View Full Version : Band saw motor won't turn at full speed after replacing blown cap

02-21-2013, 02:55 PM
Single phase 1725 rpm 110v cap start motor was taking a few secs to get up to full speed the last few days then the start cap blew out. I replaced the 150mfd cap with a 200mfd and it ran fine with no load. When I attach the belt it will only get up to 1300 rpm and the centrifugal switch will not engage. Bearings seem ok, I can spin the shaft easily by hand and it continues to turn 5-6 times and I cannot see any damage to the windings. For now, it works ok but the belt vibrates (due to lower speed I assume) and it heats up. Without the centrifugal switch engaging I may be risking burning out the start windings and/or blowing another cap. Any ideas?

02-21-2013, 03:21 PM
Is everything else along the drive train running freely?
Check your pulleys and gears , make sure nothing is binding.
How old is the motor?
Somebody might have let some magic smoke out.

02-21-2013, 04:21 PM
You may have already cooked the windings, the slow start is a give away.
Is it an R.O.C. motor, by any chance?
They are notorious for failed caps and cent. switches.

The Artful Bodger
02-21-2013, 04:48 PM
R.O.C.? is that a type of motor or where they are made?

02-21-2013, 05:02 PM
By the way, what's your band saw?

Bob Ford
02-21-2013, 05:22 PM
Your start contacts are suppose to close at low speed and open at normal speed. If they are not opening you will burn the start windings by running the motor.
From your description the start contacts are stuck( motor does not get up to speed).
Open the motor and check the start switch contacts and that the centrifugal switch is operating freely.
If the contacts are burnt stuck together they may need replacing. The centrifugal part should be free, check for sawdust grease dirt stopping the movement and clean.


Jim Hubbell
02-21-2013, 06:51 PM
I had a table saw motor show quite similar symptoms. After much checking it came down to the sil. rubber dust guard around the cent.st. switch. It would hold very fine dust between the contacts which would burn and form a series resister in the start circuit. Thru the dust guard away, cleaned the points and all is now well.

02-21-2013, 07:15 PM
R.O.C.? is that a type of motor or where they are made?

Republic of China. ;)
They don't seem yet to have mastered spit phase motor technology!

02-21-2013, 07:26 PM
Looks lile you need to get ready to consider:

- replacement motor; or

- a re-wind.

J Tiers
02-21-2013, 07:47 PM
The sticky switch may be the original cause of the blown capacitor. (and they often DO "blow"...) It's far more likely that you will blow the cap than fry the winding , at least at first. If you keep replacing capacitors, well, then..............

The R.O.C. (Taiwan) make OK motors...... it's the P.R.C. motors you really need to look out for. (aka "chicom", "red china", etc......)..

So take it apart, figure out what the problem is, and fix it if possible. If not, a "new" (to you) motor should cost you between $5 and $15 at a yard sale. There is NO REASON to spend hundreds on a new motor when so many are out there for cheap.... You can buy 10 of them before you approach the cost of a single cheap new motor.

There isn't really any reason for a capacitor motor on a bandsaw,.... they are made for better start torque, but a bandsaw doesn't really need that, and any regular "split phase" motor would likely work as well.

02-21-2013, 10:01 PM
It's likely the contacts sticking closed that caused the cap to burn. Interesting- will a capacitor start motor not come up to full speed with the start winding powered? I've never checked that out-

02-21-2013, 10:55 PM
Contacts and centrif switch were both clean and work fine with no load - revs at normal 1725rpm. Motor seems plenty strong as I cannot slow it down when I jam a piece of wood against the pulley but when I attach the belt to the lower wheel, no blade, it will not go over 1300rpm and the centrif switch will not engage. I also tried running without the centrif switch and started by spinning by hand but it will not go over 1300.

Bob Ford
02-21-2013, 11:36 PM
Next question. How good are the bearings? Could the side load from the belt be causing the armature to drag on the stator?


02-22-2013, 12:01 AM
They are not noisy and I cannot feel any play but that makes sense. I'll try a new set

Bob Ford
02-22-2013, 12:26 AM
Before you replace bearings, look to see if there are marks on stator or armature. Check if there could be drag by side loading the pulley shaft on the band saw. Somewhere you have developed enough drag to cause this problem. Ether of the pulleys could have shifted out of line causing drag. OR could you have a small cord, bad connection causing low voltage under load?


J Tiers
02-22-2013, 07:49 PM
If the motor runs well with no part of the saw connected, but it refuses to start with the saw connected to the shaft, then the problem would seem to be one of two things:

1) the saw has so much starting friction that it cannot be started by that motor. This is probably not correct, and so you need to look at the bearings, and anything that may be touching the blade and perhaps jamming it. Could be that at low RPM by hand, that item causes no trouble. Might be that the blade has to move a certain speed before the piece of material is moved to where it causes a problem.

