[TowerTalk] Rotor specs, are they a bill of goods ??

[jimlux]

Jim Thomson wrote:
> OK,  I see mentioned on most rotors... specs like   "start up torque"   and "running torque"    and also "stall torque" 
> 

Because that's how motors and gearboxes are specified, in general?

Stall torque is limited mostly by the winding resistance and the kind of 
motor.  Running torque has to take into account the losses in the gear 
train.  There will usually be some sort of time limit on how long you 
can run it stalled (for motor heating).. if the motor is internally fan 
cooled, spinning at all makes a big difference.

By looking at the current draws at the different torques (if available) 
you can figure out how much of the power is going into the losses vs 
actually driving the load.

For 3 phase AC induction motors, there's standard ratios that are used 
(Full Load Amps vs Locked Rotor Amps vs Normal Starting Amps).  7:1 is 
typical Locked Rotor vs Full Load. 3 or 4: 1 is typical for start vs run.
(this is why a slow starting capacitor start motor cooks the start cap...)

And, as you point out, there's also a "holding torque" for the brake.


The other things that affect these specs are the kind of gear train.  A 
spur or planetary gear set will usually allow the load to backdrive the 
motor.  A worm gear drive won't.

As for why inchpounds and not Newton-meters or ft pounds or other 
units.. tradition.  Small sub-fractional HP motors are almost always 
specified in inch pounds.