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Thread: electric motor question - amp draw vs hp

  1. #1

    electric motor question - amp draw vs hp

    I have a 15 HP motor on my Cyclone. When originally installed, it was right at full nameplate amperage with just the ductwork to and from the building (no branches installed yet). Now the amp draw has risen 3A on the middle leg compared to the other 2 legs. The starter's thermal overloads are starting to trip even on moderate loads - 3-4 gates open - a small portion of the initial test load. I have checked the starter, shutoff switch, and fuses to make sure they have good connections.

    So first question - Is this a sign that the motor is going bad?
    The bearings are not silent, but not too noisy.

    Second question - I have heard more than one other user report motor overload problems in similar systems. Does a moderate to heavily loaded system like this wear out motors?

    Third question - If you put a higher HP motor on the exact same load, will the amp draw be higher if motor efficiency is the same?

    Fourth - My options seem to be to replace the motor with exact same model or one with higher efficiency and higher service factor, or replace bearings and have motor shop check out the existing motor. Or both , and keep the old motor as a spare. Which would you be inclined to do?

    Thanks for any insight.
    Last edited by J.R. Rutter; 07-03-2007 at 2:17 PM.

  2. #2
    Join Date
    Nov 2006
    Little Rock, AR.
    15 HP is pretty close to the break even point of rebuild vs new. Wouldn't hurt to get a rebuild quote from the motor shop, might not be less than a low end motor of equal HP, but probably less than a high end or higher HP model. Sounds like you may be getting some insulation breakdown in one winding of one leg, possibly even a ground fault. The shop should be able to test for either case if you take it to them. But then again it could just be bearings going out, causing more drag. Simple bearing replacement with a possible winding re varnish would be the cheapest route, IF you get it done before the increased heat of the high amp draw cooks off the rest of the insulation.
    While you're getting the motor checked out you can get pricing on replacements, then make a decision. (You probably won't be able to avoid getting pricing on replacements if they have any sales force at all. )

  3. #3
    Just to check - you are talking about a three phase motor, aren't you?

    Go into the world and do well. But more importantly, go into the world and do good.

  4. #4
    Join Date
    Dec 2006
    Toronto Ontario
    Hi JR, I have a couple of questions;

    1) Initially, did you have any branch ducts connected or did you just have the main branches?

    The motor load will reduce as you add restriction, because the amount of air moved will be reduced. With all branches closed, the motor will be at lowest load, as you open branches to increase airflow, the motor load will rise.

    So, if you are at full HP rating with no branches open, you must have a very large amount of air leakage.

    2) Is your incoming voltage balanced and correct? Unbalanced or low voltage will increase motor current.

    Regards, Rod.

  5. #5
    Join Date
    Jun 2007
    Wichita, Kansas
    Quote Originally Posted by J.R. Rutter View Post
    Third question - If you put a higher HP motor on the exact same load, will the amp draw be higher if motor efficiency is the same?...
    The amperage an electric motor draws, in normal operation, is porportional to the load. The motor doesn't develop any more HP (or draw more amperage) than it needs to keep it's RPM near the rated value. So, if the overall efficiency is the same in both, a bigger motor won't draw more amperage than a smaller motor when both are driving the same load. That assumes of course, that both are the same voltage - a 120v motor will draw about twice the amperage of a 240v motor when both are identically loaded.

    I have a 5 HP motor driving a blower impeller sized for 2 HP. I've checked the amperage draw of that motor with the impeller under full load. Considering efficiencies, the amperage draw (about 8 amps at 240v) is consistent with developing about 2HP.
    Tom Veatch
    Wichita, KS

  6. #6
    Rob Will Guest
    I would check the voltage on all three legs at no load, and check voltages again with the cyclone and a few other things running. I would check on the output side of your magnetic starter or directly at the cyclone motor. You may end up taking the starter apart and cleaning the main contacts. If everything is tight and all the voltages check out, then it's probably time to change that motor.

    If you can get a 20hp in the same frame size as what you have, that might be a good move considering you are at full capacity on the 15hp. If the 15 had a little breathing room, I would say put a set of bearings in it.

    If you upgrade, do the existing wire size, circuit breaker, and magnetic starter all support a 20 hp motor?



  7. #7
    Join Date
    Dec 2005
    Du Bois, PA
    You should block all the openings in the duct work and check the amps again. With no restrictions the air through the blower will cause the motor to run faster than designed and the current will be high. With the openings blocked your current readings should be about half of rated amps. The rule of thumb we always use is under 10 hp is a throw away motor, but with the price of copper this may not hold true.

