Bleeder Resistors

[Ken Garlock]

I know I will be sorry for getting into this exchange.

1. Voltage NEVER "flows trough" a capacitor, if it did you would have a short circuit and that is why there is a voltage rating on the cap. That rating is a maximum amount of potential difference(voltage) the cap is built to work at. What you do have is an alternating voltage that is continuously changing polarity. For voltages distributed in the US that change is happening 60 times per second.

The capacitor consists of two conducting plates that is insulated from each other. That insulation may be air, paper, and materials known to be insulating, even oil. The plates are placed close to each other where one plate can cause an effect on the other when a potential difference occurs between the two. When you have an excess of electrons on one plate it is negative. At the same time, the other plate has a deficiency of electrons thus being positive. IF you have a DC voltage, that is all that happens in a capacitor, end of game and we all go home. ( One time I took a 5 Mfd oil-filled capacitor charged to 500vdc to to physics class. With the cap on the lab table, I dropped a wad of steel wool across the two terminals of the cap. The result was a loud crack like a fire cracker, as the charges on the two plates of the capacitor equalized for a resulting voltage of Zero.)

But we normally are dealing with alternating voltages. In a alternating voltage environment the polarity of the voltage is reversing at some stated rate, 60 cycles. Think a bout the sine wave. The change of polarity causes the electrons to flow from the negative plate while at the same time electrons are be accumulated on the positive plate make it now negative and the original negative plate now positive. The change of charge on the two plates is the apparent current flow through a capacitor. At NO time did voltage actually pass through the capacitor.

Yes, you can have high pass and low pass filters that consist of combinations of capacitors, resistors, and inductors. Capacitors, inductors, and resistors are fundamental parts all electrical circuits.

A bleeder resistor is just as the name says, it bleeds a stated amount of current off to its reference point, typically ground. It provides a nominal stabilizing load on a circuit.

[Dan Friedrichs]

Good explanation, Ken. The one minor thing I would clarify is:

Voltage NEVER "flows trough" a capacitor,

Voltage never flows through anything - CURRENT is what flows. Voltage is just a potential.

[Dan Hintz]

it would be dependent on TOTAL number of on/off cycles, not the number of on/off cycles in a short time.

Very true... it's easy to forget the individual trees when you're talking about the forest. I'm back to being open to other suggestions, then...

[Stephen Cherry]

The residual DC voltage across the motor start capacitor will be random as it depends on when in the AC cycle the switch opens.

El correcto. When I was putting together my phase converter, I added bleeder resistors to the start capacitors. Without them the caps will stay charged for a long time. Been zapped enough times already.

Also, as stated above, Horowitz and Hill, Art of Electronics, is a great book for someone who wants to do electronics. Not much on electrical though.

[Chip Lindley]

Ok Ok! Except to negate the discomfort of being zapped while working on your motor's capacitor or RPC, this question:

Under what circumstances, and what real damage can result from lack of a bleeding capacitor on any of my shop's 1ph, 230v capacitor-start (a few capacitor-run also) motors, ranging from 1/2hp to 7.5hp???

I am thinking my fears would be better belayed by a whole-house surge protector at the main box! But, the only lightning strikes that did damage (a cordless phone base and computer modem) came in on the phone line--not the power line.

I'm from Missouri! Show Me!

[Anthony Whitesell]

Chip this issue at the point is if the residual charge on the start cap is enough to cause damage to the switch points, contacts, or other part(s) of the motor.

The run cap will discharge when you hit the off switch as the power switch is between the line and the cap and motor, while at the same time the starter cap has been automatically disconnected from the circuit by means of the centrifugal switch. When you hit the off switch the motor starts to decellerate and eventually the partially charge capacitor re-enters the circuit when the centifugal switch closes again for the next start-up.

Nothing up stream of the on/off switch will help the situation being discussed here.

[Dan Friedrichs]

Under what circumstances, and what real damage can result from lack of a bleeding capacitor on any of my shop's 1ph, 230v capacitor-start (a few capacitor-run also) motors, ranging from 1/2hp to 7.5hp???

There are no circumstances that would result in any damage from not placing a bleeder resistor in your motor. This whole idea is ridiculous.

[Ken Fitzgerald]

Anthony.....are you talking about "run" capacitors or "start" capacitors?

[Mike Henderson]

Chip this issue at the point is if the residual charge on the start cap is enough to cause damage to the switch points, contacts, or other part(s) of the motor.

The run cap will discharge when you hit the off switch as the power switch is between the line and the cap and motor, while at the same time the starter cap has been automatically disconnected from the circuit by means of the centrifugal switch. When you hit the off switch the motor starts to decelerate and eventually the partially charge capacitor re-enters the circuit when the centrifugal switch closes again for the next start-up.

