Tetrix DC Drive Motor W739083 dynamometer data

Technical Motors
1

I got some of the W739083 Tetrix motors (12V, 152 RPM marking) to test in my lab. We performed dynamometer tests to measure the output mechanical power vs. torque, for two types of electrical source: (1) a regulated 12V laboratory bench supply, and (2) a standard 12V (marked) W739057 Tetrix NiMH battery pack. We did not measure the battery voltage; however, the (unloaded) initial motor speed when fed from the battery probably indicates variation of its voltage due to the previous test. Some charging was done between tests.

We repeated the power curve measurements for four sample motors. Three showed consistent performance, and one was slightly lower. Results follow:

With regulated 12V dc supply:

Motor 1: 10.8 Watts peak output mechanical power at ~75 RPM, 145 RPM initial motor speed
Motor 2: 10.8 Watts peak output mechanical power at ~75 RPM, 145 RPM initial motor speed
Motor 3: 10.1 Watts peak output mechanical power at ~75 RPM, 143 RPM initial motor speed
Motor 4: 10.9 Watts peak output mechanical power at ~75 RPM, 148 RPM initial motor speed

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

Based on the above, my calculations for climbing time and gear ratio would yield a recommendation for 4" wheels and 2:1 gearing, for a (theoretical) climbing time of about 3 sec if we assume the minibot mass is 5 lb, two motors are used, and the mechanical efficiency (motor shafts to pole) is 75%. That mechanical efficiency might be about right, considering that my dynomometer measurements did not include any side loading on the motor shaft, which will certainly be present in the minibot.

Of course your motors may vary from the ones we tested. However, two general conclusions are still valid:

(1) Battery state of charge will be a very significant factor for minimizing climbing time. So will mechanical losses in the power train from motor shafts to pole.

(2) Design calculations based on the published motor data are likely to be too conservative; my calculations using the published data would yield a recommendation for 3" wheels, while the test data above suggests that 4" wheels are a better choice.

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

2

Nice !

Thanks Richard for doing this test and sharing.

3

Thanks for the testing Richard.

For reference here are the published specs for the motor as taken from the PTC curriculum which uses tetrix.

TETRIX Drive Motor Performance-1.pdf (774 KB)


TETRIX Drive Motor Performance-1.pdf (774 KB)

4

3 seconds seems about right. We have 2 separate designs that travel up the pole between 2.7 sec and 3.1 sec. (depending on who is running the stopwatch)

My initial calculations stated that we could make it up in a little less than 3 seconds, so it seems that the JVN Calculator was accurate.

So, summary
4lbs bot
2:1 gear ratio
2.7 second climb time

5

Like Richard, I too live for the details. Thank you Richard for stepping up to gather and provide this information for all teams. You really have a full grasp on the spirit of this competition.

6

Thanks for the information. However, it doesn’t make sense in my mechanical mind…using a 2 to 1 gear reduction with 4" wheels. An unloaded motor runs @ 178 RPM, (2.96 revolutions second) multiplying this by the circumference of a 4" wheel. 2.96 x 12.56 = 37.17 inches per second. Based upon the math and considering the load will reduce RPM, +/- 3 seconds is plausible.

If the RPM is reduced by 1/2. +/- 6 seconds seems like the time to reach the top.

Am I missing something?

7

Sorry, I should have expressed that a little more clearly.

I meant a 2:1 speed INCREASE from motor shaft to wheel. Effectively making a 4" wheel behave like an 8" wheel mounted directly on the motor shaft.

Here’s how I did the math:

Climb height = 110 inch (2.79 meter)

Energy = height X weight = height X mass X g

Energy (Joule) = height (meter) X mass (kg) X 9.8 m/sec^2

Example: if mass is 5 lb (2.27 kg) then energy = 62.2 Joule

Maximum motor output power (assuming a full Tetrix battery) is about 15 Watt at 90 RPM – operating at different RPM will REDUCE the power.

Time to climb = Energy / Climbing Power

Climbing Power = Motor Power X drivetrain efficiency

If drivetrain efficiency is 75% and two motors are used, then Climbing Power is 30 X .75 = 22.5 Watt. So, Time to Climb is 62.2 / 22.5 = 2.8 seconds, and climbing speed is 110 inch / 2.8 sec = 39.8 inch/sec. If 4 inch diameter wheels are used, then wheel speed is 39.8 / 4*pi = 3.2 rev/sec = 190 RPM. To match this wheel speed to the 90 RPM motor speed, the gear ratio is 190/90 or ~2.

