DC injection brake, Single phase PCS 1

[Barry1961]

I have a 1/20hp, 115VAC, 60hz, single phase, PCS, gear motor with a 200:1 ratio that I would like the stop time/position to be repeatable. I would rather not use a mechanical brake so was thinking of DC injection braking. There is not much money to be spent in this project so I was thinking of trying to make a little homemade DC injection brake.

I just have a general idea of how the injection braking works. You supply a DC voltage to a single winding for a short period of time causing the rotor to try to align with that one pole. I also know that it works better at low speed. What I need is some actual numbers like........

How much voltage? What pulse rate? What kind of back EMF should I expect? Do I need to disconnect the run capacitor before DC braking? What percent of full motor torque can I expect get at full speed, half speed, quarter speed?

I know most of the answers will be variable depending on voltage, pulse rate, ect... I am just trying to get a little bit better feel to see if it is worth experimenting with. If you were going to experiment with it where would you start?

The motor is not under any load to speak of and will cycle 4 times a min. for 8 hours a day.

Barry1961

 

[Marke]

Hello Barry1961

Dc injected braking operates in the same way as AC operation in that there is current induced on the rotor due to the "slip" and as a result there is a stator field and a rotor field and torque produced by the interaction between the two. Just think of the DC field in the stator as the same as an AC stator field except that it has a frequency of 0Hz and therefore is stationary. The rotor tries to synchronise to that field.
The current in the stator is limited by the DC resistance of the stator and on a small motor like that you would probably want a DC voltage of around 12 volts as a starting point.
There is no need to pulse (PWM) the DC, in fact if you can apply a smooth DC you will get a better result. I have done it before on larger motors using standard lead acid batteries!!

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

To further Marke's excellent response, the one issue will be repeatability. That will be totally depentant upon load connected to the motor. If the load is always the same, a DC injection brake will be very repeatable. If the load varies from cycle to cycle, it will not.

And the amount of braking torque will be dependant upon the current applied. With an unlimited amount of current, the braking time can be theoretically as fast as the acceleration time Across-the-Line. However, if your brake device is only capable of a limited current output, your braking time will be longer.

Lastly, if you use a voltage control scheme for applying the DC, where a low voltage is applied and increased until the motor stops, the brake torque curve will look a lot like the torque speed curve of the motor, except a mirror image. So you will have the least brake torque at the beginning, but it will increase as the motor slows down (the field will be cutting fewer lines of force, so circuit impedance goes down as the motor slows). This means that at or near the end of the brake cycle, the load will "jerk" to a stop, just like the jerk that happens when accelerating. If you can do a current controlled DC injection brake it will be much flatter and less stressfull on the mechanical components, albeit a little more complicated to build from scratch.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Barry1961]

Thanks for quick responses!

About what rpm does the DC injection braking quit working and will it generate high voltages when applied at high rpm?

Barry1961

 

[jraef]

Providing that you allow the motor field to colapse prior to applying the DC, the answer would be Supersynchrounous by the slip percentage. Most DC injection brake systems have a delay of at least 250ms before they apply DC so that the motor field colaspses, because if it is applied while the field is still there, the DC will provide enough excitation to allow the regenerated voltage to self-excite and maintain it's ability to generate.

Without that initial motor field, the only time the motor can become a generator is if it excited AND is overhauled beyond it's synchronous speed by at least the amount of it's slip percentage. So if it is a 4 pole motor that has a synch speed of 1800RPM, and the slip speed is 1750, the slip is 3%, so the motor would need to be spun at 103% (1854rpm) for it to be capable of regenerating.

In other words, not a worry if you just wait a quarter of a second or so. Come to think of it though, with yours being a PSC motor, it may take longer because of the caps in the windings. I only do 3 phase motors so it isn't an issue for me.

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

As Marke explained the developed torque depends on the DC current applied. The temperature rise of the windings will limit the amount of current allowed. I will try similar values to the nameplate FL current.

If the main terminal lines of a single-phase S.C. induction motor are fed with DC voltage, the result is a constant magnetic field. This field is fix in magnitude and stand still in the space. (It is like a permanent magnet).
The magnetic flux intensity (Phi) is limited by the ampere turns (AT) and the magnetic circuit reluctance (R).
Phi = AT/R
The induced voltage in the rotor bars is proportional to the flux density (B = Phi/Area) and the tangential rotor speed (v).
e = -BLv
The current in the bars (I) is a function of the bar resistance (rb) and induced voltage.
I = e/rb
And the force (Fb) in one bar:
Fb = BLI
When the speed drops the braking force drops too. At locked rotor there is no induction and no braking force in the bars. If while the rotor is at standstill condition a torque is applied to the shaft, the magnetic-field will oppose (Tb) to any accelerating force.
Tb = F*rad

The DC field will develop reluctance forces due to the phenomena of alignment the stator and rotor teeth. These forces add to the rotor bars braking torque.

 

[RobWard]

I would add: (although I'm not too clued up on the theory I have fitted a few DC brakes in our plant during the last year) all the units I fitted had a limit on the number of operations per hour, usually about 6 to eight, which I assume is due to a heating effect somewhere.
I didn't trouble myself to find out whether it was in the brake unit or the motor windings. Perhaps someone will exlain that here.

You are talking about 240 operations per hour, so perhaps someone could point out where you may need some cooling

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[jraef]

The heating effect is roughly the same as in accelerating, so you need to count each brake cycle as a "start" when looking at your starts-per-hour capacity of the motor. So 6 brake cycles, plus 6 corresponding start cycles, is 12 starts-per-hour.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Marke]

During start, and during stop, you dissipate the full speed kinetic energy of the driven load in the rotor. There is additional dissipation in the stator.
The number of starts per hour is dependent on the total energy dissipated and the ability of the motor to get rid of it, so in reallity, it is very load dependant.

Best regards,

Mark Empson

http://www.lmphotonics.com