3phase motor question

Status
Not open for further replies.
Smash said:
Smash said:
they should have installed a motor starter not a lighting contractor .
Click to expand...

GoldDigger said:
The "just a loud hum" with no rotation at all strongly suggests something much worse than voltage imbalance, like single phasing. If the motor itself is in fact good, that is.
Click to expand...


And that old lighting contactor has probably been there as long as the original motor. It won't cut out like a motor starter and will let the motor draw current until the breaker trips.

If it were me I would want to make sure I had the correct power before hooking up a new motor and possibly burning it up to. He knows he is not getting correct voltage but jumps to the conclusion that the load is not balanced causing the problem.

He never says if he reads this voltage imbalance back at the panel.

People may not think about but the reason they make rebuild kits for motor staters is they do wear out.

I'm not saying the contactor is bad but I would check it out before going any farther. I have replaced a lot of contacts in motor starters and the probably got more use than those controlling an exhaust fan but I found bad contacts at an air compressor a couple of years ago.
Click to expand...
 
I'm leaning this way as well. Cheap installers will think cheap parts, so may have used a cheap 20A DP (Definite Purpose) or lighting contactor, thinking that since the motor is 13A or so, 20A is fine. It is NOT, switching a motor is VERY different from switching a resistive heater or a lighting circuit. A 20A DP or lighting contactor is usually NOT rated for 5HP @ 208VAC. So I'd be suspecting that one pole is burned out (or broken) and the motor is getting single phase power. You might read through the burned contact with a digital meter, but it can't carry any load current.

Single phasing a 3 phase motor makes it hum and not start.
 
Jraef said:
I'm leaning this way as well. Cheap installers will think cheap parts, so may have used a cheap 20A DP (Definite Purpose) or lighting contactor, thinking that since the motor is 13A or so, 20A is fine. It is NOT, switching a motor is VERY different from switching a resistive heater or a lighting circuit. A 20A DP or lighting contactor is usually NOT rated for 5HP @ 208VAC. So I'd be suspecting that one pole is burned out (or broken) and the motor is getting single phase power. You might read through the burned contact with a digital meter, but it can't carry any load current.

Single phasing a 3 phase motor makes it hum and not start.
Click to expand...
And they are not expensive to replace.
 
Electric-Light said:
My purpose wasn't suggesting to use this to correct imbalance. It's for testing. My last post wasn't really clear so I should append it.

Clamp meter wrapped around all three wires at once but without wrapping around the conduit: should read zero amp. Any reading here indicates winding faulting to ground.

The next step involves taking amps of each leg separately and writing down. The voltage imbalance could be caused by the motor in question. Visualize it as a triangular knob. You rotate the knobs as if you're turning a three pointed knob forward one corner. If the imbalance is essentially the same, it is the source. If you have a massive imbalance that follows the "knob turn", it's likely the motor with
Click to expand...
 
Last edited:
I haven't been back yet will be there on Monday. I have no intention of hooking the motor back up until the power problem is corrected. As far as I'm concerned I'm already ahead of the game with a good motor. Will post my findings. Thx for all the feedback.
 
Multimeters are designed to be invisible to the circuit. The input is around 10MOhm which will show full or near full line voltage through slightest conductivity and stray capacitance. Put the meter in DC V. Put black lead to one terminal of a 9v battery. Hold the other probe in your finger and touch the other post on the battery with your finger. You'll read substantial portions of 9v on the meter. That's how sensitive it is.

A comparatively modest load such as a 5w light bulb in parallel eliminates phantom voltage and does the same thing as the LoZ available only on fancy meters. It doesn't make the meter idiot proof because a 5w bulb is significant enough load to burn out or cause something to get activated if you were poking around controls and it wouldn't have the CAT III/IV safety because of the damn light bulb.

You said somewhere along the line the rotation is correct yet the motor is not turning?
You didn't specify the conditions and exact point voltage readings were taken. (load/supply side of? with/without motor energized. . . )

mike_kilroy said:
And I would pick a multimeter with inductance on it; I have easily found a partially shorted winding in 3ph motors more than once that way.
Click to expand...

You can also get by with a doorbell or 12v controls transformer and a 10w resistor 100 ohms or so. It works by making the actual measurement with a 60Hz current. This is the one time you'll use the 10A socket on your meter. You compare the current just powering the resistor, then the current when you put the motor winding in the current path. It's not hard to figure out impedance and inductance with math but you're just looking for difference in the amount of change between different windings. The current is not a whole lot but still over a 100 times more than what average multimeters push out. It seems simple, but power supply that can generate clean AC from a battery adds quite a bit of complexity to the DMM.
 
Well it took a while as I wasn't getting these off voltage readings all the time. Checked everything all the way down the line sometimes voltage was perfect 208 all three legs. An hour later 196 between 1 & 3 now because it's a large work area with a dozen or so mechanics doing various tasks I'm starting to lean toward a load imbalance. It was getting late and I wanted the motor off my truck so I figured I'd at least bolt it back in get the belts and pulleys squared up and hit it again with fresh eyes. As I'm putting the whip into the motor just by luck I thought I saw a different color wire a dark brown not black that I swore was the color of phase one. Long story short turns out the exhaust fan was installed in the loft above the break room now it's outside of the same wall. There is one employee still there from the previous ownership He said yea they moved that fan outside because it was too loud and would rattle through the office area suspended ceiling. Well after crawling back into this old storage loft there mounted on the inside of the exterior wall my missing disconnect. 3ph 30 non fusible service disconnect. Open it up and to my delight was burned up leg number one. Intermittent problem that might never have been found if the installer didn't run out of black thhn that day. Plus too lazy to move said disconnect outside where it belongs. Case closed and thank you everyone I still learn something all the time hanging here at Holts Family Tavern. Thx again
 
