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Unread 21-01-2011, 12:30
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Re: Tetrix DC Drive Motor W739083 dynamometer data

Some folks here on CD have asked for a bit more data, so I am adding it here.

We used another of our dynamometers to measure mechanical output and efficiency for two of the Textric DC Drive motors (units 2 and 3 from the original post), with their gearheads removed. This gives some insight to the electrical and mechanical power losses.

At 12V dc, these two motors developed peak shaft powers ~ 17.6 and 15.8 Watts, respectively -- this is about 7 Watts more than was measured when the motors were tested with gearheads. That difference is the mechanical power loss in the gearhead, at peak loading.

Also, at peak shaft powers these two motors showed electromechanical efficiencies ~ 45% and 40%, respectively. In other words, motor 2 consumed 17.6 / 0.45 = 39 Watts, and motor 3 consumed 15.8 / 0.4 = 39.5 Watts, from the 12V dc supply. So motor 2 wasted 39 - 17.6 = 21.4 Watts, and motor 3 wasted 39.5 - 15.8 = 23.7 Watts, due to heating of their respective armature windings. These are the motors' electrical losses.

My conclusion -- teams might gain an advantage in climbing power by replacing the gearheads on these motors with more mechanically efficient alternatives, but the potential benefit is probably only a few Watts (my guess is 2 to 4 Watts). The time a team would spend refitting Tetrix motors with more efficient gearheads is probably better spent working on faster minibot deployment methods.
Attached Files
File Type: pdf Tetrix_Without_Gear.pdf (46.6 KB, 806 views)
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Unread 21-01-2011, 15:58
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Re: Tetrix DC Drive Motor W739083 dynamometer data

Richard, what was the measured free-speed of the motor without the gearhead?

Last edited by Tom Line : 21-01-2011 at 16:17.
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Unread 21-01-2011, 16:10
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Re: Tetrix DC Drive Motor W739083 dynamometer data

Quote:
Originally Posted by Tom Line View Post
Rich, what was the measured free-speed of the motor without the gearhead?
At 12V dc, both motors measured about 7350 RPM with 0.4 Ampere current draw. The true Free Speed is probably a little higher and true Free Amperes are probably a little lower, since the initial dynamometer loading was not really zero -- probably about 0.002 Newton-meter.
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Unread 21-01-2011, 16:15
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Re: Tetrix DC Drive Motor W739083 dynamometer data

Ok Richard, thanks for all your work!!! One more question, if you don't mind. Any idea what the gear ratio is through that gear head?

Last edited by Tom Line : 21-01-2011 at 16:17.
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Unread 21-01-2011, 18:04
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Re: Tetrix DC Drive Motor W739083 dynamometer data

Quote:
Originally Posted by Tom Line View Post
Ok Richard, thanks for all your work!!! One more question, if you don't mind. Any idea what the gear ratio is through that gear head?
The specs posted by Greg in the first reply state that the gearbox has 1:52 ratio which matches closely to Richard's measured 145:7350.

And I'd like to add to the thanks for publishing your results Richard.
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Unread 21-01-2011, 18:46
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Re: Tetrix DC Drive Motor W739083 dynamometer data

Wow. I can't believe I missed that. Thanks.
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Unread 24-01-2011, 22:26
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Re: Tetrix DC Drive Motor W739083 dynamometer data

We were doing some Physics (yes, finally there is a purpose to that class! ) and we calculated a speed of ~22 inches a second under a 5 pound load using two Tetrix motors idk if it is relevant to anyone but...
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Unread 05-02-2011, 14:23
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Re: Tetrix DC Drive Motor W739083 dynamometer data

Hi Richard, for the results with the Tetrix NiMH battery, with voltage >14V, is there any way you could provide the info on RPM vs. torque (I'm curious how the stall torque increases with voltage--does it scale proportionately?) and current vs. torque? Then we can re-run our simulation for the various gearing and wheel size ranges we're considering. We wanted to look at whole range of motor operation at ~14V, and not just the peak power point.

I realize you were unable to attach the raw data because of file size limitations, but if there is a way you could send us a more limited set of data that would be much appreciated. Thanks very much for your help!

...............
With Tetrix battery: (voltage > 14V)

Motor 1: 16.5 Watts peak output mechanical power at ~90 RPM, 178 RPM initial motor speed
Motor 2: 16.0 Watts peak output mechanical power at ~90 RPM, 173 RPM initial motor speed
Motor 3: 14.2 Watts peak output mechanical power at ~90 RPM, 172 RPM initial motor speed
Motor 4: 15.2 Watts peak output mechanical power at ~90 RPM, 176 RPM initial motor speed

------------------

.....

NOTE: I wanted to include the raw data and plots of speed and power vs. torque; however, they cannot be attached to this post because the file size is too large (>101.8 kB).

---------------

Test Temperature: Ambient (22 degrees C)
Software: M-Test 5.0
Dynamometer: Magtrol HD 705-6N (Calibrated 01/12/2011 by NT) Dyno Controller: Magtrol 4629B (ML144)
Power Supply: Sorensen XG 150-11.2[/quote]
[/i]
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Unread 05-02-2011, 16:32
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Re: Tetrix DC Drive Motor W739083 dynamometer data

Quote:
Originally Posted by Dave Otis View Post
... (I'm curious how the stall torque increases with voltage--does it scale proportionately?)
Yes, stall torque does scale proportionally with voltage, because increasing the supply voltage will increase the stall current.

Torque-per-ampere does not scale; it is the same constant (given by the magnetic flux linkage, which of course does not change) for all voltages.

When I get some time I will create a reduced version of the measured curves for speed vs. torque and power vs. torque, at each of the test voltages for each motor. We recorded too many data points to include the raw file.
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Unread 08-03-2011, 06:01
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Re: Tetrix DC Drive Motor W739083 dynamometer data

Richard:

With regard to climb rate modeling, there is a simple approach that skips the conversion to energy.

For your example of 22.5 W net output power (two motors @ 15W x 75% efficiency:

Converting units gives 22.5W * 0.7375 = 16.6 ft-lbf/s

Divide by the weight (in pounds) that you want to lift, and you get the theoretical top speed
16.6 /5 lb = 3.3 ft/sec,
or 2.8 seconds to climb 110 inches.

The trick, as you point out, is to get the right wheel radius and gear ratio to get to the peak power point for your weight. The estimate of efficiency is key, since it needs to include all sources of friction/torque. As I learned (again) this year, mis-estimating this can put you near stall, and smoke a motor.
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