ac motor braking system


Johan Bengtsson

>First things first....... Break means to damage as in destroy, damage,
>cause (irreversible) negative change, I don't think this is what you
>mean, try Brake, which is to slow down/stop.

OOPS! sorry! of course!

>> It was, however, not what I meant. What I was talking about was that
>> if the motor is still connected to the drive, but rather when the
>> drive gets disconnected from the power grid (or if the power grid
>> fails).

>The drive may or may not be disconnected from the power grid, the real
>question is where if anywhere will the energy be dissapated, if there
>are Braking resistors the power may be converted to heat in the
>resistors, i.e. there is still an electrical path present which will
>disharge the accumulated energy. If the power grid is disconnected the
>energy may still be discharged in your neighbours
>toaster/airconditioner/HiFi system, or anything else which is locally


>> The drive would not die completely immediately (since it have
>> capacitors holding a considerable amount of energy) and if it would
>> try to stop the motor in case of a power failure it could very well
>> succed with that because of regeneration without any external power
>> availiable.
>The amount of energy the capacitors store and are thus able to
>discharge is immaterial, what is of coincern is can they absorb the
>Kinetic energy you want to change to (potential) some other form of
>energy and thus dissapate as heat.

Partly true, they will need to hold enough energy for the drive to begin stopping the motor, but since that don't take very long time the energy needed isn't that high. But if you remove them completely it won't work.

>> A too long delay would - of course - kill the drive and after that
>> there would be no hope of electrically breaking since electrical
>> power - as you say - need to be availiable.
>Kill the drive ??? what does this mean, inanimate objets are not to
>good at dying, the issue here is, is there a discharge path for the

What I mean if of course that it needs energy to work, no energy = no work done = not able to stop the motor. Ok, I am not a language expert (I have never pretended beeing one either <grin>) but of course a thing like that don't die but isn't it quite common to call it dead when it doesn't do what it is expected to do?

>> In order for it to work the drive have to be connected to the motor
>> all of the time, and operational. It could (theoretically at lest)
>> stay operational on the power regenerated for as long as it take to
>> stop the motor since there would be more energy regenerated than is
>> needed by the drive. (why would you otherwise need a breaking
>> resistor).
>It does not (necessarily) take power to stop the motor, it takes
>conversion of the stored kinetic energy to some manageable form, and
>subsequent dissapation of this energy, it does not/should not matter if
>the drive is "powered" up as the motor generates the power, the DC link
>if it is powered up should keep the electronics going, and the
>transistors/thyristors firing, and allow the dissapation of the above
>mentioned energy to proceed, by the time there is no more energy left
>to realise this, it does not neccessarily matter anymore as the motor
>is stationary.

You definitely need power to electrically stop an asynchronous motor, that is what Phil Corso tryed to say all the time, and he is completely right about that. The power is availiable since it is possible to pull it from the kinetic energy stored in the motor, but not unless you have some amount of electrical energy to begin with.

As opposed to brake with a DC current trru one winding, that will also stop the motor, but you need to feed electrical energy into the motor until it does. All this energy are then transfered to heat inside the motor.

The matter is completely different with a syncronous motor or a DC motor since they work as generators all by themself (if they are rotating of
course) and just short circuiting them make them stop rather quickly (too quickly perhaps ... but that is another story)

>Donald Pittendrigh

/Johan Bengtsson

Do you need education in the area of automation?
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]

Phil Corso, PE

In the interest of technical correctness, the following comments are intended to clear up some inconsistencies:

(1) My Wed, Jan 2, 10:44 pm post.
Plugging Method. Although I pointed out that the current will not be excessive, I want to caution readers that net-torque could be. This results from the fact that net-torque is the difference between the torque required by the load and the counter-rotational torque developed by the motor.

(2) My Fri, Jan 4, 1:46 pm post.
Dynamic Braking Method. The rule-of-thumb equivalent dc-current factors for the two star-connected cases were inadvertently crossed. The factors should be 1.225 x FLA for the 3-terminal (3-windings energized) case, and 1.414 x FLA for the two-terminal (2-windings energized) case. The factor for the delta-connected winding is still 1.225 x FLA.

(3) Johan's Fri, Jan 18, 8:05 pm post.
Your statement that all of the energy is transferred as heat, is in error. Of the total power transferred across the air-gap, part is converted to heat as rotor loss, and part is converted to mechanical power (torque times rotational speed!)

BTW, to the List... unless you subscribe to the philosophy that "what's fur free ain't worth nuthin" lets hear some feedback!

