Bleed Resistor for Capacitor Question

I think that you misunderstand the urpose of bleed resistors.

The bleed resistor is there so that, after the starting capacitor is used, it discharges within some reasonable time.

The bleed resistor was never meant to conduct a lot of current.

The time when the cap bleeds about 60% of its charge would be RC, or

0.0005*15000 = 7.5 seconds. In the next 7.5 seconds it would lose 60% of the remaining charge, etc. That means that in about a minute, a 15k Ohm resistor would almost fully discharge the capacitor. i

Ok, thanks but I am concernted about the watts. Is 2 KW enough or will it burn up?

2 kW??? As in kilowatts? Let's see. The maximum voltage that your capacitor would see is perhaps about 400V.

At 400V, and 15 kOhm, the power produced at the resistor would be

400^2/15000 = 10 watts. After 7.5 sec, the voltage would decline to perhaps 180 volts, or 2 watts. After 7.5 more sec, it would decline to perhaps 70 volts, or 0.3 watts. i

Yep, the bleed resistor is only to prevent you from getting bitten by the cap when you open up the machine to service it. Some capacitors will hold a substantial charge for several hours. One type of machine I work on has a warning on the side of the high voltage caps that they store a lethal dose of electrickery!

The 15Kohm one you mention is a good size, but anything in the range from 5K to 50K that will handle a watt or two will do.

As long as you take your time to open a machine after having the machine powered up, there is plenty of time to allow the cap to discharge through whatever bleed resistor you care to use, but remember that if you are servicing any electrical machine, the bleed resistor probably is open circuit and this will allow the cap to bite harder than a doberman! That bite can come from any piece of wire that is connected to the cap, as long as there is a circuit, so it could be several feet from where the cap is!

Stick a (cheap) screwdriver across the terminals so you know the cap is discharged before you touch wires! If it takes a chunk out of the screwdriver, you will only get a little scare!

Hope this helps, Peter

I had some caps store it for days. I am glad that I shorted them with a screwdriver before starting to work on them, just in case.

Yeppers.

i

You mean 2 watts, right? Since it's going on the start caps which should only see power for a few seconds at a time you can probably get away with the 2W, 15K resistor, but I'd size the resistor for 100% duty. Watts = volts squared/ohms; 240^2/15000 = 3.8 watts.

A much wider selection of power resistors is available from Mouser or DigiKey. I like this style if you have a place to attach the resistor to a panel...

Ned Simmons

You are WAY off base. The cap is wired centrifugal switch -> start cap -> start winding. From the way you wrote your email, I believe you are thinking the resistor goes in series with the cap i.e. centrifugal switch -> resistor -> start cap -> start winding. This is WRONG. The bleeder resistor is wired in parallel with the start cap. When you put 240 volts across the start cap it will draw 24 amps, but when you put 240 volts across a 15k resistor, it only draws

240/15000 = .016 amps, or 16 mA.

Figuring power at VI, get 3.84 watts.

So a 5 watt 15k resistor would work fine.

I used to scrounge big motor run caps from Seattle City Light (came out of old street lights) and those had multiple tab-type terminals. They often had a 10k resistor wired across the capacitor.

Note that run caps don't need bleeder resistors since they're wired across the windings of the motor. When you shut down a phase converter, any voltage that would have been stored on a run cap bleeds down through the motor windings.

GWE

Kind of hard on the screwdriver, especially if someone else borrows one of your good screwdrivers to do it - and the induhvidual also forgot to turn off the power feed to the equipment...

A much more elegant solution is to use a Wiggy voltage tester across the capacitor, and it'll jump and indicate as the solenoid coil bleeds off the charge.

-->--

I may be forgetting something in my dotage. Can someone set me straight on this one?

Isn't the bleeder only there to discharge the starting capacitor in the event the centrifugal switch fails to close after the rotor comes to a stop? Or maybe if a winding opens up?

This page shows numerous configurations of single phase motors and it looks to me like the capacitors in all of them will discharge through the motor winding(s) when power is removed. (If the centrifugal switch closes as it should.)

Is there yet another commonly used configuration which leaves the starting capacitor wide open every time power is removed?

I think anyone foolish enough to try and open up a motor while it's still powered up and running deserves to get a shock.

Jeff

>

Yes, that I understand, but the motor would be *running* then, and what kind of fool is going to open up a running motor so he can touch the capacitor terminals or leads?

When the motor stops, the start switch closes and there's then a path through the motor windings which discharges it.

Capice my question?

Jeff

In article , Robert Swinney wrote: :The OP wanted bleeders for starting capacitors. After the starting :interval, when the switch opens, the start cap would be in an open series :circuit with the start winding.

That's while the motor is _running_, and since the power is ON any safety issue about a charged capacitor is moot. Whether or not the capacitor has a discharge path while the motor is _stopped_ depends on how the motor is wired. A non-reversing motor with just a on/off switch would normally discharge the capacitor through the motor windings when the motor stops. A motor wired to a drum switch for FWD/STOP/REVERSE would very likely leave the capacitor charged when the switch was in the STOP position.

Thanks, that's the kind or reason I was looking for. I hadn't thought of how a drum switch could affect things. That makes a bleeder make sense in that application.

Jeff

All in all, this really seems like over-kill. A lack of bleed resistor would only be a problem if:

- the motor is wired with a Forward-Stop-Reverse switch, and

- the capacitor is being accessed/removed, and

- the start switch opened during the high part of the voltage cycle, and

- you don't discharge it mechanically, and

- you put fingers of both hands on opposite terminals (fingers of the same hand would give you a shock, but not across your heart).

The "expected value" of an event is it's probability of occurring times the value of the consequences. The expected value here is pretty low. I live with greater ones all the time.

Bob

On Thu, 03 Nov 2005 15:33:53 GMT, Ignoramus12686

SNIP

SNIP

Hey Iggy,

"DAYS" is nothing for a cap. And while it's a "better than nothing" safety practice maybe to do that shorting with the screw driver, it's not the best plan. I've been told by guys that I trust that it is very hard on the cap if happens to be charged, and it will definitely be hard on the tool if it's a big cap!! Better to drop a resistor across it, if there is even a suspected reason to do so, then the screw driver for your own feeling of well-being after that. We used a lot of 100 to 200 mikes at 150VDC caps (relay drop-out timers), and guys reaching in to a big cabinet would sure do a lot of head-slams and elbow whacks on the cabinet sides and doors if they weren't careful. The whack on the steel hurts a lot worse that the shock!!

I picked up a "fence charger" for a buck at an auction last week. Plugged it in today and the sucker arced right across the HV stand-off to the case. Surprised the heck out of me!! Gotta be well over 5KV!! I didn't think it would be that high!

Take care.

Brian Lawson, Bothwell, Ontario.

I have some large caps, and when I discharged them, the sound was like a .22 shot.

I agree. I may make a capacitor discharing tool for this (nothing fancy).

That's great!!!

I have 20 franceformers for sale, 9,000V, center tapped, no GFI, 30 mA, the rec.crafts.metalworking special price is $15 apiece.

i