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Motor amperage lower than name plate #201174 05/16/11 09:41 PM
Joined: Oct 2002
Posts: 830
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sparkync Offline OP
Member
I checked out a sewage pump today with questions of why the breaker was tripping every once in while (actually only twice in about 4 to 5 days) I think I may have found the problem in a wire that had a "nick" in it that fed the timeclock that operates the pump that was laying up against the rough edge of the time clock. I'm thinking that the vibration of the motor was possibly causing the wire to short out, but don't know for sure since I didn't see any burn't marks on the timeclock, (that is until I started to get a amp reading and it blew up in my face:(
But laying this thought aside, I took amp readings on the motor phases. The voltage is around 213 volts on each leg.
The nameplate at 200 volts is showing 15.4 amps. The actual amperage is only around 7.7 amps. I don't understand why the amperage is so low. I disconnected the motor wires from the rest of the circuit and checked to see if one was shorted out to ground and they were not. I checked the motor numerous times and the breaker never did trip. Any ideas why I would get so low amperage when the nameplate is calling for twice what it was pulling??
When I left I had the pump coming on and off with the timeclock like it was suppose to. The customer is suppose to check it later and give me a call. I hope it is fixed but the amperage puzzles me. Thanks for the input.. Steve

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Re: Motor amperage lower than name plate [Re: sparkync] #201176 05/16/11 10:03 PM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
Member
It's like lifting weights ... lift more, work harder.

The amperage on the pump is based upon the maximum load under normal conditions. If you're not lifting the water that high, it will draw fewer amps.

You'll still need to allow for the full current inruxh when the pump starts, though ... that's likely when the breaker is tripping. I'd see if I could go up a breaker size if the existing breaker is less than 5x the full load amps on the nameplate.

If you can take the pupm out of service, I'd try to spin the thing by hand. It ought to spin freely in BOTH directions- and you ought to be able to feel if a bearing is binding.

Re: Motor amperage lower than name plate [Re: sparkync] #201177 05/16/11 10:24 PM
Joined: Oct 2002
Posts: 830
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sparkync Offline OP
Member
Reno, I was just getting ready to post about the breaker size. Where do you get the 5x at? I am using an old code book, but I go to the table that tell the Maximum rating of the motor branch circuit short circuit device and it tells me I can go 250% of the full load amps, which in this case
250% of 15.4 amps is 38.5 amps, and if I figured it right I should be able to go to a 40 amp breaker. Tell me if I'm wrong, it's been a while since I been to code class and I don't work with motors all that much. The motor is only on a 30 amp 3 phase breaker now. I did notice at one point the in rush current was 44 amps and this could be the reason of the breaker tripping. Most other checks I did it was 33 amps which still might cause the breaker to trip. Of course this set up is a old set up and has been there for years. Question is why has it started to cause problems now, unless of course the 30 amp breaker is weaker. It does show signs of age.. Thanks again

Re: Motor amperage lower than name plate [Re: sparkync] #201179 05/16/11 10:51 PM
Joined: Jul 2004
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gfretwell Offline
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At a certain point you have to just pick up the Easter egg and believe it was from a bunny.
If the breaker is not tripping any more you fixed another one.
As for the breaker. 250% is the max. Most motors will start with a lot less. Think of all of those 12a 120v motors on 15 and 20a breakers.


Greg Fretwell
Re: Motor amperage lower than name plate [Re: sparkync] #201180 05/16/11 11:02 PM
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Posts: 830
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sparkync Offline OP
Member
Thanks Greg. I thought I was right on the breaker size. 5 times would take me to 77 amps. Awful big breaker to put on a # 12 wire.
About the egg. I hope this mystery is solved. Remember I'm working around a sewage pump, and I'm having to almost straddle part of the reservoir to work on it:(

Re: Motor amperage lower than name plate [Re: sparkync] #201185 05/17/11 12:10 AM
Joined: Mar 2004
Posts: 947
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twh Offline
Member
Centrifugal pumps draw less current if there is a restriction in the line, because it reduces the flow. Less flow = less work = less current.

Re: Motor amperage lower than name plate [Re: sparkync] #201187 05/17/11 02:11 AM
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Posts: 9,569
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gfretwell Offline
Member
They call that suction throttling. It is the same thing that happens when you put your hand over a vacuum cleaner hose.
I do think these sewer pumps are conservatively rated. I would trust the guys who install a bunch of them a year to get the breaker sizes right.


Greg Fretwell
Re: Motor amperage lower than name plate [Re: sparkync] #201188 05/17/11 08:21 AM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
Member
I apologize here not for an error, but for an omission ....

In this instance, I left an opening to confuse 'measurement' with 'rating.'

Depending on the specifics of the motor's design and the application, the momentary current inrush at starting is usually between five and six times the full-load current rating for the motor. This is both confirmed by actual measurement, as well as what the motor makers will tell you. That's where I get that "5x" figure.

The key here is 'momentary.' It's over in an instant. How long that 'instant' lasts is, again, influenced by the actual load a motor has to overcome.

Code allows us to increase our over current protection device ratings depending on the type of device. Your usual breaker doesn't have a fixed rating; the actual value it will trip at depends largely on how long the breaker sees that current. A fast-acting fuse will blow for even a very brief, very slight over current. Thus, the code allows different 'multipliers' for different devices.

In your example, that 30-amp breaker would probably trip instantly at 150 amps, but will tolerate 70 amps for half a second, 50 amps for one second, and 35 amps nearly forever.

The exact response curve will vary not only by breaker model, but slightly differ between individual breakers. That's why changing the breaker - even to one of the same rating- will sometimes 'solve' the problem.

As mentioned, though, sometimes there is a real problem. A bad bearing can cause an intermittent high load. For that matter, foreign material in the pump can bind the pump as well. Sometimes the equipment is doing its' job.

Re: Motor amperage lower than name plate [Re: renosteinke] #201189 05/17/11 08:25 AM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
Member
I DID give out bad info in ny first post.

I should have said the breaker needed to allow for the 5x inrush current at start-up .... not to use a breaker at 5x the nameplate rating. As pointed out, a 'normal' breaker that large would be a code violation.

I do hope my more recent post has explained the topic better.

Last edited by renosteinke; 05/17/11 08:25 AM.
Re: Motor amperage lower than name plate [Re: sparkync] #201190 05/17/11 09:19 AM
Joined: Dec 2005
Posts: 860
R
RODALCO Offline
Member
Your breaker rating should still be 15 or 20 Amps to protect the motor and its associated wiring.
The wrong type standard B or C breaker may be fitted as a motor rated breaker will allowe a 5 to 8 times Inom. for motor start currents for 1 or 2 seconds only, as reno says.
The type we call it is a D-curve breaker which has that characteristic.
Raymond from OKC.
Im not sure if the USA uses the same coding for MCB's.

Last edited by RODALCO; 05/17/11 09:22 AM. Reason: typo

The product of rotation, excitation and flux produces electricty.
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