BRUSHLESS SERVOMOTORS BASIC INFORMATION

A synchronous machine with permanent magnets on the rotor is the heart of the modern brushless servomotor drive. The motor stays in synchronism with the frequency of supply, though there is a limit to the maximum torque which can be developed before the rotor is forced out of synchronism, pull-out torque being typically between 1.5 and 4 times the continuously rated torque.

The torque–speed curve is therefore simply a vertical line. The industrial application of brushless servomotors has grown significantly for the following reasons:

● reduction of price of power conversion products

● establishment of advanced control of PWM inverters

● development of new, more powerful and easier to use permanent magnet materials

● the developing need for highly accurate position control

● the manufacture of all these components in a very compact form

They are, in principle, easy to control because the torque is generated in proportion to the current. In addition, they have high efficiency, and high dynamic responses can be achieved.

Brushless servomotors are often called brushless dc servomotors because their structure is different from that of dc servomotors. They rectify current by means of transistor switching within the associated drive or amplifier, instead of a commutator as used in dc servomotors.

Confusingly, they are also called ac servomotors because brushless servomotors of the synchronous type (with a permanent magnet rotor) detect the position of the rotational magnetic field to control the three-phase current of the armature. It is now widely recognized that brushless ac refers to a motor with a sinusoidal stator winding distribution which is designed for use on a sinusoidal or PWM inverter supply voltage.

Brushless dc refers to a motor with a trapezoidal stator winding distribution which is designed for use on a square wave or block commutation inverter supply voltage.

The brushless servomotor lacks the commutator of the dc motor, and has a device (the drive, sometimes referred to as the amplifier) for making the current flow according to the rotor position. In the dc motor, increasing the number of commutator segments reduces torque variation.

In the brushless motor, torque variation is reduced by making the coil three-phase and, in the steady state, by controlling the current of each phase into a sine wave.

DC SERVOMOTORS AND TRACTION MOTORS BASIC TUTORIALS

DC servomotors are high-performance motors normally used as prime movers in computers, numerically controlled machinery, or other applications where starts and stops must be made quickly and accurately.

Servomotors have lightweight, low-inertia armatures that respond quickly to excitation-voltage changes. In addition, very low armature inductance in these motors results in a low electrical time constant (typically 0.05 to 1.5 ms) that further sharpens motor response to command signals.

Servomotors include permanent-magnet, printed-circuit, and moving-coil (or shell) motors. The rotor of a shell motor consists of a cylindrical shell of copper or aluminum wire coils. The wire rotates in a magnetic field in the annular space between magnetic pole pieces and a stationary iron core.

The field is provided by cast Alnico magnets whose magnetic axis is radial. The motor may have 2, 4, or 6 poles.

Each of these basic types has its own characteristics, such as inertia, physical shape, cost, shaft resonance, shaft configuration, speed, and weight. Although these motors have similar torque ratings, their physical and electrical constants vary considerably.

The choice of a motor may be as simple as fitting one into the space available. However, this is generally not the case since most servosystems are very complex.

DC Traction Motors. These are dc series motors typically rated 140 hp, 310 V, 2500 r/min. Four motors are used in each transit car, two on each axle.

The power supply is 600 to 1000 V dc from the third rail, which is powered by 2500- to 5000-kW rectifier sets in rectifier substations located along the track. Starting and speed control are by either a cam controller or a chopper controller on board the transit car.