My Adventures in DC Injection Braking

Thread Starter

mcardoso

Joined May 19, 2020
192
Hi All,

I've been meaning to revamp my lathe's control cabinet for some time now. There were a few things I did on the original build some years ago which made me less than comfortable with the safety of the wiring. Since then, I have taken a job designing industrial control panels and I am much more aware of good practices and code requirements. I figured while I was correcting the panel, I would also experiment with a method of quickly stopping the motor using its own magnetics.

My lathe is an import ENCO 12x36 manufactured back in '94. It uses a common single phase 4-wire reversible motor with two capacitors. The motor can be started in either direction from a stop by reversing the polarity to the start winding. For those of you unfamiliar, a single phase AC motor is unable to produce any torque at a dead stop. A couple of tricks are used to start these motors, most common of which is phase shifting the AC line using a capacitor. This creates pseudo 2-phase power at the motor and gives it the torque necessary to start. A 3 phase motor does not have this issue. Once the motor gets part of the way to its operating speed, a centrifugal switch inside the motor opens and disconnects the capacitor and the so called "start" winding.

Many of these motors run in one direction, but some can be provided with separate connections for the start and run windings. By reversing the polarity of the AC line on the "Run" winding relative to the "Start", you can control the motor's direction. Again, the centrifugal switch disconnects the "Start" winding once the motor is at speed. This is typically done with a pair of contactors which are interlocked to prevent them from coming on at the same time.

The experiment came in play for when I want to stop the motor. Normally both contactors open, power is removed from the motor, and it spins down under friction. Instead, I added an additional pair of interlocked contactors labeled "Run" and "Brake". The Run contactor is pulled in whenever the motor is running forward or reverse. When the motor shuts off, the Run contactor turns off and the Brake turns on. This disconnects the motor from the AC line and connects a 24VDC 10A power supply across the Run windings of the motor. The effect is a rapid deceleration of the motor to a stop.

Here is the panel I built to do this,

1.jpg

2.jpg

From the bottom up: the power enters and is protected by 10A Class CC fuses. Terminal blocks allow easy-ish hookup of all the switches and buttons on the lathe. The right pair of contactors in the middle is the FWD/REV controls. The left pair is the RUN/BRAKE control. The big DC supply up at the top is for the DC injection braking, and in the top left is a PLC to control the timing. This could be easily accomplished with a timing relay, but I had the PLC on hand so I went for it.

The whole idea of this braking goes back to a concept known as slip. In an AC induction motor, AC voltage is applied at the stator and generates a rotating magnetic field. The rotor experiences this magnetic field and a voltage is induced in its windings. As long as there is a difference in the angular velocity of the stator's magnetic field and the rotor's mechanical angular velocity, there will be voltage induced on the rotor and torque generated. This leads to the realization that an AC motor can never match the electromagnetic field speed since there would be no difference in speed between stator and rotor, no induced voltage and no torque. The more load that is placed on the motor, the slower it rotates to generate more torque. This speed difference is known as slip.

The reverse can be used to brake the motor. When power is removed, the stator's magnetic field disappears, but the rotor remains spinning. At this point, we apply DC voltage to the stator which creates a stationary magnetic field. Now again, there is a speed mismatch between stator and rotor and torque is generated - but this time in the opposite direction, stopping the motor. All the mechanical energy is converted to heat in the rotor, so this has to be used carefully. For my lathe, I am not concerned.

Still finishing the program to time the brake, but it looks like it is working pretty well. I'm interested to see how much the stopping time is reduced.
 

Thread Starter

mcardoso

Joined May 19, 2020
192
It works! Took my lathe (again a 12x26) with a 6" chuck to spin down from 800 rpm to stopped in what looked to be around 3 complete revolutions. Certainly not as fast as stomping on a foot brake if your lathe has one, or as fast as a VFD dynamic brake, but it is a huge improvement over the stock motor and works great with single phase motors where a VFD cannot be used.

I programmed it to wait 200ms after the RUN contactor dropped out, then the BRAKE turns on for 1 second. The motor is stopped long before this shuts off. Ideally I'd have a speed monitoring encoder to tell me when the motor stops so I can turn off the brake, but this is pretty darn good as-is.
 

shortbus

Joined Sep 30, 2009
8,477
Two comments.
1. I have a similar Enco lathe but it's 13X36. The forward/reverse switch goes bad on them and is no longer available. But there are some similar to the original on EBay but have a different electrical configuration to them. But these switches are modular so by changing the position of the modules they will work if yours goes bad.

2. Not sure about the spindle on your lathe, but if it isn't a "D" nose type spindle (camlock) and is a threaded spindle, watch when you use the brake. I could cause the chuck to unscrew and fall off.

