AC induction motor- Dynamic Braking

[Kal_B]

Hello everyone,

What is the relation between resistor/s size and the amount of torque generated by a motor that is generating?
I was reading this article and it stated that " The lower the resistance, the higher the current in the braking circuit and the greater the negative motor torque. If you want to stop fast, this provides additional torque along with the DC injection braking the drive is providing (stop mode on the drive is “ramp”). "

I don't understand how increasing resistance would decrease the torque.

An AC motor enter generating mode when the rotor speed is higher that the speed of the rotating magnetic field. At this point the motor acts as a generator and negative torque is produced, how , I have not idea.

Could someone explain in as simple terms as possible how that works. There are a lot of animation and graphics of positive torque and rotation but nothing showing how negative torque is produced and how it relates to current. Buzz words that I read have to do with magnetic flux and Fleming rules and .........

Thanks

 

[KeepItSimpleStupid]

Take a permanent magnet DC motor because it' easier.
#1: Torque is proportional to current
#2: RPM is proportional to unloaded voltage (tach)
#3. A DC motor acts as a DC generator when externally driven.

A simple way of implementing dynamic braking - short the winding. The
"generator" is operating into the lowest load, itself or the resistance of the windings. Note is says "negative motor torque". it's in the opposite direction of a normal motor.

That said, you can have a 4-quadrant power supply which means you can have +I,-V and -I,+V in addition to +I+V, =OI, -V.

When you have all 4 quadrants available, you can charge batteries, you can coast, you can brake and you can drive.

I built a gantry crane for a model railroad for a client and to stop the motor instantly, I shorted the windings.

I didn't in the 80's, but PWM is generally used to control speed. So, if you apply full voltage to the motor, but with a small duty cycle, you get all the torque the motor can produce at a much lower speed.

 

[Pommie]

Could you not have asked this in your previous thread?

Mike.

 

[rjenkinsgb]

Torque is roughly proportional to current.
Forget the "negative" torque part, that just relates to the relative direction.

Acting as a generator and open circuit, a motor after disconnecting power will just coast, with speed controlled by inertia and friction.

Add a load, and energy is taken from the motor, causing it to decelerate harder. The lower resistance the load (so higher current at any given speed), the more braking effect it has.

An induction motor has no inherent magnetic field - the fields are produced by current in the stator and induces current in the rotor.

To ensure a strong field and good braking, DC can be applied across two phase connections after the AC supply is disconnected.
The third phase connection can be shorted to one of the others (so part of the stator winding is directly shorted), or connected via a resistor or capacitor to regulate the current in that phase.

 

[MaxHeadRoom78]

Another means of braking with a 3ph motor is 'plugging' or reversing two phases, it was popular at one time, but can be hard on the mechanics.

 

[schmitt trigger]

Also, by applying a DC voltage.
Edit: SORRY; I realized this was mentioned in the first post.

DC injection braking - Wikipedia

en.m.wikipedia.org

 

[Kal_B]

@Pommie I barely remember my name let alone a thread I posted almost a year ago. But that phenomenon/principle has been challenging to understand. So I though I understood dynamic braking until I ran into an application using six Braking Units and braking resistor groups/banks. Then realized I did not really understand it.​

I re-read the article and I think I misunderstood parts of it and that caused my confusion. I will do more research and reading.

Thanks
Kal

 

[Pommie]

I'm sorry but my post has been deleted. Why? Someone forgetting their previous post or not using the search is just lazy.

Posters spent time answering the original post and are now smacked in the face.

Mike.

 

[JimB]

my post has been deleted. Why?

Because the thread was degenerating into a pointless squabble.

Someone forgetting their previous post or not using the search is just lazy.

You reminded him of the old post, well done.
But there is no need to go on about it, calling him lazy.

Posters spent time answering the original post and are now smacked in the face.

Hardly, their work is there for all to see and you have brought the OPs attention to it.
Sounds good to me.

JimB