Types of brushed DC motors

DC motors have inherently straightforward operating characteristics, flexible performance, and high efficiency; these factors together with the long development history have resulted in brushed d.c. motors being used as a standard within many industrial applications.

Brushed d.c. permanent-magnet motors can be obtained commercially in the following forms:

  • Ironless-rotor motors.

  • Iron-rotor motors.

  • Torque motors.

  • Printed-circuit motors

Ironless-rotor motors

There are three elements: the rotor, the magnet assembly, and the brush assembly. The rotor is constructed as a self-supporting basket, with the conductors laid in a skewed fashion to minimise torque ripple and to maximise the mechanical strength. The conductors are bonded to each other and to an end disc or commutator plate (which supports the coil 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.

Iron-rotor motors:

Permanent-magnet iron-rotor motors have evolved directly from wound-rotor designs and the design has been refined for servo applications. Due to the location of the magnets and the large air gap which is required, these motors tend to be relatively long with a small rotor diameter; this ensures that the motor’s inertia is minimised.

The manufacturers of these motors provide features that are designed to ensure ease of application; these features include the provision of integral tachogenerators, encoders, brakes, and fans, together with thermal trip indicators within the rotor windings.

Torque motors:

The operation of a torque motor is no different to 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 conventional 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 frequency 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 performance, does require particular care in the design and fabrication of the system.

Printed-circuit motors

The magnetic-flux path has been radial in the motors considered so far. This results in machines that are typically long and thin, with the actual size depending on their output power. However, the magnetic field is axial within a printed-circuit motor, leading to a very compact motor design.

The magnets are mounted on either side of the rotor, and the magnetic path is completed by the outer casing of the motor. The commutators are located towards the centre of the rotor, with the brushes located on the rear of the motor case.

The main constraint on the length of the motor is the size of the magnets. The motor design could use either low-power ferrite magnets (to give a short motor) or Alnico magnets (to give a longer, more powerful, motor). The use of neodymium iron- based magnetic materials has allowed high-power motors to be designed with minimum lengths.