bleed resistor across start cap

[zekeman]

Can you think of any reason other than personnel safety for a bleed resistor across a starting cap on a single phase motor. If you tell me it protects the relay contacts, show me why.

 

[jraef]

I don't even see the need for personnel safety. When the motor is below 75% speed, the centrifugal switch re-closes and the start winding is once again in series with the cap, which puts your main winding in a parallel circuit with both of them and would bleed the cap energy off when power is disconnected. So in essence, your main winding IS the "bleed resistor" in the cap circuit once the motor is turned off. Being that the resistance of that circuit is low, it will discharge that cap energy very quickly. An extra bleed resistor would serve no purpose that I can think of.

Why would getting into the capacitors be considered a routine procedure anyway? It probably takes 10 minutes or more to get that deep into the motor, and it's not a typical procedure done at the motor, it's usually done on a workbench after removing the motor.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[SteveKW]

Braking effect.

 

[SteveKW]

See also...

http://www.leeson.com/servicesupport/troubleshooting.htm

 

[aolalde]

If the starting capacitor is not failing frequently and/or the application does not have frequent re-starts , you do not need a bleed resistor.

 

[itsmoked]

aolalde; can you explain why a bleeder resitor would ever be functional?? I agree with jraef here.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Will a bleed resistor only across the cap only provide braking? I would think it would need to be across the entire start winding circuit, but then again, maybe that's what he is interpreting as a bleed resistor.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

I suspect that a small capacitor intended for power factor correction was used as a starting capacitor. Possibly as a replacement for the factory original.
I have never seen a starting capacitor with a bleed resistor. The resistance of the starting winding is very low in relation to a discharge resistor.
I have used small individual capacitors for power factor correction and they do have a bleed resistor.

 

[cbarn24050]

I think it's just there to remove any possible shock hazard although I have never sen one fitted in a motor.

 

[SteveKW]

Start capacitors with bleed resisters can be found in a lot of quality single phase motors. They help increase the life of the capacitor and bleed the voltage off. When the motor is shut off and the centrifugal switch closes, the excess cap voltage can instantly brake the motor too.

 

[stardelta]

I am with Steve, I have seen plenty of bleed resistors on start caps, they have to be disconnected from one side of the cap when its value is measured/tested as it confuses the meter. I have never thought to ask myself why they are there though! Maybe its just a `European thing`.

 

[zekeman]

Where do you get a braking effect since that resistor is rather large in the first place and on shutting down it is in series with the main coil, hardly a braking situation. I also don't see how it prolongs the life of the motor.

 

[cbarn24050]

Ive had thought! when the switch opens on run up the cap could be left charged, when you stop the motor the switch closes and dicharges the cap throug the windings causing an arc at the switch contacts, this cant happen if you discharge the cap while the motor is running.

 

[SteveKW]

That is correct.
The capacitors stored DC charge energizing the start winding, as the motor slows and the switch closes, is what can brake a motor. Not the resistor.

 

[SteveKW]

The resistor adds longer life to both the start switch contacts and the capacitor. Which then prolongs the life of the motor.

 

[fsmyth]

I would say that the only valid reason for a bleed
resistor (other than safety) is that the potential
across the capacitor would be double, if it was
switched in when the line was at the opposite
polarity from the stored charge. How often this
would occur with conventional centrifigal switches
or relays is problematic - there are zero-crossing
SSR's and other methods of controlling switching
timing, but these would be a bit more expensive
than a resistor, methinks.
<als>

 

[zekeman]

"Ive had thought! when the switch opens on run up the cap could be left charged, when you stop the motor the switch closes and dicharges the cap throug the windings causing an arc at the switch contacts, this cant happen if you discharge the cap while the motor is running".

The problem with this is that since the cap is in series with the secondary coil you have an L-C series circuit and so you don't get a surge discharge but a slow measured discharge.

So far I don't see any valid answer to my original post. but I thank you for all your input.