Controlling the speed of electric motors

Industrial automation processes very often require some method of rotational speed control. Fluid power motors, either hydraulic or pneumatic driven, can accomplish this task admirably over a wide speed range with fairly constant torque. Electric motors, especially AC motors, require a little more analysis and application knowledge before the best selection can be made.

This article focusses on the advantages, costs and efficiencies, and typical applications of the following motor drive technologies:

AC-PWM Closed Loop Flux Vector Drive

AC-PWM Open Loop Flux Vector Drive

AC-PWM Inverter Drive

Brushless AC Servo Drive

Brushless DC Servo Drive.

Other speed control technologies such as standard DC motors and Switched Reluctance motors are not discussed here due to their limited application in industrial automation.

Some of the formidable terms used in the above titles need a little explaining before we tackle each of these controllers:

The terms AC and DC refer to alternating current and direct current respectively. AC current, either three-phase or single-phase, is readily available in most industrial plants and is preferred in most applications. DC current, although preferred for control situations, is expensive and hard to produce for power applications.

PWM or Pulse Width Modulation is used to control the voltage intensity on AC or DC motors. PWM converts DC voltage to positive and negative signals of varying pulse widths that average out to a voltage less than or equal to the DC input voltage.

When a control requires feedback from a device to confirm position, it is said to be a closed loop system. When a control assumes position without any position feedback signals, it is open loop.

A vector drive controls the current and frequency to the motor to maintain the magnetic flux in the motor at a constant level. This provides good torque regulation throughout the entire range.

A brushless motor uses position feedback to commutate the windings correctly, while a DC motor commutates the armature mechanically through brushes.

Trapazoidal or “six step” speed control for DC motors is a relatively inefficient and old technology. It develops a multi-stepped approximation of a sine wave of varying frequency to control the commutation of the windings. This method of speed control develops excessive currents and heat in the windings, which must be dissipated.

AC-PWM Flux Vector Drives

Flux Vector Drives, although expensive, are about 90% efficient, close to that of a full servo motor. They are used to control the speed and operating characteristics of standard “squirrel cage” AC motors. They can be operated with or without feedback (closed or open loop, respectively) but are much more efficient with a closed loop, especially at slower speeds. They are excellent and more economical for controlling larger motors up to 2500 hp.

Typical uses would be in AC dynamometers and test stands, steel rolling mills, plastic extrusion, wire drawing and large machine tools. Their advantages include a wide speed range (1000:1), servo performance, 100% torque at 0 rpm, 0.05% speed regulation, and positioning capabilities to within 1/4 of an encoder pulse. They are, however, expensive, particularly when used with a feedback encoder.

AC-PWM Inverter Drives

AC-PWM Inverter Drives use 60-Hz AC power rectified into DC which is pulse width modulated into a sine wave of variable frequency and amplitude. This allows relatively inexpensive variable speed control of standard AC motors. They make no attempt to control the current to the motor and thus torque and speed regulation are relatively poor.

The controlled speed is accurate to within 2 rpm to 5 rpm of the target speed, regardless of what that speed is. This means that the percentage error at slow speeds is far greater than at high speeds, although, on average, 3% speed accuracy might be typical. This limits the practical applications of this device to mixing, processing, pumping, machining and other operations with wider speed tolerances.

Brushless AC Servo Drives

Brushless AC Servos are typically used in applications under 20 hp that require extremely high motor acceleration and deceleration rates. The motors generally have built-in feedback devices and selection is somewhat limited, making the costs higher than the induction motor alternative. Popular uses for AC brushless motors are in canning and bottling plants, as well as for robotics and sophisticated machining applications.

Brushless DC Drives

The trapezoidal or six-step drive was the first technology used with the introduction of brushless DC motors. Although inexpensive, it is inefficient and develops a lot of heat. It has since been superseded by PWM controls and servo technology.

Summary

AC Servo technology is used primarily in machine applications, usually requiring higher accuracy and smaller motors. Flux vector drives offer two principal advantages: the same type of response as Brushless AC Servos but in larger horsepower ratings; and the use of standard, off-the-shelf motors.

The most current technology uses AC variable frequency or inverter type drives for very exacting control of the speed and torque of standard AC motors.

Ted Grove, corporate training manager for Wainbee Limited of Mississauga, Ont., is an experienced fluid power trainer. This article is the sixth in a series. Visit www.mro-esource.com and click on the Past Issues button to view Practical Automation columns from previous issues of Machinery & Equipment MRO.