Servo Drive Performance Criteria also examples where the motor designer strives to minimize the

34.7.2 Servo Drive Performance Criteria also examples where the motor designer strives to minimize the

rotor inertia. Modern permanent magnets allow the required The performance of a servo drive can be expressed in terms airgap flux to be developed with a much reduced volume of of a number or factors such as servo bandwidth, accuracy, the magnets, consequently reducing the diameter of the rotor.

34 Motor Drives 961 It is well-known that the moment of inertia of a motor

increases as the fourth power of its outer radius! Another benefit of the modern permanent-magnet material is that the motor volume is also reduced. Servo motors often have to be located in a very confined space, and this reduction in volume is an important attribute.

The ironless designs mentioned earlier bring other bene- fits in the form of reduced inductance and cogging torque. Brushed pancake ironless motors are available with armature inductance as low as 100 µH.

From Section 34.2.2, the mechanical and electrical time constants of a brushed dc motor are given by

R a J τ m = mechanical time constant =

L a FIGURE 34.70 Pancake armature of a dc servo motor. Courtesy: Printed

and τ a = electrical time constant = s. R a Motors Ltd., UK.

It is well known that for the highest load acceleration, the load inertia referred to the motor should be equal to the rotor inertia. Thus, in a matched system, the total inertia the motor accelerates is twice its own inertia. In other words, the motor inertia should also be minimized.

For a good servo motor, the ratio between the mechanical and the electrical time constants is often of the order of five or more. This allows the speed and the current-control loops to

be decoupled and noninteracting. The electrical time constant of a motor determines how quickly the motor current may be changed and hence how quickly the torque can be changed. As also mentioned in Section 34.2, drives with a reasonable dynamic performance should have an inner torque loop. This torque loop is built around current loops, for the armature for the brushed dc motor, or for the d- and q-axes currents for the induction and synchronous motor drives. Having a low inductance in the winding allows these currents to be followed dynamically changing current or the torque references with higher accuracy and bandwidth.

The cogging torque, if appreciable, causes the rotor to have preferential positions. As a result, the position accuracy of the motor may suffer. Another problem is the ripple in speed as the motor is operated at low speed. At high speed, these ripples due to cogging torque may be filtered out by the motor inertia; however, the extra loss due to cogging remains. The ironless or toothless rotor obviously produce very small cogging torque because of the absence of preferential paths for the airgap flux to establish through the rotor iron of the brushed dc motor. The surface-magnet synchronous motor also has this feature. The interior-magnet motor normally has skewed stator slots to avoid the production of cogging torque.

Servo motors often operate with frequent start-and-stop duty, with the fastest allowable acceleration and deceleration

FIGURE 34.71

A disk rotor stepping motor with ironless rotor for low during which the motor current is allowed to reach about inertia and inductance. Courtesy: Escap Motors.

2–3 times the continuously rated current. The increased I 2 R

962 M. F. Rahman et al. loss in such duty must be dissipated. This calls for adequate synchronous motor) reference frame, have transformed the

cooling measures to be incorporated in the motor housing. prospects of ac motor drives in servo applications. With such operation, it is sometimes possible to excite the

Because of the fast dynamic response requirement of servo mechanical resonance due to shaft compliance. This is avoided drives, the servo motor is nearly always driven with the through proper arrangement of the shaft position/speed sensor maximum torque per ampere (MTPA) characteristic. Field and the coupling between the motor and the sensor. A belt- weakening is normally not used. In other words, field con- driven speed sensor may be acceptable for an industrial drive; trol either directly for a brushed dc motor or a synchronous however, for servo applications, a rigid, direct-coupled sensor motor or indirectly through armature reaction (i.e. through i d mounted as close as possible to the motor armature is prefer- current control) for induction or PM ac synchronous motors able. Additionally, the speed sensor is also required to have is not used for field weakening. It is nevertheless used for regu- negligible noise. Speed signal from analog tachogenerators, lating the field at the desired level. Field weakening are mainly which were used for speed sensing until recently, invariably used for drives where the operation at higher than base speed needed to be filtered to remove the cyclic ripple/noise that with constant-power characteristic is desirable. existed. Such filtering often limits the maximum speed-control bandwidth of a drive.