Speed Control Vs Current Control

Moses

Guest
M
In DC Drive application, what difference between speed control and current (torque) control? Relation between speed and torque? Where to get knowledgebase of this?
Thank you
 
Typically, in a speed control loop, the desired speed (reference) is compared to the computed or measured speed and the error between the two is used to adjust the current reference, which controls the flow of current to the motor. The motor's current is used as a feedback within the current loop to maintain the proper current level (as well as limits for rise time and levels attained).

This puts the inner loop as a current control loop within the outer speed control loop.

In current (torque) control mode, the speed is irrelevant and not measured at all, except as a limit for overspeed protection, typically. The reference is for torque.

The speed can be determined using either an encoder (accurate) or by calculating the speed using an EMF regulator. Armature voltage will be proportional to speed times field flux. Field flux can be calculated from field current (non-linear, logarithmic, but still a function of field current) until saturation of the field laminations. The EMF regulator uses this computed speed as the feedback to compare to the reference. As armature voltage will vary with ambient conditions, especially temperature, the EMF regulator method has an inherent speed calculation error that is not reliable for precise control. Typical speed calculation errors are in the 2 to 4 percent range.

There is no real relationship between speed and torque. Speed and torque both increase with Power (speed times torque is proportional to power). However, when one starts examining the motor itself, you'll find that decreasing the field flux will cause an increase in speed and a decrease in torque, giving one a "constant power" range.

Until base speed* is reached, typically full field current is supplied in a shunt motor application, allowing armature current to dictate the torque (field flux times armature current is proportional to torque). Above base speed, field current/flux is decreased, allowing the speed of the motor to increase with a corresponding decrease in torque.

As full armature current is available in this pre-base speed range, we call this the "constant torque" range of motor operation.

*base speed = the speed of the motor with full nameplate field flux and full nameplate armature voltage.

The speed of the motor is proportional to Armature voltage divided by field flux. You can easily see that as field flux decreases, this will cause an increase in speed as well as a decrease in torque from the two formulas I've cited.

In a series motor application, the armature current and the field current are in series. As the torque will now be roughly equal to current squared (ignoring the exponential curve in the current to flux calculation), as current decreases, so will torque. Likewise, speed will increase with a decrease in current, due to the lessening of the field flux. This is why, on lightly loaded series motors, the speed will tend to increase very rapidly, often to the point of overspeeding the motor and causing damage.

The knowledgebase on this can be found in any basic DC motor text.

Does this answer your question?
 
Last edited:
USdutchman anyway

Somehow, I didm't get the logon window (I tried!!)

Anyhow,

Not withstanding the above posts, it is actually prety simple to choose between speed and torque:

1) If you have a none stretching material, you can only have 1 speed master per zone, all other drives (AC, DC Vector or ...) have to be torque controlled.

2) If youre material stretches (or if you want to elongate it), you can have multiple speed controllers (second controller x% more speed than first).

FYI: Storage solutions exists to isolate (speed) zones (called accumulators, loopers, towers etcc..).

Hope this helps
 
Steve,

You are correct, however Don's reference, as I see it, was limited to the electro-magnetic issue / torque produced by the flux, and not in reference to the inertia of the mass.
 

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