Stepper Motor Power

Discussion in 'General Electronics Chat' started by scottyjr, Oct 24, 2011.

  1. scottyjr

    Thread Starter New Member

    Aug 12, 2011
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    Hello All. I'm a bit confused about the power requirements for a stepper motor out of an Epson printer. According to the service manual it's driven by 42V and the resistance of each coil is 5 ohms. That seems like way too much power for a printer carriage drive. The stepper is a EM-233 (STP-42D135-01); I've been unable to locate a data sheet for it. The stepper is driven by a SLA7043. The service manual describes that IC as supplying a 'constant current source' controlled by 3 bits of data. Does that have anything to do with keeping 8 amps of power from flowing through the coils of that stepper? I have a feeling I'm missing something here as far of the power to that stepper. Would appreciate any input. - Scotty
     
  2. crutschow

    Expert

    Mar 14, 2008
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    They likely use a high voltage to reduce the effect of winding inductance and rapidly ramp the winding currents when each winding is energized. The constant current circuit then limits the winding current after it reaches its rated value.

    If it doesn't state what that current is then you may have to experimentally determine the value.

    If the motor is still in the printer than you should be able to measure the voltage across a winding at the end of the pulse with an oscilloscope. Then just divide that voltage by the winding resistance to get the current.

    If it's already out of the printer then you could use a variable DC power supply to slowly increase the current in one winding until the motor has a holding torque comparable to the operating torque you need for your application.
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Stepper motors have a very predictable power in watts per phase, that will be the same (ie very close to the same) for all motors in that same size package.

    If you compare your motor to catalogue photos of same-sized motors on the internet that will tell you watts per phase and you can work out the voltage and current based on that.
     
  4. scottyjr

    Thread Starter New Member

    Aug 12, 2011
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    Thank You both for your responses. - Scotty
     
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Would that really give you a meaningful measurement? I think the winding resistance only applies with the motor at a dead stop or with the shaft locked. if it was moving at all, I think it would void the measurement.
     
  6. jimkeith

    Active Member

    Oct 26, 2011
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    Hi Scotty,
    I agree with Carl that the high voltage is used to force the current quickly--when the set current is reached, the IC pulse width modulates the voltage to keep the current under control.

    This is a simple crude technique that you can use to determine winding current:
    Power one winding via a variable voltage supply
    Observe the 'break away' torque by turning the shaft with your fingers
    Keep increasing the current until you feel no subsequent increase in this torque
    This is roughly the current rating--note that this current is not all that critical
     
  7. crutschow

    Expert

    Mar 14, 2008
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    The motor moves in steps and thus stops at some point in the step waveform. You should be able to see that from the voltage waveform across the stepper. You would use the voltage at the end of the step, at which point the motor should be stopped.
     
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  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Practically all of the modern printer stepper motor drivers use a chopper IC, so even when stopped the voltage across the motor coil will be the full PSU voltage chopping on and off via PWM.

    The best way to determine motor volts/amps is by package dissipation as that remains quite predictable (ie compare the motor to other same-package motors with known specs). :)
     
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