Speed Reducer or Increaser, which is better ? 5

[edison123]

An ID fan running speed is 850 RPM. We have two motors – one with 720 RPM (8 pole) and other 990 RPM (6 pole). Which motor will give an overall better efficiency – the 720 RPM motor with speed increaser or the 990 RPM one with speed reducer ? Forget VFD’s.

 

[electricpete]

It may be somewhat academinc, but I would like to provide a better discussion of why acceleration time is the same for different speed motors (same horsepower), neglecting differences in motor inertia, and relative shape of motor torque speed curves.

Let the two speeds of interest be speed 1 (880rpm) and speed 2 (770rpm).


N = speed
Trated = Torque at rated power
Taverage = average torque over the speed range 0 to N
t = acceleration time
N = speed
J = inertia
Tload – load torque assumed zero during start for simplicity

t1 = J1*N1/(Taverage1)

Trated2 = Trated1*(N1/N2)
Assuming similar torque vs slip curves:
Taverage2 = Taverage1 * (N1/N2)
J2=J1*(N1/N2)^2

t2 = J2*N2/(Taverage2)
t2 = [J1*(N1/N2)^2] * [N2] / [Taverage1*(N1/N2)]
t2 = J1 * N1/Taverage1
t2 = t1

The quantity of interest is not likely starting time, but the ability to start the motor safely. As noted before, the lower speed motors have ability to start larger inertia's, BUT as jomega pointed out the inertia will also look bigger to the lower speed motor. I'm not sure which effect in general is more important.

 

[electricpete]

NEMA Inertia limits for standard squirrel cage motors (not sure about wound rotor) are given in table computed based upon the following (ref ANSI C50.41 and NEMA MG1 section 20.11)

Load WK2 =+A*hp^0.95/(rpm/1000)^2.4 - 0.0685 *hp^1.5/(rpm/1000)^1.8

If I am not mistaken, it's telling me the max load inertia is roughly propotional to 1/speed^2 for a given horsepower. Since the inertia seen will also be proportional to 1/speed^2, there doesn't seem to be a reason to prefer one over another based on motor safe starting considerations. But you should investigate starting carefully for yourself.

 

[electricpete]

Another error in the torque computation excercize above that concluded there would be an overload:

The fan torque requirement was computed from 250hp, should be computed from 230hp. That will give you a little bit of margin to keep the motors less than full load amps. If you still feel you are too close to full load amps (and the process allows), you can change the ratio to decrease the speed sligthly..... fan power will change according to speed^3.

 

[jbartos]

Suggestion to electricpete (Electrical) Jan 6, 2003 marked ///\\I was under the general impression that wound rotor motors (with rotors shorted) have less capability to accelerate large loads than comparable squirrel cage motors. The reason being that squirrel cage rotor can withstand a lot of heating, wound rotor can withstand much less due to insulation. Correct me if I'm wrong.
///The wound rotor induction motor has a lot of heat dissipated in the motor starting resistor during the motor start. In fact, the wound rotor starting resistor is sometimes used for the motor speed control. However, the starting resistor has to be properly designed with respect to the wound rotor motor rotor and stator parameters.\\\