2) A motor problem that causes low starting torque......Maybe the capacitor is the wrong value....... maybe one of the start windings is open, etc, etc.

A starting problem would still let the motor have good torque when running, it would be only a problem when starting.

02-23-2013, 04:29 PM
I took the motor apart, bearings are like new, as is everything else, expect, I noticed a little soot on the start windings but as I said, it starts up ok except when the belt is attached, then it won't go beyond 1300 rpm. If I start it with the belt on loose, it revs up to 1725, then as I tighten the belt it drops to 1300rpm.

I then removed the blade to isolate the lower wheel and shaft. same thing, only revs to 1300, BUT, if I remove the lower wheel, it gets up to full speed - 1725rpm. So it seems that small extra load of the lower wheel is too much for the motor. The lower wheel shaft bearings seem ok, no play no noise or drag, wheel spins for a good 10 secs with a crank of one finger.


J Tiers
02-23-2013, 05:08 PM
Verify that the motor is set for the correct voltage.

If set for a higher voltage than you have, it will be very weak.

Bob Ford
02-23-2013, 11:51 PM
The pictures of the motor makes me suspect the sooty area on the run winding where all the loom is bundled.
All the rest looks good. How hard would it be for you to cut the string and investigate. It looks like it could be starting to fail in that area. If the wires are starting to short, but not damaged. Use a wood or plastic tool to separate enough to re insulate. You may need something like this. http://www.mcmaster.com/#electrical-insulating-varnish/=lm1ggc You can lace it back together with cotton string and coat with the varnish.


02-24-2013, 12:02 AM
Maybe JT has put his finger on it- the motor starts and runs, coming up to speed, but has little torque. Barring friction and loading effects, which you have investigated, there seems little else except that it's wired for 220 and running on 110. It's probably too late to compare the normal running temperature to what it's doing now, but if it is running on a lower voltage than it's wired for, it might be running cooler under no load than it otherwise would. If it's getting hotter under no load than it normally would, then that would be the clue- internally it's using up more energy than it should be, leaving less for shaft power. Shorts in the wiring could easily cause this.

J Tiers
02-24-2013, 12:18 AM
Looking at the pics, the sooty area is adjacent to the opening to the box, in which the capacitor seems to live. A blown capacitor, if it literally did "blow", could easily account for that.

I do not believe in any shorts in the wires that might cause heating....... Picture 6 and picture 2 do not agree..... the other end of the coils is clean and shows ZERO evidence of any heating that I observed. if there is a shorted turn, generally the whole thing gets hot, and there isn't any question about it.

Check voltage setting..... if it only got to 1300 and should have gone to 1750, that is around 66% or so of full speed, and very credible as the switch RPM, so switch may never have cut out the capacitor........ if it stayed there for too long, the starting capacitor might have gotten rather a beating, and may have blown on account of it. That means it may do so again, maybe while you have your head over it looking for problems....!

If it IS wired for higher voltage, the start winding will get low voltage, and be weaker than it should be, along with the general weak behavior.

If you have a clamp-on ammeter, check running current unloaded at 1750 rpm. I would expect it to be about 40% of nominal full load current, or a bit more. If it is a lot less, then the idea of wrong voltage or the like is made more credible.

Bob Ford
02-24-2013, 12:20 AM
Go back to 1st post he has been using it blew a capacitor then started having the problem. I doubt the motor is hooked up wrong voltage. It is common to wire new motors for the higher voltage so if a person runs them on the lower voltage they fail to work well. If new were wired for lower voltage and they tried to run on higher voltage they windings would burn quickly. Although from the pictures this might be new.


Bob Ford
02-24-2013, 12:27 AM

He may be losing one or two turns in that coil as he says it runs uneven. Once the short gets worse then he will lose the coil and it will really show up. I used to wind motors generators for a living.


02-24-2013, 02:13 AM
`Definitely wired for 110 and the cap did blow with a bang and smoke which may account for the soot. I think I'm running out of options though. I'll have another look at those sooty windings but they are very tightly varnished together. In any case, it will still be useful for lighter duty as a disc sander or grinder.

Bob Ford
02-24-2013, 08:11 AM

Before you tear into the windings can you check voltage at the motor when under load. There is the possibility that you have a poor connection somewhere from the panel to the motor. If you have good voltage at the motor when running then you have a motor problem. If you have poor voltage at motor when running you have a feeder problem and you need to measure voltage at the outlet the machine plugs into. If voltage is good at this outlet with motor under load. Problem is between outlet and motor. If it is a feeder problem could be as simple as push in connections in wall outlet.


