Results 1 to 10 of 10

Thread: Motor Starting Voltage Drop

  1. #1
    Join Date
    Nov 2014
    Location
    TX
    Posts
    161

    Motor Starting Voltage Drop

    Greetings,

    I was wondering if there is a rule of thumb for motor starting voltage drop calculation.

    Thank you in advance

  2. #2
    Join Date
    Jun 2006
    Location
    Ohio
    Posts
    21,385
    I will have achieved my life's goal if I die with a smile on my face.

  3. #3
    Join Date
    Feb 2016
    Location
    Illinois
    Posts
    488
    Quote Originally Posted by Electriman View Post
    Greetings,

    I was wondering if there is a rule of thumb for motor starting voltage drop calculation.

    Thank you in advance
    Not sure exactly what you're looking for, but a standard NEMA design B 3 phase induction motor typically has a starting (locked rotor) current of 6 X FLA using across the line starting.

  4. #4
    Join Date
    Dec 2012
    Location
    Placerville, CA, USA
    Posts
    18,377
    Quote Originally Posted by retirede View Post
    Not sure exactly what you're looking for, but a standard NEMA design B 3 phase induction motor typically has a starting (locked rotor) current of 6 X FLA using across the line starting.
    And you would probably want to keep the voltage drop to 20% or less on a dedicated circuit, rather than the 3%/5% guideline for running current.
    If the circuit is shared with other loads, in particular lighting, then you may need to try harder to keep the starting voltage drop low to prevent annoyance rather than a safety hazard.

  5. #5
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    33,521
    Significant voltage drop that occurs on the service, feeder, or if there would be other loads on the branch circuit is what you want to avoid.

    Otherwise drop more then 3-5% during starting isn't ordinarily a bad thing if it doesn't effect other things. It sort of is a soft-starter to some extent, as long as the drop isn't so much the motor can't accelerate the load.

  6. #6
    Join Date
    Dec 2012
    Location
    Placerville, CA, USA
    Posts
    18,377
    Quote Originally Posted by kwired View Post
    ... It sort of is a soft-starter to some extent, as long as the drop isn't so much the motor can't accelerate the load.
    Specifically, a reduced voltage start.

  7. #7
    Join Date
    Dec 2008
    Posts
    468
    See-for instance:
    EPRI POWER PLANT ELECTRICAL REFERENCE SERIES VOL.6 MOTORS EL-5036-V6 pdf
    ch.6.5 MOTOR LOAD CHARACTERISTICS AND STARTING DUTY

  8. #8
    Join Date
    Jun 2006
    Location
    Ohio
    Posts
    21,385
    Quote Originally Posted by Julius Right View Post
    See-for instance:
    EPRI POWER PLANT ELECTRICAL REFERENCE SERIES VOL.6 MOTORS EL-5036-V6 pdf
    ch.6.5 MOTOR LOAD CHARACTERISTICS AND STARTING DUTY
    It appears you are not aware that most people do not have access to that document.
    I will have achieved my life's goal if I die with a smile on my face.

  9. #9
    Join Date
    Dec 2008
    Posts
    468
    You are right, Smart $. 5 years ago was for free. Now, it is not.
    Since here are about 20 pages I cannot send it.

  10. #10
    Join Date
    Apr 2009
    Location
    Williamsburg, VA
    Posts
    4,244
    Quote Originally Posted by GoldDigger View Post
    And you would probably want to keep the voltage drop to 20% or less on a dedicated circuit, rather than the 3%/5% guideline for running current.
    If the circuit is shared with other loads, in particular lighting, then you may need to try harder to keep the starting voltage drop low to prevent annoyance rather than a safety hazard.
    Even if the circuit isn't shared with lighting, excessive voltage drop on the system can cause havoc with VFD and, oddly enough, larger motors. To wit: one WWTP I worked at long ago completed a 10MGD to 30MGD upgrade. Site lighting included 6 26kW (13 2k HID bulbs ea) lighting towers... which all came on at once (156+kW dead load). Voltage drop on the service was enough that when the lights came on, the VFD centrifuge (250HP main drive, 100HP back drive iirc) kicked off line, and restarting it usually dropped a few towers, or an aeration blower (350HP+). Little motors like mixers and small pumps couldnt have cared less about the dip, but the big loads were not happy with it... and the operators (us), well, we were seriously unhappy with it.

    Only got fixed when one shift, the evening operator started the centrifuge, the towers came on, knocked it off line, he restarted it, lights went down, lights came back up, centrifuge went off again... and stayed off. He didn't try restarting again. Mgmt was seriously unhappy. I, uh, he explained. Lighting towers finally set to stagger start, eqpt happy, operator happy, mgmt happy. EC not happy they had to add eqpt to make it work, but at that point, I didn't care for or about the EC.
    Electricians do it until it Hertz!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •