Bleeder Resistors

[Aaron Rowland]

I think this is important enough to mention again. Over at the Workshop site I discuss some details under "how many times do you stop/start your cyclone?"

Graiinger sells some 15K/2watt resistors for it for verification. I recently burnt out a very expensive motor on my Euro band saw. The bleeder might have prevented this. On my new Baldor motor you can be sure I will add this 15K/2 watt resistor or a light bulb. I didn't bother on this bandsaw because I hate it and rarely use it. A bad mistake. I have a elcheapo little one made in Taiwan that works just fine.

[Lee Schierer]

Okay, I'm in the dark why do you need a bleeder resistor?

[Anthony Whitesell]

I've not heard of this before, but I'm willing to think on it a while.

In the meantime, please atleast use resistors with some safety factor. These 2 watt jobs are a tad small. Not to offend those that know but the basics are:

Power = Voltage * Current and Voltage = Current * Resistance.

Thus Current = Voltage / Resistance and therefore Power = Voltage * Voltage / Resistance

The point is:

For the 15K resistors on a 220V motor, Power = 220 * 220 / 15000 = 3.23Watts

Worst case would have the voltage high +10% and the resistor low by 10%, pushing the wattage up to 4.34W.

The capacitor has very little to do with sizing the resistor. If you use a lower resistance the capacitor will discharge faster but the current will increase with the power increasing proportionally and simulataneously.

[Ken Fitzgerald]

I'll bite too.....why would you need a bleeder resistor? What voltage are you trying to bleed off of what component?

[bob hertle]

Purpose of the bleeder resistor, on a capacitor start motor, is to bleed off the charge on the capacitor before shutting off the motor. When the centrifugal switch contacts close on spindown, the cap, if still charged, will dump its charge across the windings causing (sometimes violent) shudder. Baldor engineering recommended to me, 200 ohm, 20 watt wirewound resistor. 15 K ohm sounds high and if you start and stop frequently, the time constant resulting from the 15 k ohm may be too long to fully discharge the cap. I may be full of it though, so I'll defer to any electrical engineers on the forum! I can tell you that I have (2) 1.5HP Baldors, running with the 200 ohm for about 15 years with no problems other than one replaced capacitor. Another useful trick to get rid of the shudder, is to utilize a switch with an extra pole or set of contacts, and switch the line between the capacitor and the start winding.

Bob

[Dan Hintz]

Ken,

I asked a similar question in the thread that spawned this one. From the Oneida rep quoted, it sounded to me like the problem with multiple on/off cycles was arcing at the contacts of the centrifugal relay. I also questioned the point of a bleeder resistor on the cap if the relay was the cause, but after reading Bob's reply here, I can see the advantage of it. I don't know the value of the cap, but I have my doubts it would be big enough to provide enough back-EMF on the windings to cause it to shudder, but it's certainly enough to arc-weld the relay's contacts.

At least it's something else to research...

[Ken Fitzgerald]

Capacitors store DC not AC. That capacitor should not have any voltage stored on it. I wonder if it's a series limiting resistor and they are limiting rush current from back-EMF when the contacts close?

[Dan Friedrichs]

Various points:

1) 15k would be about right, and higher would actually be better. The time constant of a "normal-sized" 1-2hp motor starter capacitor and a 15k resistor would be about 10secs. That countdown starts once the centrifugal switch opens as the motor is turned on, though. Unless you are running the DC for less than 10 seconds at a time, I'd use a much, much larger value.

2) That doesn't matter, though, because this whole thing is a stupid idea. Here's why:

a) The problem with starting induction motors frequently has NOTHING to do with the centrifugal switch. When was the last time you've seen a motor fail because the centrifugal switch welded itself shut from arcing? I've never seen it. The centrifugal switch can get stuck open or shut from dust, etc, but I've never seen contacts welded together from arcing. Have you?

b) The reason that capacitor-start induction motors are not supposed to be repeatedly started and stopped is that they have a starter winding (connected to that starter cap). This winding is generally only a few turns, and is often higher-resistance wire. Because of this, the current density is much higher than in the primary winding, and simple I^2R losses in this winding cause it to get quite hot. Frequent starting will preferentially heat this winding, causing potential motor failure. This is the ONLY REASON that induction motors are not supposed to be started frequently.

c) If the bleeder resistor was a good idea, the motor manufacturers would include it. You don't even need a 2W resistor. A 100k resistor with 120V across it will only dissipate 0.1W. A quarter-watt resistor costs $0.001 or less.

[Chuck Wintle]

I think this is important enough to mention again. Over at the Workshop site I discuss some details under "how many times do you stop/start your cyclone?"

Graiinger sells some 15K/2watt resistors for it for verification. I recently burnt out a very expensive motor on my Euro band saw. The bleeder might have prevented this. On my new Baldor motor you can be sure I will add this 15K/2 watt resistor or a light bulb. I didn't bother on this bandsaw because I hate it and rarely use it. A bad mistake. I have a elcheapo little one made in Taiwan that works just fine.

what are you talking about? more detail would work well.

[Frank Drew]

Sorry about the motor, Aaron. What band saw is it (what make?), and why don't you like it?

From the title of your thread, my first thought was some kind of hemostat, something to stop the bleeding if you stuck your finger up against something sharp!

[Aaron Rowland]

Purpose of the bleeder resistor, on a capacitor start motor, is to bleed off the charge on the capacitor before shutting off the motor. When the centrifugal switch contacts close on spindown, the cap, if still charged, will dump its charge across the windings causing (sometimes violent) shudder. Baldor engineering recommended to me, 200 ohm, 20 watt wirewound resistor. 15 K ohm sounds high and if you start and stop frequently, the time constant resulting from the 15 k ohm may be too long to fully discharge the cap. I may be full of it though, so I'll defer to any electrical engineers on the forum! I can tell you that I have (2) 1.5HP Baldors, running with the 200 ohm for about 15 years with no problems other than one replaced capacitor. Another useful trick to get rid of the shudder, is to utilize a switch with an extra pole or set of contacts, and switch the line between the capacitor and the start winding. ================================================== ================================================== ==============================

Bob

You are 100% right, Bob. You know your stuff. I had my hands full keeping it simple and still got some really off the wall comments Tell these guys to argue with Grainger or Baldor , not me.. In one of my posts I bumped the number to 10K/3 watts.. I use a 50 watt light bulb with a diode in one leg so they last for ever It's fun to watch the light(s) work. I need verification for ideas like this because who am I? So I use Graingers statement on their site. It will work for most aps. To much baloney out there to trust any one except people like Baldor. They are the very best. Sure wish I had something in writing published by them on this.

[Dan Friedrichs]

To much baloney out there to trust any one except people like Baldor. They are the very best. Sure wish I had something in writing published by them on this.

If this was a good idea, why wouldn't Baldor be including it?

[Aaron Rowland]

Won't go there. See my posts on bandsaws. Thanks for the question however.

[Ken Fitzgerald]

Aaron....explain why it's needed.............capacitors don't store AC voltage so you wouldn't need a bleeder.....come on....get specific .......

Now if they are trying to limit current that would be something different......but........not a bleeder.

[Mike Henderson]

It's baloney alright. Dan nailed the reasons in his earlier post. Plus, the cap is across AC as Ken pointed out, not DC, so you're not going to have much of a residual charge on the cap when the starting switch kicks out at startup.

I'm just amazed that these "urban legends" get passed along without anyone questioning whether they make any sense or not.

Mike