J Tiers
02-24-2013, 11:20 AM
A shorted turn normally gets really hot......pretty darn fast..... but nothing is burnt, or even overheated-looking, at the other end, and any sort of dead short should make itself known pretty quick. Apparently you have been messing with it enough that if it were suffering from shorted turns it ought to be reasonably obvious.

A problem that dead-shorted across one coil, when the 4 pole motor has 2 in series, would make it run oddly, but it would also draw a lot of current. There wouldn't be sufficient back EMF on that winding, and there would be a good deal of induced current, so it probably would not come up to speed. but it would be the same at no-load.

Working at no-load and not at "some" load, indicates more of a low current low torque situation. Wrong voltage is a common issue, open windings, wrong connections, etc are others.

because this motor BOTH starts slow, AND has low torque when at full speed, BUT doesn't overheat, it really suggests that there is an open, OR that the voltage is low/motor is set up wrong.

Motors can't be diagnosed well without a current reading, they run on current..... Sorta like checking health of a person without checking pulse, breathing and temperature.

If it has been running the machine OK, then it may be on the right voltage.... agreed... wasn't clear to me, but it is implied in the statements.

A capacitor that is losing capacity can end up blowing, but may not, it may just quit. But it WILL lead to a slow start. A motor that fails to get up to speed, may blow the capacitor... it will eventually, it's just a question of whether it does it sooner or later, and whether you fix it first.

The windings look so good, and no particular damage is visible, the connections look good, so failing proof otherwise, one would need proof of a short etc.

Basically, if a single-phase motor has:

1) all windings working correctly, not shorted or open

2) a good start cap (if it is a cap-start)

3) a good working start switch

4) is not jammed, nor overloaded

5) proper power supplied

6) proper connections

It kinda has to work..... so if it does not, one of those has to be missing/not true/etc.

At this point, instead of being "out of options", it's time to diagnose the thing right.

The very first things anyone needs to deal with motors is a clamp-on ammeter, and a multi-meter with AC volts and ohms. These are sometimes in one unit, I have a little "digital" Amprobe meter that does all three, and it is often handier to take along than a DMM and clamp-on meter, etc. I prefer a meter with a needle for many things, but it does work OK.

What you need to know is that the windings are all working, with some reasonable ohm value for each winding, that you have full voltage to the motor, and what current it draws, particularly when it is running no load.

02-24-2013, 01:55 PM
Republic of China. ;)
They don't seem yet to have mastered spit phase motor technology!

There a bit behind on making anything of rubber too. :)

02-24-2013, 03:55 PM
I really appreciate the help so I'll give it another shot but I think I have eliminated everything that is practically repairable, hence the "out of options" comment.

I bypassed the switch and tested the voltage directly at the motor terminals as 120vac. Attached is the wiring diagram. I checked resistance for the 2 windings as follows: (starting cap was removed for the test)
Winding A: Yel-Red - zero
Winding B: Blk-Gry - infinity (Is this a problem?)
Between red and blk I get 100k ohms (also a problem?), yel and gry I get infinity

I have no proper ammeter at the moment, will try to get one.


02-24-2013, 05:37 PM
Found it! Perseverence pays off sometimes.....



3 wires burnt apart, possibily from the cap blowing but I suspect it was something else as it was running a little slow before the cap blew. Anyways I'll try to jumper or solder them back together and see what happens.

J Tiers
02-24-2013, 06:25 PM
The open winding is certainly a hint, as you may have noticed!

Reason #1 for an ohmmeter and so forth.

Not clear as to whether you mean you have 3 "ends" or 6 ends (3 wires burnt apart). I suspect "6 ends".

Make sure that each pair of ends you connect do not already have conductivity to other wires. You don't want to create shorted turns. You still might have a problem, you need to be sure to connect all in series correctly.

Insulate the $#@!~ out of the ends.

I give it a 60% chance..... no telling what else may be an issue.

That must be a "PRC" (red china) motor, the ROC (Taiwan) manufacturers have figured out that you should varnish the wires together so they don't vibrate and do what likely happened there.

I must say, that is less damage than I would have expected.... accounting for my disbelief in shorts....... the suspicion is that the short was pretty much across one entire coil, could be wrong.

might be worth "glyptal" coating the visible wires, once you determine that the fix is good (assuming it is). Might cut the chances of it happening again soon.

02-24-2013, 07:56 PM
Son of a B---h! It works! There were 3 pairs, 6 wires to join. I soldered on jumpers and used shrink wrap. It is a Taiwan motor and there was varnish, string most everywhere, but those 3 wires had a little too much slack.

Thanks again for your help

J Tiers
02-24-2013, 08:53 PM


Make sure those loose wires are glyptaled/varnished/epoxyed/whatever it takes to be sure it won't happen again.

02-24-2013, 10:25 PM
Good for you, glad you got it running. Single phase motors are a bit harder to troubleshoot than 3 phase. Now you have a bit of experience too!