Scalar control - What does it actually mean?
Many times we see that the term ‘scalar control’ is used in incorrect way or being misinterpreted. In this article we will briefly discuss scalar control – what does it actually mean? How does it differ from a field oriented control? What advantages and limitations does this type of control bring?
Look into history
In order to have a better understanding it is useful to look back at the historical development of speed-controlled electric drives. Few decades ago, modern medium voltage AC drives were not yet available (lack of suitable power electronic devices). However, variable speed drives were required and there was a fairly good solution: DC based drive systems. High power thyristors were already there and thyristor-based rectifiers allowed a smooth and accurate speed control of DC machines. In fact, the control was so precise and relatively simple that it quickly became the benchmark (in some areas you would still find DC motor drives today – not just legacy products but also brand new equipment). The rotor of the machine was supplied from the main converter while the stator field was controlled by an excitation converter. Through stator flux and rotor current speed and torque of the machine were controlled.
After some time the power electronics was mature enough to enable high-power AC drives to evolve. Nonetheless, to make the medium voltage AC drives a real success their ability to control speed and torque had to be at least as good as the one of DC drives. Not an easy task at all. The breakthrough of this challenge basically came with the introduction of field-oriented control.
Scalar versus vector control
Now let’s go back to scalar mode. We know from physics that there are quantities characterized by magnitude and direction while other quantities only have a magnitude but no direction. The first group are vector quantities (vectors) while the latter one are scalar quantities (scalars).
Therefore, the scalar control is just maintaining the ratio between output voltage (rms value) and output frequency. However, it does not mean that the ratio Us/fs is constant in the entire range. Scalar control respects the field weakening range etc.
Structures of scalar control
There are numerous control structures that can be classified as “scalar”. They range from primitive implementations up to more sophisticated control architectures.
In the simplest realization the input is just the desired stator frequency. Another block, e.g. a look-up table, generates stator voltage corresponding to the stator frequency. This block can compensate a resistive voltage drop (especially relevant at low speed or low stator frequency) or the field weakening point (limiting the maximum stator voltage). The structure does not have any slip compensation. Thus, the control inherently works with certain speed error which is load dependent.
This simplest implementation can be extended by several additional blocks. For example, the resistive voltage drop can be calculated online based on the actual stator current. Another block can generate pre-programmed speed ramps for the start-up. Generally, limiting the rate of change of stator frequency is a good idea that minimizes the risk that the machine is drawn into the unstable part of torque-speed characteristic.
Scalar control does not have to be “primitive”. In fact, more advanced scalar modes exist. They can have a speed controller (closed-loop structure) hence eliminating a steady state speed error. These schemes limit the rotor frequency in such a way that the critical slip is not exceeded. Consequently, the motor always operates in the stable part of torque-speed characteristic.
Closed-loop scalar mode allows fairly good dynamic operation and high speed accuracy. Nevertheless, the transient operation remains problematic.
Performance of scalar mode
Scalar control is generally known to have somewhat limited performance, particularly when it comes to dynamic behavior. For many applications this is perfectly fine and scalar control can be used there without too many restrictions. For example variable speed fans – they often have significant inertia so that the speed cannot change too fast anyway.
Scalar control is not automatically open loop control
The term scalar control is sometimes used to describe an open loop (speed) control. However, this is not quite correct (see the principal diagrams in Figures 1 and 2). In fact, scalar control schemes may be both open loop (without feedback) or closed loop (with speed feedback). The latter one has improved dynamic performance that can satisfy also some more demanding applications.
Advantages and limitations of scalar control
1. Advantages
Main advantage of scalar control is its simple and straight-forward implementation. It does not require complex algorithms hence reducing the performance requirements of the microprocessor. Also, scalar control is easy to parameterize and faster to commission.
2. Limitations
As mentioned, limitations appear in several cases:
- High dynamics is required ⇒ Scalar control is not as dynamic as field-oriented control (although closed-loop scalar control can perform quite well)
- Precise speed control is required ⇒ Asynchronous machines inherently have slip that is not speed-dependent but rather load-dependent
- Uncontrolled transients ⇒ This is the main limitation of scalar control. Response to disturbances may be heavily oscillatory and badly damped
Remark
This blog post provides basic information. We have just scratched the surface a litle bit. For a full experience consider purchasing our premium subscription.
References
[1] VFD control -Introduction, https://mb-drive-services.com/vfd-control-introduction/
[2] Torque-speed characteristic of asynchronous machine, https://mb-drive-services.com/torque-speed-characteristic-of-asynchronous-machine/
