Cheap DC Injection Braking

[SteveGass]

Any suggestions for the cheapest (a few dollars) way to implement DC injection braking (or other braking) in a motor control circuit? All we need in the circuit is on/off control and we are considering a triac or a relay. I have wondered if we could simply drive the triac during one half cycle of the AC line voltage to simulate DC and achieve essentially free braking if we used a triac. In preliminary tests with a diode to mimick this, the motor continues to drive if I switch directly to the diode-treated power with the motor still at full speed, but stops very quickly if the diode-half-recitified power is not applied until the motor has slowed significantly, i.e. turn power off for a second or two and then reapply line power with the diode in series.

The two motor groups we are working with are 1.5hp 120/240 capacitor start, 3-5hp single or 3ph 240/480. The machine that I am controlling the motor in has a microcontroller, so it is quite cheap to control the switching times of the triac.

 

[cbarn24050]

Hi, a few dollars! you need a thyristor not a triac, a flywheel diode, and a contactor.You will need to phase control the thyristor to control the current. 3x FLC is about the usual current for braking.

 

[SteveGass]

cbarn,

Thanks for the suggestion. I think I understand the contactor - cheaper than a triac I suppose, and the thyristor is used like I was thinking to use half the triac, but what is the "flywheel diode" and how is it arranged in the circuit? Also, do you have to have a current sense resistor, or can you simply limit the on phase to a small enough section of the half cycle and know that current will be about right?

 

[cbarn24050]

The diode allows the current to flow during the off part of the cycle. If you don't know what it is then you are not competent enough to build 1. Buy something! this is no area for beginners.

 

[SteveGass]

cbarn,

I'm not looking to design it myself, but rather understand the operation to so that I can help plan overall design with someone more knowledgeable. I'd be happy for any suggestions of a knowledgeable design consultant in this area as well. But back to the question, where is the diode letting the current flow during the off portion of the cycle and what for? It seems like the thyristor is going to shut off when the current goes to zero and you won't have any more current until you turn it on again.

 

[jbartos]

Suggestion: Visit

http://home.hiwaay.net/~davida1/pwm.htm

http://www.bostongear.com/pdf/installation/beta2_angle_bracket.pdf

for more info