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#49951 - 03/21/05 11:18 AM Troubleshooting 3Ø Motors
DougW Offline

Registered: 06/08/03
Posts: 1083
Loc: North Chicago, IL
Got the circulating pump motor up and running - found out after church that there's a separate furnace (forced air) for the sanctuary. And of course, the blower motor's a 3Ø... behind an access panel in the basement.

Got power going to it, but it doesn't run. D?C'd it from the line, ohmmetered it, and have no resistance between phases - a little resistance shows if I maually spin the motor, but hard to read and spin at the same time. No contiuity to the case on any lead.

I'm a resi & light commercial spark - not a big motor guy - HELP!

Other than the fact that the inside "looks melty" (dig my technical jargon ), it's not as badly scorched on the outside as the circ motor was. And since it's bigger (7.5 HP v. 3/4 HP) and significantly heavier, I can't just pull it out for a look see.

Any help is appreciated,


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#49952 - 03/21/05 01:02 PM Re: Troubleshooting 3Ø Motors
Dnkldorf Offline

Registered: 12/12/04
Posts: 1091
Loc: nowhere usa
Doug, here is a link to a basic 3 phase motor schematic.

The first drawing is probably the one you have.

You should have a measureable amount of resistance between phases, but the thing that bothers me is you stated you have resistance when you spin the motor by hand.

I'm no expert here, but I am guessing you have (2) open windings in the motor. I have to guess also that when you spin the motor, the scale may be jumping from a voltage being produced when you spin it.

Hopefully, someone with alot more know-how than me will jump in here and correct me.

If you have no resistance to ground on the motor, that is a good thing, otherwise the breaker should trip upon powering it up. I am assuming this isn't happening?

Let us know what you find.


#49953 - 03/21/05 02:11 PM Re: Troubleshooting 3Ø Motors
FWL_Engineer Offline

Registered: 05/15/04
Posts: 53
Loc: Barnet, Hertfordshire, UK
Doug, it certainly sounds like the motor is open circuit across at least two of the windings. Measurinf resistance across a motor when spun by hand is not a good sign in this case, as it implies there is some induction going on.

I would rip the motor out and bin it, the costs of rewinding a 7.5hp against the cost of replacing, and the time involved...more economoical to replace..well it is here in the UK anyway
Click me if you dare \:\)

#49954 - 03/21/05 02:26 PM Re: Troubleshooting 3Ø Motors
wa2ise Offline

Registered: 11/29/02
Posts: 769
Loc: Oradell NJ USA
When you say "no resistance" do you mean zero ohms (dead short) or infinate ohms (open circuit? I (and probably others) think of "zero quantity" when hearing "no quantity". Saying "no voltage" would mean "zero voltage, not "infinite voltage". "no resistance" would imply to me "zero ohms", zero ohms = dead short, not open circuit. This sort of thing can cause communications problems if someone says of a hot wire "no resistance to ground" meaning "dead short" but the guy at the circuit breaker panel thinks he meant to say "open circuit" thinking it's okay to turn its breaker on. And then Bamb!

#49955 - 03/21/05 06:01 PM Re: Troubleshooting 3Ø Motors
Dave55 Offline

Registered: 05/08/04
Posts: 666
Loc: Crystal Lake, Illinois, USA
Be sure to check the rotation anytime you disconnect the wires. Three phase motors are reversable.


#49956 - 03/21/05 06:56 PM Re: Troubleshooting 3Ø Motors
e57 Offline

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
I'm in the same boat with wa2ise! Your meter shows 0.0001 and is beeping, or shows "OL"? Ohms may be too high for some meters to read, depending on the motor, and type. Get the data plate info, and you can get the resistance various laod amperge values from the manufactuer many times, to check against.

Reguardless it don't work! I assume you have checked voltage? Phase to phase, and ground with the contactor closed past the over-loads? (Motor disconnected, as not to read through it, or other motors.) Considering it would be pretty expensive to change this out and find that it was a control issue. If you have, you can safely assume the motor is done.

3 phase can be a little funny to trouble-shoot when control work is in the equation. As voltage for lack of better words to describe it, can be read through other things, especially motors. Shut everything off, control wise, so the machine is idle, with the power on. Find the contactor for the motor in question, and hold it down physically, or with the coil voltage, and check voltage with the leads disconnected at the motor. For safety reasons I do this with wirenuts on the leads and stick my probes in the wirenut. (No loose live conductors. Gloves and mask, work safe, etc.)

