Page 147 - Electric Drives and Electromechanical Systems

140 Electric Drives and Electromechanical Systems

motor is an actuator which can be directly attached to the load it drives. The motor’s
design is identical to a brushed motor and has a permanent magnet (PM) field and a
wound armature. The motor can be used in either in applications that require position or
speed-control systems. torque motors are designed for three different types of operation:

Speed control systems that require high torque at low speeds.
Applications that require torque control at high speed for positioning, rate, or
tensioning applications.
Applications that require a high stall torque for positioning applications.
Direct-drive torque motors are particularly suited for servo-system applications
where it is desirable to minimize size, weight, power and response time, and to maximize
rate and position accuracies.
The operation of a torque motor is no different from that of an iron rotor machine;
however, there are two significant constructional differences. Firstly, the number of
commutator segments and brush pairs is significantly greater than is found in a con-
ventional motor. The large motor diameters permit the use of a large number of
commutator segments, with two or more sets of brushes. This design allows a machine
to have a torque ripple which is considerably lower in magnitude and higher in fre-
quency than a conventional brushed motor, and, depending on the motor size, this can
be as low as 500 cycles per motor revolution at two per cent of the average output torque.
Secondly, since the torque motors need to be directly integrated into the mechanical
drive chain to maximise the stiffness, they are supplied as frameless machines (with the
rotor, stator, and brush gear being supplied as separate items) and they are directly built
into the mechanical system. This form of construction, while giving excellent perfor-
mance, does require particular care in the design and fabrication of the system. The
selection of a torque motor is no different from the selection of any other type of motor,
and a detailed consideration of the torques and the speed is required. Since the motor is
supplied as a frameless system, considerable care is required during the mechanical
design and installation. In particular, the air gap must be kept at a constant size by
minimising any eccentricity, and, during the installation, the stator’s magnets must not
be damaged or cause damage. Motor diameters in excess of 1 m are possible, and the
present level of technology allows torque motors to provide speeds as low as one
1
revolution in 40 days (1.17 10 5 rev min ) if a suitable drive system is used.
5.2.4 Printed-circuit motors

The magnetic-flux path has been radial in the motors considered so far, Fig. 5.4A. This
results in machines that are typically long and thin, with the actual size depending on
their output power. However, if the magnetic field flows along the motor’s axis as within
a printed-circuit motor, a very compact motor design results, Fig. 5.4B. The magnets are
mounted on either side of the rotor, and the magnetic path is completed by the outer
casing of the motor. To maintain rotation power the magnetic field direct is alternated,