Smash said:
Well it took a while as I wasn't getting these off voltage readings all the time. Checked everything all the way down the line sometimes voltage was perfect 208 all three legs. An hour later 196 between 1 & 3 now because it's a large work area with a dozen or so mechanics doing various tasks I'm starting to lean toward a load imbalance. It was getting late and I wanted the motor off my truck so I figured I'd at least bolt it back in get the belts and pulleys squared up and hit it again with fresh eyes. As I'm putting the whip into the motor just by luck I thought I saw a different color wire a dark brown not black that I swore was the color of phase one. Long story short turns out the exhaust fan was installed in the loft above the break room now it's outside of the same wall. There is one employee still there from the previous ownership He said yea they moved that fan outside because it was too loud and would rattle through the office area suspended ceiling. Well after crawling back into this old storage loft there mounted on the inside of the exterior wall my missing disconnect. 3ph 30 non fusible service disconnect. Open it up and to my delight was burned up leg number one. Intermittent problem that might never have been found if the installer didn't run out of black thhn that day. Plus too lazy to move said disconnect outside where it belongs. Case closed and thank you everyone I still learn something all the time hanging here at Holts Family Tavern. Thx again
Click to expand...

:thumbsup:

Always good to hear a resolution to a problem. ty for the follow-up

Holt's Family Tavern.. could be a good bar where there is a large number of IBrEW folk.... :D

(not a slam or anything against unions, just a convenient abbreviation for this post!)

...if Apple hasnt trademarked the name iBREW yet (and that is a dig at Apple).
 
Smash said:
Well after crawling back into this old storage loft there mounted on the inside of the exterior wall my missing disconnect. 3ph 30 non fusible service disconnect. Open it up and to my delight was burned up leg number one.
Click to expand...

Thanks for the update, and glad you found it.

In the future use your amp clamp to help identify open conductors. It is more reliable than a voltage test.
 
208 is 120 to ground on each leg.
A set of 3 decoration lamps (like the ones in my avatar, but filament bulbs that run at 120v) connected all at once i N to L1, N to L2 and N to L3 doesn't tell you the exact voltage but they tell you what's happening in real time. When you use single phase test equipment you're only looking at one after another.

A simple filament light bulb based instrumentation is/was the tool used to phase match the generator before throwing the switch to connect to the grid. Watching the pattern as you see all three bulbs at once tell you a lot of story. An intermittent contactor or disconnect will cause it to flicker which is often too fast to see on most clamp meters.
 
Electric-Light said:
208 is 120 to ground on each leg.
A set of 3 decoration lamps (like the ones in my avatar, but filament bulbs that run at 120v) connected all at once i N to L1, N to L2 and N to L3 doesn't tell you the exact voltage but they tell you what's happening in real time. When you use single phase test equipment you're only looking at one after another.
Click to expand...

What does that have to do with this thread?
 
Electric-Light said:
How the same tool can be used to diagnose a problem like the one discussed in this thread. It was more to confirm the value of stupidly simple tools that are often overlooked.
Click to expand...

No, not really as those will all light with the readings the OP described.

The issue is back feed through the motor winding and why a voltage tester is not only tool needed.
 
iwire said:
No, not really as those will all light with the readings the OP described.

The issue is back feed through the motor winding and why a voltage tester is not only tool needed.
Click to expand...
Wye connected lamps would have had a change in brightness levels of individual lamps. May not tell you what the problem is though, just that there is a problem.

Kind of like the TV ads Lifelock has run more recently, here is one of them if you haven't seen it.

https://www.ispot.tv/ad/Ant5/lifelock-fix-it
 
kwired said:
He made a crude three phase tester out of light emitting decorations.
Click to expand...

iwire said:
No, not really as those will all light with the readings the OP described.
The issue is back feed through the motor winding and why a voltage tester is not only tool needed.
Click to expand...

He said the motor would immediately trip the breaker so he was probably taking the unloaded voltage. You're measuring voltage with a 10,000kohm load when you use a multimeter which makes it prone to picking up ghost voltage. 6W holiday incandescent lamp will draw 0.05A or has an impedance of 2.4kohm when lit up. This draws 4,000 times more amps from the circuit than a multimeteIt does exactly the same thing LoZ does. The affected leg won't light up or will be dim or flicker. If it does manage to light up normally without the motor connected, the affected leg will drop out when it is repeated with the motor present.

Filament light bulb is a PTC that has about a 10:1 ratio and it's a simple but a useful instrument because it responds quickly to power glitches. Usually the very first power related issue is flicker. Industrial motor circuits don't share power with lights but when you hook up a power quality meter and press "flicker" it will let you see the same thing you see by hooking up three light bulbs, but they can still have issues with phantom voltage.

Power quality meter is more convenient to hookup and can capture something that happens when you're not there. You certainly won't have to rig up 12v controls transformers connected to some brake lamps nor do you have to worry about those hot light bulbs burning you.
 
ActionDave said:
Diagnose what? How does hooking up three lights provide any help finding a bad connection in a closet?
Click to expand...

The light bulb that sips power off through the bad connection would lose power/flicker and you can see all three phases side by side which is not possible with a clamp meter/dmm.
star connect. midpoint to neutral or ground. if the contator is burned out on pole 1, the bulb 1 won't light up or will be much dimmer than the rest.

motor terminal 1 bulb 1
motor terminal 2 bulb 2
motor terminal 3 bulb 3
 
Last edited:
Status
Not open for further replies.
Top