What's the procedure for picking heater's? 2

[Lovison]

Last person who called me said he picked heaters by taking the FLA of the motor and adding in the service factor.

Now that sounds find - but what about Starting Amps how does the heater compensate for that in rush and not blow?

Simple I know but a simple answer will make me more informed in the future.

Thanks

Wayne

wlovison@toyopumps.com

 

[peebee]

Re: "What's the procedure for picking heater's?"

See NEC 430.31 through 430.44. I suspect most of your motors will be covered by NEC 430.32.


Re: "what about Starting Amps how does the heater compensate for that in rush and not blow?"

That's the whole point of the heater, it takes some time to heat up, it's got "thermal inertia". So a short-term overload won't trip it, nor will short term inrush currents. That same current, applied over a longer duration, will give it enough time to heat up and finally blow.

 

[electricpete]

go to

http://www.geindustrial.com/cwc/library?famid=35

Scroll down to look at the following detailed document:
GET-2681L Heater selection guide

 

[electricpete]

If you use different manufacturer heaters you may want to look at their guidance.

 

[jbartos]

Suggestion: The manufacturer catalog guidance might also be used since there are various subtleties in the heaters selection. One very minute, however important item, is whether or not the overload relay is attached to the contactor or is mounted away as a stand alone item. This makes difference on the heater rating since the overload relay attached to the contactor experiences heat from the relay contacts.

 

[bigbillnky]

There are actually several different varities of overload trip units available, and each provide different trip characteristics dependent on the application. The most common probably is "molten alloy", with dashpots, bi-metalic, and electronic being others. In order to provide the best protection while avoiding nuisance tripping, you must consider the ambient temperature difference between where the starter/overloads are and where the motor is located. Service factor is another variable that must be considered. Each manufacturer has their own guidelines for proper sizing of their overloads, although all follow the same basic principles. The electronic trip overloads are the best for most applications and most can be customized for the particular application. Good luck.

 

[jbartos]

Suggestion: If the electronic overload protection is applied, then there are more features available than with the traditional overload relays. The electronic overload relay protection may become expensive and may make the product with the motor expensive, if all features of these relays are used.

 

[peebee]

On a side note: bigbillnky made reference to the molten alloy overloads. The way they work is pretty cool.

The alloy is more or less similar to solder, it's just a metal that melts at a relatively low temperature, but also at a very predictable and repeatable temperature. It's very similar to the alloys used in the pop-up turkey thermometers. Different alloy blends give different melting points. With the turkey thermometer, the alloy holds a spring-loaded plunger in place; when it melts, the plunger is released and it pops out.

With a motor overload heater, instead of a plunger, there's a spring-loaded ratchet. The ratchet shaft sits in a pot full of the alloy. When the heater melts the alloy, the ratchet can spin, and that trips the motor stop circuit. After the alloy cools and re-hardens, you push in the reset button, which winds the ratchet up to the next notch and tensions the spring again.