Any advice would be great. I've got a 220v a/c motor. Runs a industrial saw. With no blade on it, motor spins up and runs. When a blade is placed on it, just hums unless you help it to start spinning. If any load is placed on the motor/blade, it blows the fuse. Could this be a bad capacitor or windings? Thanks for any help.

 

It could be either. It could also be a faulty start switch.

Most Capacitor start motors will work with the capacitor shorted out, try that but don't leave it on more than a couple of seconds if it doesn't start.

Roy

 

The most likely problem is the "start" capacitor!

Regards, Phil Corso ([email protected])

 

Roy, the fact that it starts when unloaded, and its direction of rotation is correct indicates that there is current in the start-winding resulting in sufficient rotational torque. Therefore the centrifigal switch must be OK, otherwise start-winding current would be zero!

The fact that it doesn't start with the blade indicates that the torque is insufficient to overcome increased breakaway torque resulting from the blade's weight!

An earlier thread covered how torque is the result of the run and current magnitudes, and the angle between them! Thus, your conclusion about the cap being shorted is, at least directionally, correct. That is, if the cap is shorted, minimum start-winding current flows!

Regards, Phil

 

Phil,
From the original I assumed also that the capacitor is open circuit. Since most of the small cap start motors I have seen have been regular split phase motors with a capacitor added to increase the starting torque it's reasonable to assume by shorting the capacitor out that the motor will start as a normal split phase motor.

By "normal split phase motor" I mean one with 2 windings where a centrifugal switch opens up at speed to de-energize the start winding.
It may be a "capacitor run" motor of course, without a starting switch.
Hopefully Marinel will let us know what he found.

Regards,
Roy

 

Marinel... what was your fix?

Phil

 

I've ordered the capacitors but have not been able to install them yet. Soon as I do, I will post the results, thanks again.

 

Simply replace the capacitor with a new one. Take care of correct rating, as printed on the body of the old one. If you are not getting a capacitor of exact values, you may use another one with higher working voltage, but the capacitance (UF rating) should be the same.

Thanks,
MAGK

 

Responding to MAGK's 16-Jul-08 (02:00)... changing the rated voltage, will change the effective capacitance value!

I suggest you read thread #1026194594 "Motor Cap-Run Theory", dated 10-May-04.

Regards, Phil Corso ([email protected])

 

Phil, MAGK's post said (UF rating) (microfarad rating) should be the same.

The rated voltage is what it will operate at.
Changing the voltage will change the amount of energy stored not the capacitance value.
A capacitor rated at 10 uF 1000 volts will store the same energy as one rated at 10uF 100 volts if operated on 100 volts.

Roy

 

Also an electrolytic capacitor operated far under it's voltage rating may increase its capacitance value somewhat over time as the dielectric film reforms. After protracted use at lower voltages or storage its voltage capability may decrease as well. This can be a problem when caps long in storage are put into service.

I haven't seen directions for reforming the dielectric for a while but they used to suggest you regrow the dielectric after long storage to prevent blowing up capacitors. There is a lot to these electrolytic capacitors, they aren't that simple and bulletproof.

Regards
cww

 

Certainly not. I have few capacitors in my collection, having same microfarad rating but diffrent working voltage rating.

It is similar to resistors having same ohmic values but different wattage rating.

Thanks,
MAGK

 

Voltage ratings are for determining the safe working ac voltage of a capacitor (WVAC) as the dielectric will start to degrade, according to most manufacturers, at as low as 10% over their ratings so a little more is always better. Many capacitor manufacturers used to sell 280 volt ac capacitors for motor start and run applications and the most popular rating is now 370 volts ac.

One of the big three things that determines the capacity of a capacitor is the dielectric constant not its dielectric strength.

Lets not mix kVAR and microfarads.
This is from an old reference book but it may help with some of the confusion.

For 208 Volts 61.31 MFD / KVAR
For 240 Volts 46.05 MFD / KVAR
For 480 Volts 11.51 MFD / KVAR
For 600 Volts 7.37 MFD / KVAR

I hope this helps,
William Hinton

 

Responding to several recent posts in light of Marinel’s question about a single-phase motor...

1) Start-Capacitor Rated at Line Voltage.
These are power-factor units, and it is dangerous to use them for capacitor-start motors!

2) Start-Capacitor Voltage Rating.
Has no-one noticed that their voltage ratings are significantly higher than the motor’s rating? It has nothing to do with a “reforming” phenomenon or compensation for decreased voltage capability. It is do with the fact that when a capacitor and inductance are series-connected, as is the case with a capacitor-start motor, then the voltage across the capacitor could be much greater than the nominal rating of the motor!

3) Effective Capacitance.
Please notice I said effective capacitance! As any power EE knows that in a power circuit consisting of apparatus having different voltage ratings, then, their reactances must be adjusted to a common base voltage. Such adjustment alters its “effective” capacitance, as was shown in the earlier referenced thread.

Regards, Phil Corso ([email protected])

 

I'm having a similar problem with an AC motor(s) on a reel to reel tape machine. With no tape reels on the machine(Unloaded), the motors themselves seem to be working ok, though the take-up reel often needs to be given a hand turn to get started. When loaded, the take-up reel simply can't pull the load, neither can the supply reel during rewind. So the problem seems to be one of the motors lacking adequate torque. The motors are rated at 60V. During operation each is receiving 17.8V. Unfortunately without schematic I do not know if this is the correct voltage. I've checked the start cap with an ohmmeter. Both sides seem to charge ok. Definitely no short or open. Could this cap still be too weak for proper operation? Or is there anything else that may be responsible for losing adequate torque/voltage to the motors?

Thanks - JA

 

William,

What old reference book did you get this from? I am involved with power factoring and this was a great help to ball park the amount of Cap/kvar for cost purposes.

Liston