### Author Topic: mechanically coupled DC motors  (Read 3131 times)

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#### dmta

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##### mechanically coupled DC motors
« on: March 28, 2013, 02:14:31 am »
Hi all,

I am building a DC motor control demonstration panel. Apart from controlling the speed I also want to simulate variable load acting on the DC motor. Hence I thought of coupling another DC motor to the existing one and rotating it in the opposite direction just like in the figure.

If we say that motor A is the motor to be controlled (clockwise) and B is the torque generator, even though B is given power to rotate anti clockwise it will always be rotating in the opposite direction (because of A). This will cause B to heat up and I fear that it would get burnt.

The goal is to somehow dissipate the current generated by B. Can you all help me.

BTW both motors will be driven by H-bridges and both will have speed measuring ability.

Thank you !!!!!!!!!!!!!!

#### NiHaoMike

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##### Re: mechanically coupled DC motors
« Reply #1 on: March 28, 2013, 02:57:49 am »
On the motor that is being used as a generator, leave the high sides off and PWM the low sides.
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#### vlf3

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##### Re: mechanically coupled DC motors
« Reply #2 on: March 28, 2013, 08:57:13 pm »
Have you thought about using an Eddy Current Brake... mount onto the motor shaft, an aluminium disc; the diameter is
determined by the amount of breaking required, along with two fixed magnets, or electromagnets; these are set
between the disc outer rim area = N to S pole fields through the disc, set as close to the disc surface as possible...

With electromagnets you could vary the field strength, and alter the applied load; heat would build-up within the disc
and be dissipated in-air and through the motor rotation... this system was used by Philips in there video recorder,
to engage the tape spools, by shifting the motor/magnets into the spool discs.

Some experimental work or testing would be required; otherwise see Wiki:

http://en.wikipedia.org/wiki/Eddy_current_brake

#### Smokey

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##### Re: mechanically coupled DC motors
« Reply #3 on: March 28, 2013, 09:21:13 pm »
Coupling two motors together (with a proper coupling) with one acting like a dynamic load works really well.  If you use a good properly sized motor and amplifier the thing that is going to overheat is most likely the amplifier driving the load motor before the motor itself over heats.  Either way, all good motors and amplifiers will have a wattage rating.  Good motors will also have a temp sensor and trip point signal coming out.  Stay within the spec of both and implement an over-temp trip signal and you shouldn't have any problems with things blowing up at least.  You can always go up to a larger wattage motor and/or amplifier for the load side than you are using on the driving side.

Another way to do it if you are just looking for a load is to not use an amplifier but connect the load motor leads together with a big-honkin power resistor or two.  Easy to heatsink and have airflow on that resistance load, but it's not as dynamic without switching in a different value resistor.
If your motor/amplifier can't handle the wattage, you need a bigger motor/amplifier.

#### SeanB

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##### Re: mechanically coupled DC motors
« Reply #4 on: March 29, 2013, 10:04:54 am »
What also works if you want a load is to use a stepper motor as an alternator, with the AC output being dissipated in a few power resistors or light bulbs as a simple load monitor.

#### mazurov

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##### Re: mechanically coupled DC motors
« Reply #5 on: March 30, 2013, 04:06:40 am »
This is the load I made a while ago -> https://www.circuitsathome.com/measurements/dummy-load-for-brushless-motor-controller  . I simply shorted all windings on the "load" motor together to get maximum mechanical resistance; if less than maximum is desired resistors could be used in series with windings.

#### Galenbo

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##### Re: mechanically coupled DC motors
« Reply #6 on: March 31, 2013, 12:23:27 am »
The goal is to somehow dissipate the current generated by B. Can you all help me.

Please specify the Power of the motors.
Answers are very different when for 12W of 500kW motors.

In industrial installations, I worked on 1MW installations where dissipation of the power was done by submerging electrical plates in water so they produce vapour.

On the other hand, for arduinic setups you could better use 2 rubber wheels that touch eachother and rotate thereby in opposite direction.
By pressing them closer together the power will be easily dissipated in rotative momentary deformation. No need for a generator.

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