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110V DC Motor controller design needed

Dig

Hi all,

 

I recently broke my Grizzly X3 mill's electric control board. I have no idea how.

 

The motor it drives is rated 110V DC @ 6.7A.  A replacement board from Grizzly is in the $250 dollar range. So, I was thinking of just making a controller myself.

 

My original plan was to rectify wall, 110VAC, power to DC and run the mill at full-tilt boogie. But, starting the motor out at full power/full speed is a great way to break it real fast.

I need a slower, gradual, start for the motor.

 

Any thoughts on how to do this?

 

Dig

  • 0

    Hi Dig,

     

    What are the operation conditions of the motor? Does it only need to spin in one direction? If that is the case then it would seem that all you are interested in is speed control.

     

    In general terms the solution is going to involve an H-Bridge driver circuit, and then the control circuitry to control the motor. The first part is pretty standard and simple to understand, the control circuitry is where things get hairy.

     

    Speed control can be as simple as a single transistor a PWM signal at it's gate, or extremely complex requiring a microcontroller performing park transforms in order to maintain the speed and torque of a motor.

     

    Check out the following app notes from Microchip:

     

    AN1078 and AN1160 will give you an idea of how complex this subject can be

     

    AN857, AN885, AN894 give more fundamental info. I have assumed that the motor being controlled is a brushless DC motor, the app notes assume multiple phases. If the motor is a single phase then things get a little simpler.

     

    Look forward to hearing your thoughts.

     

    Jorge Garcia

  • 0 in reply to Jorge_Garcia

    Jorge Garcia,

     

    Thanks for the reply. Yes, one direction is fine for now. But ultimately, I would like to control both direction and speed.

     

    I should have listened to you. Here is what I did.

     

    I needed 110V DC, so I rectified the 110V AC coming out of the wall. I placed a low ohm ceramic potentiometer (from a large industrial machine) in the circuit to reduce the voltage at the start, so I could slowly ramp up the speed.

     

    I turned it on, nothing happened. Then smoke poured from the capacitors. The stench was horrendous.

     

    What do you think happened?

     

    And do you have a one way circuit in mind for this motor?

     

    Dig

  • 0 in reply to Dig

    Hi Dig,

     

    Sounds like an overvoltage failure to me. Remember that when you rectify 110 VAC you don't get 110VDC. You actually get 110 * sqrt(2) VDC which is approximately 155.6 VDC. Did you check that your capacitors can handle that kind of voltage?

     

    I have attached an LTspice file that shows two circuits one is an H-Bridge circuit which gives the potential of speed and direction control. The other circuit is simpler but only allows for the possibility of speed control. LTspice is free to download from Linear tech's website so you should have no problems opening the file.

     

    Speed control could be achieved using PWM on the inputs of the driver circuits. I didn't want to overwhelm you with info so I only sent the driver stages, a quick search on google will show you dozens of ways to generate a PWM signal.

     

    hth,

     

    Jorge Garcia

    Attachments:
    HBridge.asc.zip