About Electric Motor Control
Credit to Author: Francois Malrait| Date: Fri, 22 Nov 2019 20:21:20 +0000
What is a Motor Control equipment?
An electric motor is part of power conversion chain from electrical source to mechanical system. More than delivering a mechanical power to a system, it’s providing a mechanical torque at a mechanical speed.
- Using a Direct-on-Line:
We do not control neither motor speed nor motor torque. According to load torque and mains supply, you get a torque response on mechanical system, and a current consumption on the electrical motor part. All equipment must be defined to accept high transients. - Using a soft starter – thyristors bridge:
We start to offer freedom degrees. Voltage applied on motor is built from pieces of mains voltage. It offers the capacity to manage mechanical control during transient phase. Equipment may be defined to accept less current transient as well as less mechanical stress. - With a variable speed drive:
We propose full freedom degrees to define voltage to apply. Electrical Power Conversion device is usually made of three parts: diodes/thyristors rectifier stage, DC stage, IGBTs 2-level inverter stage. Drive, as Power electronics device, provides a full capability to apply a desired voltage on electric motor. Counter parts are impacts on efficiency, current harmonics on mains supply, voltage waveform on motor: common mode voltage, dv/dt, overvoltage. There is plenty of power conversion options to be considered for those extra features: AC current input filter, sinus filter, etc.
There are not only power conversion devices but also cyber-devices able to be connected in automation systems, functionally delivering services on customer applicative system: control, protection and monitoring. They have the capability to address problems in different time-scales with efficient analytics: right algorithms in front of right amount of data.
What does Drives Motor Control mean?
Drive Motor Control System Engineering
Let’s have a look on the motor control system, figure 1. We have command information, mains supply, drives, motor, and application.
Figure 1: Drive Motor Control System and sub-systems
On a functional point of view, figure 2, we may breakdown the system into physical parts and their control functions.
Figure 2: Drive Motor Control System Functional view
It’s a theoretical approach based on time scale and bricks approach. Each level has its own time scale. So, a time scale control (aka cascading control) makes sense. It’s an efficient way to address Drives Motor Control system on a flexible manner, on a power range from 0.1 kW to 50MW, on a voltage range from 200V to 13.6kV, and more.
On a technical point of view, each control part may be named accordingly to academics naming or state of the art:
- Power Control: Pulse width modulation, Space vector Control, etc. From 16kHz to 500Hz
- Electrical Motor Control: Direct Flux oriented Vector Control, Indirect Flux-oriented vector Control, Volt-by-Hertz, etc. From 2kHz to 10Hz.
- Mechanical Control: Torque control, Speed control with PI, PID, PIDF, feed-forward, RST, Model predictive control, etc. From 1kHz to 0.01Hz.
Drive Motor Control System naming
Of course, it’s possible to mix the topic: you may generate a voltage profile (electrical control level) without any direct mechanical control. Obviously, motor will deliver a mechanical work that may reach your objective of (low) performance or (high) robustness.
You may name a power control strategy with the mechanical word. Not sure that a pump is requesting a direct torque control with a response time of 1ms to start with a 30s acceleration time. Surely a pump will prefer a quality torque control that will take care at maximum of its mechanics.
On a user point of view, technical naming is not the topic.
- At the level of Application, for instance a pump, you are asking if the system is acting well to deliver the right pressure for the flow demand … if you manage properly the best system efficient point.
You are requesting a Pump Control Law - At the level of Mechanical system, you are asking if the system is acting well to deliver the right speed whatever the load is.
You are requesting a Speed/Position Control Law - At the level of Electrical Motor Part, you are asking if the system is acting well to deliver the right torque what ever the electrical parameters are.
You are requesting a Torque Control Law - At the level of Power electronics Part, you are asking if the system is acting well to deliver the right voltage under power electronics uncertainties.
You are requesting a Voltage Control Law.
To stay generic at the level of Drive-Motor bundle, we may choose Quality Torque Control naming. Behind ‘Quality word, users must understand that a drive embeds the best technology at the right level of your need: Robust and Performant, achievable by settings.
What is Electrical Motor Control?
Electrical Motor Control transforms torque reference into voltage references (Figure 3). These references are converted into real voltages applied to electric motor.
The objective of Electric Motor Control is that Electric part of motor delivers an electromagnetic torque at the right dynamic, at the right static accuracy compared to torque reference (Figure 3).
Figure 3: Action vs Objective
Applying control of systems theory methodology request to:
- Write a dynamic system representing the system to control
- In terms of dynamic states, input, output: based on physics
- In terms of parameters:based on datasheets, or through identification specific procedures
- Verify existence of equilibrium
- For instance, with a volt-by-hertz law, Low speed – Full torque area is not accessible.
- Assess natural stability
- For instance, Around low stator frequency value, system is naturally unstable
- Check controllability/observability
- Build a controller
- Structure must provide a stable ‘closed-loop’ system in the operating range
- Gains must be set to get ‘closed-loop’ system as performant as desired
Electrical Motor Control Performance is 4 steps.
As far as it’s possible to inverse your system, meaning able to calculate what input corresponds to your desired output, it’s efficient to do it in order to be performant.
- Prepare your route (and apply!) is more efficient that relying on a controller saying where to go at each crossroad.
Your system needs to be stabilized and controlled around its trajectory.
- Manage your car to avoid accident, be able to switch to a secondary road to avoid a small perturbation.
Limitation on your input, on your system, must be considered to recalculate an accessible trajectory on a stable way.
- In front of a working area on a motor way, you adapt your speed, and then may need to calculate a new fallback route to achieve initial objective.
Trajectory must be accurately followed, if there are uncertainties within the controller, within the real system, that may impact the accuracy, they have to be rejected.
- I wanted to see Antwerp, we saw Hamburg again (J.Brel): route is done to go to the objective.
For electrical motor, it’s the same story. Performance comes from 4 elements:
- Voltage feedforward terms accuracy: acting vs reacting
- Current stability control
- Voltage limitation and Anti-wind up stability
- Torque application accuracy by self-rejection of parameters uncertainty.
Electrical Motor Control Performance Value.
Performance is relative. Same result for some people will be excellent and for other bad… because performance without reference is not quantifiable.
Key value for performance is to do what you expect. This is a generic value that allows you to ask you the right question: what trajectory you want to track– more than what set point you want to reach.
Electrical motor is an equivalent filter of voltage to provide current, and torque. By electrical motor control you may adjust its bandwidth very high in ideal case:
- with a feedforward term (1): ~infinite, or just less due to imperfect accuracy
- with control gains (2): in accordance with sampling time
So, what is the limitation ? Sampling time, and power electronics!
The calculated voltage reference is converted into real voltage by Power electronics control stage. Direct control or not, for a motor rotating at 1500rpm-50Hz, the voltage is mainly a 50Hz signal! Obviously according to your way to control power electronics switches, you will add a different harmonics content on your voltage: we may say that it’s a direct torque control of high frequency undesirable ripple.
8 kHz control sampling time will recalculate a voltage reference each 0.125 ms. Reasonably it will be transferred in terms of torque in 3 to 8 steps: 1 ms response time on a torque step! The same control law will also be able to respect slow torque ramp to start as slow as needed your pump.
To continue to draw a parallel with the route, Direct Torque Control on Power electronics is equivalent to constantly accelerating and braking to arrive in same time than the one going at the average speed.
Let’s choose Quality Torque Control.
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