Results 1 to 10 of 10

Thread: 50HP drive on a 75HP motor

  1. #1
    Join Date
    Mar 2006
    Location
    Boston, MA
    Posts
    995

    50HP drive on a 75HP motor

    I have a 50HP ABB drive and 75HP pump which I'd like to pair up on a temporary basis. Vendor is telling me that there is no reason why it wouldn't work; the obvious draw back being that you won't get the full capacity of the pump but that it is a code violation.

    Is it? and if so, why is it. If you can set the electronic overload appropriately, why would this be in any way unsafe?

    Thanks,

    Mike
    Mike Shields, PE
    Boston, MA

  2. #2
    Join Date
    Aug 2010
    Location
    Texas
    Posts
    292
    Quote Originally Posted by mshields View Post
    I have a 50HP ABB drive and 75HP pump which I'd like to pair up on a temporary basis. Vendor is telling me that there is no reason why it wouldn't work; the obvious draw back being that you won't get the full capacity of the pump but that it is a code violation.

    Is it? and if so, why is it. If you can set the electronic overload appropriately, why would this be in any way unsafe?

    Thanks,

    Mike
    What do you mean by "electronic overload"?

    Just like the vendor said it won't be unsafe, you just won't be able to run the motor at full load capacity. However, most VFDs have some overload capacity meaning you may be able to run at motor full load intermittently.
    "Because it's there!"
    George Mallory

  3. #3
    Join Date
    Sep 2008
    Location
    UK
    Posts
    12,154
    Quote Originally Posted by LMAO View Post
    What do you mean by "electronic overload"?

    Just like the vendor said it won't be unsafe, you just won't be able to run the motor at full load capacity. However, most VFDs have some overload capacity meaning you may be able to run at motor full load intermittently.
    And it will also be down on efficiency. If that matters.
    Si hoc legere scis nimium eruditionis habes.

  4. #4
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    16,987
    Quote Originally Posted by LMAO View Post
    What do you mean by "electronic overload"?
    Presumably he is referring to the built-in overload protection in the VFD.

    I would be a little concerned that the no load current of the motor might exceed the overload setting because the PF is really low on some motors at low loads, but they draw considerable current.
    Bob

  5. #5
    Join Date
    Jul 2006
    Location
    San Francisco Bay Area, CA, USA
    Posts
    8,188
    Quote Originally Posted by mshields View Post
    I have a 50HP ABB drive and 75HP pump which I'd like to pair up on a temporary basis. Vendor is telling me that there is no reason why it wouldn't work; the obvious draw back being that you won't get the full capacity of the pump but that it is a code violation.

    Is it? and if so, why is it. If you can set the electronic overload appropriately, why would this be in any way unsafe?

    Thanks,

    Mike
    Code violation possibility: 110.3... Suitability for installation and use... The VFD is listed and labeled for a maximum 30HP motor, you are not using it for a 30HP motor, you are using it for a 50HP motor, ergo it is not "suitable".

    BUT... you CAN set up the drive to "current limit" at the maximum rating of the DRIVE so that it never is allowed to deliver more than the current normally associated with 30HP to the motor. So you COULD make an argument that you are NOT in fact using it beyond its listed purpose. Whether or not an AHJ will buy that argument is basically a craps shoot...

    On that note, I have done it many times very successfully, with the limitations as you described. The usual situation is that, to use you your example, I had a 30HP motor and VFD on something like a pump and the motor failed. The only available motor for a few weeks was a 50HP. Install and adapt the 50HP motor, program the VFD to never exceed 30HP, but I already knew that 30HP was sufficient because the 30HP motor had worked. It does cost you in efficiency, so you will want to put in the right motor again as soon as possible.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  6. #6
    Join Date
    Dec 2015
    Location
    Texas
    Posts
    173
    Quote Originally Posted by Besoeker View Post
    And it will also be down on efficiency. If that matters.
    Boston appears to be under the IECC 2015 if this installation is even applicable to requiring it; the only thing I could think of regarding the decreased efficiencies of the motors would be under IECC 2015 C405.8

    C405.8 Electric Motors (Mandatory). Electric motors shall meet the minimum efficiency requirements of Tables C405.8(1) through C405.8(4) when tested and rated in accordance with the DOE 10 CFR 431. The efficiency shall be verified through certification under an approved certification program or, where a certification program does not exist, the equipment efficiency ratings shall be supported by data furnished by the motor manufacturer.

    Tables indicate most values to be around ~93% efficient +/-1% depending upon RPMs and Open Drip Proof versus Fan-Cooled Motors.

    I don't believe you meet this requirement if it's a temporary installation and not new construction.

  7. #7
    Join Date
    Dec 2006
    Location
    Eastern Oregon
    Posts
    3,638
    We do a lot of stuff that isn't exactly code legal for temporary.

    As long as it's safe, won't kill anyone, or damage property, I don't have a problem temp'ing something in. This is with the customer fully aware of the situation though.

    So, a 50hp drive gets put on a 75hp motor. What's the worst that'll happen? The drive will fault. That is not hazardous to life or property. I'd temp it and not think twice if someone asked me to do it. The drive only cares about FLA, so I'd set a current limit in the drive to limit motor speed if necessary and call it good.

  8. #8
    Join Date
    Aug 2010
    Location
    Texas
    Posts
    292
    Quote Originally Posted by Cow View Post
    We do a lot of stuff that isn't exactly code legal for temporary.

    As long as it's safe, won't kill anyone, or damage property, I don't have a problem temp'ing something in. This is with the customer fully aware of the situation though.

    So, a 50hp drive gets put on a 75hp motor. What's the worst that'll happen? The drive will fault. That is not hazardous to life or property. I'd temp it and not think twice if someone asked me to do it. The drive only cares about FLA, so I'd set a current limit in the drive to limit motor speed if necessary and call it good.
    This is the definition of a common sense response. I love this site but sometime people just go way overboard with codes and regulations. It is just a temporary setup and even if it were permanent I can argue it'll be fine.
    "Because it's there!"
    George Mallory

  9. #9
    Join Date
    Jul 2006
    Location
    San Francisco Bay Area, CA, USA
    Posts
    8,188
    Quote Originally Posted by LMAO View Post
    This is the definition of a common sense response. I love this site but sometime people just go way overboard with codes and regulations. It is just a temporary setup and even if it were permanent I can argue it'll be fine.
    Well, I’ve done this probably a dozen times myself, code violation or not. I rarely get an AHJ involved in emergency fixes, especially those that are cobbled together with parts on-hand at the moment.

    But, he SPECIFICALLY asked if it was a code violation, so I specifically responded why someone might think so. Whether or not that was relevant is a separate question that was not asked.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  10. #10
    Join Date
    Sep 2008
    Location
    UK
    Posts
    12,154
    Quote Originally Posted by petersonra View Post
    Presumably he is referring to the built-in overload protection in the VFD.

    I would be a little concerned that the no load current of the motor might exceed the overload setting because the PF is really low on some motors at low loads, but they draw considerable current.
    Typically, no-load current is about 30% of FLC.
    Si hoc legere scis nimium eruditionis habes.

Posting Permissions

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