Sizing IEC starters for pump motors 1

[tulsar]

Can you advise how you size your IEC starter sfor pump applications. Seems to be two schools of thought. Allen Bradley says it is based on horsepower while IEC says it is sized using AC-3 ultization catergory. Given the high amperage of submersible pump motors especially grinders it would seem to make sense to size on AC-3 given that there is only 4 to 6 starts per hour

 

[RRaghunath]

IEC 60947-1 defines AC-3 duty as "Squirrel cage motors: Starting, switching off motors during running". Thus, any squirrel cage type induction motor starter needs to be AC-3 duty as a minimum.
Further rating the starter in accordance with HP rating of motor is a basic thing any way. So, it is not either this or that, it is both.
In case the motor can be started more frequently than 3times an hour, it could be better to go in for AC-4 duty starter.

 

[tulsar]

Thanks raghun. In your opinion doesnt the AC-3 rating cover off the characteristic as it is LRA starting and FLA stopping. We have found that pump motors have high amperage in comparison to the horsepower ie 45 amp FLA for a 5 HP 230/1/60 for grinder style. Selecting based on horsepower would undersize the contactor or starter. The Allen Bradley 5 HP 230/1/60 IEC contactor has a maximum rating of either 30 or 37 amps (100-C30 or 100-C37). Usin horsepower would undersize it. I would like you ropinion on this

 

[jraef]

HP in a motor is a shorthand way of stating torque at a given speed. It is NOT a measurement of electrical power. You can ESTIMATE the electrical power, including the current component of it, from HP, but that still is an ESTIMATE.

Current carrying components, such as contactors, are sized based upon the amount of current they can handle without raising their own temperature to the point of destruction. But when contactors are rated for motor switching, they are ALSO rated for their making and breaking capacity, which is harder on them than just the thermal effects of the current. So what A-B is telling you is that you CANNOT just look at the current rating of a contactor, you must ALSO look at the HP rating, because that is where they factor in the making and breaking duty issues. But that does NOT absolve you from making sure that you are STILL sizing a contactor for the motor FLC. The AC-3 rating system then is just a little more accurate in the selection process because that says this is the CURRENT rating of an inductive load that can be safely switched.

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[RRaghunath]

tulsar,
jraef has explained nicely. I can add a little more.
The HP rating of a motor is telling us about the shaft power of the motor and not the input power. That is the reason, the contactor should not be sized based only on HP rating.
The HP rating of the motor, if divided by efficiency of motor and the power factor of the motor can give us the input power.
Then, this input power, when converted to the current should be the minimum required rating of the contactor (we add 10% margin to it and select next higher size contactor).
AC-3 duty, once specified takes care of the making of starting current (which typically is 6-8times the full load current) by the contactor.
Hope it is helpful. Sorry, I couldn't reply you earlier.