In my home shop I have a Delta 31-695 sander which I received second hand. It has a 120VAC capacitor start motor. The from the circuit I have, the capacitor stays on while the motor is running. So far, I have replaced the motor, relay, capacitor (several capacitors), and toggle switch, which would be all the electrical components. The wires are in good shape. The relay coil appears to be solid #10 or #8 wire, so it should handle the current.

Now that I've replaced all the components, the sander will work fine for not much more than 30 seconds and then the capacitor will fail. It smokes and splatters oil and goey black smelly stuff. I have the following diagram that has one ambiguity that Delta tech support cannot clear-up for me: is the capacitor fed from the line or load side of the relay coil?

120v switch relay
blk---o/ o------o^^^o--------black motor wire
? ?
|---+
?
| ___ cap
+-o o---|(---red motor wire

wht--------------------------white motor wire

grn---------(machine ground screw)

So...
1. Why would a circuit with all new components result in the capacitor failure?
2. Is it important which side of the relay coil the capacitor power is fed from?


Thanks, Anthony Kerstens P.Eng.

 

The possibilties are
1. the motor is not picking up speed or
2. the centrifugal switch is not suitable for that speed.
If the motor runs at 1400 rpm at full speed and you install a 2800 rpm switch, the switch will never open. To test if the switch is at fault, connect another switch external to the motor and open it once the motor picks up speed.
3. the capacitor voltage rating is not 110 v ac.
Since a coil is connected in series with it, the actual voltage appearing on the capacitor will be around 150 -200 volts. Make sure the AC working voltage on the capacitor is higher than the actual applied voltage.

Best regards,
Sekar

 

The motor start capacitor is only designed to be connected in the circuit for a few seconds then the start circuit is opened by the start switch. If the start capacitor is connected in the run circuit it will fail. The capacitor simply shifts the phase(time)of current for the start winding so the magnetic North and South are happening at a different time than the North and South of the run winding. The motor may start and run with the capacitor in the start or run circuit but if you want long capacitor life, you need a motor RUN capacitor as it is rated for continuous duty. The motor run capacitor must have a WVAC (working volts AC) above your operating voltage, 280WVAC for a 240 volt AC system. The root cause of the problem may be some confusion of the original wiring or wiring diagram. The general or typical wiring colors are: BLACK as the feed to the start switch with the capacitor connected after this switch being in series with the START winding, The RED wire is on the other side of the start winding, BLUE as the feed to the run winding with YELLOW on the other end of the RUN winding. GOOD LUCK!

 

Hi there,

The capacitor should be wired in series with a centrifical switch mounted on the shaft of the motor.

It is quite possible the centrifical switch is not opening once the motor is up to speed. This will definitely cause your capacitor to blow up. The capacitor is meant to give the motor a starting torque and then be disconnected once the motor is at speed. Try lubricating the switch. If you pull off the back cover of the motor you should see the switch which will include an assembly with springs.

Malachy

 

Hi Anthony

If the capacitor is < 20mfd. it's a run cap and I have no idea. Probably winding shorts or something either applying too much voltage or possibly drawing too much current through the cap. If however, it's > 20mfd. it's a start cap and should not be in the circuit once the motor is up to speed. If it is, it will soon overheat and spew bad stuff all over. The cap is switched out with a relay or a centrifical switch arrangement. You will have to determine how they are doing this and why it isn't happening once the motor gets up to speed. There have been a sufficient number of schemes used to do this that a little more info would be required to guess right.

hth
Regards

cww

 

I suspect the capacitor is only supposed to be in the circuit while the motor is starting. When the motor gets up to speed, the current through the relay coil (if I have interpreted your schematic correctly) decreases to the point where the relay drops out and removes the start winding from the circuit. I would guess that the capacitor should be fed from the line side of the relay coil. Putting it on the load side may provide enough extra current throught the relay coil to keep the relay from dropping out.

************************
Michael Griffin
London, Ont. Canada
************************

 

The capacitor is 270-324mfd. Delta tech support tells me that there are no switches in the motor, only the external relay. They tell me it's just a plain induction motor. The external relay stays continuously energised because the coil is wired directly to the machine switch. So, why bother having relay, unless its sole purpose is to delay the capacitor circuit closure slightly?

This is what has me puzzled. I have a circuit diagram and a part number from the Delta BOM. They can't have designed this for the capacitor to explode?

Anthony.

 

On December 2, 2002 05:05 pm, Anthony Kerstens wrote: <clip>
> The capacitor is 270-324mfd.
> Delta tech support tells me that there are no switches in the motor,
> only the external relay. They tell me it's just a plain induction motor.
> The external relay stays continuously energised because the coil is
> wired directly to the machine switch. So, why bother having relay,
> unless its sole purpose is to delay the capacitor circuit closure
> slightly?
<clip>

Yes, but I believe you indicated that the relay is also in series with the main winding. The current through the relay will therefor vary according to the motor speed. Since the starting current of the motor is quite high, the relay should pick up. As the motor accelerates the current will decline, and at some point the relay should drop out.

This method is obviously quite sensitive to motor current, so adding an addtional load to the relay coil by running the start winding current through it may add enough ampere turns to prevent the relay from dropping out at full speed.

--

************************
Michael Griffin
London, Ont. Canada
************************

 

Responding to Anthony Kerstens, Mon, Dec 2, 5:05 pm, query:

Are you sure that you haven't overlooked a resistor-like or fuse-like device? If so it is actually a positive-temperature-coefficient (PTC) resistor.

Some manufactures had switched to this device to replace electro-mechanical centrifugal switches. Typically, it was wired in series with the start-winding capacitor, or the relay that switched the cap or winding out of the circuit. Two of the more well known manufactures of PTC devices are Murata-Erie, and Spraque.