And a question. Do you get a *pattern* in your finish cuts? No matter what feed you use? I did and switched my motor to a 3phase one with a VFD. Big difference.
 

Thread Starter

mcardoso

Joined May 19, 2020
192
Two comments.
1. I have a similar Enco lathe but it's 13X36. The forward/reverse switch goes bad on them and is no longer available. But there are some similar to the original on EBay but have a different electrical configuration to them. But these switches are modular so by changing the position of the modules they will work if yours goes bad.

2. Not sure about the spindle on your lathe, but if it isn't a "D" nose type spindle (camlock) and is a threaded spindle, watch when you use the brake. I could cause the chuck to unscrew and fall off.

And a question. Do you get a *pattern* in your finish cuts? No matter what feed you use? I did and switched my motor to a 3phase one with a VFD. Big difference.
My forward/reverse switch is a pair of large microswitches which hit cams on the start lever shaft. I don't know if I can get originals, but Grizzly sold a nearly identical lathe to mine (G9249). I was able to get all the change gears from them that I was missing. Not much I couldn't get, but it did take 8 months for them to arrive from overseas. I would expect to have moderate success getting them there. If not, I could probably adapt any microswitch.

Mine is a thread on spindle (2 1/4"- 8). There is certainly a chance of it unthreading but the braking isn't that violent. I also tighten the chuck onto the spindle with a 4' pine 2x4 in the chuck jaws as a cheater bar. I don't crank on it, but definitely snug it up beyond hand tight.

I do get a pattern but it is not significant. The belts in my lathe were shot when I got it, so I replaced them with off brand Fenner link belts. Weird things, but I think the live up to their purpose and really reduce vibrations. I think part of the issue is if the motor belts take a set after sitting unused for a while, and the other half is the speed fluctuations due to the single phase motor. I've been very happy with this lathe.
 

shortbus

Joined Sep 30, 2009
8,477
The link belts was the first thing I did too. I wish my for/rev switch was micro switches, the plastic in the modular switch gets brittle with age and then stop working in mine. I too am happy with the lathe . I have 3 different lathes,the Enco, a 9" South Bend and a 12"x 60" Logan. Both of those I put DC motors and motor drives on. The Logan was originally a line shaft drive from back in the 1920 -30 era.
 

Thread Starter

mcardoso

Joined May 19, 2020
192
That's awesome! I bet that Logan is a real beauty.

I have the lathe, a benchtop mill converted to CNC with some Allen Bradley servos, and a large Seiko/Epson SCARA robot that I've been messing with during quarantine.
 

bloguetronica

Joined Apr 27, 2007
1,434
Nice! Allen-Bradley material, which is of good quality.

Have you tried how the DC braking is working? For the 24V you inject, does the current stay below the nominal value at full stop?
 

Thread Starter

mcardoso

Joined May 19, 2020
192
Nice! Allen-Bradley material, which is of good quality.

Have you tried how the DC braking is working? For the 24V you inject, does the current stay below the nominal value at full stop?
The braking is working nicely. The spindle spins freely by hand with the brake off and feels like I’m dragging it through molasses with the brake on. It doesn’t lock it in position but rather resists an increase in speed.

I measured the current draw on the 24V supply just now with a Fluke clamp meter and got 11.3A at steady state. It is hard to get a reading when the motor is spinning down since it is over so quickly, but it looks to hit 11A. Haven’t seen anything higher. The power supply is rated for 15A for 3 seconds so this is fine.

The motor’s FLA is 7.1A at 240V so this needs to be an intermittent operation or the motor will overheat.
 

goghie

Joined Oct 13, 2020
1
I would worry about overheating of components. There is not enough of space for natural ventilation. Luckily for you, you are using metal enclosure mounted on metal body of machine.
 

Thread Starter

mcardoso

Joined May 19, 2020
192
I would worry about overheating of components. There is not enough of space for natural ventilation. Luckily for you, you are using metal enclosure mounted on metal body of machine.
Believe it or not, I actually did the heat load calcs for the components and it came out kind of close but perfectly fine for a non-ventilated enclosure. It also helps that I am not running the PLC power supply or the DC 24V power supply anywhere near max load. The highest heat load actually comes from the PLC itself.

After running the lathe for an hour or two, the box still feels room temperature or extremely close to it.

