New open-loop speed control extends AC drive use

A Control Techniques product story
Edited by the Engineeringtalk editorial team Mar 8, 2006

Rotor flux control uses simple, accurate, mathematical motor model to calculate a simulated speed and position feedback, letting the loop be closed without mounting and wiring an encoder physically

A new, extended method of open-loop speed control devised at Control Techniques is available on its flagship range of AC drives, Unidrive SP.

Rotor flux control (RFC) has advantages where a combination of dynamic performance, stability and speed accuracy are required without encoder feedback and where low audible noise is important.

"This is another first for Control Techniques", said Product Manager Richard Smith.

"Control Techniques is recognised as a world leader in closed-loop technology".

"Now, with RFC, we can provide a simulated speed feedback that gives excellent speed accuracy; certainly the best results without actually having the cost of direct feedback from an encoder".

Millions of pounds have been spent by the drives industry trying to develop the perfect control algorithm to achieve accurate and dynamic control of a standard AC induction motor without measuring its speed - yet, to date, closed-loop control, with its extra costs of wiring and a costly encoder, has been the only choice for some applications.

Conventional V/F control is adequate for many applications, and it is low-cost, but the speed of the motor changes significantly when the load and temperature vary, so that good speed holding and control at low speed becomes impossible.

Open-loop vector control, which models some aspects of the motor, is better, giving good speed holding, but only moderate dynamic performance.

Direct torque control (DTC) improves dynamic performance, but does not allow the user to modify the switching frequency resulting in significant audible noise from the motor plus additional motor losses.

RFC addresses all these drawbacks, using a simple, yet accurate, mathematical motor model to calculate a simulated speed and position feedback, allowing the loop to be closed without the need to mount and wire an encoder physically.

The simplicity of the algorithm lets the calculations be performed synchronously with the speed and current loops within the drive, resulting in a characteristic performance that is dynamic, with speed accuracy and a stable operation even with troublesome light loads.

RFC control delivers full torque in less than 0.5ms, (compared with four times this for DTC) significantly increasing the dynamic performance of the drive and motor and extending the bandwidth of the system.

Commissioning is easier, with the drive able to measure many of the key parameters required; the others being obtained directly from the motor's rating plate.

The switching frequency is selectable, making the drive suitable for applications where low audible drive noise is important, such as lifts.

"RFC is not suitable for every application", said Smith.

"Demanding systems, where accurate speed and torque control is required down to zero speed, will still require feedback devices".

"It is, however, a big step forward that will allow open loop drives to be used in more applications where, in the past, only a closed loop drive would be able to perform".

"And it's now a no-cost feature on Unidrive SP, allowing the user to select the best control method for their application, including V/F, open-loop vector, RFC and true closed-loop control".

Because RFC is based on closed loop technology, the drive is able to switch seamlessly between true closed loop-control and the RFC mode; useful in many applications, such as for spindle motors with a wide speed range, where an encoder is needed for low speed positioning and performance, but at higher speeds the encoder is unable to transmit coherent data, or for adding an inbuilt level of encoder redundancy at no additional cost.

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