motor reversal

[rancid rock]

ah ok thx man,

i'll work on this a bit and get back to ya.

 

[bananasiong]

BRAKING:
But if a DC motor has it's two terminals connected together it will brake. So if you want the motor to coast to a stop, just disconnect power. If you want it to brake, disconnect power and connect the two terminals together.

Can a DC motor brake?? Stop immediately?? Is it really that simple? just connect two terminal together??

 

[Nigel Goodwin]

Can a DC motor brake?? Stop immediately?? Is it really that simple? just connect two terminal together??

Not IMMEDIATELY, but it does stop very quickly, it's a standard technique that's been used for years and years.

Easy to demonstrate, just get a DC motor in your hand and spin the motor, it spins quite freely - now put a short across it, and spin again, it's now FAR harder to turn.

 

[dknguyen]

When the motor spins it generates a back-EMF. If you short the two terminals, then it forces the voltage difference between the two terminals to zero. A motor only has zero back-EMF when it's not spinning so it kind of forces the motor to stop (something like that, anyways I think).

 

[bananasiong]

How about this type of stopping?
10 or 01 is given to stop the DC motor, will it stop quickly? Or just same as removeing the supply, slowly?

 

[dknguyen]

How about this type of stopping?
10 or 01 is given to stop the DC motor, will it stop quickly? Or just same as removeing the supply, slowly?

Looks like it does the same thing as shorting the circuit terminals. It stop quickly.

Disconnect motor power & short motor terminals = fast brake
DIsconnect motor terminals and leave terminals unconnected to each other = coast to a stop.

 

[bananasiong]

Looks like it does the same thing as shorting the circuit terminals. It stop quickly.

Disconnect motor power & short motor terminals = fast brake
DIsconnect motor terminals and leave terminals unconnected to each other = coast to a stop.

Yes, thank you so much.

 

[rancid rock]

ah thanks heaps fellas this is really starting to make sense

there are many H-Bride types i have only just discovered....

i would have thought they all do the same job....

 

[bananasiong]

When the motor spins it generates a back-EMF. If you short the two terminals, then it forces the voltage difference between the two terminals to zero. A motor only has zero back-EMF when it's not spinning so it kind of forces the motor to stop.

is this explaination correct??

 

[dknguyen]

It's kind of a hand-wavy explanation. Here is an article I found online.

Q. How does a motor brake work?
A. Doug Ingraham, Lofty Pursuits

When a DC motor is spinning with the speed control turned off it is acting as a generator. With the prop wind milling the motor (generator now) is producing a voltage at the motor terminals but is doing no work. If you place a short across the motor terminals the motor must now work hard to try to generate the same voltage across a dead short. This will cause the generator (motor) to slow down. The motor short is provided by an electronic switch in the speed control.

So this would suggest that after you disconnect the power from the motor it is free-spinning on inertia and acting like a generator. If you short the motor terminals, it is like the generator is driving a low-impedence load (which is really hard to drive). Since the motor must work so hard to produce a large enough current across the short circuit to achieve the "generator-voltage" across the short-circuit, it slows down.

I think I incorrectly used the word back-EMF when I should have been using the voltage generated by a motor when it free-spins.

 

[bananasiong]

It's kind of a hand-wavy explanation. Here is an article I found online.

So this would suggest that after you disconnect the power from the motor it is free-spinning on inertia and acting like a generator. If you short the motor terminals, it is like the generator is driving a low-impedence load (which is really hard to drive). Since the motor must work so hard to produce a large enough current across the short circuit to achieve the "generator-voltage" across the short-circuit, it slows down.

I think I incorrectly used the word back-EMF when I should have been using the voltage generated by a motor when it free-spins.

Thank you very much!!