×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Cheap DC Injection Braking

Cheap DC Injection Braking

Cheap DC Injection Braking

(OP)
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.

RE: Cheap DC Injection Braking

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.

RE: Cheap DC Injection Braking

(OP)
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?

RE: Cheap DC Injection Braking

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.

RE: Cheap DC Injection Braking

(OP)
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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources