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Single phase 1/8HP motor wiring 2

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DynahoeDave

DynahoeDave

Electrical
Dec 14, 2018
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IHi everyone,
I recently acquired a 1960's Bridgeport milling machine. The cross feed has a motor driven feed mechanism.
The motor Nameplate:
Master
Alternating Current Motor
Serial DV6265 Style 245574
Type PA Frame 5217 H/P 1/8
Volts 208-220 Cycles 60 / 50
Amps .7 Phase 1
RPM 1725 / 1425
Code M Temp. Rise Cont 50 C

It has 2 Black #16 AWG wires. Also, 2 blue #18 wires, and a red #18 wire. One of the blue is connected to a 71-90 MFD 110-125V capacitor. Metal can, rubber terminal end with 2 lugs. The other lug is connected to the red wire.

It also has a KB Electronics solid state motor switch [model KB-6] . One line connection goes to the yellow [line] wire of the KB-6. The other line connection goes to one of the black motor wires. The loose blue wire goes to the KB-6 red / start wire. The remaining black wire goes to the KB-6 Blue main wndg wire.

I have this connected to 240VAC line. It is drawing around 4 amps, which is really high for such a small motor, and way over the .7 on the nameplate. I have a clamp on ammeter, and it shows 4amps in the run winding, and a pulse during start in the start winding, and in the capacitor wiring.

I removed one end cover, and the windings do not look discolored, burned, etc. The bearings are good, and all of this is with no load on the motor.

I did a few tests trying to determine if a winding is bad, or somehow they had been swapped or miss wired, as the wiring in the machine was kind of a hack job in places.

I left the black wire connected to one line input. I disconnected the Motor switch. Wired a 100W incandescent in series with each of the 2 windings, and connected the other side of the lamps to neutral, to give it only 120V. Both lights light up nearly full brightness, motor does not turn or buzz. If I momentarily bypass the lamp on the start winding, it spins up, and the other lamp dims, as I would expect. Actually, the start lamp dims also, but not as much. And it will continue to run with the start circuit open. If I then bypass the lamp in the run winding circuit, the motor continues to run, hum gets a little louder, current goes up to about an amp. Obviously, if I were to go to 240, the current would go up to the 4 amps I measured at the start of realizing something isn't right. It almost seems like it would be happy to run on 120VAC - although I don't know if the torque would be there. But with it wired to the KB-6, it doesn't do anything with only 120V applied.

Alll of my wiring description is based on how it was when I got it, so there could be an error in the assumptions. I'm hoping to find someone familiar enough with this sort of motor to confirm / help narrow down anything I'm missing.

[the main [3 phase] motor on the milling machine I got a VFD for, and it's working great]
 
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You stated a short pulse of current in start winding which implies it's working as it should. You stated it is drawing 4 amps, is that while connected to gear box or unloaded?
I haven't seen where a motor winding would decay just barely enough to leak excessive current without opening the protection but read it's possible and since it's an older motor just maybe it could be failed insulation. Have you checked motor winding resistance to frame of motor?, better yet a meggar test would be helpful.I am curious to see reply's on this possibility though and will watch to see what the E.E' s think.

Chuck
 
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So what you have is a Capacitor Start /Induction Run motor that does not have an internal centrifugal switch to take the starting capacitor out of the circuit. That’s what the KB-6 does, it is a “potential relay” that looks at the back emf from the start winding to know when to switch out the capacitor. The likely scenario here is that your KB-6 is fried and no longer functioning. KB stopped making those decades ago now so anything still in use (or available on FleaBay) is very old and prone to failure. The good news is that you can get other motor starting potential relays, just not that one. Do a search for a “Supco potential relay”, they are available on Amazon fairly cheap. They are not fully solid state like the KB-6 was, but that’s probably a good thing. There was a reason KB pulled them off the market...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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Ok, additional info - I recently got the machine. Had to go through everything to get it set up and running. I had run the motor & gearbox for a few minutes, to see how things work, etc. and noticed how hot it had become in that fairly short run. The wiring was a hack job. It had a motor starter, but it was not even wired normally to the 3 phase motor, just the much smaller single phase one I am asking about. Somewhere along the way, I noticed the heaters were way over the nameplate amp rating, so I bought heaters based on the nameplate. They shut it down fairly quickly. So I measured the current, and found the 4 amps. I can turn the gear drive without the motor easily by hand. I have not found schematics for the machine. Other similar model machines don't have the same electrical setup at all, so what I have was messed with by someone before I got it. But it's really just 2 motors. 3 phase 1HP main, and this small one for the cross feed.
Oh, and the 3 phase one had a reversing switch.

