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Ken Leung
11-15-2001, 05:04 AM
I've been trying to calculate the motor power of globe motor with gear box...

According to the manual, Globe motor with gear box have the following specs:

No load speed = 87 RPM +- 1
Stall Torque = 150 in-lb

If Motor power = ½ Free Speed X ½ Stall Torque

and

Stall Torque in N-m (1 in-lbs = 0.11 N-m)

Then the motor power = 1/2 * 9.05 Rad/s * 1/2 * 16.5 N-m
= 37.33 Watt...

But according to the motor presentation in white paper, Globe motor with gearbox have a motor power of 50W...

Am I doing this wrong? Am I lacking some additional info to calculate the correct motor power?

Joe Johnson
11-15-2001, 12:17 PM
The specs the FIRST gives are the specs that Globe put on there print which, for reasons that are not clear to me, are at 10.5 Volts.

If you scale both the stall torque and the free speed by 12/10.5 you will get:

37W (12/10.5)^2

which is 48W

Close enough to 50W for me.

Joe J.

Kit Gerhart
11-18-2001, 02:48 PM
Has anyone done actual dyno tests on some of these motors to confirm that the "½ Free Speed X ½ Stall Torque" based calculation is reasonably accurate? I assume that the calculation should be fairly close to actual output, but I'm curious as to how close. The actual output might be higher than a calc. based on stall torque because the commutator timing is set to optimize the motor for some rpm greater than zero in the "forward" direction.

Joe Johnson
11-20-2001, 10:33 AM
The dyno tests we have on the power sliding door motors we use match the theoretical curve very well in the middle 80% of the curve. The last 10% on either side of the curve are close but quite the linear form that the simple models predict.

As for me, the calculations are good enough to get me in the range I want to be in. Designing to the last decimal place is always dangerous and often disasterous.

As to FIRST robots, there are plenty of other much larger unknowns to deal with than the difference between the simple motor model I use and the actual motor performance.

Joe J.

Ken Leung
11-21-2001, 11:41 AM
I never quite figured it out... I know that the window lift motors are used to move car windows up and down... Seat motoros for the power seats... And Sliding door motors for Van doors...

So, what exactly do the globe motors do in real life?

Joe Johnson
11-21-2001, 01:43 PM
The Globe Motor's "Day Job" is to shift from 4WD to 2WD on GM's trucks (specifically, Tahoes, Blazers, & Jimmys to name a few).

My understanding is that the motor also is used in Ford's as well but the motor/gearbox combo that we use is the GM model.

The same style gearbox and motor is used in 100's of applications that Globe has sold programs for including an satellite pointing application. The only way these motors can get as cheap as they are is for the investment of developing a motor family and a motor assembly line can be spread out over many many motor programs.

So now you know.

Joe J.