  8. #8
    Join Date
    May 2005
    I agree with Rob Will's comments but would also add to check for tight connections on all your wiring. I would also check for voltage imbalance with the motor not running. This could indicate your problem may be elsewhere.

  9. #9
    Join Date
    Mar 2003
    SE PA - Central Bucks County
    Quote Originally Posted by Mike Henderson View Post
    Just to check - you are talking about a three phase motor, aren't you?
    Indeed he is...
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  10. #10
    Hi J.R. Have you checked the rating of the heaters in the starter? Example from the name plate Full Running Amps FRA (20.0) X Service Factor SF (1.10) = Service factored Amps (22.0) This should be the heater rating. Most heaters have a range, say (22.0/24.0) or something like that. There is usually a chart on the inside cover on the starter. More air flow more amps rpm will not vary, it may sound like it, motor slows it's overloaded.


  11. #11
    Join Date
    Dec 2005
    Du Bois, PA
    The slowing under load is true under a normal load such as a pump or a saw. This is not true with a blower or fan load, if there is an un restricted air load going over the fan it will cause the motor rpm to increase and the amps will go up. If running with no restriction the motor may only run for minutes before tripping out.
    The connections or contacts were bad this will cause a problem also but if the motor was pulling full load amps at original start up this would indicate too much air flow from the beginning.

  12. #12
    Thanks for the info so far. It was a crazy day yesterday, and I'm home today, so I have not had a chance to check voltages. Max nameplate draw is 37.5 @ 208. Last time I checked my voltage was more like 207.

    Some clarifications:

    The motor is running OK with all gates closed. Yes there are some leaks, but not bad. Amp draw on the highest leg with nothing open was around 24. Seems like every gate that is opened ups it by 3-5 A, depending on size. This system came to me used, so I don't know if the motor is original or replacement, or what maintenance may have been done.

    I do understand about blower motors - the worst case that we tested on startup was just main lines to/from building - about 25-30 ft of 12" and 14" pipe - open on both ends for max air flow. We stood by to shut it off if needed, but it went to 37 A and moved a lot of air ;-) So something has changed since then. Baldor suggested that the bearings may be running dry, so I lubed and got some improvement

    I did find that taking the sarter cover off to cool the overloads a bit kept it from tripping so often. The overloads are sized correctly according to the electrician who installed them. The motor only has a 1.0 SF and 86 EFF, so I may just replace with comparable 1.15 SF and 90 EFF, then see if the old motor can get away with new bearings and a dip to keep as a back-up...

    I don't think that a larger motor will work after reading Rob's post. The #6 that is in there is the max that the conduit will allow, and I don't want to redo everything...

    If you think of anything else please post it.

  13. #13
    The overload is bad.

    If I understand correctly the nameplate says 37 amps and you are drawing 24 amps or so and the overload is tripping. That to me says all is good with the motor. Your other tests indicate all is well with the motor with the exception of bearings and it is sized properly for the task at hand.

    Have your electrician replace or repair the overload since it is tripping well below the rated current draw of the motor and you might as well replace the bearings.
    Last edited by Richard Butler; 07-04-2007 at 2:30 PM.

  14. #14
    Rob Will Guest
    I have a 15 hp motor with a 1.0 SF and an 89.5% eff. It has a FLA rating of 41.5A @ 200V. If we multiply 41.5 x 1.15 we get something in the neighborhood of 48A. That's what the NEC calls for on a 15 hp 208V 3ph motor circuit (48 amps).

    37 or 38 FLA sounds about right for your motor at 230V rather than 200(208).

    Perhaps your heaters are set for the motor's 230V rating (?).

    If that's not the case and you do indeed need to upgrade to a 1.15SF motor, your #6 wires should still be ok. You're right, 20 hp would require bigger wires.


  15. #15
    Restating the problem:

    Motor overload trips because the amperage draw is higher now, with 4-5 gates open, than it was with no ductwork hooked up 6 months ago. Seems like a winding is going bad, but will check voltages today to see if the local power is signifigantly out of balance.

    An industrial control / electrician working at my neighbors candy factory made a cold call Tuesday to introduce himself. He's coming back today to check out a shaper motor break and will take a look at the DC as well.

    I'll summarize findings later. Thanks for all the good input!

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