Page 145 - Electric Drives and Electromechanical Systems

138 Electric Drives and Electromechanical Systems

and the commutator segments) by an epoxy resin. This form of construction produces a
rotor that is compact and of low weight and inertia. The motor is assembled around
a central permanent magnet, which supports the main motor bearings and the outer
housing. The outer housing protects the motor and it also acts as an integral part of the
magnetic circuit. The commutators are located on a plate attached to the rear of
the rotor, while the brush assembly is supported from the main housing. The brushes are
manufactured from precious-metal springs resulting in a low-contact resistance
throughout the motor’s life and they ensure that the motor will start when a very low
voltage is applied. Because of these design features, the ironless-rotor, d.c. brushed
machines are limited to powers of less than 100 W; however, high output speeds are
possible; and, depending on the motor type, speeds in excess of 10 000 rev min 1 are
available.
The selection of an ironless rotor motor for an application is, in principle, no different
than for any other type of motor; however, one important additional constraint is
imposed by the self-supporting nature of the rotor. If the power rating is exceeded, the
excessive rotor temperature will result in the degrading of the bonding medium, and the
winding will separate at high speed. This can be prevented by careful consideration of
both the thermal characteristics of the motor and its application requirements.
The power, P d , generated in the rotor is given by
2
P d ¼ I R a (5.2)
a
where I a is the root mean square armature current and R a the armature resistance. From
this value, the temperature rises of the rotor windings above the ambient temperature,
T r , can be calculated from,

T r ¼ P d ðRt r h þ Rt h a Þ (5.3)
where the thermal resistance from the rotor to the housing, Rt r h , and from the housing
to the ambient, Rt h a are obtained from the manufacturer’s data sheets. As long as the
rotor’s temperature is less than its specified maximum, their will be no reliability
problems. For a system designer, ironless rotor d.c. machines have a number of distinct
advantages including:
The speed-torque, voltage-speed, and load-current characteristics are linear over
the operational range of the motor.
Due to the uniform magnetic field and the relatively large number of commutator
segments there is no magnetic detent or preferred rotor position. In addition, this
form of construction results in minimal torque ripple over the motor’s speed
range.
The use of precious-metal brushes results in low brush friction, and hence a low
starting torque. The high quality of the contact between the brushes and the
commutator reduces the electromagnetic interference, EMI, and radio-frequency
interference, RFI, to a minimum.