Tutorial -- Motors and Drives

By: Frank Severance, Western Michigan University

With the dynamic speed of new technologies sweeping the motion industry, here's a two-part refresher course that examines core components and systems. The second part will appear in the June issue.

The part of a motion control system that actually does real work is the actuator. Whether electric, hydraulic or pneumatic, this is the device that puts the "motion" into motion control. Electric actuators are motors, of which there are three basic types. These are the DC motors which require a DC power supply, the AC motors which require an AC power supply, and the stepper motor.

Each motor type has a number of possible designs, each with their particular strengths. However, the most common motor in general use is easily the AC induction motor. This device essentially goes at a constant speed but is rarely used in servo applications. Even so, it is powerful, reliable and relatively light weight, making it most useful in industrial operations. At the same time, it is easier to control a DC motor than an AC. Therefore, in servo applications requiring feedback control, the DC brush motor is used more often than any other. Its more modern sibling, the DC brushless motor, is more reliable and also more expensive, so the tradeoffs continue. Finally, in cases where open loop control is sufficient, the stepper motor is very attractive for point-to-point solutions, especially if the torque requirements are not too severe.

The above generalizations are far too superficial. There do exist AC servomotors and powerful DC motors. For that matter, all motors do not rotate ­ some are linear. Even the AC versus DC classification is not absolute since universal motors can go either way! Thus, no perfect classification scheme exists.

Drives

Probably the most misunderstood component of a motion control system is the drive. Every company and every country seem to define it differently. Yet its purpose is simple, to serve as an interface between the controller and the actuator.

From an engineering point of view, a drive is a black box similar to Figure 1. A single control signal and the power source serve as inputs to produce appropriate multiple actuation waveforms.

Inevitably the control signal, whether digital or analog, will be a small (on the order of 5 volts) signal with little power (current in the milliamp range). The power signal, whether AC or DC, will be large since this is the power that actually drives the actuator. From the two inputs, the drive itself is a power electronics module that will create device-specific waveforms by which to power the motor in proportion to the control signal. For instance, in the case of a DC motor, one actuation signal might power the magnetic field windings of the motor and another variable waveform would change the armature current as directed by the control signal. In the case of an AC motor, the control signal might vary the frequency of the power signal, thus requiring AC-DC and DC-AC conversion to produce a single actuation signal of fixed power but variable frequency. In the case of a stepper motor the actuation signals would likely be a sequence of several signals, each of which would energize an appropriate coil at the appropriate time in "bang-bang" fashion.

It should be clear from the above description that a drive cannot be generic. Its role is that of an interface between the controller and the actuator itself. As such, a drive must match the control signals (voltage and power levels) and signal type (analog or digital). It must also match the power supply type (AC or DC) and power, voltage and frequency requirements. From this, the drive produces power conversion, amplification and sequencing of appropriate waveforms to power the actuator's expectations. Again, with the wide variety of actuators, virtually every motor has its own drive requirements.

The Motor Sizing Problem

Aside from the technology of the actuator itself, the primary problem on the motion side of motion control is that of sizing the motor (or other actuation device). One must find the best motor that will perform the required task, thereby optimizing engineering design. This requires two pieces of information. First, the old truism "Know thy load" is ever true. There is no way by which to design a solution unless one first knows the problem. Specifically, the moment of inertia and friction characteristics of the load, motor and linkages are required for any motor sizing requirements.

Motors are traditionally characterized using torque-speed curves, one of which is shown in Figure 2 along with a corresponding current-torque curve. Such curves express a good deal of information. For instance, in the example shown it is clear that as the torque increases, the speed must decrease. However, as the speed decreases, the current increases. This might not be intuitively obvious, but it is true in general. Indeed, generally we can say that the speed is directly related to the back EMF voltage and the torque is directly related to the current.

The torque-speed graph also gives the no-load speed (where the torque is zero) and the stall torque (where the speed is zero). Once the torque requirement is known the speed can be inferred from the graph and the power requirement calculated from P=ST for power P, speed S and torque T. Data sheets and motor name plates typically list power/speed so that sizing can be readily accomplished.

Part 2 - AC, DC and stepper motors.

For more information, contact Dan Jones, President, AIME, 509 Marin St., Suite 221, Thousand Oaks, CA 91360. 805-496-2621. If you found this article to be helpful, please Circle 490 on the reader service card.