VFD application to standard duty motor 2

[roryrobb]

Customer has 3 each 75 hp Reliance standard duty motors, 4 pole, 460 vac, tefc that run from ROM stacks of coal for train loading.

They need to start blending these stacks and want to slow down the feed on the belts based upon sulphur content etc.

The worst scenario the engineers can see is slowing to 10 hz which at this speed will require only 5 hp and only for a maximum of 2 hours while the train is loading.

Question: Besides the obvious concern about a non-inverter duty winding system, will cooling be a factor for this type of time duration at this minimul load since air over will be reduced to 1/6 of original volume?

 

[jraef]

Yes. Add blowers to those motors that run independently of the motor power whenever the VFD is in the "Run" mode. That way you will not need to worry about any operating speed.

Curious, how did they come up with 5HP at 10Hz on a constant torque application like a loaded conveyor belt that required 75HP at 60Hz? Are they simultaneously reducing the amout of coal fed onto the belt as the speed is reduced? And even if so, what about the material that was already on the belt when the decision was made to slow it down? That should theoretically require the same torque as it previously did until the heavier section passes the load-out. Something doesn't sound right in those numbers.

HP = Tq * Sp/5250 So at 75HP and 1750RPM, Tq was 225ft.lbs.
To get the same Tq at 292rpm, the HP would be 12.5, assuming the same load.

Not that it will make any difference in your application, the motors and drives will still work fine as long as you cool them properly. Just curious. Subvert the dominant paradigm... Think first, then act!

 

[electricpete]

jraef is probably right as usual, but to me it's not a straightforward answer. Yes cooling flow will be reduced 1/6. Perhaps heating (approx no-load heating) will be aprox 1/2 of full-load heating.

TEFC conducts heat to the fins where it is removed to the air. Even with no air flow it seems conceivable that natural convection heat removal from fins might get you 50% of the heat removal that you get with full air blowing (assuming similar stator temperature). And you do still have 1/6 air blowing. And you will only run that for 2 hrs. I'm sure Reliance could do a study on that without too much trouble.

 

[GusD]

Hi good day

My experience tells me that you would have trouble
operating TEFC motors at 10hz on a constant torque application without overheating same.External cooling blowers might be necessary.




GusD GusD

 

[mdonner]

In constant torque application the power is reduced with speed (+ add excitation and losses).
Check control range in motor data sheet. If it is lower than the required 1:6 the motor will heat up (assuming the cooling fan is motor shaft driven). You will need external cooling.
Note that airflow is reduced proportonally to the square of the rotatinal speed.