Weird braking circuit

Appearing below (hopefully) is a circuit I found in an old print for an equally old machine. It's the drive circuit for an indexing table.

CR 200 is defined as the "Indexer Run Relay". CR201 is described as the "Indexer Brake Relay". The motor does not appear to be braking.

Naturally, I'm going to change the rectifier and test relay 201. But I've been at this ten years, and have never before seen a DC rectifier used in a 120 VAC circuit for braking (or for anything else, for that matter).

Can somebody explain to me how this works? Seems like the motor, the rectifier, heck SOMETHING should be getting damaged here...

Okay, screw that...

Image didn't work, here's an attachment. Regular bitmap.

When you run 120 VAC across a full-wave bridge, you get 90 VDC. There are thousands of 90 VDC brakes of the backs of motors.

Warner electric used to offer a combination clutch/brake unit and a controller to go along with it. You connected relay contacts to terminals on the controller to switch the 90 VDC between the clutch and the brake. CR200 could be supplying power to the clutch, CR201 powering the brake.

You might also have an electrically released brake. That would be spring-loaded to set the brake when power is removed, a 90 VDC coil that needs to be energized to release the brake.

Well, I opened your file. It looks like CR200 cuts in the 120 VAC to the singl-phase motor. When CR200 is off, and CR201 is on, you're feeding DC voltage to the motor. That will certainly stop the magnetic field from rotating and act as a brake.

Some VFDs use DC injection braking, but they don't recommend applying it until the motor slows down. They also caution you not to leave it on very long after the motor stops.

I've seen this used before on motion control applications - as far back as the 70's. When you put DC on an AC induction (squirel cage) motor it comes to an abrupt stop. That is what is going on here. The stopping is very rapid - it scared me the first time I saw this system applied, and couldn't understand why they didn't chew up motors like popcorn, but they used it for years. I don't know the procedure for determining the torque produced, but I'm sure some one here can provide it if you need the info.

A way to think about DC injection is that you are literally sending a 0Hz signal to the motor, stopping it.

Udc = sqr(2)* U / (3.14/2) - (saturation voltage for 2 diode 1.4V) with full wave rectifier.

120VAC means 106.6 VDC and with half wave rectifier 53.3 VDC
230VAC means 103 VDC with half wave rectifier.
110VAC means 97.6 VDC and with half wave rectifier 49 VDC

Great Inductive reactance smooth curren with Voltage average and
it is usuallu stamped to rating plate.

I have seen dc injection braking like this a few times. The DC is usually injected for a set time until the motor is stopped and locked in the dc field. As the motor slows down, you can see it breaking through the field sort of like a piston engine coming to a stop.

The motors I have seen it on are usually three phase high inertia motors.

Thanks to all

Steve : Yep, no brake, just a straight shot of DC into the coil. Never seen the like. It's not a VFD, just a basic AC motor running a cam-based indexing table. I get the impression, since CR201 is NO and under separate control from CR200, that current is being applied briefly to the motor, not continously.

Tom : 70's? That's about the right vintage And yes, I wondered the same thing - how this sucker keeps turning with that kind of abuse!

Rick : Thanks for the tip! Makes sense.

Sepp : Thanks for the math Hopefully, I won't need it. I turned the issue back over to maintenance with the suggestion they replace the rectifier. Seems like a good place (for them) to start.

Thanks everyone!

TM

It is really just a milder form of plug stopping or reversing. The load is just twisting of your shaft if the load is balanced. So side loads or such that stress the bearings. As long as the shaft is the right size, it should work great. Sounds bad, though.

Properly done, DC injection braking is a cheap and non-damaging way to use your motor as a deceleration brake. Normally, the DC is inserted on only two phase leads so only two of the three motor stator phase coils are magnetized. And, as pointed out earlier, it is the same as putting 0Hz into the motor inducing it to run at 0 rpm.

The amount of DC current into the motor determines the braking torque. So, if full wave rectification of 120VAC is too much braking, just add a little resistance in series until you get the braking torque you desire. Max braking current is at least motor nameplate AC amps and often can be higher by 20-30%.

It is important to understand that DC braking can not be used for holding purposes. Just as a motor has slip when motoring, it also has slip when braking. It follows that, at zero speed, the rotor is no longer magnetized and cannot develop any torque. You need a mechanical brake to hold a stationary load in position.

One other important difference. On an inverter using a decel ramp, the motor is forced to follow the ramp down regardless of how heavily loaded it is. DC injection is simply braking torque so, on light loads, the stop will be rapid while, on heavy loads, the stop will be longer. If you need a predictable stop time, DC braking is not the answer.

You can buy stand-alone DC brake kits to add to an existing magnetic starter system. They are very inexpensive. Most any manufacturers that makes softstarters (Benshaw, ABB, Motortronics, etc.) also make DC brakes. They can also be combined with softstarts economically. DC braking on inverters is usually limited to slowspeed stopping (typically 5hz and below). I've never understood why some (most) inverter manufacturers don't permit DC braking from full speed. On many machines, the only fast stop is the infrequent E-stop and full speed DC braking would be perfect for many of these applications. And it would be essentially free!

Hmmmm! I might have just answered my own question!