Phil Corso, PE
(Boca Raton, FL)

Johan Bengtsson

That would be true - if it were not for the fact that torque times rotational speed, with the correct signs where they should be, you get a negative power when you reduce the speed, ie the the energy are not transfered to the load as an increased speed, but rather the oposite. This means that the mechanical energy (rotational speed times inertia) does have to go somewhere (there is a law against destroying energy) since it is not only hard to brake that law but it is supposed to be impossible the energy simply becomes heat - if you don't do anything else with it - such as regenerate it to electrical power or something.

So by electrically braking without regeneration all the energy you feed to the motor + all mecanical energy stored in the motor and load will end up as heat. And will do so inside the motor (apart from the small amount lost in feeding cables, friction elsewhere, cooling fan and so on - of course)

Btw, if the law actually could be broken it would be possible to create energy too - a very good idea but unfortunately not something anyone know how to do.

/Johan Bengtsson

Do you need education in the area of automation?
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Hey!!! Just a minute there? What is the solution for the 3/4 hp radial saw????? I've got a 1 hp (120v) table saw ("contractor saw") that I would like to retro-fit with a brake... The thing takes too bloody long to coast to a stop, and I get a bit impatient sometimes, and I've only got 1 set of fingers... I am looking for a device to do this. I have no luck finding such a device on the
web - everybody seems to be talking about multi-horsepower, high voltage, multi-phase stuff, with the braking as part of the overall speed control.

If possible, please contact me directly: [email protected]

> Schmooot... If you are referring to the 04-Jun-02 post, the answer is yes. Do you need details?

Yes I would like details please.

I've got a 3-phase machine that I converted to single phase by changing the motor and minor wiring. The electronics are happy on 220V single phase. And my single phase 220V motor runs the machine just fine.

I would however like to utilize the brake system that is built in. It consists of the start contactor opening (disconnecting the 2 lines)and the brake contactor closing (connecting one line and the + lead of a rectifier into the one winding.)

If I can utilize that brake contactor, and/or the rectifier as well it would be nice. Or if I have to use an external brake of some sort I can run it off the brake contactor as well.

It is a farm duty motor with about 8 wires coming out of it.....the diagram shows most of them either not used or joined together
Schmooot... two questions:

1) Is the motor also reversible?

2) Can you send me the Schematic, so that I can modify it to include the "Brake" circuit?

Phil Corso Cepsicon[at]AOL[dot]com)

Here is the schematic off the motor housing. It is reversible.

Also shown is where I attached it to my machine...couldn't be bothered to change the picture so just ignore that

I have tried swapping the wires to reverse the motor while it was still spinning (hoping for a brake and subsequent direction change) but it just spooled back up again in the same direction.
Schmooot... regarding the comments you posted:

1) Can't Reverse Motor While Running.
A single-phase fractional motor can be reversed by switching the power supply leads to either the 'run' winding or the 'start' winding. But, while operating at normal speed the 'start' winding is disconnected from the supply by a centrifugal switch. If reversal is attemped, the 'start' winding effect is negated until the motor slows down enough to re-close the switch. But, at that point the available torque could be too low!

2) Dynamic Braking Problem.
The nameplate reveals the motor is a Dual-Value Capacitor motor; having both a 'Run' and 'Start' capacitor. Hence, braking by DC-current injection, while doable, is more complicated than for the typical motor. The reason is the 'Run' capacitor must be isolated and any residual charge "bled" off.

I will contact you off-list with additional information.

Regards, Phil Corso
Follow up to Schmooot... Attached is a schematic circuit showing how to alter the Dynamic Braking Circuit (originally provided for a 3-ph induction motor) so that it can accommodate the replacement single-phase, 2-Value Capacitor (Start and Run) Fractional Hp motor! Modifications to the original Schematic are shown in red. Following is a description of major components:

o C1, Run Contactor (original.)
o C2, Brake Contactor. The original, 3xN/O contacts, are designed for AC-duty and are not suitable for DC-duty. I suggest replacement with one having 2xN/0 and 2xN/C contacts, or 2xSPDT contacts.
o CS, Start-Capacitor (original.)
o CR, Run-Capacitor (original.)
o Rs, Discharge-Resistor* for CS-cap (new.)
o Rr, Discharge-Resistor* for CR-cap (new.)
o RB1, and RB2, DC-Injection Resistors* for deceleration-time adjustment and to limit RUN-winding current (new.)
* Type, Size, and Wattage to follow.

Regards, Phil Corso
Hesham… The reason for contacting me off-forum is that I need detail about your application.

The schematic in Schmooot’s case covered a 1.5 Hp, single-phase, dual-voltage, start-cap, run-cap, motor, which had been substituted for a 3-phase motor. The conversion was more involved because of the very large run-cap.

An aside… follow-up to the Schmooot case: The conversion was successful! Originally, the motor took about 30 seconds to stop. The modification resulted in a stopping time of less than about 4 seconds, without exceeding FLA!

Phil Corso