Bleeder Resistors

[Ken Fitzgerald]

and here's the gentleman's reply...........

Ken,

Baldor's experience is that is some applications, frequent starting, long coast down, etc, the start capacitor will discharge as the motor passes from the run winding to the start winding (approx. 1400 RPM on an 1800 RPM motor) creating a shuddering effect as the motor tries to "restart". The addition of a bleed resistor mounted on the start capacitor can mitigate this in varying degrees depending on the application. We recommend a 10k ohm 5 watt resistor mounted between the terminals of the start capacitor. We also recommend a three-pole contactor that is wired so the start winding is completely out of the circuit when power is removed. This allows for a coast down with no regeneration through the start circuit regardless of the charge remaining on the start capacitor.

Baldor has always advised that the starting frequency (number of starts per
hour) of a single phase motor is dependent on the starting capacitor being energized for a maximum of 60 seconds per hour. This is sometimes hard to quantify, fans and pumps do not require high starting torque so the motor comes up to speed quickly with the start capacitor in the circuit for a very short time. Harder to start loads would require the capacitor to be in the circuit for a longer period of time. You are also correct in your comment that frequent starting increases temperature rise within the motor and can also be detrimental to a long service life but the capacitor is really the limiting factor on starts per hour.

It seems reasonable to believe that eliminating the effects of the capacitor discharging during coast down would be better for the motor than to experience the effects of regeneration. We do not have any data that estimates the increase in capacitor, switch or motor longevity.

I hope I have answered your questions. If you have further questions please let me know and I will try to answer them.

This is a copy and paste from the guys email.

So based on what he says...The resistor paralleled to the capacitor may reduce shudder when start capacitor is placed back in the circuit by the centrifugal switch BUT....a 3 pole contactor that immediately removed the start winding when power was removed would eliminate the problem entirely.

It's complex........


and.... Arguing with an engineer is like mud wresting with a pig....ya' get muddy and then you realize the pig's enjoying it......

[Dan Hintz]

the start capacitor will discharge as the motor passes from the run winding to the start winding

I take this to mean when the centrifugal switch disengages?

[Phil Thien]

and here's the gentleman's reply...........

Baldor has always advised that the starting frequency (number of starts per hour) of a single phase motor is dependent on the starting capacitor being energized for a maximum of 60 seconds per hour...

Does the starting cap wear out faster if it is used more than 60 seconds per hour? That is, does it get hot and gooey or something?

IOW, will the start capacitor work 5000 times whether you do them one after another every five minutes, or one per year for 5000 years? Or will doing them one after another every five minutes make the start capacitor provide 1000 cycles instead of the intended 5000?

[Anthony Whitesell]

Baldor's experience is that is some applications, frequent starting, long coast down, etc, the start capacitor will discharge as the motor passes from the run winding to the start winding (approx. 1400 RPM on an 1800 RPM motor) creating a shuddering effect as the motor tries to "restart".

I actually understand it to mean when the start cap switch re-engages during the slowdown to be ready for the next start up. Do you agree?

[Ken Fitzgerald]

I take this to mean when the centrifugal switch disengages?

Dan.....to me it means when the starting cap re-engages the starting winding during slow down as the centrifugal switch closes.

Does the starting cap wear out faster if it is used more than 60 seconds per hour? That is, does it get hot and gooey or something?


IOW, will the start capacitor work 5000 times whether you do them one after another every five minutes, or one per year for 5000 years? Or will doing them one after another every five minutes make the start capacitor provide 1000 cycles instead of the intended 5000?

I don't know.

I actually understand it to mean when the start cap switch re-engages during the slowdown to be ready for the next start up. Do you agree?

Yes


To me what it means is that there is a possible charge on the capacitor when the centrifugal switch opens up when the motor is first turned on and is coming up to speed. During spin down after power is shut off the residual charge may cause a motor to shudder as the capacitor is reconnected to the start winding by the centrifugal switch closing. A resistor in parallel may help reduce or eliminate that shudder but putting a 3 pole contactor in the circuit so that the starting motor winding is removed totally from the circuit when power is removed would completely eliminate the chance of a shudder.

He states they have no data estimating increased life in any of the components as a result of either of these two acitons.

[Aaron Rowland]

I agree with Bador . A very simple mod to solve a problem with a simple resistor. I and others, like Bob, have been using this trick for years. This is the best way for a novice to modify the motor. The more advanced mod uses a method to actually remove the starting wind and cap from the motor during coast down. The cap needs to return to zero voltage for start up. Before start up the start winding and cap is reconnected. That's what I did with my light bulb trick but in a simplified manner as I mentioned. I won't post that circuit because it is a bit complicated.

I've never posted my credentials but I'm well educated and versed in the field of electronics. I made a very good living at it ,have some patents relating to my specialty, and sold my company to a well known multinational who moved it off shore. I am also a bit bemused by the rancor here about a simple resistor. As Caspar noted, read Trismegistus, relax and enjoy life ,it's short. Wood working is the best hobby in the world. My apologizes to Ken if I seemed a bit smug.

[Phil Thien]

I don't know.

Ken, that is unacceptable! You're the man with the answers, not the "I don't knows."

I was listening to Car-Talk once and they were advising someone about buying a former taxi or chauffeured car. The mileage wasn't that high, but they cautioned the caller that the engine probably had a lot of hours form idling (waiting) on it. But they (Click/Clack) didn't seem that concerned.