Interesting about the KB-6. I figured it's basic function, what it has to do. Could not find anything about them. I'll look for a modern replacement. Yes, the KB-6 does seem to function as I would expect, allowing power to the start winding for a short time on startup. But I can also manually start the motor, and it still seems to me that if I hook it up to 240, it will draw 4 amps. I can try this, and report back...

I can also check the leakage to the frame with a megger type setup, and resistance, etc.

All of this investigation is with no load on the motor other than turning gears, which I can also move by hand easily. Now that I have it removed from the gearbox to check the winding condition, I'll confirm the current / loading = zero, etc.
 
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If the KB-6 is indeed switching out the start winding and the motor, unloaded, is drawing 4A, the motor is either working too hard because of bad bearings, messed up gears (since it hands easily).

If nothing mechanical is apparent then definitely megger it, as that's probably the problem.

Keith Cress
kcress -
 
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Update..
No short to the frame. Using ohm meter open circuit, or my capacitor tester, which uses moderately high voltage [100s], current limited, to check for leakage, kind of like a megger.

No load, no friction, ball bearings good, spins and coasts freely, motor sitting on the bench.

Re checked with the motor loose, 240 V draws 4amps.
Rigged a transformer with taps, 180 V draw is about 2 amps. At least this might let me use it for now.... 'Till it dies the rest of the way...

Voltages in between, current is higher as voltage is higher.

I guess I'll be looking for a good used motor, find a modern one to adapt, or maybe a modern power feed add on. I can't imagine having this one re wound would be a low cost option.

 
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Huh! now I'm puzzled, the only thing left is a turn to turn short. In a three phase motor I would separate the winding's and compare resistance of one to the others. I don't know how to test for that in a single phase motor. One of the others on here may make some suggestions.

Thank you for the update. If you can borrow a meggar and run 500V through it that would be a more definitive test to be sure.

Chuck
 
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That is a two speed motor.
The speeds are not 2:1 so it will be two separate windings.
If both windings are energized they will be fighting each other.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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An easy way to verify a turn/turn short is to look at stator voltages when you rotate the rotor.
Any scope can be used. Even a simple volt-meter.

image_fq9avc.png



Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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Most single phase motors are three-phase. Some are two-phase. The technique works just as well on a two-phase winding. Only shaded pole motors have a single phase winding. And they ere not even 1/8 HP. A few watts, mostly.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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Bill did we miss something? Two speeds 1725 rpm @ 60hz and 1425 rpm @ 50 hz but same winding, correct?

Gunnar, That technique would work great on three phase setup because you can separate the windings and connect test leads in pecker head, However with windings
internally connected on a single phase motor how would you make test lead connections?

Chuck
 
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Chuck
If there's a will - there's a way. That's what my pennalist scout leader told me many years ago.
It is usually possible to connect to the windings. And you do not need to separate them.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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Use an ohmmeter to check the continuity. Verify which wire pairs are connected to a winding.
When the windings are identified check the impedance of each winding by energizing the winding in series with an incandescent lamp and checking the current and the voltage across the winding.
Lt us know what has continuity with the the red wire.
There are exceptions but the start winding generally will have a higher resistance and lower inductive reactance than the run winding.
There are a lot of different designs on small single phase motors.
For every rule of thumb there are lots of exceptions.
However, with a number of common arrangements, if the run winding is interchanged with the start winding the current and the heating will be much higher.
Does this motor reverse in normal operation?


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Gunnar

I think I understand what you are saying, Even though the winding's are connected in series The OP can connect a scope and when the rotor passes the pole with a shorted winding the
difference will be apparent at that rotor position. Would this be a true statement Gunnar?

Chuck
 
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Gunnar's method has merit but there are issues.
The majority of North American single phase motors have the start winding displaced 90 electrical degrees from the run winding.
With most of those motors the start winding is not identical to the run winding.
You can expect the run winding to produce a different voltage than the run winding.
A shorted turn will still show a low voltage but it is a judgement call if a low voltage is an indication of a shorted turn or a normal condition.
A comparison to an identical known good motor is a good method if an identical known good motor is available.


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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No, Chuck

You will not have any idea where the rotor "frozen" flux is. But if the windings are in series, there is a common point where they meet. Measure from there. The one with low voltage is the one that is shorted.

Actually, it doesn't matter which one. You still need to rewind (or scrap) the motor. But if both (or all three) readings are within around 10% then you can safely assume that there's no short-circuited turns. And look for other problems.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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