Hi,

I am trying to understand fully on how a DC motor could damper a rotating force applied on its rotor when no voltage is present in the circuit. Automatic swing autodoor operators have a DC motor fitted with gearbox to drive the swing door opened and a spring applying a force on the opposite direction to close the door at low speed. What I would like to understand is that the DC motor actually dampens the speed induced by the spring. This dampening is operational dead or live and can be adjusted with a potentiometer faster or slower. If the polarity of the motor is reversed, the dampening is applied to the other direction depending if the door opens inward or outward. I have always wondered on how this was working, certainly to do with having electromagnetic forces being reversed and having to fight against each other between the armature and the rotor??? Bu this is only a suggestion and would appreciate more info. And also, how would the potentiometer affect this phenomenum to adjust the speed. Thanks

 

I guess the motor is being used as a generator. When power is taken like this it makes the motor harder to turn. Again I guess the pot is adjusting how much is being generated, possibly by controlling the field current.

 

Hi,

I am trying to understand fully on how a DC motor could damper a rotating force applied on its rotor when no voltage is present in the circuit.

Well there is a voltage in the circuit , if the input terminals are shorted the motor becomes a generator , voltage is generated internally in the coils by changing magnetic flux and the power is dissipated in heating the coils .

 

Well there is a voltage in the circuit , if the input terminals are shorted the motor becomes a generator , voltage is generated internally in the coils by changing magnetic flux and the power is dissipated in heating the coils .

That's interesting OZ, therefore the DC motor would be used as a motor to drive open the door and as a generator to control the closing speed against the spring. Therefore, I understand that the voltage generated by the rotation created by the spring would induce magnetic flux to dampen and control the closing speed. But could you develop on this though? I need to understand fully please

 

This is the whole premise behind Dynamic braking, a DC motor generates when motoring, the voltage level is almost equal to the degree of applied voltage the would be required to produce this rpm value.
Max.

 

Max, you just nailed it on the head, thanks so much for your input. Now I know the terminology for this theory, dynamic braking, I can search for it on the internet. I have already and it makes sense now the science is explained behind the phenomena.

 

There is also regenerative braking, but usually quite a bit more involved in its application.
Max.

 

There is also regenerative braking, but usually quite a bit more involved in its application.
Max.

ok thanks. I make a note

 

Motors can certainly operate as generators, with some types doing far better than other types. Brush type motors, as discussed, make very good generators, an ability which serves not only dynamic braking, but also for regenerative braking. This has often caused problems with variable speed drives, where a sudden decrease in the commanded speed turns the motor into a generator and causes the supply voltage to rise beyond normal.
If any folks doubt that a regular induction motor can produce a lot of braking torque without any outside power source, there is a simple experiment to prove it. With the motor sitting on the floor, power it up and let it reach full speed, and then unplug it and quickly short-circuit the pins of the plug. Usually the motor will flop over and roll across the floor. The effect is caused by the braking torque transferring from the rotor to the stator.

 

DC injection braking is a typical way to brake a AC motor, and used to be quite common.
Also plugging the motor is another way.
Max.

 

DC injection certainly works but it is a lot more complex than simply shorting the motor terminals. Some folks are totally amazed to see a motor roll over when the plug shorting is done..

 

DC injection certainly works but it is a lot more complex than simply shorting the motor terminals. Some folks are totally amazed to see a motor roll over when the plug shorting is done..

While what you say is true, it isn't very effective for when there is a load that can still power the motor by inertia. Once the residual magnetism is gone from the "shorting" it stops working. Where the DC injection can control braking over a longer and more controlled time.

 

For the theory, look into Emil Lenz and his law. Some have called this "the fifth Maxwell Equation".

https://en.wikipedia.org/wiki/Lenz's_law

ak

 

An article for the Chevrolet Bolt electric car says you can drive in L using only one pedal. Push it and it goes, release it and it stops. I wonder how they prevent it from going and stopping at the same time when you want to go slow.

 

Differentiation. Slow pedal movement in the push direction is interpreted as desiring forward motion. Slow movement in the release direction is interpreted as wanting to slow/stop.

ak

 

I have wondered the same thing: How to keep it from braking when all I want to do is coast? But then, I find the single pedal approach to be very poorly advised, bordering on "really stupid", in fact. Probably a concept derived by folks who have never ridden a bike. Sometimes, even many times, coasting is the right thing to do. Why rush up to a stop light at full speed, for instance? Aside from saving fuel, anybody who has ever had a brake line burst knows why not.

 

When a DC electric motor is coasting then it is a DC generator. Without an electrical load it "cogs" each moment a magnet passes a coil. But if the generator has a load or is short-circuited then it must work hard an is a very strong brake. With less electrical load (a higher resistance than a short) then its braking action is less.

The single pedal on an electric car is not simply full power or full brakes. The pedal gives adjustable power and adjustable braking and the car's computer must change it to "coast" when at halfway, instead of normally providing half power when at half-way.

 

. Without an electrical load it "cogs" each moment a magnet passes a coil.

That would have to be an exceedingly (unusually) LOW pole count motor.
Max.

 

A great example of dynamic braking is on a diesel-electric train locomotive. The diesel engine runs a generator that powers traction wheels. During dynamic braking, the traction wheels (which are simply electric motors) are used as generators. The power generated by them is fed into giant air cooled resistors.

Google "diesel locomotive braking resistor" for some great information on the subject.

 

A great example of dynamic braking is on a diesel-electric train locomotive. The power generated by them is fed into giant air cooled resistors.

Here is an example in a old post of mine https://forum.allaboutcircuits.com/threads/railroad-switch.143045/page-2#post-1211264
See post # 38.
The Resister banks are under the roof cooled by very large fans.
Max.