I am looking for bleed resistors for my 5hp 240v single phase
compressor motor start capacitors. It draws about 24 amps.
There are 2 capacitors in parallel giving close to 500 uF.
Since P=VI I figured P=240*25 = 6,000 w or 6kW
And V=IR or R = V/I or 240/25 = 10 ohms.

But Grainger only supplies Capacitor Resistors at 2 watts and 15 k ohms
Can I use this resistor?
http://www.grainger.com/Grainger/wwg...5421&cc item=

Thanks
Steve

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

"Ignoramus12686" wrote in message
...
On 3 Nov 2005 06:22:47 -0800, gtslabs wrote:
I am looking for bleed resistors for my 5hp 240v single phase
compressor motor start capacitors. It draws about 24 amps.
There are 2 capacitors in parallel giving close to 500 uF.
Since P=VI I figured P=240*25 = 6,000 w or 6kW
And V=IR or R = V/I or 240/25 = 10 ohms.

But Grainger only supplies Capacitor Resistors at 2 watts and 15 k ohms
Can I use this resistor?

http://www.grainger.com/Grainger/wwg...i=xi&ItemId=16
11755421&ccitem=

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


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

In article 1131029717.405898.173180
@f14g2000cwb.googlegroups.com, says...
Ok, thanks but I am concernted about the watts. Is 2 KW enough or will
it burn up?


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...

http://www.mouser.com/index.cfm?
handler=displayproduct&lstdispproductid=195904
&e_categoryid=354&e_pcodeid=71013

Ned Simmons

gtslabs wrote:
I am looking for bleed resistors for my 5hp 240v single phase
compressor motor start capacitors. It draws about 24 amps.
There are 2 capacitors in parallel giving close to 500 uF.
Since P=VI I figured P=240*25 = 6,000 w or 6kW
And V=IR or R = V/I or 240/25 = 10 ohms.

But Grainger only supplies Capacitor Resistors at 2 watts and 15 k ohms
Can I use this resistor?
http://www.grainger.com/Grainger/wwg...5421&cc item=

Thanks
Steve


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

On Fri, 4 Nov 2005 01:11:36 +1000, "Bushy Pete"
wrote:


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!

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.

-- Bruce --

--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.

gtslabs wrote:

I am looking for bleed resistors for my 5hp 240v single phase
compressor motor start capacitors. It draws about 24 amps.
There are 2 capacitors in parallel giving close to 500 uF.
Since P=VI I figured P=240*25 = 6,000 w or 6kW
And V=IR or R = V/I or 240/25 = 10 ohms.

But Grainger only supplies Capacitor Resistors at 2 watts and 15 k ohms
Can I use this resistor?
http://www.grainger.com/Grainger/wwg...5421&cc item=

Thanks
Steve



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.)

http://www.lmphotonics.com/single_phase_m.htm

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

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."

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.

Bob Swinney
"Jeff Wisnia" wrote in message
et...
gtslabs wrote:

I am looking for bleed resistors for my 5hp 240v single phase
compressor motor start capacitors. It draws about 24 amps.
There are 2 capacitors in parallel giving close to 500 uF.
Since P=VI I figured P=240*25 = 6,000 w or 6kW
And V=IR or R = V/I or 240/25 = 10 ohms.

But Grainger only supplies Capacitor Resistors at 2 watts and 15 k ohms
Can I use this resistor?
http://www.grainger.com/Grainger/wwg...5421&cc item=

Thanks
Steve



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.)

http://www.lmphotonics.com/single_phase_m.htm

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

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."

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.

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


--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."

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.

--
Bob Nichols AT comcast.net I am "RNichols42"

Robert Nichols wrote:
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

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."

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

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.

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.