistance losses in the wire and eddy currents in the rotor.

However the motor would not be able to start spinning, at all, without some capacitor coupling the second set of coils to the line to create a second phase. Motors with just a run capacitor are able to start themselves spinning because the run capacitor creates the second phase, but a run capacitor does not create a very powerful second phase, and such motors can’t start against much of a load. A grinder is fine in this situation, because it is very easy to turn at low speed. A fan is another example of this, and fan motors that use capacitors often use only one, primarily as a run capacitor.

If the motor has to start against a heavy load, for example if it is turning an air compressor that might be trying to start on a compression stroke, it needs more capacitance at first to have a strong enough second phase. That’s what the start capacitor is for.

Induction motors running on one phase only are somewhat like like piston engines trying to start with the piston at the top or bottom of the stroke. They need some force at an angle to the phase they do have available.

There are other ways of creating a separate phase that don’t use capacitance, like having a shaded pole or using a split phase winding, but they aren’t as efficient or powerful as capacitors. Better still is to use a three phase motor, which is why motors bigger than somewhere around 1 to 5 horsepower are usually three phase, but you need more wires back to the power plant for that.

Much more knowledgeable people have already replied, but I’ll stick this in anyway, as a simplified version.

Capacitors are added to devices for a variety of reasons. Sometimes the capacity is only chosen for cost concerns, but sometimes it is chosen because it is necessary for how it interacts with the rest of the circuit.

When you don’t know exactly why the capacitor is there, the logical thing to do is to replace it with the same VALUES, even if the size and shape is wrong.

Greetings

Here is a link to a wikipedia article:
https://en.wikipedia.org/wiki/Motor_capacitor

This link takes you to a cite that explains the different methods of starting a single phase electrical motor. The article covers most of the different methods mentioned in this thread and has diagrams for each. The article can help you understand the particular motor your are working with:
http://what-when-how.com/electric-motors/single-phase-induction-motors-electric-motor/

Zuer-coli

It’s most likely a run cap. A start cap will be bigger because it’ll store energy and dump it to the motor for it to start moving. It takes more energy to move it from a standstill than it is to keep it running.

When you put a load on the motor the run cap kick in.

Yes, this is the same kind the OP is looking for. I don’t like the trend of smaller voltage, smaller package size run caps in PSC Motors. I’ve been running into these for a while now to my dismay.
The 10μfd cap in the Amazon post would be a better replacement if you can make it fit.

If you have the same capacitance in a lower voltage, you could buy four of them, connect them in a series-parallel square, and double the voltage.