Basics of the Electric Servomotor and Drive - Part 2: Permanent-Magnet Brush Motors

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[Part 1 of this article covers basic magnetics, definitions of the motor control system elements, and an overview of electric servomotors.]

15.5 Permanent-Magnet (PM) Brush Motors
Electric motors create torque with flux from two sources: the armature and the field. In permanent-magnet motors, the field flux (Φ_F) is created by magnets, as shown in Figure 15-12 for a four-pole brush motor. The magnetic path for the feld flux is the circular equivalent of the magnetic circuit of Figure 15-3, where the magnets provide F, the feld strength, and the circuit reluctance is the sum of the reluctances of the magnets, the back iron (steel on the outside of the stator), the rotor steel, and the air

Figure 15-12. Field flux path for four-pole permanent-magnet brush motor (armature windings, which are on the rotor, are not shown).

Figure 15-13. Armature flux path for a four-pole brush motor.

gap; it is necessary to have an air gap between rotor and stator to allow mechanical clearance for the moving rotor. Note that the rotor windings are not shown here, to simplify the drawing.

15.5.1 Creating the Winding Flux
The second flux that must be created for torque is that from the armature windings. Figure 15-13 shows the flux created from the armature windings (Φ_T)¹ in a four-pole brush motor. Follow the flux path and notice that flux travels from the rotor between the magnets, through the back iron, and then again between the magnets to return to the rotor. This is the proper path for flux to generate torque from the windings.

15.5.2 Commutation
The armature of a brush motor has many windings, and only some portion of those windings is excited with current when the motor is in any one position; the process of selecting the proper windings in a given rotor position is called commutation. Brush DC motors use mechanical commutation, so the drive needs no knowledge of motor position to regulate torque.

In brush motors, a mechanism of brushes contacting a commutator (see Figure 15-14) controls which windings are excited in any given position. Figure 15-13 shows only the windings that are excited in that position, assuming the commutation mechanism has selected the winding that yields maximum torque.

Figure 15-14. Partially disassembled brush motor showing commutator.

15.5.3 Torque Production
Electromagnetic torque is created by the interaction of the field flux and the armature flux. It is proportional to both according to Equation 15.7:

T_E Φ_T × Φ_F × sin(θ_E)                        (15.7)

where θ_E is the electrical angle between the field and armature flux. The four-pole motor of Figure 15-15 has the flux from Figures 15-12 and 15-13 overlain on one cross- sectional view. Only the flux in the air gap is shown. The angle between the two flux vectors in Figure 15-15 is 90° (electrical). This is equivalent to 45° (mechanical) for this four-pole motor.

Footnote:
1. The armature flux is often referred to as the "torque producing flux" and so is named Φ_T. This is a misnomer, in that torque is produced by the interaction of the two fluxes Φ_T and Φ_F.