Potential back/emf problem driving motor?

Discussion in 'General Electronics Chat' started by fastwalker, Nov 29, 2009.

  1. fastwalker

    Thread Starter Active Member

    Jun 24, 2009
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    Guys,

    I'm not that experienced in driving large DC motors. At work my project is to monitor the current drawn by a variable speed DC motor. The motor is a 141V DC motor from Panasonic. It has on-board power electronics so you can control the speed with a 0-5V signal. I'm using a 1000uF filter capacitor across it to stabilize the voltage. The motor draws about 1A when running.

    I'm measuring the current across a shunt resistor on the low side, using a TI INA-195 shunt amplifier to get a nice voltage out of the mV range. This chip seems to work fine, but it looks like I've blown up a couple of them in one experiement where I was turning the motor on and off about every 60s. Could I be getting a voltage spike when turning off the motor? Should I add a diode across the motor to protect from back/emf voltage spikes? If yes, what kind of diode should I use?

    Thanks for any help.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yes. If it's a brushed universal-type motor, when the power is disconnected and the motor is winding down, it will act as a generator. [eta] the following sentence is not correct: The voltage output will be in reverse polarity from what was being applied to it.

    That would help signficantly.
    You could use a 1N5402 to 1N5408 rectifier diode. They're rated for 3A at 200 to 1KV, depending on the last digit of the part number; 2=200v, 8=1kv.

    If your 1000uF capacitor is a polarized aluminum electrolytic capacitor, the reverse EMF may have damaged/destroyed it already. if it's non-polarized, you might be OK.
     
    Last edited: Nov 29, 2009
  3. fastwalker

    Thread Starter Active Member

    Jun 24, 2009
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    0
    Thanks for the response, very helpful.

    I did not expect a large back EMF as I am controlling the on/off via using the analog speed control to the onboard power electronics, as opposed to simply cutting the DC 141V off. I will add the diodes suggested and may also need to implement some kind of soft start/stop in the speed signal I sent to the motor.

    Thanks again.
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    I really hate to disagree with you Sgt., even partially. Here are my thoughts.
    Unless something causes the motor to spin backward, the terminal polarity of the motor will be the same as the polarity of the voltage driving it. If the polarity actually reversed, a diode across the motor would not be a practical concept since the diode would be either forward biased during run or forward biased during spin down. A properly placed diode across the motor WILL however, absorb the momentary inductive kick of the initial field collapse. The concept of Back EMF or Counter EMF is that the motor, while running, is actually generating a voltage that will oppose the CURRENT flow through the motor To do that, the motor will need to generate a voltage that is of the same polarity as that which is applied. As load is applied, the motor tries to slow down, therefore the generator effect is reduced and since the Back EMF tries to reduce, more current is drawn and torque is increased.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    BillB3857,
    Thank you for noting my incorrect statement. You are correct. It is important that corrections be made when erroneous information is posted.

    The coffee hadn't taken effect yet. ;)
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348

    I thought it must have been something like that!:) I usually end up learning something FROM your posts!
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
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    He did say the motor has onboard power control electronics, operated by a 0v-5v signal. Those electronics are most likely some type of SMPS that might have its own inductor etc.

    I think to safely turn this motor "on and off about every 60 secs" you should ramp it down with the control signal over a period of time, likewise ramp it up to turn it on.
     
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