Which one

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I have two 1hp 480v gear motors located within 20 feet of each other performing the same function and identically wired. Motor A failed an insulation test after 20 plus years. Replacement was Code C vs B, output speed is higher but FLA is lower. Three amp magnetic only breaker ahead of starter A would not hold on start of new motor. I temped motor A to motor B starter and all works fine. Ordered replacement reconditioned CB because new of same mfg is no longer available. Still won?t work. Moved CB of motor B to A. Works great. Neither old or new CB will now start Motor B. Both motor circuits were megged and starters checked. Both motors are now running with a borrowed CB and motor B actually has the higher inrush current of the two. (Fluke Power Quality meter)

I now have three 3 amp breakers that worry me. Are the two that now will not start either motor working correctly? Or the one that starts both the problem?
 
Tom,
Those types of breakers normally have an adjustable instantanous trip setting. Are they all set to the same value?
Don
 
I'm assuming that the motor starters don't provide overload protection and that you're using the breakers for this?

What is the nameplate FLA on each motor?

steve
 
This happens a lot.
The new motors are probably designed for higher efficiency. To do that, they often reduce the air gap as well as change the winding materials and methods to reduce losses. The side effect of that is that the newer motor designs have higher instantaneous inrush currents. There is probably nothing wrong with your old breakers, but the NEC now allows higher magnetic trip settings as a result of these motor design changes; up to 20 times FLA (instead of the old 13 times FLA). Bottom line, just get a new breaker for that new motor.
 
You stated that the output speed of new motor is higher. How much higher? If the new motor runs twice as fast it will be trying to perform twice the amount of work and current will be affected accordingly. Twice the work means twice the power required.
Know What I Mean, Vern
 
ptonsparky said:
Starters provide overload protection and both adjustments are to the max. New motor is 1.6 amp, old is 1.9 amp
Click to expand...



The reason that I asked about overload protection is....

If the starter provides overload protection for the motor, you can use a higher amperage breaker to supply it.

Since the motor is protected from overload by the starter, the breaker is only for short circuit protection.

Read NEC 430.32(D)1, 430.32(B)(1), 430.32(A)(1), 430.32(C) and 430.52

I'm assuming (again) that the starter overloads haven't tripped?

steve
 
Tom,
What is the magnetic trip range of you 3 amp breaker? I just checked on one and its mag trip range is from 9 amps at the lowest setting to 33 amps at the highest setting. That should be high enough for any 1 hp, 3 phase, 480 volt motor.
Don
 
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