mechanically coupled DC motors

[dmta]

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 !!!!!!!!!!!!!!

 

[ronv]

If you drive both of them hard, then yes they will get hot. If they are permanent magnet motors you might consider braking one by shorting the armature. If you use pwm you can vary the load. You may still need to be careful about heating.

 

[KeepItSimpleStupid]

If you can find a surplus brake your good: https://sdp-si.com/eStore/Direct.asp?GroupID=429

Current determines braking force. Tension controllers have to go slightly negative, though.

 

[dmta]

Hi every one !!!!!

I am VERY SORRY for this late reply.

Even though I asked this question a while back I am trying to implement it just now and I have a problem with selecting the motors.

Should I chose same motors for A and B or are there other factors that I have to weigh in?
Also, should I choose high power motors to get practical load resistor values?

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

 

[Ramussons]

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.

Why not use the coupled motor as a DC generator and use an external Resistive Load?

Ramesh

 

[Dean Huster]

I've done this before in my electronics classes. The second DC motor makes a wonderful generator and load. If you short the driven motor's leads, you can often bring the driving motor to a dead stop, enough that if you leave it that way, you'll burn it up. I used DECENT DC motors, not toy car motors. I like the 12-volt variety with carbon brushes, the more commutator segments, the better. Adding a rheostat to the "output" of the driven motor allows you to vary the load. Using a lamp bank, switching in parallel lamps is good too, as it gives a visual indication if you vary the speed of the driving motor.

 

[kinarfi]

Another form of braking would be to mount a copper or aluminum disk on the motor and an electromagnetic close to the disk, it will induce a current into the disk which will create a magnetic field that opposes the electromagnet, the closer and stronger the electromagnet, the more the braking action.

 

[dmta]

hi and thank you to to all three of you !!!!!!!!!

dear Dean Huster,

What was the power of the motor and do I have to consider about the torque output capability of the motor too? and did you use the same motor for driving and to generate torque ?

Dear kinarfi,

What if I just change the magnetic field of the electro magnet and not decrease the distance ? will it still work ?

Thank you

 

[kinarfi]

I imagine changing the field will work just as well as changing the distance, however, I haven't ever really built one yet, I saw it at a state fair once and one of the guys I worked with was going to mount the copper dist to his drive line to use to slow his truck down while coming hill with his boat in tow. Aluminum is cheap and easy to work with, I may just give it a try for my own experience.
The electric brakes on a trailer work via varying the strength, so I'm sure it'll work here also.
Please let me know it you try it,
Thanks,
Kinarfi

What motor or motors are you wanting to use?

 

[misterT]

Are you simulating change in inertia (mass) or friction? There is a big difference. If you increase friction, it is harder to accelerate, but much easier to decelerate. This is because all the energy put in the load is dissipated. If you simulate change in inertia, then energy is precerved and decelerating is also hard (you can do regenerative braking). Simulating change in friction is "easier" than simulating change in inertia.

 

[kinarfi]

dmta , I cut a ≈ 6" x ≈3/16" disk of aluminum and spun it with DC motor out of a dirt devil vac which pulled 2 amp at 3 volts and when I held a magnet close to the spinning disk, it just about stopped it and the amperage went up to 10 - 20 depending on how close I held the magnet, the magnet is out a server hard drive and is very powerful, I also use a circular Super Neodymium Magnet and had the same results, lessor magnets did not have nearly as dramatic effect. Did not try electro magnet.
Kinarfi

 

[dmta]

Thank you kinarfi for your reply. Really appriciate what you did and I will try it my self !!!!!!!!!!

Thank you also misterT !!!!!!!!!

I am trying to simulate a constant or varing load acting on a motor and how the motor controller (a PID) can be used to drive it at a constant speed and etc. That is why the torque generator.

How about I don't change a load and get a constant load resistor and connect it to the torque generator using PWM, will it work ?

Regards

 

[kinarfi]

What type & size of motor do you plan on using, just the simple PM with 2 leads or something more complicated?

 

[dmta]

Hi kinarfi

I'm just using two PM DC motors.

thanks

 

[kinarfi]

Here's how I would do it if I used PWM, or you could get a low ohm, high wattage reostat

 

[gary350]

Most motors can handle locked rotor current for 90 seconds before they become too hot and need to be turned off. If you can do your experiment in 90 seconds you do not need to worry about over heating. The motors will need to cool for several hours before you can do another experiment.

Put both open frame motors in an oil tank for cooling. You can get 3 hp from a 1 hp motor by doing this. Now you have a cooling system to deal with. If the motors are not open frame motors oil can not get to the windings for cooling.

 

[misterT]

Most motors can handle locked rotor current for 90 seconds before they become too hot and need to be turned off.

Depends how much current is going through the winding.. it does not make any difference if the motor is locked or turning.
All motors have maximum continuous current that they can handle continuously. This current is limited by the thermal characteristics of the motor.
They also have absolute maximum current that cannot be exceeded. The motor windings will burn if you exceed the absolute maximum. This current is limited by the physical construction of the windings/commutator.

Then there are ways to calculate periodic operation characteristics, if you want to "overdrive" the motor (drive the motor above the maximum continuous current).

 

[dmta]

Dear kinarfi, gary350 and misterT, thank you all for replying !!!!!!!!!!!!!!!!

@ kinfari,

Yes that is the way that I am thinking of using PWM to control the load. I suppose I can use a relay and another mosfet + power resistor if I want to do bidirectional load control.

@ gary350,

Since this is a demo board I won't be running the motors for too long. If the heating becomes a very big issue I will have a temperature sensor and cutoff power to motors if they get too hot. But in that case I will have to mount the sensor on the motor cover.

@ misterT,

The biggest drawback in my case is that I do not have the datasheets for the motors (or part numbers). Can you give me some idea on this periodic operation characteristics?

Thank you again to all of you !!!!!!!!!!!!!!

 

[misterT]

The biggest drawback in my case is that I do not have the datasheets for the motors (or part numbers). Can you give me some idea on this periodic operation characteristics?

Usually you know the cycle how the motor is driven and you can use that to optimize for motor selection. If you do not know how the motor will be driven (because it is used by students etc.) then you have two options: 1) Limit the current to the "max continuous current". and 2) Monitor the motor temperature and shut it down if it's "too hot".

Maxon has great self-learning material: https://www.maxonmotor.com/maxon/view/content/academy

EDIT: Look at pages 51 and 52 in this pdf: https://www.electro-tech-online.com...cation2Fpdfrealnamemaxon_Formelsammlung_e.pdf

 

[gary350]

The windings will heat up faster than the heat is able to transfer through a metal case to a sensor.

Locked rotor current is the industry standard for checking maximum load current. Even if the motor is turning at some slow speed it might be = to or less than max load current.

Another industry standard is maximum load current is usually 3 times the maximum run current listed in the motor tag. If the tag says max run current is 5 amps then locked rotor current should be about 15 amps. You can test it yourself and see. Lock the shaft so it can not turn then turn on the motor with and amp clamp or amp meter in the circuit. Take a quick 5 second reading. These will tell you a lot about the design of the motor. Most good motors will be 3, some manufactures might be about 2.5 to 2.7, some cheap Chinese motors are 2.

I use to work at the GE Motor Winding Factory in Murfreesboro TN all the motors that came off the assembly line were tested for, locked rotor current and lightning test. If the motor was going to short out and self destruct we want it to do it now, not after the customer buys it. One of the tests was a 270,000,000. amp 5000 VDC capacitor bank that would be charged up to a certain amount or power for each size or type motor then discharge through the motor windings to simulate lightning. If the windings shorted out or to the metal case it was junk.