Soft Starter Motor Starter 40 Hp 200 VAC

Discussion in 'General Electronics Chat' started by Elk Hunter, May 13, 2013.

  1. Elk Hunter

    Thread Starter New Member

    May 13, 2013
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    The soft starter I'm using is a Siemens, it has a settings for maximum starting current and overload current. The motor nameplate FLA is 122 amps. The motor bogs and does not reach synchronous speed until I have raised the starting current to 425 amps. This equals 3.5 times the nameplate current. I have never had to set the starting current this high, in the past lowering the starting current just prolonged the time it took to get to speed. My three phase voltage dips from 205 VAC to 191 VAC. Does anyone know what could cause this high starting current with a soft starter? (This motor is being used in a hydraulic elevator pump unit.)
     
  2. #12

    Expert

    Nov 30, 2010
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    Do you mean the motor has worked properly before, but it changed recently? That would say the motor is damaged. If you are trying a different motor, you have to keep in mind that all motors are "stalled" at the first moment of starting and they will try to allow LRA (Locked Rotor Amps). No surprise to see LRA at 5 to 7.5 times FLA.
     
  3. Elk Hunter

    Thread Starter New Member

    May 13, 2013
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    The motor has been changed recently, the orginal motor did not have a data tag so we used the data from another elevator on the same job. The starter has been changed from a Nordic to a Seimens. I never observed the operation of the orginal motor. The slow starting of the new motor caused me to investigate the operation. I discovered that the motor is drawing 115-130 Amps empty car up. The building hired an electrical consultant and it's his opinion that the wire size to the disconnect is undersized. It is #2 AWG THNN. I am going on the assumption that the orginal motor had worked properly, but I can't reconcile the bogging on start and the high running current. It appears something has changed but I can't determine what the problem is.
     
  4. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Is there a bypass valve so that the motor isn't trying to start with the hydraulics under load?
     
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
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    If it's drawing more than rated FLA under normal minimum load condition, then something is wrong, probably mechanically. Do your due diligence and verify that the motor is wired for the right voltage (Is it a dual voltage motor?)

    I suspect mechanical or hydraulic load issues - why are you replacing the motor? Did the other one burn up from habitual overload because nobody ever maintained the thing mechanically?

    Not being there and looking at it, I'm not going to disagree with what sounds like the most knowledgeable guy on-site, but I would question why the wires weren't too small before, but they are now.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    In your first post, you said that your three phase voltage dips from 205 VAC to 191 VAC. Is that measured at the input or the output of the soft-start unit. If it is at the input, you may have a loose connection somewhere in the feed line. If it is at the output, that just means that the soft-start unit is doing the limiting based upon current control. Low voltage on a 3 phase motor will result in high currents, overheating, low output torque, etc, etc.
     
  7. gerty

    AAC Fanatic!

    Aug 30, 2007
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    #2 THHN is good for 130 amps, but what is the length of the wire from the service source to the actual motor? If it's very long you'll have a voltage drop. I agree with the previous posts and I believe their assessments would be more likely than this one.
    I'm just throwing this out here on the off chance the motor you selected is in fact larger than the one that came off of it. And since a larger motor draws more current, the voltage drop would be an issue.
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
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    I just had another thought.... What is the rated RPM of the motor? If the original was 1765 (when did they change from 1750??) and the new one is 3500+ (when did they change the speed on various motors?? Grainger shows anything from 3515 to 3590RPM), that would make a MAJOR difference when driving a hydraulic pump.

    Also, is the pump a compensating pump or fixed displacement? If fixed displacement, is the relief valve working properly? If it is a compensating pump, is the compensator set properly?
     
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  9. Elk Hunter

    Thread Starter New Member

    May 13, 2013
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    I have a Square D quick Motor Data Calculator I've had it for years, not to say it is the last word in wire size and fuse size selection. It says a 40 HP 200VAC motor should have 1/0 wire to it with a 174 Amp fuse or circuit breaker. Also the wire run from the building sub panel is a good 150' away from the disconnect. I had an hydraulic elevator engineer calculate the HP for an elevator of my size, speed, and capacity and he said I should have a 50 HP motor. I asked the building to have their panels checked with infrared diagnostics. The building is 20+ years old and they have never checked their circuit breakers or connections. The motor is the correct RPM because the elevator has a positive displacement pump and the car is running the correct speed in the up. I measured the voltage drop on the input side of the starter. I am starting to think that this motor is and has been undersized from installation. Originally it had a wye/delta starter assembly which would not have shown the bogging as the soft starter does. I'm not sure what effect running a large motor above FLA's will have on the motor, you would have to expect it would shorten it's service life. Elevator companies have always lived by the motto "Cheaper is Better We Want The Best".
     
  10. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Are elevators not regulated by some governing body? This seems like a serious safety violation if it was designed with the motor to run higher than FLA. Hard to believe they could get away with that.
     
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  11. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    You say it was previously wye/delta start. Was this configuration replaced by a softstarter, or was the softstarter inserted into the existing wye/delta. Does your replacement motor have six or nine leads. What is the nameplate voltage of the motor.

    If #2 THHN is only rated for 130A, you're undersized. Your wire must be 1.25xFLA. Overcurrent depends on the type of fuse or if it's breaker.

    It depends on the duty rating of the motor. A continuous duty motor run intermittently will have plenty of life. Starts, and their subsequent thermal stress are the danger. Extended starts or frequent starts should be avoided.
     
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