Overload protection

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rfkjr

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My question is about using one motor overload to protect multiple fractional hp fan motors.

Can I use one adjustable overload device to protect 3 fan motors? They are ½ horsepower motors, 480 volt, 3 phase. My thought was to check the running amps of all three and set the overload just above my reading. The full load of each motor is 1.1 amps. Operating they each draw .8 amps. If I set the overload to 2.5 it would trip if any one motor went over the 1.1 rating.
 
rfkjr said:
... Operating they each draw .8 amps. If I set the overload to 2.5 it would trip if any one motor went over the 1.1 rating.
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In general the motor overload protection cannot be set higher than 125% of the motor nameplate current. 430.32(B)(1)
 
I appreciate the replies thusfar. Simply put, I am trying to get 1/2 hp 3 phase 480v fans up with minimal wiring to replace 120v fans throughout a plant. It is easy to install a box with one correctly sized contactor, an adjustable overload, and power multiple fans with one run of conduit, on one circuit. I was not sure if the overload requirements for fractional HP motors are different from the larger motors I am accustomed to dealing with. I wouldnt ask this question about a 5, 10 25 etc. hp motor. Could I install an overload (or even fuses as I have seen fuses used for overload protection on small motors), at each fan to protect each fan individually and still power from the same circuit?
 
Tony S said:
Why would you want to do that?

It may save money in the short term but a burnt out motor will be more expensive than two extra O/L units.
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Just for the record: I agree that the application should have one overload for each motor - but not for the reason you state.

Q: How does setting up the overload cause the motor to burn out? Hint: Changing the overload setting does not change the load on the motor whatsoever.

ice
 
Lets put it differently:
Each motor must have its own INDIVIDUAL protection against running overload conditions. If the motor has that built-in, as many fractional HP motors do, then you are NOT required to provide external OL protection. So IF your small motors say something like "Thermally Protected", or just "TP", then all you need is a contactor. If not, then you can have one contactor, but each motor must have its own OL relay, and since an OL relay doesn't actually stop the current flow, all it can do is open the coil of the contactor and shut down ALL THREE fans.

An alternative would be to use 3 individual Manual Motor Starters down stream of the contactor. That way if one tripped, it only disconnects that one motor. It also gives you a way of locking out one motor to work on it without having to shut down all of them.
 
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