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Thread: A smokin' hot motor

  1. #41
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    Jun 2016
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    Owner is out of town and doesn't want to deal with motor until he comes back. I'm just hoping he's not going to try and blame me for the new motor burning up. (We don't know if it did yet.) I'll take a little blame, but seems to me the engineer for thing should have specified heaters, and the original installer should have known it needed heaters. If the thing was properly overcurrent protected in the first place, I would not have even been there. On the plus side, I now know a lot more about motor protection.

  2. #42
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    Quote Originally Posted by Coppersmith View Post
    Owner is out of town and doesn't want to deal with motor until he comes back. I'm just hoping he's not going to try and blame me for the new motor burning up. (We don't know if it did yet.) I'll take a little blame, but seems to me the engineer for thing should have specified heaters, and the original installer should have known it needed heaters. If the thing was properly overcurrent protected in the first place, I would not have even been there. On the plus side, I now know a lot more about motor protection.
    Most general purpose single phase motors already have integral overload protection. This one being designed specifically for pumps (from what I recall) however likely needs additional overload protection.

  3. #43
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    Quote Originally Posted by kwired View Post
    You must have outrageous utility bills, I figured 7.5 hp even if only 80% efficiency would only cost me maybe around 70 cents an hour to run.
    I get roughly the same as you. Maybe he meant $30 a day and lives in an area where electricity is 20-30c/kw-hr
    Electricians do it until it Hertz!

  4. #44
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    Quote Originally Posted by kwired View Post
    Most general purpose single phase motors already have integral overload protection. This one being designed specifically for pumps (from what I recall) however likely needs additional overload protection.
    To this point, it's also possible that this is not the original motor, and the original one DID have integral thermal protection, so the original circuit design was legit. Then when it went bad (again, probably because someone opened a valve and repeatedly overloaded it), nobody noticed that the replacement motor did NOT have integral thermal protection and needed a different circuit. It's hard to know the complete history of something like this if you were not there from the beginning.
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  5. #45
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    Jun 2010
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    Do you guys not have an ammeter?
    If it's drawing much more than the name plate, then you stop there.

    You'll have an idea if it's dragging if you run it dry briefly and observe or listen how it slows down.

  6. #46
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    Quote Originally Posted by Electric-Light View Post
    Do you guys not have an ammeter?
    Jraef mentioned measuring the current in post #3 and, of course, it's the obvious thing to do.
    Si hoc legere scis nimium eruditionis habes.

  7. #47
    Join Date
    Mar 2014
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    South Florida
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    Too late to measure the current

    Any update on this? I'm interested to know the result.

    Quote Originally Posted by Coppersmith View Post
    I'll take a little blame, but seems to me the engineer for thing should have specified heaters
    You shouldn't take the blame for lack of motor thermal protection, unless you designed the electrical system. Trying a 70 amp breaker to get the thing running wasn't a great idea and you may have to answer for that. The electrical designer should take blame for misrepresenting the work scope (failure to identify code required equipment) and redesign to provide motor thermal protection. Either way, you should absolutely get paid if you have to supply and install a motor controller.

    If an EE designed the motor feeder circuit, they should have made sure overload protection was provided, either integral to the motor or external. If they didn't know this they shouldn't be consulting, imo.

  8. #48
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    Jun 2016
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    Quote Originally Posted by drktmplr12 View Post
    Too late to measure the current

    Any update on this? I'm interested to know the result.



    You shouldn't take the blame for lack of motor thermal protection, unless you designed the electrical system. Trying a 70 amp breaker to get the thing running wasn't a great idea and you may have to answer for that. The electrical designer should take blame for misrepresenting the work scope (failure to identify code required equipment) and redesign to provide motor thermal protection. Either way, you should absolutely get paid if you have to supply and install a motor controller.

    If an EE designed the motor feeder circuit, they should have made sure overload protection was provided, either integral to the motor or external. If they didn't know this they shouldn't be consulting, imo.
    I agree. I'm waiting for the client to call me back to work on it (and kinda hoping he never does.)

  9. #49
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    Quote Originally Posted by drktmplr12 View Post
    Trying a 70 amp breaker to get the thing running wasn't a great idea and you may have to answer for that.
    My answer to that is Square D's motor calculators recommend an 80 amp breaker for a 7.5 HP single phase motor, and that NEC allows up to 100 amp before it must fail to start on that before you can upsize it even more.

  10. #50
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    Mar 2014
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    Quote Originally Posted by kwired View Post
    My answer to that is Square D's motor calculators recommend an 80 amp breaker for a 7.5 HP single phase motor, and that NEC allows up to 100 amp before it must fail to start on that before you can upsize it even more.
    I can see that argument being made, except the calculator (slide card, right?) states that 80A thermal mag is for short circuit protection only and depends on an enclosed starter with overload heaters for thermal protection. Taken without this context, that argument can work. However, it would be cherry picking information.

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