What could make a motor not achieve full RPM's?

[AaronH]

Howdy all, Some of you may have read my post from a few days ago regarding a schematic I needed. Well, I'm still working on that same motor. I haven't found the schematic specifically meant for this motor, but I think I have found enough general ones that apply to be dangerous. Anyway, I've been messing around with this motor for a couple-three days now. This motor is a Delco, 3/4 horse, 115/230 volt, 1 PH, 1725 RPM, thermal protected I believe and capacitor start (it's got a start cap on it anyway). There are a total of 5 leads that I can switch around for high or low voltage. Here's the problem, I believe I have identified low and high voltage connections. However I can only get 900 RPM according to my hand held tach. Mysteriously, that is half of the proper RPM. Perhaps that's coincidence, but it seems to be a clue to me. Just to eliminate doubt, I checked my air compressor and bench grinder with the tack and they were right on the money. I have disassembled the motor to see how it's wired inside and ohm things out. Here's what I found. There is a stand-off inside the connection box that is connected to the cap. The other side of the cap connects to a winding, the other end of the winding connects to the centrifical switch which is then connected to another stand-off. I believe this is the start winding circuit. The start winding itself is around 2.5 ohm's. Ok, now there are two other sets of wires. Each pair of wires connects to it's own seperate winding. Both of those windings are around 1.5 ohm's each. I am guessing those two are the run windings. Here's ultimately what I have done. I have unwired the run windings from the start circuit and thermal overload (which has a total of three lugs). Then I connected the two run windings in parallel. Then I spun the motor by hand (remember, no start circuit wired in right now) and applied 115 volts to the run windings. The motor spins ok, but it doesn't seem to get up to speed. I noticed the motor has somewhat of a vibration. Being dumbfounded I connected the run winding in series and tried the experiment again just because. This time the motor would not continue running on it's own wired this way with 115 volts (which makes sense). Mechanically it seems to spin freely. I even oiled the bushings. As far as my eye can tell, no bent shaft. Would a bent shaft prevent full RPM's anyway? I even powered each run winding individually. Both times it tried to run, but just didn't quite have enought to keep going. It performed the same for both windings individually. None of the windings are grounded to the case or each other. Non of them "appear" burnt or shorted. Sorry about the long post, but this is really bugging me. Anyone want to take a shot at this one? I can accept the fact the motor may be junk, but why? I need to know for my one peace of mind. Thanks in advance for any thoughts.

 

[d23]

AaronH

Not trying to be an idiot, but:

1)
Try the motor connected in the proper fashion witout any load connected to it. You may have a compressor problem.

2)
Contact DELCO direct. They do not want problems any more than you do.

David

 

[AaronH]

This motor is currently not mounted on anything. I am simply bench testing. It is not associated with my air compressor in any way. I simply used the motor on my air compressor as well as my bench grinder to check my hand held tach to make sure it was accurate. Unfortuantely, there is no Delco anymore. They were bought by Lincoln about 10 years ago and Lincoln disolved the single phase line as soon as they bought it. So, there is no OEM support possible.

 

[automatic2]

Try this for contemplation. You may actually have a two phase motor. Your cap alters phasing. An incorrect value or faulty cap will affect rotational qualities.

 

[jbartos]

Suggestion: 900RPM could be a result of the 230V winding connection to 115V.

 

[SteveKW]

Howdy all, Some of you may have read my post from a few days ago regarding a schematic I needed. Well, I'm still working on that same motor. I haven't found the schematic specifically meant for this motor, but I think I have found enough general ones that apply to be dangerous.

Won't argue with that.

Anyway, I've been messing around with this motor for a couple-three days now. This motor is a Delco, 3/4 horse, 115/230 volt, 1 PH, 1725 RPM, thermal protected I believe and capacitor start (it's got a start cap on it anyway). There are a total of 5 leads that I can switch around for high or low voltage. Here's the problem, I believe I have identified low and high voltage connections. However I can only get 900 RPM according to my hand held tach. Mysteriously, that is half of the proper RPM. Perhaps that's coincidence, but it seems to be a clue to me. Just to eliminate doubt, I checked my air compressor and bench grinder with the tack and they were right on the money. I have disassembled the motor to see how it's wired inside and ohm things out. Here's what I found. There is a stand-off inside the connection box that is connected to the cap. The other side of the cap connects to a winding, the other end of the winding connects to the centrifical switch which is then connected to another stand-off. I believe this is the start winding circuit.

Looks good so far.

The start winding itself is around 2.5 ohm's. Ok, now there are two other sets of wires. Each pair of wires connects to it's own seperate winding. Both of those windings are around 1.5 ohm's each. I am guessing those two are the run windings.

Good here.

Here's ultimately what I have done. I have unwired the run windings from the start circuit and thermal overload (which has a total of three lugs). Then I connected the two run windings in parallel. Then I spun the motor by hand (remember, no start circuit wired in right now) and applied 115 volts to the run windings. The motor spins ok, but it doesn't seem to get up to speed.

Here is what you need to try. Swap the leads of one winding and see how it runs!

