Compresssor motor capacitor

[used red MN]

The motor on this compressor just hums when turned on, and will turn slowly buzzing loudly if you give the compressor pulley a spin. I understand that this could be caused by the capacitor. My question is how close does the MFD of the replacement cap. need to be. I have another 1 hp motor that I took out of service for heating up under a load. I have yet to look at the cap. on it, but wondered if there is a MFD range that it would have to fall in to be a replacement. Does the label in the picture on the one off the problem the compressor show it as 540-648MFD? TIA!

 

[OH Boy]

yes that is likely the reason the motor isn't running right. That would be a 600 microfarad capacitor. 540 would be -10 percent. 648 would be +8 percent. look for a 600 microfarad one to replace it.

 

[Russ from MN]

Hers is a simple troubleshooting test, take the belt off so there's no load, hook the 2 wires connected to the capacitor together. Then turn on the power and see if it starts, if it does the start switch is OK, and you need a new capacitor. My experience is they don't need a capacitor to start, it just helps when starting a load.

 

[Steve@Advance]

The MFD numbers need to be close, a little over or under it will still work.

Under, it won't have as much starting power. Over will have more starting torque but will be hard on the starting contacts and the start windings if started repeatedly or failed to start because of high starting load.

The voltage can be higher, but not lower.

Like Russ said, try it with the cap wires tied together. If it still won't try to start, the start contacts are bad or the start winding is open. Could also be a bad bearing letting the rotor drag.

 

[buickanddeere]

The start capacitors increase the phase angle or phase shift between the run windings and the start winding .
The wider the phase angle and the higher the current, the more starting torque the motor will have .
The higher the voltage rating the more resistant the start capacitor is to damage from high voltage spikes .
Anyone who tells you that the voltage rating and mfd rating can not be increased . They do not understand electrical motor theory .

 

[MarkB_MI]

As you've probably guessed, the "540-648 MFD" value is a range. 648 is 120 percent of 540. So the cap is expected to maintain at least 540 microfarads over its life, but when new should be closer to the 648 value. Yes, you can go a bit bigger, but the replacement cap may not fit under the motor housing.

Note that there are start capacitors and run capacitors. Start capacitors have high values for their size, but are intended for intermittent service. Yours is a start capacitor.

Here's one that appears to be an exact replacement for yours. (Assuming the title is right. In typical Amazon fashion, the description is for a different device.)
540 648 starting capacitor

 

[showcrop]

When I was servicing machines, I would replace probably 150 start winding switches for every capacitor that I replaced.

 

[Texasmark1]

.....as long as you don't get too far off the "Quadrature" peak for the locked rotor, input starting current. V = L di/dt and I = C dv/dt for which the motor
was (should have been) desgned.

 

[buickanddeere]

(quoted from post at 10:08:17 12/16/19) .....as long as you don't get too far off the "Quadrature" peak for the locked rotor, input starting current. V = L di/dt and I = C dv/dt for which the motor
was (should have been) desgned.

Better to have over size start capacitors and reduce the length of time the start windings are in service to accelerate the load .
The only exception would be where a particular load requires a "soft" start.

 

[gears]

Where were you guys when my RV AC went out and I had to replace my start and run capacitors - "new" used compressor had a start timer circuit inline with the run capacitor, the ac unit was newer and had a starter circuit - I put both inline and popped the start cap (it could've been bad due to age and weather too).

So what does one of the piggyback SuperStart capacitors do for a motor? Increase the phaze angle difference at startup for more torque and then drop itself out of the loop once the armature is up to speed?

 

[Steve@Advance]

"So what does one of the piggyback SuperStart capacitors do for a motor? Increase the phaze angle difference at startup for more torque and then drop itself out of the loop once the armature is up to speed?"

It's common for refrigeration systems to come with only a run cap and a load sensing start contact, no start cap.

The Superstart kit simply adds a start capacitor and start relay to the motor for more kick to get it going. It's especially helpful with the old piston type compressors.

 

[guido]

Hello Steve@Advance,

AND its resistance increases ones the unit is running, taking it out of the circuit,

Guido.

 

[MarkB_MI]

The "hard start" capacitor is actually a second run capacitor that's installed in parallel with the original run capacitor to provide additional starting torque. There are several designs, but the most common one has a positive temperature coefficient thermistor in series with the capacitor that reduces the current through the capacitor a few seconds after startup, effectively taking the hard start cap out of the circuit.

The PDF at the link below explains them in great detail.
Hard Start Capacitors

 

[buickanddeere]

(quoted from post at 19:18:13 12/16/19) The "hard start" capacitor is actually a second run capacitor that's installed in parallel with the original run capacitor to provide additional starting torque. There are several designs, but the most common one has a positive temperature coefficient thermistor in series with the capacitor that reduces the current through the capacitor a few seconds after startup, effectively taking the hard start cap out of the circuit.

The PDF at the link below explains them in great detail.
Hard Start Capacitors

Yes, no and maybe . I see those hard start kits adding phase shift to the start windings not the run windings .
Sealed AC compressors are peculiar as they often leave the start winding energized through a capacitor during normal operation.
The standard issue split phase motor used on air compressors etc . No current flows though the start windings and start cap . Once the centrifugal switch opens at approx 2/3 of rated operating rpm .

 

[Geo-TH,In]

"My question is how close does the MFD of the replacement cap. need to be."

You answered your own question:
Does the label in the picture on the one off the problem the compressor show it as 540-648MFD? TIA!

Capacitor is between 540-648 MFD.
A little larger won't hurt.
You may need more if it doesn't want to start.
In winter the cold oil makes it harder to start.
I'll loosen belt so it slips a little on start.
Cheaper to replace a belt vs a motor.
Or I'll replace motor pulley with a smaller one.
geo.

 

[used red MN]

(quoted from post at 08:43:35 12/16/19) The motor on this compressor just hums when turned on, and will turn slowly buzzing loudly if you give the compressor pulley a spin. I understand that this could be caused by the capacitor. My question is how close does the MFD of the replacement cap. need to be. I have another 1 hp motor that I took out of service for heating up under a load. I have yet to look at the cap. on it, but wondered if there is a MFD range that it would have to fall in to be a replacement. Does the label in the picture on the one off the problem the compressor show it as 540-648MFD? TIA!
......
Went back to refer to this post and I see that somehow the picture I posted of the capacitor has been swapped to a picture of cookies baking in an oven. Lol! That is not even my picture or a picture I have saved anywhere. Oh the mysteries of the YT forums. My wife does like those baking stones though. Thanks all for the replies and info!