Motor Control Solutions Put You In Total Control

Looking back in history, we know that first came the wheel, followed some time later by the motor . Both accomplish work and both are still in existence today with little variation… until recently. Motors are a part of our everyday lives and, in fact, consume an estimated 50% of our energy on this planet. It is also a fact that 80 – 90% of motors lose enormous amounts of energy while running.

Electric motors operate by exploiting the relationship between magnetic flux and the flow of electricity. One can control the work that a motor does simply by turning it on and off, but this is not a precise way to control a process. Starting and stopping can be hard on a motor if it's done repeatedly and rapidly because it can overheat, damaging the motor and possibly burn it up.

Variable speed drives were the next step in the evolution of the motor. The shaft of a motor rotates at varying RPMs to accommodate load changes. Instead of switching on and off, a continuous control signal is applied. The amount of work the motor does is capable of matching only what is needed with little waste. Now, instead of “letting off steam” to accommodate a lower work load with the same one-speed motor, you have the ability to save energy by backing off in a controlled fashion rather than expending the extra energy in venting, braking, heat, or some other means.

What's next?

Let's take that one step further. It may seem like we have squeezed as much energy efficiency out of it as we can by carefully matching the motor's speed to the work of the moment, but not so. Electrically speaking, you can save yet more energy by controlling power expended in a motor by separating current into components and controlling them individually. To do this, you need to perform complex mathematical calculations to get real world results involving imaginary numbers, torque, frequency, phasors, vectors , and a pinch of math from that lecture you slept through.

A vector drive can accomplish the calculations and control of the components that make up electrical current powering the motor.

Applying this concept to a motor's stator current, the two electrical current components are the magnetic flux component (also called the magnetizing current) and the torque component. Torque is merely turning force and torque current can be thought of as load current. Resistance to spinning causes load current to increase in response. Higher amplitude voltage produces more torque. The other component, magnetizing current, establishes the magnetic field in the windings so a motor will turn. Incidentally, rotation caused by magnetic attraction was first documented in 1269 and included a rudimentary description of the first motor design.

For vector drive control, each current component is mathematically represented by a vector and then calculations are made for optimal control on each component separately. The components are added together and the final “vector” (a mathematical construct that helps us keep track of the complicated math) then is applied to tell the motor what to do next. This all happens very fast, on a continuous basis, since vector drive processors require less than a couple of microseconds to complete a calculation and issue the resulting command. In the end, you have extremely accurate control regarding the application of force to instantaneous requirements of the process at hand.

What's the Vector, Victor?

The vector drive is a control function, not a type of motor. The vector drive enables a motor to consume up to 50% less energy than the typical AC induction motor . The hardware cost is nominal, so there is little added cost in extra hardware for a vector drive. It will need a microcontroller or DSP and perhaps some form of feedback, but little more equipment. So is there a trade-off for this energy savings? It's complicated.

Complex mathematics are implemented through software and the characteristics of the motor have to be understood and matched in the control algorithm. The good news is that many companies provide software tools and examples with microprocessors tuned for vector drive motor controls, but tuning a control algorithm and matching the characteristics of the motor to a dynamic process load needs to be a consideration beforehand. For one thing, the control algorithm has to account for the response of the individual motor to the load. This can be calculated without sensor feedback if you know the impedance of the motor and can carefully match the algorithm to the performance of the motor. The performance of a motor changes with respect to operating temperature, for instance, but performance characteristics should be available in the specifications.

If your application is too dynamic with respect to changing load, extreme operating temperatures, or other harsh conditions, you may do better using actual feedback from the motor with current sensing amplifiers, encoders, or Hall Effect sensors and the like. It's handy to know what the temperature is inside the motor case as well, since overheating is an alarm condition. The feedback is used to dynamically adjust the control of the motor.

New opportunity

The vector drive performs complex calculations and controls the motor for high performance, dynamic response, and most importantly energy savings. However, a good deal of forethought has to be applied with the decision to use a vector drive.

Vector drives are not in common use yet, but mastering the techniques of the vector drive is a great opportunity to tap into a vast supply of energy savings since motors (especially AC induction motors) are so ubiquitous. As motors with vector drives start replacing worn out motors and new factory designs incorporate motors with variable and dynamic capabilities, the momentum in growth in this area will pick up as the learning curve reaches critical mass.

By Lynnette Reese, Mouser Electronics