Re: DC shunt motor speed control
From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 01/31/05
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Date: Mon, 31 Jan 2005 09:57:43 -0800
John Popelish wrote:
> abhinav wrote:
>
>>hi there
>>well, i have a little doubt here, kudos to any one who could clear it
>>:)
>>
>>well i need to control a DC shunt motor above and close to base speed,
>>and so i figured that field control to be optimum
>>i figured out a strategy to adjust the field resistance by adjusting
>>the duty cycle of a FET connected in shunt to it. I eeve figured out a
>>way to vary the duty cycle of the FET in relation to the control
>>voltage .
>> now my problem is that i'm quite quite unsure of the system's
>>behaviour during startup as the control voltage is generated of the
>>speed of the motor
>>Any pointers?
>>any help would be gratefully accepted!
>
>
> Since your control system raises the duty cycle for lower speed, it
> should provide maximum duty cycle for start up. The pitfall is that
> there is an L/R time constant for the current in the highly inductive
> field windings and if you apply full armature voltage at the same
> moment you apply full duty cycle to the field, there will be a period
> of time during which the field current will be low, and the armature
> will draw lots of current that produces little torque. There may need
> to be a small time delay between the application of field voltage and
> the application of armature voltage to reduce this inrush of armature
> current.
>
John: you have that backward. As the field is decreased the motor speed
will tend to increase because of reduced back-EMF. You'll also see a
drop in the motor torque constant, so the field will go exactly the
wrong way under heavy loads. You'll also have a case (starting at zero
speed is the obvious case) where under heavy load decreasing the field
voltage will decrease the available torque and let the motor slow down,
so a monotonic speed/field relationship will cause positive instead of
negative feedback.
Abhinav:
I would probably try to do this by monitoring the motor speed and
putting a lower limit on the field current (or PWM) that I'd allow.
This should make sure that you have enough torque without the armature
going into runaway, and you'd presumably rise above the limit speed in
"normal" operation. This will give you maximum field current at start
up and during disturbances, and if the motor can spin at the desired
speed at all you should be able to find a function that'll get you there.
Of course your design will have to be fairly conservative to make sure
that the lowest field current generated will actually get the motor
no-load speed high enough. An alternative would be to sense the
armature current and increase the field current anytime the armature
current rises above some threshold. This should keep your motor
accelerating any time it needs to, and if you're careful allow it to
transition to field-current control when you get close to your target speed.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
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