Regards,
Phil Corso, PE
(Boca Raton, FL)
[[email protected]]

 

Hi Anthony

That is definitely a start cap and the relay must be set up to sense the drop in current once the motor hits speed. Or it could be a thermal time delay relay of some sort. Or if a cap start cap run motor it could be a potential relay that senses back EMF I'm sure you've been around enough to sense when the tech support guy knows less about it than you do. With what we know, ( I don't have your diagram handy ) the start cap should be in the circuit only until the motor is up to speed. The relay makes no sense unless it is for this purpose. So here's what you've got. Your motor has two windings. A start winding which goes in series with the cap and relay. And a run winding which is powered directly from the switch. The other end of both windings will go to the common (unswitched) line. Make your wiring consistant with this. If the start circuit isn't deenergized at speed, the relay isn't doing it's job. In my part of the world, these relays are hardware store items. But, I live in a rural area. In more civilized areas, you will get blank stares when you ask at the hardware store.

Here's a link to some explanations and circuits. I don't see your situation exactly diagrammed but, it is touched on in the explanation. Some power tools are special cases because they spin up slowly.

http://www.leeson.com/products/techref/sphase.htm

Regards

cww

 

Thank you all for the responses.

I tried a new capacitor last night wired to the line side of the relay and it survived.

I suspect Michael is right: the relay must only operate on the motor starting current.

Anthony Kerstens P.Eng.

 

Anthony,

You mention that the start capacitor "smokes and splatters oil and goey black smelly stuff" when it blows. Does the cap actually catch fire? You know that these start caps are advertised as being "self-healing", and that they are always supposed to "fail safe", or fail open. Judging from what you wrote, this is not always true. ould you tell me a little bit more about this failure? I have an air conditioner that caught fire, and the only logical source of the mishap is a start capacitor disgourging its contents and burning.

Best regards,
George

 

I don't think it actually flamed, but it did definitely smoke. An even if it was self-healing, I don't think I would have trusted it.

 

Hi:

I have a puzzling problem with the 120V 1Ø capacitor-start motor on my old table saw. One day I went to turn it on and got only a quiet click. I checked the capacitance on the start capacitor which read within range (110 mF). Circuit: the motor has three leads, black, red, and white. Resistance (I have no inductance meter) is ~5 ohms black-to-white, ~20 ohms red-to-white, ~25 ohms black-to-red -- from which I infer that the red is the start winding and the black the run. Replaced the contactor (start relay) -- no effect. Local motor shop says that the rotor winding is okay.

I replaced the contactor with a momentary switch, turned on both windings (the main direct from line, start through the capacitor) by depressing the button and turning on the switch. Motor hums furiously and the lights dim, but no motion.

Now the fun part. I can get the thing to run IF I PUT THE CAPACITOR ON THE RUN WINDING. If I switch the motor leads so that the momentary switch feeds the capacitor which feeds the run winding, and the main switch feeds the start winding directly, and I depress the button and flip the switch, the rotor revolves. It sounds a little weak and slow, but it spins. If I then release the button, the motor slows down a little but keeps running on the start winding alone.

What's going on here? Any ideas?

Thanks.

Craig Appel
[email protected]

 

There might be a short circuit in the motor windings.

Meir

 

> So, why bother having relay, unless its sole purpose is to delay the capacitor circuit closure slightly? <

A contactor (relay) is often used so that the heavy wiring for the motor current doesn't need to be brought to the user operated switch

 

I have one of these units - same problem.

These units do not use a centrifugal switch like most ac motors.
Instead, the start relay depends on gravity to work. The inrush current is high causing the relay armature to raise up and make contact for the start windings. Then as the motor picks up speed, the current draw is less and the relay no longer has enough force to hold the armature up. It is supposed to fall and disconnect the start windings. Unfortunately, when the machine is loaded heavily, the relay engages again and the motor stops. When this happens, turn off the sander fast to prevent motor burn up.

DELTA NEEDS TO SOLVE THIS PROBLEM AS THERE ARE SEVERAL POSTS BY PEOPLE WHO HAVE THIS PROBLEM. AND --- DELTA NO LONGER OFFERS THE START RELAY AS A REPLACEMENT PART!

 

I am not sure this is the same problem that I have, but I think so. I have the same unit. It runs for 10 to 15 seconds and then trips the 15 amp breaker. I can smell that something is overheating and I think it is the capacitor. I think if I had plugged the sander into a 20 amp circuit it would have burnt something out, but since only 15 amps it just tripped the breaker. When I first bought the unit I had no problems. It suddenly started to do this. Should I try replacing the capacitor or is it this mysterious relay that is no longer availabe.

 

I have 6 of the machines at my facility...I have replaced parts in the past to remedy the same issue. What I did do one time to fix the relay problem was add another 24" of copper wire from a burned up relay to a brand new one by soldiering it in.. It fixed the problem until the motor went bad again. The capacitor some parts vendors offer, isn't the correct one to match the Delta factory motors. I've been buying factory replacement motors and they come with a matching capacitor. They are pricey but getting a new machine seems impossible right now. Also don't let the motor run more than 5 seconds if you're hearing a growling sound. It's burning the motor up as long as you leave it running. I'm still experimenting on a fix because these units are a pretty BIG part of our manufacturing process.

>I am not sure this is the same problem that I have, but I think so. I have the same unit.

 

Andrew... unless you use a start-capacitor equal in size and voltage to the original one, you could expose the new capacitor, as well as the start-winding, to over voltages that will result in failure.

I suggest you search Control.Com archives for other threads that discuss the reason. If you are unable to find helpful information, then submit nameplate data! Also, include what you know about the "fixes" that were tried, to date.

Regards, Phil Corso ([email protected])