8

Great information on motors/drivetrain for the minibot! Thanks!

Some rough analysis by FIRST Team 241 of the video at the Kickoff: The running bot seemed to have:

  1. direct drive
  2. 3" wheels
  3. two motors

It also seemed to climb the entire length of the tower in just under 5 seconds.

That would seem to imply PI*3" per revolution = 9.4" or 12-13 revolutions in 5 seconds or about 144 RPM under load of the minibot.

We don’t have the ftc-020 First Choice kit yet and the abbreviated list of parts does not detail the wheels included. I see a grainy picture associated with the kit but I really could not make out 4" wheels.

We plan to run experiments as soon as we get the kit. Hopefully we will get them soon.

minibot.jpg
minibot.jpg1273×543 114 KB



minibot.jpg
minibot.jpg1273×543 114 KB

9

Thanks for the data Richard!

4"x1:1 versus 3"x1:2

(4"x1:1 is a 4" wheel with a 1:1 ratio, direct driven; 3"x1:2 is a 3" wheel with a 80T gear on the motor and a 40T gear on the wheel)

Since mass will affect every race, is the added mass to get a 1:2 gearing on a 3" wheel worth the complexity versus a direct drive 4" wheel? I don’t think so. My charts show that a 3"x1:2 is only 0.1 seconds faster than a 4"x1:1 since adding gears adds weight (gear mounting, extra axle primarily). I’d suggest trying it with both and seeing what you gain from the flexibility in the design from 4"x1:1.

Edit – Whoops – the calculator didn’t factor in another efficiency drop due to the extra gears – so now 3"x2:1 is just on par with 4"x1:1.

10

so from what I see everyone is planning on a tall gear ratio (to theoretically increase wheel diameter)

I am very uneducated on electrical motor performance but so far I thought Max performance was somewhere near the motors free speed (although I do understand you get full torque at any RPM assuming your motor does not burn up) if this was the case wouldn’t you want a shorter ratio than 1:1 essentially creating a slower and higher torque drivetrain to ensure the motor was operating at peak RPM.

I’m pretty sure I’m wrong based on Richard statements

11

Colin -

You are likely thinking of a different type of motor. Maybe an internal combustion engine?* DC motors behave very very differently.

Take a look at the spec Greg posted, it will clear things up.

For a given voltage, a DC motor has the most Power output at about half of its free speed. Torque is highest at stall, and decreases linearly to zero torque at free speed.

12

after looking at the graphs for the thousand time I sort of understand how to read them I see that maximum efficiency at 88 RPMs

I had the idea about Max torque at zero RPMs from electric dragsters
But I guess that doesn’t translate maybe brushless motors?
Anyhow I’ll gear it to hopefully bog to 88 RPMs
Thanks for all help folks just goes to show 20 years of school rarely matches up to simply asking a question and accepting that you are clueless in the subject.

ps Wankel’s are awesome i Have a snowmobile with one

13

Wow, thanks Richard!

Could you please post a little info on how you got the data? I might make a few freshman recreate this data.

Also, would you mind emailing me the raw data?

14

Great work. thanks for the test and info

15

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.

Tetrix_Without_Gear.pdf (46.6 KB)


Tetrix_Without_Gear.pdf (46.6 KB)

16

Colin,

You are confusing power and torque. Your statement about maximum torque at zero speed is absolutely correct. However, power is equal to torque multiplied by angular speed. Since the speed is zero at max torque, the power is zero hence the efficiency is zero.

This is why you will read about these motors burning out is you stall them at full voltage.

At the other end of the curve, the torque is zero at “free speed” and, once again, the power is zero.

For DC brushed motors, the max mechanical power is always very near 50% torque and speed (exactly 50% in a perfect, theoretical world). The max efficiency will always be at a lower torque (higher speed). Exactly how mush lower is the question…

20 years was not wasted. The laws of physics still hold and all is good…

Regards,

Mike

17

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

18

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.

19

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?

20

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.