DC shunt motor speed control

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.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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.

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John Popelish

A number of years ago I designed and one of my uncles sold, in his golf cart shop, a "hot rod" kit for the people in Sun City who drive their carts on the streets to go to the grocery store.

Series wound motor. Just switch in a shunt across the field, and top speed increases to 25MPH ;-)

Before some liberal wienie panics... 4-wheel carts.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Damn, you are right, of course. Sorry for the mental lapse.

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John Popelish