Anyway, always assume motor is good until you check the control work. One bad fuse, over-load, heater or contactor, could make it a very expensive change out, only to find you have 2 good motors now. You have no idea how many times I have slapped myself saying, "Pull fuses out to check, stupid."
Mark Heller
"Well - I oughta....." -Jackie Gleason

#49957 - 03/21/05 08:05 PM Re: Troubleshooting 3Ø Motors
jkraft Offline

Registered: 05/05/03
Posts: 75
For a standard 9 lead dual voltage motor you put the power on 1, 2, 3. At low voltage connect 7 to 1, 8 to 2, and 9 to 3, with 4,5,6 connected together. At high voltage connect 4 to 7, 5 to 8, and 6 to 9. When connected you will always have continuity between phases but never to ground.

#49958 - 03/21/05 09:13 PM Re: Troubleshooting 3Ø Motors
rusty Offline
Junior Member

Registered: 12/22/02
Posts: 7
Loc: aurora, ne. usa
Doug, is this motor thermally protected (internally)? May be a chance of the protector being open if you're not getting any resistance across the coils. Still going to be difficult to check without getting the motor open.

#49959 - 03/21/05 09:23 PM Re: Troubleshooting 3Ø Motors
DougW Offline

Registered: 06/08/03
Posts: 1083
Loc: North Chicago, IL
It was no resistance between phases. Sorry - should have been more specific, but was on one of the church's (slow...486) computer on a break.

As jkraft estimated, the wires were (line) = motor

(Yellow) = 3 & 9
(Red) = 1 & 7
(Black) = 2 & 8

Color me embarassed!

Luckily I had e57's "moment of zen" before picking up the new $651 motor.

Just as a last thought, I went downstairs to verifiy that there was in fact power going to the motor in the first place.

(I'd had to crawl through the vent shaft to get to the blower, so I'd shut off everything I could find first to avoid frappe'd or fumed sparky. Church was built in 1930's, and the ventilation intake system was built-in as a part of the structure. Probably a permit required space nowdays.)

Went back, turned power on, and as I was heading back into the duct, It suddenly dawned on me that someone had installed an override switch to turn the fan from 'auto' to 'on'. Realizing that a regular 15A snap switch doesn't control 3-phase (more than once), I looked inside the D/C, and sure enough, there are a few black 14 AWG conductors sneaking around the back of the phases.

Tracing out the conduit through several wall / duct penetrations revealed what appeared to be a duct panel was, in fact, an old relay / control enclosure, with *you guessed it) a single newer C-H 3-ph starter relay that had tripped.

Reconnected the motor, threw the D/C, and reset the relay (even though the 'trip' indicator wasn't visible, the reset button was protruding.... heard the happy rumble of the fan starting up, and then felt the rush of a 5' x 2' centrifugal blower's output... I'd forgotten to close one of the access hatches near the heat exchangers!

All appears to be working now, but I told the bookeeper to keep the already drafted check handy for a few days, just in case.

As an additional thing, I went through and labeled the main and the appropriate motor and control D/C's for the boilers / furnaces, as well as the access hatches.

Thanks to everyone for the input and assistance!

BTW - Here's the info off the (heavily corroded) plate: (Luckily Dayton = Grainger. The "Z" in the ID was actually a "2")

Dayton 3ph. LR-13758
ZN985-1 7.5 208-220 440
1740/1440 20.0 10.0
60/50 LS254U
P 1.15

The original motor's (1930's) sitting in a corner - I'll try to get some data or the plate and some pics!

#49960 - 03/22/05 11:34 AM Re: Troubleshooting 3Ø Motors
Dnkldorf Offline

Registered: 12/12/04
Posts: 1091
Loc: nowhere usa
Well, he got me on that one.

Terminology is an important thing.

When he stated "he has power going to it", I assumed he checked the voltage at the motor leads.

The other thing I guess I better learn is the "no resistance" point.

I always figure everything has resistance, however small, resistance is still there, even a "dead short" had resistance, it is just incredibly small.

For me, I like the continuity, and no continuity terminology.

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