current controlled motor 3

[jackxie]

When I read papers, I often met the descriptions such as; the linear motors are current controlled three phase motors. I wonder what does the current controlled mean. What are the differences between current controlled and voltage controlled mode? What are their advantages and disadvantages? I appreciate your comments and directions. Thanks a lot.

 

[Warpspeed]

With voltage control, there is no real way of knowing what the actual instantaneous motor running current is going to be. Current will vary hugely with load, (and the degree of motor slip).

When current control is used, that effectively controls motor output torque and (obviously) motor current. Under extreme load, motor voltage will automatically fall, and as the motor accelerates, voltage will automatically rise. But torque and motor acceleration will be predictable, and remain almost constant without the possibility of overloading the motor. That gives a much nicer and safer control characteristic, and the motor can safely operate over a wider range of slip rates.

 

[DickDV]

While this is a question that engineers and scientists could debate for months, in general, from a practical standpoint, AC induction motors are frequency-controlled devices as to speed. That is, the frequency of the AC power controls the motor speed within a couple of percent. This would be true for linear AC induction motors as well.

As to torque, loosely speaking, AC induction motor torque is controlled by the current. Motor lead current serves two functions--magnetizing the motor and producing torque in the shaft so the torque control is diluted by the magnetizing current especially at lighter loads. The ratio of the applied voltage divided by the frequency actually controls the available torque more directly. Ultimately, the actual motor torque is a complex thing including both the above and the applied load to the shaft.

I know of no three-phase AC motors that are voltage controlled.

 

[blacksea]

You read about AC or DC brushless motors with standard three phases wye (star) connection. The servo with such motors has inner current loop with PID-filter as a rule.

 

[cswilson]

The terms "current-controlled" and "voltage-controlled" here refer to the method of control, no the type of motor. In electric motors, the torque produced is proportional to the current -- other things being equal. The motor will also produce a back EMF voltage proportional to its velocity.

We live in a world of voltage sources, so to obtain current control, we need a measurement of the actual current, and a feedback loop that outputs the voltage necessary to obtain the desired current. This obviously adds cost and complexity, but does provide several important performance advantages:

1. The feedback loop can dynamically and temporarily apply very high voltages to get quick current changes, effectively reducing the electrical time constant for faster responses.

2. The feedback loop will automatically compensate for the generated back EMF, apply voltages above and beyond that to get the desired currents.

3. The above two factors means that you are much more directly controlling current, and therefore torque and acceleration, of the motor, providing much better dynamic physical response.

4. Performance is much less sensitive to physical changes such as increasing resistance with temperature.

5. Control of current is your best protection against over-current conditions.

The above is true for DC or AC motors, for synchronous (e.g. brushless servo) or asynchronous (induction).

Standard volts/hertz VFDs are voltage-controlled systems. Vector-control systems, with or without shaft sensors, are current-controlled (even if marketed as VFDs). Servo-control systems are virtually always current-controlled, except maybe at the very low end.

Curt Wilson
Delta Tau Data Systems

 

[jackxie]

Thank all of you very much!! I got more than what I expected. Your help and directions are highly appreciated.

Jackxie