• State
  • Locked Locked
  • Replies 5 replies
  • Subscribers 15 subscribers
  • Views 1919 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UC2625 Speed Control

mc172
Prodigy 20 points

I've built the reference design as shown in the UCx625-EP datasheet (the one with the 51kΩ resistor between VREF and SSTART). The only deviation being that I've used different output devices. I'm struggling to achieve any form of speed control using either the potentiometer setup as shown or by supplying a voltage to pin 1 directly. 

Currently, pin 1 is working like an enable switch with a threshold of 2.5 V precisely. Varying the voltage applied to pin 1 between 2.5 V and 5 V makes no difference to the speed of the motor.

If I connect VREF (+5 V, pin 2) directly to E/A IN(+) (pin 1), the result is the same, as is having the potential divider with the voltage set anywhere between 2.5 V and 3.125 V (the maximum theoretical voltage possible with the 3k resistor, 1k potentiometer set to maximum and 4k resistor).

Am I missing something?

  • 0
    TI__Expert 7445 points
    Hi MC172,

    Let me look into it and will provide you feedback on it.
  • 0 in reply to Ramesh Khanna
    Prodigy 20 points

    Ramesh,

    Did you get a chance to have a look at this?

    Thanks.

  • 0 in reply to Ramesh Khanna
    TI__Expert 7445 points
    Hi MC172,

    Motor speed is controlled by the voltage across the motor and motor torque is controlled by the current through the motor. The same can be said for solenoids. We can model a motor as a series resistor and speed-dependent voltage source, sometimes called the back EMF.

    The higher the current, the more the drop on the resistor. The higher the speed, the higher the back EMF. When the motor is spinning up, it is applying torque to the inertia, which requires current.
    When it at a steady speed with zero load, there is no torque, so no current and no voltage drop on the resistor. The only drop is the back EMF.

    If you want to regulate speed, you need to close a loop around the tachometer output. You can also control speed by regulating the voltage applied to the motor, but this is an open-loop control that will give speed error depending on motor loading (friction, force delivered, wind resistance, etc.)

    Controlling pin 1 is like controlling torque, not speed.
  • 0 in reply to Ramesh Khanna
    Prodigy 100 points
    Hi Ramesh,

    I stumbled across this thread as I was trying to achieve the same functionality that MC172 explained above. Based on what you said, the datasheet then incorrectly states that Pin 1 can be used for speed control (this should be fixed on page 19).

    Does TI have any reference designs for closing the loop around the tach output for basic speed control? With an RC filter on the tach output, we would have the average voltage (aka motor speed) and should be able to use this. My question is how/where to tie it back into the IC for speed control (probably some form of PWM)? I'm a little confused on the E/A pins 1, 27 and 28 and PWM IN and RC-OSC pins 26, 25 that note they can be used for PWM feedback loops.

    Could you help clear this up and provide any advice?

    Thanks!
  • 0 in reply to HP77
    Prodigy 100 points

    Ramesh,

    Can you provide any insight?