Web searches have led me nowhere, and a Man hates to ASK for "directions" But,
using a 3 ph. Size 1 magnetic motor starter, to run a 1 phase 220v. 5 hp motor rated at 25 Full Load Amps, using 2 heaters instead of three. ( Q. is each heater the 25 FLA value?? or only half that value?? ) I have Square D and GE starters available. THANKS for your expert input!
The 25 FLA value.
It would be better if you used all 3 heaters. Use any 1 heater on one leg and the remaining 2 heaters in series on the other leg.
Art
The rule is "one heater for each current leg". You're right in that score. Pick a heater value the next size larger than your full load rating for starters "no pun intended". The ambient temperature of the controller housing can affect the trip point on overload switches, so there is room for adjustment if you have problems with tripping. There is nothing wrong with using a 3ph starter for single phase. Gives you a spare contact for running auxiliary equipment, like a dust collector or coolant pump. Good luck, Bugs
Yes, use the 25A current value to size your overload heaters. The FLA values for any motor type indicate the current which flows in each phase. I.E. - You would still use 25A to size heaters if the motor was 3-phase.
That being said, the actual heaters you need may not be labeled with "25A" - the current they're labeled with should be slightly greater than the full load current of the motor to which they're applied. Most heaters are manufactured for a 'range' of FLA motor currents.
You'll also need to consider which starter you're using (more precisely, which overload relay is used in the starter), as each manufacturer makes heaters for use with their specific equipment. Look up the manufacturer on the web, you can usually find heater table references. You need to use heaters specific to the actual overload relay model in your starter. You can also find overload heater tables referenced by equipment distributors, such as Graybar and Grainger.
If you really get detailed about this (some might say this is a bit AR for non-industrial applications), you can size heaters for a difference in ambient temperature between the motor and the starter locations. But, that's usually not significant unless it's a large differential.
For a single-phase motor, you can use just two heaters, and two poles of a three-phase starter/contactor. A more protective setup would use all three phases of the starter - just run one leg of the motor circuit through two poles (in series connection) of the contactor and overload relay.
Hope this helps, John Sellers
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