I'm just posting this because I hope someone knows whether start caps get hot/gooey from frequent starting. Of if they just have X starts to their life, and then they're done.

Because if the entire point is frequent starts/stops was going to kill the cap sooner but I still get 5000 starts/stops, then big deal. If I'm going to significantly reduce its life cycles, then I'd be more concerned (but probably not).

[Ken Fitzgerald]

Phil,

I have more questions than answers and the older I get the more I realize how little I ever did know.

I can't accurately state whether it would get "sticky and gooey".

I have seen electrolytic capacitors fail in several ways...explode spewing electrolyte all over the place (these you'd think shorted just before exploding and then opened up as a result of the explosion) ......open internally without any physical evidence (in the case of a starting capacitor this you would think would cause a motor to not start properly....wrong direction or not start at all leading to a circuit breaker trip, fuse blowing or a motor burning up) ....change values of capacitance without any physical evidence....short, explode and catch fire(in this case we actually measured the shorted capacitor after the fact)......

Based on the recommendation that you don't run starting caps more than so many seconds per hour, I'd say you would shorten the life by doing it more often and therefore more seconds per hour. However, the important question is whether you would shorten the life of the capacitor or the motor?

[Anthony Whitesell]

Based on the recommendation that you don't run starting caps more than so many seconds per hour, I'd say you would shorten the life by doing it more often and therefore more seconds per hour. However, the important question is whether you would shorten the life of the capacitor or the motor?

I can agree with this. (you beat me to writing the question)

Last Thanksgiving I incurred a catastrohpic capacitor failure myself. The onboard electrolytic capacitors for the motherboard of my computer went "pop". I spent an hour trying to figure out why the monitor wasn't working until the LOML told me she heard a series of pops earlier in the evening. When I opened the case the side panel was covered in goo. So much for that computer, but atleast it went out with a bang.

[Ken Fitzgerald]

Anthony while stationed at NAS Meridian, MS in the early 70's , I worked air traffic control maintenance. We took care of all the equipment used the joint FAA/Navy ATC there except for the FAA's search radar. We also maintained all the electronics equipment in the weather forecasting office there. They get a lot of thunderstorms there. A lightning strike near a runway resulted in lightning running in on some cables into a piece of equipment in the weather forecasting office. Several electrolytic capacitors exploded. It took a long, long time and all kinds of solvents to clean the electrolyte out of that one chassis. Terribly messy!

[John Coloccia]

and here's the gentleman's reply...........

*snip*
...but the capacitor is really the limiting factor on starts per hour.

That was always my understanding as well.


re: grounding PVC

I suspect the wire wrapped PVC works because it gives a lower impedance discharge path than YOU. It periodically zaps the wire instead. That would be my guess.

re: shuddering and resistors
I don't have a dog in this fight. I'm just wondering how this works. Let's get down to the nitty gritty, and someone correct me where I'm straying.

When the centrifugal switch is open, the run windings only are in the circuit. When the switch closes, the start cap and start windings are introduced into the circuit. Now on startup, these two windings are in parallel with the AC power source, but on shutdown the AC source is disconnected, and you end up with a very classic, series circuit consisting of:

2 coils
1 capacitor
some resistance (from the wires)
and a switch.

This is like analyzing that circuit in college where you charge up the cap, flip the switch to close the circuit, and watch the fun.

So the run coil's field is collapsing, driving the current and charging up the cap. The cap is at some unknown potential so it will do something too, potentially driving current through the coils. Ultimately, you will get a damped oscillation between the inductors and the caps.

If this were driven by the cap, I would expect to be different every time, no? I have to think that it's driven by the run coil, and that the resistor serves to damp the oscillation. Maybe one of the EE's here can think about this. I'm already running late.

[Anthony Whitesell]

re: shuddering and resistors
I don't have a dog in this fight. I'm just wondering how this works. Let's get down to the nitty gritty, and someone correct me where I'm straying.

When the centrifugal switch is open, the run windings only are in the circuit. When the switch closes, the start cap and start windings are introduced into the circuit. Now on startup, these two windings are in parallel with the AC power source, but on shutdown the AC source is disconnected, and you end up with a very classic, series circuit consisting of:

2 coils
1 capacitor
some resistance (from the wires)
and a switch.

This is like analyzing that circuit in college where you charge up the cap, flip the switch to close the circuit, and watch the fun.

So the run coil's field is collapsing, driving the current and charging up the cap. The cap is at some unknown potential so it will do something too, potentially driving current through the coils. Ultimately, you will get a damped oscillation between the inductors and the caps.

If this were driven by the cap, I would expect to be different every time, no? I have to think that it's driven by the run coil, and that the resistor serves to damp the oscillation. Maybe one of the EE's here can think about this. I'm already running late.

That sums it up pretty well until you get to the field collapsing part because the resistance in the coil is so low the LC aspect will quickly be damped out and oscillations not a factor. Plus we are dealing with AC motors so the steady discharge of the RUN cap will not count. The problem becomes when the pulse of juice (half of an AC cycle) is introduced to the start winding when the start switch closes and reenters the circuit.

From the experience with my jointer, the shut down thud appears to have a different intensity each time I turn it off. I could help but notice it last night while I was down in the shop.