If I'm understanding this right you need heaters to protect a motor from overload because they are overfused to compensate for start-up in rush current, right? Can you wire a motor without the heaters?
I don't wire motors to often and I'm doing one for a three phase air compressor. I know all about the tables in 430. Thnks for the help.

Heaters protect against overload currents, fuses/breakers protect against short circuit currents.

430 gives you the maximum ratings for both.
Newer starters (solid state, electronic) have adjustable range overloads, more convenient for the installer.

John

John, Do you have any idea how much one of the newer starters are? Or who makes a reliable one?
I got a qoute for a square D encloser w/heaters for $280. Can I wire the motor without the overload protection? It will be wired with #10s and on a 30A breaker(FLA is 12.5). The motor was $300 dollars by itself, do I need to spend another $300 to protect it?

Read 430.31, 430.32 for NEC requirements.

The Sq D solid state is approx the same cost, or +10 to 15 bucks over the heater type.

Be forwarned, you need nameplate info (FLC) to get the correct adjustable range.

IMHO, a starter w/OL is a investment, even IF it's not required by NEC.

John

Bypass the "heaters", and you might as well just use a contactor. By definition, a "starter" is only a contactor with overload protection.

Can you do it? Sure. But why? Shouldn't you fix the problem- and not remove the thing that's covering your tail?

As for the electronic types....besides overload protection, they also provide protection against low voltage , single phasing, and circuit imbalance. A real improvement, if you ask me.

Bot, as to your original theory- you've confused apples and oranges.
The fuses, or circuit breakers, are oversized to allow for start-up currents.
"Heaters," on the other hand, respond much, much slower, so this is not an issue. You generally have deaters that are set quite close to the normal operating current of the appliance. Only a sustained overload will make them trip.

If you only need to manually start and stop a motor, as well as provide the necessary protection, would something like this be suitable: http://web1.automationdirect.com/ad...tor_Starters/45mm_Frame_Width/BM3RHB-013

I can't figure out if this is a totally stand alone device which integrates motor protection, or a device which must be mated to a contactor. It is pretty clear that the above device _may_ be mated to a contactor, and that one could create a full manual starter with proper overload protection for quite a bit less than $300.

-Jon

Winnie, the prices mentioned were for NEMA- size starters. I agree that they are often much more than what is needed.

What you show is an "IEC" style component. These are mated to matching IEC contactors, and the assembly is a starter. They are very usefull, especially with smaller motors.
This item mounts on a DIN rail inside an enclosure. There is also a need to mount switches / push buttons, and wire them in.

What really gets my goat is that NFPA 130 won't allow me to use thermal overload protection in subway emergency ventilation fans. It seems a darn shame because most of the time they are used won't be for emergencies. I never got a clarification as to whether or not I can use overloads other than thermal.
Joe

" Can you wire a motor without the heaters?
I don't wire motors to often and I'm doing one for a three phase air compressor"

Why would you not use heaters??? The starter (contactor & overload block w/ heaters) are very inexpensive, compared to the price of replacing the motor.

The fuses protect the cable & the heaters protect the motor (overcurrent & from single phasing)

For an application with a compressor, I would recommend a NEMA starter, I prefer the Furnas (Siemens) Class 14 starters with the ESP electronic overload.. as when it comes to compressors, they tend to cycle quite a bit and I would not trust the IEC starters as they need to be oversized

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Allen Bradley makes a very nice and easy to field assemble wall mount style starter. It comes with nema starter, main breaker, heaters, contactor, control transformer, terminal blocks for control circuit and internal wiring, and a set of N.O. and N.C. contacts for run indication or interlocks. Another nice feature is the enclosure comes with a lockout device and power disconnect handle built in. The enclosure is about the same size as a square-D 100 amp disconnect and can be ordered with all nema ratings on the enclosure.

{bold emphasis mine}

Joe, I think the reason is the same as for fire pumps (Art. 695, 1999 NEC, latest edition I have), in that it is far more important in that application to have the motor operate to failure than to have it shut down in the middle of an emergency situation, thus adding to the hazards created by the original emergency. In fact, a fire pump is one of the few exceptions that allows a tap ahead of the service disconnecting means!

My local motor repair shop has shown me the results of a motor used in a fire pump that was operated to total failure (It was big, IIRC 25hp) and it had to be scrapped! The combination of eddy current heating and burning/arcing of the windings totally destroyed the laminations of the stator.

The rotor also showed some peculiar effects that I can't even describe.

Now as to the part I made bold here, isn't that a violation of NFPA and other codes? I had thought that those types/classes of fans (like in some large buildings) were to be used only in emergencies? The whole point of those fans is to back up the primary ventilators in the event of fire or in your case, excessive fume build-up.

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[This message has been edited by mxslick (edited 03-26-2006).]

[This message has been edited by mxslick (edited 03-26-2006).]