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.

MOTOR TORQUE DEFINITIONS BASIC AND TUTORIALS

The torques described in the following paragraphs are listed in the Standards. The minimum values are given in Table 20-1.

∗The torque values with other than rated voltage applied are approximately equal to the rated voltage values multiplied by the ratio of the actual voltage to rated voltage in the case of the pull-out torque, and multiplied by the square of this ratio in the case of the locked-rotor and pull-in torque. †With rated excitation current applied.

Locked-rotor torque is the minimum torque, which the synchronous motor will develop at rest for all angular positions of the rotor, with rated voltage at rated frequency applied.

Pull-in torque is the maximum constant-load torque under which the motor will pull into synchronism, at rated voltage and frequency, when its rated field current is applied. Whether the motor can pull the load into step from the slip running on the damper windings depends on the speed-torque character of the load and the total inertia of the revolving parts.

A typical relationship between maximum slip and percent of normal Wk2 for pulling into step is shown in Fig. 20-11. Table 20-1 specifies minimum values of pull-in torque with the motor loaded with normal Wk2; these values are given below. (See also Table 20-1.)

 

FIGURE 20-11 Typical relationship between load inertia and maximum slip for pulling synchronous motors into step.

Nominal pull-in torque is the value at 95% of synchronous speed, with rated voltage at rated frequency applied, when the motor is running on the damper windings.

Pull-out torque is the maximum sustained torque which the motor will develop at synchronous speed for 1 min, with rated voltage at rated frequency applied, and with rated field current.

In addition, the pull-up torque is defined as the minimum torque developed between standstill and the pull-in point. This torque must exceed the load torque by a sufficient margin to assure satisfactory acceleration of the load during starting.

The reluctance torque is a component of the total torque when the motor is operating synchronously. It results from the saliency of the poles and is a manifestation of the poles attempting to align themselves with the air-gap magnetic field. It can account for up to 30% of the pull-out torque.

The synchronous torque is the total steady-state torque available, with field excitation applied, to drive the motor and the load at synchronous speed. The maximum value as the motor is loaded is the pull-out torque, developed as a power angle A = 90 DEG.