I have a 24x36" panel for my CNC mill with 6kW of servo drives inside. It too is unventilated and only registers a small temperature rise under operation on the internal thermostat. It is surprising how much heat is expelled by a steel box purely with convective non-forced cooling. It helps that the servo drives have fans on them which stirs the air inside the enclosure. That actually plays a significant role in how much heat is expelled by the enclosure walls.
 

mscott89

Joined Dec 10, 2020
1
Hi All,

I've been meaning to revamp my lathe's control cabinet for some time now. There were a few things I did on the original build some years ago which made me less than comfortable with the safety of the wiring. Since then, I have taken a job designing industrial control panels and I am much more aware of good practices and code requirements. I figured while I was correcting the panel, I would also experiment with a method of quickly stopping the motor using its own magnetics.

My lathe is an import ENCO 12x36 manufactured back in '94. It uses a common single phase 4-wire reversible motor with two capacitors. The motor can be started in either direction from a stop by reversing the polarity to the start winding. For those of you unfamiliar, a single phase AC motor is unable to produce any torque at a dead stop. A couple of tricks are used to start these motors, most common of which is phase shifting the AC line using a capacitor. This creates pseudo 2-phase power at the motor and gives it the torque necessary to start. A 3 phase motor does not have this issue. Once the motor gets part of the way to its operating speed, a centrifugal switch inside the motor opens and disconnects the capacitor and the so called "start" winding.

Many of these motors run in one direction, but some can be provided with separate connections for the start and run windings. By reversing the polarity of the AC line on the "Run" winding relative to the "Start", you can control the motor's direction. Again, the centrifugal switch disconnects the "Start" winding once the motor is at speed. This is typically done with a pair of contactors which are interlocked to prevent them from coming on at the same time.

The experiment came in play for when I want to stop the motor. Normally both contactors open, power is removed from the motor, and it spins down under friction. Instead, I added an additional pair of interlocked contactors labeled "Run" and "Brake". The Run contactor is pulled in whenever the motor is running forward or reverse. When the motor shuts off, the Run contactor turns off and the Brake turns on. This disconnects the motor from the AC line and connects a 24VDC 10A power supply across the Run windings of the motor. The effect is a rapid deceleration of the motor to a stop.

Here is the panel I built to do this,

View attachment 222482

View attachment 222483

From the bottom up: the power enters and is protected by 10A Class CC fuses. Terminal blocks allow easy-ish hookup of all the switches and buttons on the lathe. The right pair of contactors in the middle is the FWD/REV controls. The left pair is the RUN/BRAKE control. The big DC supply up at the top is for the DC injection braking, and in the top left is a PLC to control the timing. This could be easily accomplished with a timing relay, but I had the PLC on hand so I went for it.

The whole idea of this braking goes back to a concept known as slip. In an AC induction motor, AC voltage is applied at the stator and generates a rotating magnetic field. The rotor experiences this magnetic field and a voltage is induced in its windings. As long as there is a difference in the angular velocity of the stator's magnetic field and the rotor's mechanical angular velocity, there will be voltage induced on the rotor and torque generated. This leads to the realization that an AC motor can never match the electromagnetic field speed since there would be no difference in speed between stator and rotor, no induced voltage and no torque. The more load that is placed on the motor, the slower it rotates to generate more torque. This speed difference is known as slip.

The reverse can be used to brake the motor. When power is removed, the stator's magnetic field disappears, but the rotor remains spinning. At this point, we apply DC voltage to the stator which creates a stationary magnetic field. Now again, there is a speed mismatch between stator and rotor and torque is generated - but this time in the opposite direction, stopping the motor. All the mechanical energy is converted to heat in the rotor, so this has to be used carefully. For my lathe, I am not concerned.

Still finishing the program to time the brake, but it looks like it is working pretty well. I'm interested to see how much the stopping time is reduced.
Good luck with the project buddy, looks interesting - posting so i can get notified on the final result!
 

Thread Starter

mcardoso

Joined May 19, 2020
192
Good luck with the project buddy, looks interesting - posting so i can get notified on the final result!
Project is all done, been using the lathe for a while now with the braking system and it is really nice. It will brake to a dead stop in all but the highest gear (1200 rpm). I could modify the braking time to increase the braking action, but then it sits there for a while if the motor was in low gear (stops VERY fast). A speed sensor could be used to monitor the system and brake until stopped, but that adds complexity and the PLC has every input currently used.

I haven't done threading on it, but this is where the braking will be most useful. Lots of stopping and starting in a confined workspace.

The PLC takes 30 sec to boot up after the lathe is turned on which is a bit annoying but not a big deal.

One other bonus is that I was able to code the action of the ESTOP to be such that if the ESTOP is hit while the run lever is on, then clearing the ESTOP will not restart the motor. This is not safety rated since it is software controlled in a non-safety rated PLC, but for my use, it is much better!
 
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