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Motors and voltage drop

Chris Pearson
I feel that I ought to know the answer to this one ...


Typically my woodworking machines draw about 6 times the rated current on start-up. So when checking for voltage drop, should I use the rated current (i.e. when on load) or the starting current?


In my particular case, there is no problem because the origin is in the workshop. However, a friend has told me that he has used a whole drum of cable (4 mm 2 for two circuits) and the biggest machines are furthest away. I would be interested to see whether he may expect starting problems. Least cost is his main priority in life.
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    525.203 says that voltage drops higher than the usual 5% etc are permitted during motor starting & similar provided the v.d. it still within what the equipment will tolerate. So I'd say use just the rated current.


    That does beg the question where other equipment will also be affected (e.g. when sharing a submain) whether you'd need to consider that equipment's tolerance too...


      - Andy.
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    I have come across this one myself with a spindle moulder which had a braked motor. These have an internal brake and it is disengaged by the motor field as long as there is sufficient voltage, but there wasn't, which resulted in blown fuses as the motor wouldn't start with the brake engaged! Some much thicker cables did fix the problem, but it was rather confusing for a bit. Brakes on woodworking machines are now common for H&S, so could well happen with a new machine.
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    AJJewsbury:

    525.203 says that voltage drops higher than the usual 5% etc are permitted during motor starting & similar provided the v.d. it still within what the equipment will tolerate. So I'd say use just the rated current.


    That does beg the question where other equipment will also be affected (e.g. when sharing a submain) whether you'd need to consider that equipment's tolerance too...


    Andy, thank you. We all know about the lights dimming when a big motor is started. Now why didn't I look there myself? ?


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    You need to be careful - if dropping 5% at nominal load, then at start up (assuming your 6 times figure is correct - my experiance is that for some kinds of motor that may actually be a bit low) you may drop 30% - which is enough for the no-volt trip on some designs to operate. Equally a cable 6 times the size needed is wasteful being 6 times oversize.

    I'd suggest  a half an half approach and work to have no more than 20% drop on start-up of the largest motor.

    The modern fix to this problem is the variable frequency drive.

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