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Discussion Starter · #1 ·
I was checking the amperage on a three phase 1hp motor on a speed drive , motor was running fine ,equal voltage on all three legs. a-2.1a b-2.1a c-0 amps im not sure why c has no amps
 

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THE "BIG RED MACHINE"
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is the clamp on your amp meter closing tightly, I don't think the motor will be running if there was n o current on one leg.
 

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Does the motor sound a little loud or gravely?
If so a phase is out.
A 3 phase motor will continue to run if a leg drops out,, but it rarely will ever start.

On older motor starters we only protected 2 legs with o/L's.
 

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Donuts > Fried Eggs
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What was the speed when the measurement was taken? Has the motor been starting and stopping? What type of meter did you use?

I agree with wirenuting it's possible it's single-phasing, but it's rare to see a loaded motor continue to run like that long-term even without the extra protection in most VFDs, and it seems likely the drive would've kicked it off.
 

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Discussion Starter · #5 · (Edited)
is the clamp on your amp meter closing tightly, I don't think the motor will be running if there was n o current on one leg.
yes it was closed all the way
Does the motor sound a little loud or gravely?
If so a phase is out.
A 3 phase motor will continue to run if a leg drops out,, but it rarely will ever start.

On older motor starters we only protected 2 legs with o/L's.
no it ran and sounded fine no noise whatsoever
What was the speed when the measurement was taken? Has the motor been starting and stopping? What type of meter did you use?

I agree with wirenuting it's possible it's single-phasing, but it's rare to see a loaded motor continue to run like that long-term even without the extra protection in most VFDs, and it seems likely the drive would've kicked it off.
there are a total of three motors on this drive the other two motors have amp readings on all three phases we changed this motor out last night and the new one has amp readings on all three i will wire the one motor to line voltage to see what it does. the meter is a BK percission it 's the only one that reads amps on vfd driven motors correctly that i have
 

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The motor does't heat up? Current should be the same for all three phases. You can give a motor parameters (plate on motor)?
 

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Donuts > Fried Eggs
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Just out of curiosity, did you check the feeder coming from the drive before it splits to the three motors to see if C phase was running 2 amps lighter? If you were running multiple motors I guess it's possible the drive couldn't see the single-phase condition.

What are the motors powering?
 

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Big John said:
What was the speed when the measurement was taken? Has the motor been starting and stopping? What type of meter did you use? I agree with wirenuting it's possible it's single-phasing, but it's rare to see a loaded motor continue to run like that long-term even without the extra protection in most VFDs, and it seems likely the drive would've kicked it off.
I missed the drive part of the OP's post.
If 3 are on the drive and only 1 is reading zero, ring that phase out. There is a possibility that you have an open in the conductor or windings.
I would believe the motor would be hot unless it is small and lightly loaded. A small air handler with a discharge air return from an exhaust fan can pin wheel and take most of the natural load off.
 

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I missed the drive part of the OP's post.
If 3 are on the drive and only 1 is reading zero, ring that phase out. There is a possibility that you have an open in the conductor or windings.
I would believe the motor would be hot unless it is small and lightly loaded. A small air handler with a discharge air return from an exhaust fan can pin wheel and take most of the natural load off.
And that's a good possibility for why the motor appears to be starting. It's true that a 3 phase motor will not likely start with single phase, but if you have 3 motors all feeding the same load, such as fans feeding a common plenum, the air flow created by the other two fans may create enough draft to start the 3rd fan spinning, then once spinning it keeps spinning even though it is single phased.

In theory, the motor OL relay should take it out, but only if the load on that fan is high enough. If the other two are doing most of the work, the OL for this fan may not be seeing high enough current (yet) to notice.
 

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JRaef said:
And that's a good possibility for why the motor appears to be starting. It's true that a 3 phase motor will not likely start with single phase, but if you have 3 motors all feeding the same load, such as fans feeding a common plenum, the air flow created by the other two fans may create enough draft to start the 3rd fan spinning, then once spinning it keeps spinning even though it is single phased. In theory, the motor OL relay should take it out, but only if the load on that fan is high enough. If the other two are doing most of the work, the OL for this fan may not be seeing high enough current (yet) to notice.
I was hoping you would show up and give the answer.

There is one drive were they do disable the o/l protection block. I think it is eaton, I'm not sure, I'll look it up later. I'm waiting on a job interview now.
 

