Rotor Flux Control (RFC) is seen as a major advantage in applications where a combination of dynamic performance, stability and speed accuracy are required without encoder feedback and where low audible noise is important. The advanced method of open-loop speed control devised at Control Techniques (CT) is a standard feature on its flagship range of AC drives, Unidrive SP.
“Rotor Flux Control provides a simulated speed feedback that gives excellent speed accuracy without actually having the cost of direct feedback from an encoder,” says CT product manager, Richard Smith.
Vast amounts 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.
Several methods of open loop control exist: variable frequency (VF) is the simplest and lowest cost to implement; however the speed of the motor changes significantly with load and temperature so that good speed holding and low speed operation is not possible.
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) offers improved 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 of 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 physically mount and wire an encoder.
The simplicity of the algorithm allows the calculations to be performed synchronously with the speed and current loops within the drive, resulting in a characteristic performance that is dynamic, with excellent speed accuracy and a stable operation even with troublesome light loads.
Control Techniques claims that RFC control delivers full torque in less than 0,5 ms, (compared with four times this for DTC) significantly increasing the dynamic performance of the drive and motor and greatly extending the bandwidth of the system.
An important feature of DTC is that the switching frequency is selectable, making the drive suitable for applications where low audible drive noise is important such as lifts and other building system drives.
CT acknowledges that RFC is not a solution to every application. Systems where accurate speed and torque control are required down to zero speed will still require feedback devices. It does, however, allow open loop drives to be used in more applications where, in the past, only a closed loop drive would be able to perform.
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