DC injection braking of a 3 phase motor is theoretically very simple. When power is removed from the three phases, a DC current is injected into two of them. Current seems usually to be between 1 and 2 times the full load current, and period of injection is either set to approx 'time to full speed' for a DOL starter, or is cut off when the rotor is detected as stopped.
OK fine, but commercial units are very spendy - several hundreds of pounds apiece. So why not make one? Has anyone gone down this line and survived to tell the tale?
The motivation is that I have two 5.5 HP and one 7.5 HP motors that I want to brake
Andrew, Like you I have thought about this on and off but not done it for a AC motor but did brake a 1hp DC motor with part of an electric fire element connected across the commutator and used the rectified DC across the load to indicate when the rotation was slow enough to automatically restart it in the opposite direction. You might be able to use something similar to control the duration of the DC injection for your motor. Another way to avoid timers might be to use a capacitive discharge system as the source of power for injection. I've not done any sums on this and the capacitor(s) might work out rather large to brake such large motors. A commercial unit I came across retrofitted to a 16" planer was quite crude and simply injected the DC for 20-30 seconds on the basis that the motor much have stopped by then. In practice the motor had stopped well within the required 10 secs and it just hummed and buzzed for the remainder of the time. I did not see any high power ballast resistor in side so I don't know how the current was limited.
Sorry I can't be more helpful but I would be interested to keep in touch on the project.
An interesting problem. If you are running the motors via an inverter, many of them do have programmable braking capability, but you usually need to add a large air cooled dump resistor. This is sold as an accessory in a nice box, but it could be built using off the shelf ali heat sink resistors if you can figure out the value and rating.
Another possibility might be to short the motor windings together, as the motor will act as a generator offline and will absorb some power as it spins down short circuited. This might be quite easy to try, using a changeover contactor, where the off contacts are just wired together, though it might be a good idea to include a current limiting resistor to start with to protect the contacts.
Dc will work, but I would use a variac ahead of the rectifier initially, to see how many volts are actually needed and you will need a changeover contactor, or some means of synchronising 2. My guess would be that you wouldn't need anything like
440v rectified and more likely between 50 and a couple of hundred volts input to the rectifier.
Finally, you could swap 2 of the 3 wires and drive the motor at a lower voltage or otherwise limit the current in some way, but this would need a timer to stop the motor running in reverse...
My TOS lathe has DC braking built it, It's not a module but part of the control box using, and this is from memory a bridge rectifier across the low voltage transformer and a timer switch. Doesn't stop it dead but it's only using 24v AC x 1.414 so 34 volts DC into two of the phases.
It also has extra auxiliary contactors on the forward reverse relays to prevent the two voltages meeting. Probably stop better if it had
In one of the books by the late Pentagrid, AKA Jim Cox, dc injection is mentioned. He wrote that using either a capacitor or DC can be used to stop the motor quickly. Interestingly to me he wrote that the DC voltage doesn't need to be anywhere near the AC operating voltage. In Electric Motors 2nd edition, page 112, Jim writes that the DC current can be up to two or three times the operating current of the motor. The voltage only needs to be about 1 tenth of the operating voltage. On page 113 are shown 3 circuits. I would check out this book from the library or borrow it from someone to get the full text and then you will understand enough about how it works and be able to apply it to your particular motor. I'd loan you my copy but I'm too far away. For a real world test I tried connecting a 12 volt car battery to a coasting 120 volt single phase grinder motor. It really did slow the motor down in a hurry. There are two wheels mounted on the grinder motor so there was lots of flywheel energy stored compared to a bare motor. Eric
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