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  1. #1
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    When do stepper motors need to draw maximum power?

    hello

    apologies for length of this post.

    it is nothing complex, it is just that i am trying to find out what is the maximum power drawn by stepper motors in my system.
    The system is like a domestic shower, where valves in the hot and cold pipes are moved with stepper motors.

    There is one valve in each pipe...
    -and one stepper motor for each valve.

    The stepper motors are as follows:- (UBB1/5, 6V, 18.5 Ohms per phase)
    http://www.auf-vertrieb.business.t-o...de/pdf/UBB.pdf

    We will be driving the stepper motors with this driver (NJM2673)

    http://www.cn.njr.com/PDF/NJM2673_E.pdf

    ..its an efficient switch mode stepper driver.

    Currently, the steppers are running from power provided by a 50Hz transformer based PSU.
    -it is my job to replace this 50Hz transformer based PSU with an SMPS.

    -However, first i must know what maximum power the stepper motors draw.
    -then i can design the SMPS.

    The engineer who spec'd the 50Hz transformer based PSU has left the company, and we do not know if he calculated the necessary power properly.

    There is an added complication............

    supposing that the steppers draw a very high peak power for a very short time in their operation.
    -it could be argued that the SMPS would not need to be rated for this peak power, because the energy stored in its
    output capacitors could (if they were big enough) supply this short interval of high power.

    ...however, this is not the case for us......our output capacitors must stay at least at 36V and no less at all times,
    because otherwise they will not have enough energy in them to allow the steppers to close the water valves in
    the event of a mains power failure.

    -so our SMPS must not suffer transient drops in output voltage when the steppers endure their maximum power operation.

    i.e. whatever is the steppers peak power draw...that is the power at which the SMPS will be rated.

    So, ....i am getting nearer my question...................

    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    QUESTION 1 :-

    Out of these following stepper motor operations in this system, which one will result in the stepper motor needing to draw the highest peak power?............

    (-in all cases , please assume that water under the maximum pressure is trying to flow through the pipe........)

    1.... Moving the valve with the stepper at constant angular speed whilst closing the valve
    2.... Moving the valve with the stepper at constant angular speed whilst opening the valve

    3..... Accelerating the valve at with the stepper at maximum d(angular speed)/dt whilst closing the valve.
    4.... Accelerating the valve at with the stepper at maximum d(angular speed)/dt whilst opening the valve

    5.....Moving the stepper motor from stationary to open the valve, when the valve was initially fully closed
    6.....Moving the stepper motor from stationary to close the valve, when the valve was initially fully open

    7.....Moving the stepper motor from stationary to open the valve, when the valve was initially half open
    8.....Moving the stepper motor from stationary to close the valve, when the valve was initially half open

    9....Holding the stepper motor stationary, whilst the valve is half open
    10....Holding the stepper motor stationary, whilst the valve is fully closed
    11....Holding the stepper motor stationary, whilst the valve is fully open

    12. ..."Pinch-Off"...that is, that instant in time when the valve is just about to completely close the valve

    13...some other operation

    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    QUESTION 2:-

    At the moment , i am thinking of putting the stepper through all of the above actions, and continuously monitoring the
    Voltage Rail that supplies the stepper motors, and continuously monitoring the current that flows in the stepper windings.

    I am just hoping that i don't miss the peak power instant.

    Do you know of any good oscilloscopes with data logging facilities so that i can save the voltage and current data
    such that i can pick out the maximum power?

    •   Alt27th August 2010, 21:14

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  2. #2
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    Re: When do stepper motors need to draw maximum power?

    Hi

    Stepper motor will draw max current when stall! or when winding get short

    All the best

    Bobi



    •   Alt28th August 2010, 11:53

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  3. #3
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    Re: When do stepper motors need to draw maximum power?

    you should ask what is the maximum power the stepper motors draw and for how long will this occur. there are different solutions that are more cost effective and practical. under normal operating conditions ... draws the most current at start up.



    •   Alt29th August 2010, 22:47

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  4. #4
    FvM
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    Re: When do stepper motors need to draw maximum power?

    In constant current mode, as apparently intended, the power increases with motor speed.
    The load torque in contrast won't influence it, as long as the motor current isn't changed
    by the controller intentionally.

    In other words - you choose the maximum power by setting the motor current and speed.



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