5HP MOTOR WITH RUN CAPACITOR

[2571]

Please be patient with me,
We have an older 16" Dewalt Radial arm saw we use for building pallets. On three separate occasions now, we have blown the run capacitor out of the mounts. Motor was recently taken to rewind shop and had the end bells machined because of damage form bad bearings. We had thought this was the suspect to the capacitors blowing. However once again we let the magic oil and smoke out. The motor in question is a 5hp 230 single phase. My direct question is there is a small transformer looking item in the pecker head, wire in series with the cap. Is this in fact reducing the voltage to 120V? In addition, I would like to hear your interpretation on how a run capacitor functions in the motor and what could cause it to blow.

 

[jraef]

The small "transformer" looking device is probably a potential relay. Do a Google search on how single phase motors work, that will explain the role of the capacitors in allowing the motor to turn when the 180 degree magnetic fields would normally just oppose each other. In the descriptions of 1 phase motor operation you will read about centrifugal or speed switches used to determine when to switch out the starting capacitors. An alternative to using a mechanical speed switch is to use what is called a potential relay, which determines when the motor has reached nearly full speed by looking at the back emf (voltage potential) on the motor. If the potential relay is not functioning, the start cap could end up stayting in the circuit and creating a resonance with the run cap, which ultimately leads to the run cap overheating and letting out the magic goo. The thing is, your motor rewind shop should have tested and replaced it IMHO.

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[waross]

I suspect that the thing is a thermal overload device, probably a Klixon or clone. I have found that the most common cause of run capacitors blowing is that they have been replaced with start capacitors. Many start capacitors will not withstand continous duty.
With respect, JRaef, I see a lot of potential relays on refrigerators and small hermetic units in general, but I have not seen one on a saw motor.
Are you sure that the capacitor is not a start capacitor? Run capacitors are becoming more common on air conditioners but I don't often see them on saw motors, particularly older saws.
Transformer to 120 volts? Not enough space in the junction box for a large enough transformer.
Function of the run capacitor? Power factor correction and incidentaly transient surge suppresion. The capacitor doesn't affect the motor directly but does reduce the current somewhat with the result of slightly higher voltage to the motor. The exception is a permanent split capacitor motor, but 5 hp. seems a little large for a PSC motor. What do you say JRaef. Have PSC motors gotten larger without me seeing them?
The fact that there is a device in series with the capacitor also suggests a probability that it is a start capacitor.
respectfully

 

[jraef]

waross,
I thought of either a OL device or maybe even a braking device, but he said it is wired in series with the capacitor and that is the only thing I could see being in series that way. I know what you mean about not seeing potential relays on saw motors, but I have seen them mentioned in a number of machine tool and wood working DIY forums of late touted as a way of replacing worn out centrifugal switches. Seems to be a sort of sub-culture solution to the fact that older motors are beginning to get difficult to repair because the manufacturers are consolidating and not supporting old designs. By the way, I was thinking of a Cap-Start Cap-Run 1 phase motor, which is what you often see in the 5HP range as far as off-the-shelf motors go.

2571,
Here is one of the types of articles I was referring to regarding 1 phase motor options. Scroll down to the Capacitor-Start Capacitor-Run design, that is what I think you may have if you have a set of caps labels as "Running" caps. If you just thought they were running caps and didn't really know for sure, they might be starting caps as waross said, shown in the Capacitor-Start Induction Run design.

http://www.iprocessmart.com/leeson/leeson_singlephase_article.htm

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[itsmoked]

I'd bet good money that is a brake device being described. Because of the exposed large blades they started in with braking on RA saws early on.

They started with manual brakes. You had to push the mechanical brake on the motor.(I have one.) Then they went to automatic ones based on relay stuff.

Also 5Hp on a single phase RA saw?!?!? Sounds about 2X bigger than I could ever imagine. A 5 HP single phase motor is [!]HUGE[/!]. I run an air compressor with a 5HP 1ph motor - it weighs almost 200 pounds and is 20" in diameter and 25" long. Maybe Dewalt went the way of Sears and blatantly lies about the actual HP though.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Keith, they make them. Remember, these are 2 pole motors so they are smaller than a 4 pole of the same HP.



