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IndustryArena Forum > CNC Electronics > Servo Motors / Drives > Servo Motor voltage & current when disc. from Inverter
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  1. #1
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    Question Servo Motor voltage & current when disc. from Inverter

    I have a basic questions in general on a AC Servo Motor.
    1. Does spinning the servo motor by hand, generate current or voltage?

    2. When the servo motor is running at its maximum speed, and i remove the inverter from the AC motor. Will it still have current and voltage on it? What will it be?

    3. When it is coasting after disconnected from an inverter, by applying brake to the motor, what will the current and voltage (if there are any) looks like?

    4. If the motor is disconnected from the inverter, what will the servo motor do if i short both lead together?

    5. If i were to short the motor lead, what current do i see from the servo motor?

    Hope someone can clarify this for me.

    THanks!

  2. #2
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    Re: Servo Motor voltage & current when disc. from Inverter

    AC servo motors are for practical purposes identical to alternators so yes they will generate a voltage as they decelerate which will be comparable to the voltage that would be needed to drive it at that speed. The theory is more complicated than that of course there will always be a voltage drop due to internal winding resistance. If you were to short out the windings whilst the motor is running which is not a good idea then the motor would stop very quickly. In practice load resistors are used to give a controlled deceleration. The actual voltage and current that will be developed will be dependent on the actual motor,. I am not sure why you use the word inverter which are not generally used to drive AC servo's but either way you should never disconnect a servo motor from the servo drive or inverter while it is still running or coasting down because high voltages spikes can be developed which can cause damage. If you short out the terminals/wires and just try and turn the shaft by hand you will notice that the shaft will be much harder to turn even at slow RPM's at higher speeds it can cause mechanical damage.

  3. #3
    Community Moderator Jim Dawson's Avatar
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    Re: Servo Motor voltage & current when disc. from Inverter

    Quote Originally Posted by AmazingServo View Post
    I have a basic questions in general on a AC Servo Motor.
    1. Does spinning the servo motor by hand, generate current or voltage?
    It generates voltage. Think of a water pipe (conductor), if there is no pressure (voltage) then there is no water flow (current). If there is a valve in the water system then that restricts (resistance) the flow.


    So if you turn the motor shaft voltage will be generated, if there is a path for current to flow then there will be current flow in the conductor. It might be possible to generate enough voltage to damage the winding insulation if the motor is turned fast enough without the windings beings shorted or connected to something.


    2. When the servo motor is running at its maximum speed, and i remove the inverter from the AC motor. Will it still have current and voltage on it? What will it be?
    It depends on the resistance of the system. Disconnecting the inverter from an operating motor can cause a voltage spike that may damage both the motor and the inverter.



    3. When it is coasting after disconnected from an inverter, by applying brake to the motor, what will the current and voltage (if there are any) looks like?
    That would be a more proper way to do it. The energy stored in the system would be disipated as heat in the brake.



    4. If the motor is disconnected from the inverter, what will the servo motor do if i short both lead together?
    It will stop as fast as it can, but this is not the proper way to slow down a motor.



    5. If i were to short the motor lead, what current do i see from the servo motor?
    It depends on the resistance of the system. A short would cause a lot of current flow which would be disipated as heat in the motor windings, and could possibly damage the motor windings.



    Hope someone can clarify this for me.

    THanks!
    The bottom line is: Do not disconnect the motor from the inverter, simply send the stop signal to the inverter, and if the inverter is set to ''coast to stop'' apply a mechanical brake if you like.
    Jim Dawson
    Sandy, Oregon, USA

  4. #4
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    thanks everyone.
    i guess when i say disconnect from the motor, what i meant is using "safe torque off" on the inverter first, then disconnect the motor from the inverter using ac contactor.
    After i disconnect the motor from the inverter (STO & AC contactor open), would shorting the motor lead do anything to the motor still since there is no current flow from the inverter to the motor anymore.

  5. #5
    Community Moderator Jim Dawson's Avatar
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    Re: Servo Motor voltage & current when disc. from Inverter

    If you short the motor, all of the stored energy in the system will be dissipated as heat in the motor windings. Not the best way of slowing down the motor.

    Maybe it would be helpful if you would tell us what your end goal is. Stopping the motor as quickly as possible?

    What is the motor power?

    What is it being used for?
    Jim Dawson
    Sandy, Oregon, USA

  6. #6
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    Re: Servo Motor voltage & current when disc. from Inverter

    Quote Originally Posted by Jim Dawson View Post
    If you short the motor, all of the stored energy in the system will be dissipated as heat in the motor windings. Not the best way of slowing down the motor.
    It is ok to do that if there is a resistance in series to absorb the energy

    if there is no resistance in series the motor won't slow down very quickly, and some say you can demagnetize the magnets, of which i am skeptical.

    here's an interesting example:

    I have a 30 pole permanent magnet 3 phase motor I made from a 36 slot 1/3rd hp induction motor core. it has 1.9 cubic inches of neodymium magnets in it and when run as a generator, it can stall a 1/2 hp 850 rpm induction motor directly coupled. its windings are 1 ohm each, meaning 3 separate 1 ohm windings.

    The motor/generator is capable of dumping 600-800 watts or so at the time it stalls the 1/2 hp induction motor, on the order of 6 amps in the coils, wasting 108 watts in the coils. (i cannot remember the actual numbers, its been a few years)

    But if you short circuit the generator, it will produce 9 amps of current in the windings and consume only 81*3 or 243 watts of power from the 850 rpm motor. no more, no less.. regardless of the rpm. the minimum rpm needed to generate those 9 amps is only on the order of 150 rpm, and i've shorted it out at 1750 rpm as well. same 9 amps.

    if i were to have done a better job winding the machine, i could probably have fit at least 50% more copper in the machine.. which would mean that when shorting out the motor the maximum watts that could be dumped into it.. would be on the order of 150-170 watts.. compared to 800 watts when optimally loaded as a generator.

    A much better designed servo will likely have an even sharper cut off where without any resistance, above some rpm they will be nearly effortless to turn.

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