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AC EQUALS DC


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New control techniques give ac induction motors performance equal to that provided by dc servos. Again, microprocessors make this new capability possible.
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Motor
Control
System response

Additional control functions
DC servo motors
AC induction motors and vector control

The need for servo drives is constantly expanding, and with it, the types of servo systems are increasing. For many years, dc servos were the standard. About ten years ago, brushless dc became commonly accepted. In the last few years, yet another servo drive has emerged and is serving new needs for fast and accurate positioning. Termed ac vector control, these units power conventional ac induction motors-as large as 100 hp-and offer response speeds as fast as most conventional dc servos: acceleration rates of more than 600 radians/sec squared.

Motor
An ac vector controlled servo motor can be a conventional ac squirrel-cage, induction motor. For higher performance applications, typically 600 to 3000 radians/sec squared, motors designed specifically for servo applications are used. Such motors have low inertia rotors, with high torque to inertia ratios. These motors are built with two types of construction:

long, small diameter rotors
rotors made with aluminum core and steel laminations
This latter design uses aluminum to hold the steel laminations, in which the rotor bars are cast, rather than using solid-steel laminations, Figure 1.







Figure 1-Low inertia rotor built with aluminum inner core and steel laminations.

A speed feedback device, either digital or analog, is mounted on the motor. In some cases, a digital unit is installed to provide feedbacks for two purposes:

speed feedback for the motor controller
machine position feedback for the position controller
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Control
The power control section is a standard PWM (pulse width modulated) amplifier, Figure 2. It converts plant ac power to constant voltage dc then into pulse width modulated ac, which powers the motor.




Figure 2-Pulse width modulated (PWM) power controller used for ac vector controlled servo.

The brains of the controller-the regulator and firing circuits-contain a microprocessor-based control and highly sophisticated programming. These new controller capabilities are responsible for giving a rugged ac motor the desirable characteristics of dc motors. Here's how:

The power applied to the dc motor windings produce a rotating magnetic field that has two components. The first induces current flow in the squirrel cage rotor. This current then produces a rotor magnetic field that serves the same purpose as the permanent-magnet field in a brushless dc motor.

The second component reacts with this rotor field to produce a torque and turn the rotor.

The microprocessor treats the field and torque producing components separately. This technique enables an ac motor to produce a linear and rapid control of motor torque for high-performance positioning. (See DC servo motors)

The motor parameters-winding resistance, inductance, capacitive coupling, air gap, etc.-effect the motor response to electrical changes. Therefore, some ac vector controllers require entering the motor parameters into the controller program.

Other controllers offer adaptive tuning. With this capability, the controller produces a series of step response changes. The speed feedback device provides rotor speed data to the controller. The controller then compares the rotor speed with the command steps and tunes the regulator for that particular motor.

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System response
One of the most important criteria in selecting a servo system is how fast the system responds to commands. Two major factors are frequently used:

controller capabilities
total system response, which includes the motor
Controllers are rated in amperes, volts, and speed of response called bandwidth. This is measured by applying a sine wave input at various frequencies and comparing the output to the input. The usual unit for quantifying the bandwidth is radians per second. At low frequencies, the output stays in phase with the input and the magnitude remains constant, Figure 3. However, as th input frequency increases, the output lags the input; and the output magnitude decreases.




Figure 3-Typical response (bandwidth)curves for an ac vector contoller.

Total system response depends on how fast the motor shaft accelerates after a step change to the controller input. The units are usually expressed in radians per sec squared. Servo motors usually accelerate at 600 to 5000 rad/sec squared depending on the motor rating and design.

Typically, a 50 hp motor ac motor connected across the line requires 2.5 sec to accelerate from standstill to 1800 rpm. With AC vector control, the same AC motor will reach 1800 rpm in less than 0.3 sec.

Additional control functions
Industry demands more than drives with fast response. For example, machine reliability is greatly increased if the jerk or G factors are kept to a minimum, Figure 4. Thus, the load acceleration pattern is as equally important as the time it takes for the acceleration phase.




Figure 4-Acceleration profiles with and without jerk control.

Another feature needed for increased machine uptime is diagnostic information. With a microprocessor, the controller can check itself and give warnings.

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DC servo motors
Two basic types of dc servo systems have been in use for several years: conventional dc and dc brushless.

DC servo motors
Long the mainstay for servo systems, the dc motor offers good basic operation, and is frequently the most economical. Conventional dc motors are built with a stationary magnetic field surrounding the armature windings. This stationary magnetic field is produced by permanent magnets, or field windings, mounted just inside the motor frame.

Current is applied to the armature windings through brushes and a commutator, this combination keeps the armature magnetic field perpendicular to the stationary field. This, in turn, develops a torque on the armature causing it to rotate.

The armature current is proportional to the motor torque. This linear relationship plus the fact that the current can be changed very quickly (typically in a few milliseconds), give dc motor systems high performance velocity and position-control capabilities.

Brushless dc motors
When compared to a dc motor, these motors appear to have an inside-out construction. Permanent magnets are mounted on the shaft and the windings are stationary. The power amplifier, rather than a brush and commutator assembly, electronically commutates the current in the stationary (stator) windings. However, the objective is the same as in dc brush-type motors-keep the two magnetic fields perpendicular to each other.

A transducer, mounted on the motor shaft, produces a rotor position feedback signal that the electronic commutation circuit uses to align magnetic fields properly.

Because the current is pulse width modulated and looks like ac to the motor, some manufacturers use the term ac brushless to describe these electronically commutated motors.

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AC induction motors and vector control
The main prime movers of industry, three-phase ac induction motors are built with stationary (stator) windings. Applying three-phase power to these windings produces a rotating magnetic field. This rotating field serves two purposes. It induces current flow in the squirrel-cage rotor, which in turn, produces a magnetic field in the rotor. (This field serves the same purpose as the permanent magnetic field of a brushless dc motor.)

The second function of the rotating magnetic field is to interact with the induced rotor field and produce a torque, which turns the rotor.

In general purpose applications, each of the three ac phases is 120 electrical degrees apart. The motor operates with typical characteristics: It operates slightly below synchronous speed with light loads; and when fully loaded, the speed slips to 3-5% below synchronous speed.

If an adjustable frequency controller (inverter) is installed, the motor speed can be adjusted by varying the applied voltage and frequency. While this is an added feature, it does not offer the fast response required for servo applications.

To achieve the needed response as well as controlling speed, three quantities of the electrical power are varied to the ac motor:

Frequency, which determines the motor speed
Voltage to keep the volts per Hertz constant (this relationship must be maintained to prevent overheating the motor, yet it must be large enough to enable the motor to develop sufficient torque)
Phase relationships (vectors) between the three electrical phases.



By controlling these vectors, the two magnetic field components-magnetizing and torque producing-are in turn controlled. This method operates the motor with the response needed for servo applications, typically acceleration rates of more than 600 rad/sec squared.




The torque producing component can be changed quickly, just as current through a dc armature. However, the field producing component varies slowly in response to changes in its command and is similar in characteristics to the wound field in a dc motor.