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Thread: Inverters for AC motor speed control

  1. #21
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    Quote Originally Posted by Chuck(G) View Post
    I've got some fractional horse brushed motors (4 lead) that have DC nameplate ratings, but are called "SCR motor". Don't ask--I can't figure that one out myself.
    Silicon Controlled Rectifier perhaps?

  2. #22
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    Quote Originally Posted by Agent Orange View Post
    Silicon Controlled Rectifier perhaps?
    Sure, but like those old "transistor batteries" it doesn't really make sense. The motor doesn't have any SCRs and the batteries don't have transistors.

  3. #23
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    Quote Originally Posted by Dwight Elvey View Post
    I still don't know if it is a AC induction motor or
    an AC motor with brushes....
    Before your post, I didn't know the difference either!
    Thank you Dwight...I have to do some more reading....

    ziloo

  4. #24
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    Quote Originally Posted by Agent Orange View Post
    Silicon Controlled Rectifier perhaps?
    This might shed some light on it: https://amatrol.com/coursepage/elect...trol-training/

  5. #25
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    Okay, that makes sense--they're not referring to the motor specifically, which is, a shunt-wound DC motor, but rather the control system, which uses SCRs with a variable conduction angle control to convert AC to DC. Okay, I get that.

  6. #26

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    I find it interesting that most people think of regulating a motor by putting a variable resistance in series with the overall current ( or pulse width ). On a AC/DC motor with a field winding, the field winding can have a high resistance ( lots of turns of wire ). The strength of the field is proportional to the ampere-turns. This means on can have the strong field with just a smaller current. So, one can regulate the speed of the motor by less power controlled in the field winding. This is typically how the fan motor in your auto gets the different speeds. At first one would think the more current in the field winding, the faster the motor would go but it is just the opposite. The stronger the field, the slower the free speed of the motor is. Of course, if you are concerned about stall torque, the stronger field has the stronger stall torque.
    Yet another interesting way to control a typical DC motor is to look at the back emf. On pulse controlled speed systems, the dead time between pulses, with no current flowing, is the back emf. For a fixed magnetic field, it is proportional to the speed of the motor. When on analyses the loss in a motor, it is the resistive part. If one knows the resistive part of the motor, that includes brushes and winding, one can calculate the speed of the motor by measuring the current and the voltage going into the motor ( actually I left a small amount out and that is the inductance to the windings that is usually quite small unless the motor is spinning really fast ). If one knows the resistance of the motor, and the current, using IR=E, one just subtracts the this voltage from the measured voltage. One is left with the back emf. This is the speed of the motor ( for a constant magnetic field ). For small DC motors, this is a handy way to make a speed control without resorting to pulse width controls. It only requires some resistors, a couple of op-amps and a series power transistor. This of course only works well for a lightly loaded motor.
    Dwight

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