Three phase motor controller

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If its less than 1hp, (I think thats correct, could be 1/2 hp) you can use a 3phase hp rated switch and the breaker serves as the ocp, (wich has to be correctly sized).
 
DavisIMI said:
If its less than 1hp, (I think thats correct, could be 1/2 hp) you can use a 3phase hp rated switch and the breaker serves as the ocp, (wich has to be correctly sized).
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:Dwere it only that simple......

You need to look at 430.Section III. There are many varibles.. . 1 hp or less, continuous duty or not, automatically started or not, permanently installed or not.
Each of those situations can effect the answer.
 
bassmanleo said:
Is a motor contoller required for a 3 phase motor? Please explain.
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I presume that you talking about the question of magnetic contactor. The answer of course is no. Having said that, you have to have a device that deactivates the main current path both in the case of overload and short circuit. There are manual devices, more popular in EU, that do that where the manually operated MCP contains adjustable thermal and magnetic elements and thus fulfills the NEC 430 requirements.

Of course the selection does not go up to 500HP:D
 
weressl said:
In the case of single phase motors it may be an in-line thermostat.
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Yes, true, but he said 3 phase in the very beginning. I don't know of any self-protected 3 phase motors, it would be very difficult to avoid single phasing unless something opens at least 2 phases at the same time.

Choices for 3 phase motors are:
  • NEMA style Manual Motor Starter, fed by CB or fuses,
  • IEC style Manual Motor Starter / Motor Starter Protector (MSP), which includes CB function,
  • Magnetic Motor Starter, fed by CB or fuses (or IEC MSP)
  • Solid State Motor Starter, fed by CB or fuses,
  • VFD, fed by CB or fuses.
  • Motor rated 3 pole switch and fuses, but only under very specific circumstances as mentioned above, tricky to implement and risky for the motor.
 
Three phase motor follow up

Three phase motor follow up

Thank you for all of the replies. Some more details: I have two different motors, both exhaust fans.

1 - 3/4HP - 480V - 3ph
1 - 1-1/4HP - 480V 3ph

There would be a fused disconnect at each fan motor. The fans would be controlled by a 3 pole relay connected to a 24V HVAC control panel with a low voltage switch override.
 
3 phase motor overload

3 phase motor overload

I was told that, on a 3 phase motor, the motor could overload a single phase, and a fused disconnect will not protect the motor. Apparently, a motor controller has heaters rated at the exact amperage as the motor, which provides proper overload protection. Any comments?
 
Your 3 pole "relay" is your "controller".. (I'm not sure I've seen an actual "realy" with HP ratings that high--but lot of things I;ve not seen)

that said: as covered most professionly by Jraef.

how do you plan on addressing overload protection ?
 
bassmanleo said:
I was told that, on a 3 phase motor, the motor could overload a single phase, and a fused disconnect will not protect the motor.
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Sort of. If a motor overloads, it is usually only 1 of the 3 fuses that blows first. If it does, and for some reason the load on the motor drops off at the same time, the motor can continue to run on single phase power without blowing either of the remaining 2 fuses. If that happens, the motor can burn up without exceeding the fuse rating. When will the load drop off if one fuse blows? On a fan or pump! How?
  • Load on a centrifugal machine like a pump or fan is based on flow.
  • Flow is based on speed and requires torque to maintain it.
  • When a 3 phase motor is single-phased, the torque drops to 58% of normal.
  • Less torque, same load, more slip, lower speed.
  • Lower speed in a fan means less flow.
  • Less flow means less load, so motor current is reduced, often to a level below the fuse melt curve.
  • Motor continues to run.
  • But 1 phase to the motor means a severe current imbalance.
  • Current imbalance means added heating (negative sequence current).
  • Motor burns up even though the current is below the fuse threshold.

Apparently, a motor controller has heaters rated at the exact amperage as the motor, which provides proper overload protection. Any comments?
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Yes, well, sort of. A motor thermal Over Load Relay, PLUS that 3 pole HVAC relay, is what we would call a "magnetic motor starter". See above. "Motor controller" is too ambiguous of a term. Technically, that HVAC relay all by itself is qualified as a "motor controller" because it is controlling the motor. But it is NOT protecting the motor. Control + protection = "Motor Starter".
 
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