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Old 11-07-2011, 01:18 PM   #1
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Question Voltage and Amp readings on a blower motor

Problem: The vacuum of the blower motor assembly was not strong enough on one of the machines.

We crawled under the machine's flooring and took the j-box cover on the motor off. This is a 480 V, 3 phase motor. The nameplate said 4.25 Amps. Japanese motor.

We used three different meters, a fluke 377, a fluke T5-600, and a Klein equivalent to a Fluke T+Pro and the voltage read about 400 volts leg to leg, A-B, B-C, and A-C. We amp clamped with the T+ Pro equivalent and each leg was drawing about 1.3 amps. We shut off the power and read resistances through the windings and they varied considerably. We were thinking that we had a bad motor.

The local outside electrician was called in by the plant manager and he used his high dollar fluke meter and he read 480 volts phase to phase. He read the resistances through the windings and they were all about equal. He did not amp clamp it, but simply said "The motor is good."

He pulled off an exhaust hose and someone had put a screen in the exhaust of the blower and it was plugged up with trash. We cleaned the screen and all is well.

I can't understand why we would read low voltages, such a low amperage draw, and unequal resistances through the motor.

I would just attribute it to a bad meter, but all three?

What did we do wrong? A lesson please? Is there something peculiar about blower motors?

Thanks in advance,

JerryinPA
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Old 11-07-2011, 05:57 PM   #2
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are you sure the motor was in the same state that it was when the measurements were taken ? were the measurements taken from the exact same locations ? the same discos were in the same positions ?
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Old 11-07-2011, 06:08 PM   #3
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I'm guessing from the sound of it you used this Klein tester:

What jumps out at me is that neither that, nor the T5-600 are true RMS meters (I think Fluke 377 is a typo? Not sure what meter that is.) If, for some reason, the voltage wave-forms to the motor were significantly distorted I would expect all those meters to read low.

The "high dollar" Fluke the electrician used almost certainly was a true RMS meter and would give a much more accurate reading under those circumstances.

Also, testing winding resistance requires a pretty precise resistance meter. Something I would not expect those testers to be able to do.

Honestly, I attribute this to the fact that those are basic testers and not the greatest in-depth trouble-shooting tools for industrial work.

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Old 11-07-2011, 06:08 PM   #4
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Can't answer on the voltage part. Maybe change batteries in the meter you were using to check voltage. Did you measure in the same location as the outside guy? The amp draws indicate to me that with the plugged screen, your motor wasn't doing any work. When you cleaned the screen, you were able do move air (do work).
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Old 11-07-2011, 06:22 PM   #5
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Quote:
Originally Posted by JerryinPA View Post
Problem: The vacuum of the blower motor assembly was not strong enough on one of the machines.

We crawled under the machine's flooring and took the j-box cover on the motor off. This is a 480 V, 3 phase motor. The nameplate said 4.25 Amps. Japanese motor.

We used three different meters, a fluke 377, a fluke T5-600, and a Klein equivalent to a Fluke T+Pro and the voltage read about 400 volts leg to leg, A-B, B-C, and A-C. We amp clamped with the T+ Pro equivalent and each leg was drawing about 1.3 amps. We shut off the power and read resistances through the windings and they varied considerably. We were thinking that we had a bad motor.

The local outside electrician was called in by the plant manager and he used his high dollar fluke meter and he read 480 volts phase to phase. He read the resistances through the windings and they were all about equal. He did not amp clamp it, but simply said "The motor is good."

He pulled off an exhaust hose and someone had put a screen in the exhaust of the blower and it was plugged up with trash. We cleaned the screen and all is well.

I can't understand why we would read low voltages, such a low amperage draw, and unequal resistances through the motor.

I would just attribute it to a bad meter, but all three?

What did we do wrong? A lesson please? Is there something peculiar about blower motors?

Thanks in advance,

JerryinPA
In reverse order:
If the air flow was blocked on a centrifugal fan, there is no load on the motor. In a centrifugal machine, load = flow. No flow = no load / low flow = low load. So that alone would explain the low current reading.

Low voltage readings? How is the motor powered? A VFD by any chance? If so, an inexpensive meter is useless on the output side, they have trouble interpreting the complex waveform.

If it doesn't have a VFD, I would have said low meter battery (just pulled that boner on Saturday, talk about embarassing...). All 3 meters reading the same? That's curious, I have no immediate answer for that. Bad connections with the probes and you connected to the same bad spot with all 3 meters?
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Old 11-07-2011, 06:24 PM   #6
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Originally Posted by CJE View Post
Can't answer on the voltage part. Maybe change batteries in the meter you were using to check voltage. Did you measure in the same location as the outside guy? The amp draws indicate to me that with the plugged screen, your motor wasn't doing any work. When you cleaned the screen, you were able do move air (do work).
Crosssed in posting no-mans's land. Great minds think alike though.
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Old 11-08-2011, 08:59 AM   #7
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if motor is running at rated speed (check with a tachometer), the motor is ok, maybe the blower is not or ducts are blocked.
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Old 11-08-2011, 09:44 AM   #8
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I can confirm what Jraef said about a plugged duct. I have seen this alot at a lime plant plant. When the cooler pit material gets compacted in the screen the blower motor will pull lower current because its just spinning the metal fan around in a circle with no work being done.
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