Our "DC Motors" article

[audioguru]

Who writes that stuff, anyway?

According to the article, in the 1st "Common Motor Applications Problems", if you take a tiny little low-torque DC permanent-magnet motor, and use additional batteries in parallel, then its torque will be increased! Oh yeah?

Not only that, but in the 2nd Problem that is answered, if I add a gear-train to the DC motor, so that its torque-load is decreased to conserve power, then the vehicle's speed will increase. Oh yeah?

Our article is here:
http://www.srl.gatech.edu/education/ME3110/primer/motors.htm

[ante]

audioguru,

The first point, only under certain condition it can be true. If the current draw is high and there is a voltage drop the motor looses some of it

[MP]

Who writes that stuff, anyway?

George W. Woodruff School of Mechanical Engineering
Georgia Institute of Technology
Atlanta, Georgia 30332-0405

I see nothing strange about this article. It reads like a basic textbook "motors 101" course.

Sorry you do not agree with it, but it is a good article.

MP

[audioguru]

But the article is just plain Wrong with those two errors.

[MP]

I think it is the way you read these things. I see nothing wrong in this article. If your motor is drawing too much current, then it is slowed down. When you add the gear train, it does speed up the motor because of the shift of the load. Again, if you do not have enough current, then adding an extra battery can help. This article is not plain wrong. These are not errors. The only error here is your interpretation of what you read.

MP

[audioguru]

Ante, MP,
If you re-read the article very carefully:

1) "On the other hand, if you need more torque, you can add more batteries in parallel with the existing ones, and your batteries will provide more torque at the same voltage. This is the way to go if your batteries are already supplying the rated voltage to the motor".
The article should say that by adding batteries in parallel, then the motor's run-time will increase, and its torque and current will remain the same.
Also, when they talked about "adding batteries in series with the ones you already have", they missed the point that the motor's torque as well as its speed will be increased.

2) "If your machine was alreading moving forward, adding a gear train will reduce the torque load on the motor, but the vehicle will move faster, and the motor will end up consuming the same amount of power".
The article should say that the vehicle will move slower, consuming less power.
They missed the point that a motor with a reduced torque-load created by the gear-train will draw less current.

They look like gross errors to me, because they have it backwards.

[MP]

1. Torque is directly proportional to current.
2. If you take the strain off a motor, it will move faster.
3. A gear train will not change current requirement. It will only shift the load.
4. Look at the torque/power formulas for motors.


I have read the article. There is nothing wrong in it.

MP

[audioguru]

It is obvious that adding batteries in parallel to existing ones will not increase the torque of an electric motor, especially with the voltage remaining the same, as they say.

It is also obvious that adding a gear-train to reduce the motor's work-load will result in the vehicle moving slower, because less work is being done, and therefore its current will be reduced.

The article is wrong in both statements.

[MP]

whatever. ::) Don't read them.

MP

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