[calcnerd]

Hi. The motor start capacitor on my tablesaw finally died. I'm looking to get a new one but am having trouble finding the exact one. I attached a picture. The ones I see on amazon for 150MFD don't have the screw terminals but the 200MFD does. Would it do damage getting the 200?

Thanks

200MFD Capacitor on Amazon

 

[afx]

Hi. The motor start capacitor on my tablesaw finally died. I'm looking to get a new one but am having trouble finding the exact one. I attached a picture. The ones I see on amazon for 150MFD don't have the screw terminals but the 200MFD does. Would it do damage getting the 200?

Thanks

200MFD Capacitor on Amazon

I'm going to assume it's a single phase motor.

To heavily generalize what the MFD does on a starting capacitor is it essentially takes the power being given to the motor and stores it so that when the motor starts up and requires more power to begin turning, it has enough electricity to keep the timing correct.

So what putting a capacitor with a higher MFD rating will do is give the motor slightly (very slightly) more torque while it spins up because it has more power available to it. This is probably just fine. However, it COULD increase the wear on the motor because it's handling more energy then it was designed for. I highly doubt this would be a problem though. It also means the cap will run a little hotter and if your motor has a housing for the cap that wasn't designed for higher heat, it could shorten the life of the cap. I doubt 50MF would generate that much more heat though.

Personally, I would just solder the leads to a non-screw-type lead and call it a day.

 

[calcnerd]

Thank you. Just what I needed to know.

 

[afx]

Thank you. Just what I needed to know.

I'm glad you bought it. I made it all up.:laughing:

 

[GeorgeC]

Instead of looking a ebay you should try a brick and mortar store. Do you have a Grainger near you? If not there may be any number of other stores near you that have what you need.

George

 

[calcnerd]

Instead of looking a ebay you should try a brick and mortar store. Do you have a Grainger near you? If not there may be any number of other stores near you that have what you need.

George

I ran to Grainger at lunch today and unfortunately the guy that was working was unable to help me. He told me he had no electrical knowledge at all and didn't see anything that matched up in his computer. I just ordered one on Amazon.

 

[knotscott]

It's not unusual for caps to be specked out at +/- 20%. IIRC, a 150mfd cap is only 0.000150 farads for calculation purposes (one you EE's please correct me if I got it wrong!). Changing it by 0.000050 farads is literally decimal dust. As long as the voltage rating is at least what the original cap is, the actual capacitance tolerance is pretty darn big. It's also pretty easy to adapt to leads to fit different configurations.

 

[TomC]

I'm going to have to take exception on what a staring cap does on a single phase motor. Basically a single phase motor has no rotation on the magnetic fields between the poles. They just change from north to south with no rotation. On a single phase motor with the starting capacitor removed if you can get it spinning by hand it will run in either direction you spin it. The starting capacitor make the motor act like a two phase motor on start up. I can't remember but I believe it put the voltage on the run winding and start winding out of phase. If the run winding and start winding are identical putting the start capacitor in the run windings will cause the motor to run backwards. I don' t believe you will have a problem with a slightly larger staring capacitor. Just get as close as you can.
Tom

 

[tvman44]

You should be fine as the tolerance on those capacitors is +/- 20%.

 

[rrich]

Please correct me if I'm wrong.

I always thought that the capacitor was something to start the motor spinning. After the motor is turning fast enough a centrifugal switch drops the capacitor out of the circuit.

Also I thought that the capacitor is in series with the coil of the motor and is tuned to be a resonant circuit. (Series) This causes a greater current draw during starting.

 

[TomC]

Please correct me if I'm wrong. I always thought that the capacitor was something to start the motor spinning. After the motor is turning fast enough a centrifugal switch drops the capacitor out of the circuit. Also I thought that the capacitor is in series with the coil of the motor and is tuned to be a resonant circuit. (Series) This causes a greater current draw during starting.[/QUOT

Yes the capacitor is there to get the motor spinning. The capacitor is in series with the start winding to make the motor think its has two phases. Once the motor is up to speed the centrifugal switch disconnects the start windings and capacitor. The reason the staring current is higher is the rotor is starting from a stopped position. Three phase motors ( no capacitors) have a high starting current. On some very high torque motors the capacitor and second set of winding stay in the circuit the entire time the motor is running. An example is a air conditioning compressor. These motors use a large oil filled capacitor.
Tom

 

[rrich]

Thank you TomC.

An important point that we haven't discussed is the MFD value. (a.k.a. Micro Farads )

The value is important because as being part of a tuned circuit, a higher current is drawn through the starting coil and capacitor. The higher current gets the motor spinning and thus the centrifugal switch prevents the motor from overheating.