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Discussion Starter · #11 ·
Big John said:
Just out of curiosity, did you check the feeder coming from the drive before it splits to the three motors to see if C phase was running 2 amps lighter? If you were running multiple motors I guess it's possible the drive couldn't see the single-phase condition.

What are the motors powering?
I did not check the feeder that's a good idea , the motors are running a conveyor system
 

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Discussion Starter · #12 ·
JRaef said:
And that's a good possibility for why the motor appears to be starting. It's true that a 3 phase motor will not likely start with single phase, but if you have 3 motors all feeding the same load, such as fans feeding a common plenum, the air flow created by the other two fans may create enough draft to start the 3rd fan spinning, then once spinning it keeps spinning even though it is single phased.

In theory, the motor OL relay should take it out, but only if the load on that fan is high enough. If the other two are doing most of the work, the OL for this fan may not be seeing high enough current (yet) to notice.
The three motors run a conveyor which is in a big loop but they are not mech. Connected I will get the motor and try to connect it to line voltage and see what happens
 

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The three motors run a conveyor which is in a big loop but they are not mech. Connected I will get the motor and try to connect it to line voltage and see what happens
Well, if it's on a conveyor and there is no mechanical link to move that one motor if the others are running, it's hard to imagine that it really is single phasing if it is starting from a dead stop. Generally, a 3 phase motor getting only 2 of the 3 phases will not spin. Sometimes it happens out of shear luck, but not often. It's more likely to be some sort of measurement error. You can't have zero current in one phase and have the motor not exhibit sings of single phasing.
 

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You might be getting an incorrect reading, amp clamp trying to read that chopped up wave, maybe you should connect leads and use ammeter function on your meter. In my limited experience, every motor that has single phased burns up in a very short amount of time.
 

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Discussion Starter · #15 ·
Well, if it's on a conveyor and there is no mechanical link to move that one motor if the others are running, it's hard to imagine that it really is single phasing if it is starting from a dead stop. Generally, a 3 phase motor getting only 2 of the 3 phases will not spin. Sometimes it happens out of shear luck, but not often. It's more likely to be some sort of measurement error. You can't have zero current in one phase and have the motor not exhibit sings of single phasing.
i have worked on a lot of motors this was new to me ,when i get a chance i will reconnect the motor and take a vid and pics
 
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often a motor that is single phasing will get very hot and it is evident when you remove the cover on the pecker-head.
also ive seen some 3 phase start running backwards when single phasing
before you hook the old motor back up to power do a continuity test on the t leads
if the motor is ok all three readings should be very similar with little difference, But if you get drastic differences then you have either a short or an open winding

a short would likely trip out the vfd
but since the vfd is running multiple motors it may not be seeing the one single phasing from an open winding as the other guys have stated
 

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Sounds like single phasing to me. You may want to check the connection at the motor itself. Quick connects are prone to problems.
 

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Side question: does a regular amp clamp meter work when measuring output current of a drive if it's running at less than 60 Hz? Don't know if frequency would have an impact on an amp meter.
 

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:)
Side question: does a regular amp clamp meter work when measuring output current of a drive if it's running at less than 60 Hz? Don't know if frequency would have an impact on an amp meter.
Measure? Sure, they will all measure, something. ACCURATELY measure? Different story. Ironically, the oldest analog clamp-ons, work better than the newer inexpensive digitals, but for real accuracy, my general rule of thumb is that the accuracy improves with the number of digits to the left of the decimal point in the price tag. 3 digits and under, not likely to be worth a damn. 4 digits and over, that's when you start getting useful info. I used to say the price point for accuracy was $3k, but I've seen some good meters for less than half of that now. I have a Fluke scope meter, it works fantastic. Cost me $5k though, so if you are not having to check output current on VFDs a lot, it's hard to justify that expense.

Most of the time, the most accurate current reading you are going to get will be on the VFD display itself. It of course gets better with price as well, cheap little throw away drives likely only give you average current of the 3 phases. But vector capable drives must use highly accurate current measurements to make it all work, so whatever the display on those reads is going to be as good or better than anything you will see on a clamp-on.
 

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Discussion Starter · #20 ·
erics37 said:
Side question: does a regular amp clamp meter work when measuring output current of a drive if it's running at less than 60 Hz? Don't know if frequency would have an impact on an amp meter.
A BK persission (sp) is what I ve seen work
 
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