I noticed the motor has somewhat of a vibration. Being dumbfounded I connected the run winding in series and tried the experiment again just because. This time the motor would not continue running on it's own wired this way with 115 volts (which makes sense). Mechanically it seems to spin freely. I even oiled the bushings. As far as my eye can tell, no bent shaft. Would a bent shaft prevent full RPM's anyway? I even powered each run winding individually. Both times it tried to run, but just didn't quite have enought to keep going. It performed the same for both windings individually. None of the windings are grounded to the case or each other. Non of them "appear" burnt or shorted. Sorry about the long post, but this is really bugging me. Anyone want to take a shot at this one? I can accept the fact the motor may be junk, but why? I need to know for my one peace of mind. Thanks in advance for any thoughts.



Let us kow how she runs now.

 

[jbartos]

Suggestion: After so much of experimentation, there may be need to measure the motor insulation resistance or impedance since there could be some sneak path for the current to flow. Also, does the centrifugal switch open properly? Are capacitors in a good condition?

 

[dcdtn]

It is possible for a 4 pole motor (1800 rpm @ 60 Hz) to emulate an 8 pole motor (900 rpm @ 60 Hz) by reconnecting the windings. For proper 4 pole operation, winding direction should alternate around the stator. If winding directions are the same, an 8 pole motor can be created.

 

[stardelta]

As its a dual voltage motor I think the run winding connections may be incorrect. The run winding will have 2 halves, each with 2 leads, a total of 4. The halves will be connected in series for the high voltage and parallel for the low. If one half of the winding is reversed (in relation to the other)its unlikely it will reach full speed, likewise if only 1 half is fed. My suggestion is to ignore the start winding and caps completely , connect the run windings in parallel and feed it with 115v, at the same time quickly attempt to accelerate the motor in some way and see how it runs. A good way (allbeit a bit risky) is to wrap a length of rope round the shaft and pull it in the way you would start an outboard motor,(TIP, use a short piece!). Direction is unimportant. I would expect the motor if connected correctly to get up to speed and run smoothly at about 1/3 to 1/2 its rated current. If it is not, reverse 1 of the run winding circuits and try again. If the run winding is sound I would expect it to now be running correctly.

 

[GusD]

I agree with DCDTN.
The most logic explanation is that this motor is hooked up as an 8 pole motor.It could be that your motor may very well be a 2 speed motor(900/1800) which is a pretty common.If the applied voltage was wrong,most likely it would blow fuses or trip breakers. GusD

 

[SteveKW]

If you look closely... I tell him how to reconnect it back to four poles.

 

[DKB1943]

The motor speed could be low if the capacitor value has reduced by ageing. Please check the capacitor value by simply conneting it across 230/115V and measuring current though the capacitor. Use Voltage/Current (Xc) =1/w*C. Capacitors are known to lose value by upto 20% in 5 years and result in reduced torque and burn out problems too, on single phase drives for airconditioners, fans etc.

 

[wired1]

question on previous post . . .

DKB1943: Is your statement about cap aging a general statement for all types of construction (metalized film, polypropylene, etc.), or are some types better than others in this regard?

thanks in advance for any feedback . . .

 

[jbartos]

Suggestion: Sometimes during testing, the windings may become somewhat damaged. E.g. a short between windings. Is the motor experiencing any elevated temperatures?

 

[stardelta]

2 speed 1 ph motors are not very common here in the UK and in fact I cant recall ever even seeing one. I am curious to know how a dual voltage, dual speed 1 ph motor will be externally connected. Will the motor have 2 seperate run windings (one for each speed) or do they feature a pole change (Dahlander) type winding or even a part winding? Lets assume the motor in question has 2 run windings, that means there must be 8 leads brought out to enable dual voltage operation and thats before start windings, switches etc. Another consideration is the operating conditions of the capacitor, a 115 volt machine requires 4 times the capacitance of a 230 volt machine. This is overcome by connecting the start circuit to the mid point of the run circuit when connected for the higher voltage ensuring the voltage in the start circuit remains at 115. One thing is for sure, its going to be complicated inside the terminal box!

 

[SteveKW]

2 speed 1 ph motors are not very common here in the UK and in fact I cant recall ever even seeing one. I am curious to know how a dual voltage, dual speed 1 ph motor will be externally connected. Will the motor have 2 seperate run windings (one for each speed) or do they feature a pole change (Dahlander) type winding or even a part winding?
Typically, two speed spa pump motors I see every day have two separate run windings.

Lets assume the motor in question has 2 run windings, that means there must be 8 leads brought out to enable dual voltage operation and thats before start windings, switches etc.
Making a two speed motor DUAL voltage complicates it too much. It leaves the door open for a winding to be connected low voltage and have circulating currents in one winding while the motor runs in the other.

Another consideration is the operating conditions of the capacitor, a 115 volt machine requires 4 times the capacitance of a 230 volt machine. This is overcome by connecting the start circuit to the mid point of the run circuit when connected for the higher voltage ensuring the voltage in the start circuit remains at 115. One thing is for sure, its going to be complicated inside the terminal box!
Not too difficult here ... the motor ALWAYS starts on HIGH speed regardless of if it is powered to the low speed. Stacked contacts on the centrifugal switch insure this!

 

[stardelta]

Thanks for the info Steve