Note that the brake is mechanical.

[url=http://www.jraef.com]JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
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[itsmoked]

Thanks Jeff. It is 6000+ watts. I see the light.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[peebee]

Just a bit off-topic....

I bought a Shop-Vac the other day which stated "5.5HP, 14A" on the box, it's 120-volt single-phase with a standard 15-amp plug.

Turns out that 5.5HP is a "peak" rating (but you gotta dig to figure that out)... and that's about the only way you'd ever pull 5.5HP out of a 15-amp 120-volt single-phase plug. I'm not quite sure exactly what "peak" HP is defined as, but I imagine you're pulling close to LRA when you see "peak" HP, and I suspect that if you divided the "peak" HP by 6 or 7 that you'd be a lot closer to a real (continuous) HP number.

I suspect a lot of motorized consumer goods are advertised by their "peak" HP rating.

 

[jraef]

That has been discussed at length here in the past. Your suspicions are correct. Consumer goods are not bound by things such as NEMA or IEEE when it comes to marketing terms such as "peak HP". The only thing they can't lie about is the electrical rating, either in watts or amps, because UL requires that on every electrical device. Read the watts, divide by 746 to get "real" Horsepower.

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[davidbeach]

Read the watts, divide by 746 to get "real" Horsepower.

and then multiply by 0.5 to 0.75 to factor in efficiency and get an even more realistic value for the horsepower that might actually be doing something.

 

[itsmoked]

Yes it is blatant lying because HP infers work and most of that "peak" is just heat dumped into the windings during the first 1/4 second of LRA.

Sears started with these blatant lies about 10 years ago and now many others have jumped on the wagon. I should really take this to our District Attorney who hates this kind of false advertising and actually prosecutes about 3 of these type cases a year against various national suppliers.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Must be slow there in SC Keith.

Apparently this has been done already though. This link is to a Google html version of a pdf, the original of which is gone now (Google stores the html search version longer apparently). It describes a class action lawsuit brought against Campbell-Hausfeld, deVilbiss et al on this very issue.
Google link

Just in case Google drops it now that the original is gone, here are the salient points.

NOTICE OF PENDENCY OF CLASS ACTION AND PROPOSED SETTLEMENT
TO:
ALL PERSONS IN THE UNITED STATES WHO PURCHASED BETWEEN AUGUST 1, 1996 AND JANUARY 30, 2004, AN ELECTRIC AIR COMPRESSOR MANUFACTURED, IMPORTED, LICENSED OR DISTRIBUTED BY CAMPBELL HAUSFELD, DEVILBISS AIR POWER CO., INGERSOLL-RAND CO., OR COLEMAN POWERMATE, INC., AND WHICH IDENTIFIED ON ITS LABEL A HORSEPOWER THAT IS NOT CONTINUOUS RUNNING HORSEPOWER. ...
<snip>

Plaintiffs allege that Defendants labeled, promoted and sold consumer air compressors with electric motors as
having higher horsepower motors than they in fact contained. Plaintiffs allege that the Defendants knew that these air
compressors actually contained smaller electric motors and nonetheless falsely labeled and promoted the higher
inaccurate horsepower, in violation of the Illinois Consumer Fraud and Deceptive Business Practices Act, and comparable
laws of other states. Plaintiffs allege that Defendants’ conduct caused them and other Settlement Class Members
damages. In the Litigation, Plaintiffs sought damages and an order of the Court prohibiting the labeling of peak
horsepower in the future. Defendants deny the allegations, deny liability, and deny that any Plaintiff or Settlement Class
Member is entitled to damages or other relief. The Court has not ruled on the merits of any of the claims or defenses
asserted by the parties in the Litigation

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[itsmoked]

Ah that is sweet!! Thanks jraef. Now I can stop feeling guilty about it. There was a settlement - class action.

Hmmm I wonder why they persist in false advertising.

Thanks, I must read it all.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I remember a discussion years ago in regards to a powersaw that appeared to be rated on the basis of peak torque times rated speed. Considering that the peak torque may be at about 30% of rated speed, this took the first lie based on intermitent ratings to a new level that was not attainable in fact.
respectfully