Nothing up stream of the on/off switch will help the situation being discussed here.

Unless you're turning your tool on and off VERY quickly, the starting cap will be discharged by the time you turn the motor off. When you start the motor, the start cap is in the circuit. At some point, the motor is close to rated speed and the centrifugal switch opens and takes the start cap out of the circuit. The start cap has a fairly high leakage current so it starts discharging across the dielectric. In about a minute most of the charge is likely dissipated and the cap is essentially discharged (the exact time depends on the leakage current). So when the centrifugal switch closes as the motor spins down, there's no charge in the starting cap to do anything unless you turned the motor off immediately after turning it on.

Mike

[Ken Fitzgerald]

Whew........it's gotten interesting.....

I will apologize for any mistaken comments I made. The last electronics engineering theory classes I attended was in 1976 so I may have used some terms loosely. My apologies.....

I realize current flows.....voltage is a potential. I started using oscilloscopes in 1969. I know what a sign wave looks like. Until my retirement last month I carried a good Tektronix digital storage scope, an RF measurement kit, had a vector impedance meter assigned to me, have used spectrum analyzers, had a Fluke 87 assigned to me...used a Tesla meter when ramping supercon magnets to 1.5T or 3.0T....I can tell you how to make a time-domain reflectorometer using a scope, a pulse generator and a known length of cable.....

I may have used some theory terms improperly and I apologize. I haven't designed anything since I developed a fix for an "X" band precision approach radar...logrithmic IF strip upgrade in 1973.......a simple diode and capacitor improved the safety of this radar used to land air craft in bad weather......

However...in theory...both Aaron and Dan are correct.......


Yet Dan is more correct IMHO.

In theory when the centrifugal switch opens there could be a charge on the starting a cap and a resistor in parallel would held decrease if not eliminate this charge....Aaron is right in theory......


However...in the discussion about why Oneida recommends I not turn my 3HP DC on and off more than 6 times an a hour....Aaron's theory doesn't work but Dan's does. The possible damage to the contacts on the centrifugal switch would be a product of the total number of times the switch opened and closed with a charge on the capacitor.....

The amount of heat in the starting windings on the motor would be a product of the number of times current flowed though those windings in a given amount of time.

Now....back to my Bud light.....and wondering how I managed to turn 3 darts for a birthday present for my oldest son.....and they all came out measuring 10.5 grams...incredible luck!

[hank dekeyser]

Ummm, wow ! WHat an absolute crock of spit. If the motor "needed" it- Baldor would put it on at the factory- period -

[Steve Rowe]

Popcorn please. Has anyone mentioned the advantages of 3 phase power yet?

[Ken Fitzgerald]

Popcorn please. Has anyone mentioned the advantages of 3 phase power yet?

No.....and please don't bring up quantum physics either....

[Chip Lindley]

....I am thinking my fears would be better belayed by a whole-house surge protector at the main box! But, the only lightning strikes that did damage (a cordless phone base and computer modem) came in on the phone line--not the power line.

Chip this issue at the point is if the residual charge on the start cap is enough to cause damage to the switch points, contacts, or other part(s) of the motor.

The run cap will discharge when you hit the off switch as the power switch is between the line and the cap and motor, while at the same time the starter cap has been automatically disconnected from the circuit by means of the centrifugal switch. When you hit the off switch the motor starts to decellerate and eventually the partially charge capacitor re-enters the circuit when the centifugal switch closes again for the next start-up.

Nothing up stream of the on/off switch will help the situation being discussed here.

Anthony, my point was: I should be more concerned with a lightning strike doing harm to my motors, than any worry over bleeding resistors.

Most commercially-rated motor starters have contacts heavy enough to withstand the arcing that normally accompanies make/break cycles. This definately may not be the case with "economy" starters. Contacts in a Square D Size 1 mag starter are about four times the size of the tiny contacts (rated for 5hp!) on my former Grizzly drum sander. Motors or starters made to painfully minimum specs cannot be expected to compete favorably over time with commercial-grade NEMA-rated stuff. Same with centrifugal motor contacts. I am not generally including European machinery in my deride, but you never know! They may be importing parts from China too!

I would wish the OP to elaborate on what exactly failed in his European BS motor. A total windings fry? Start winding fry? Start contacts fry? Or blown capacitor(s)? Inquiring minds wanna know!

[Steve Rowe]

No.....and please don't bring up quantum physics either....

Ahh, but the Heisenberg uncertainty principle would most certainly apply in this thread.