Electrical Machines - Questions and Answers
 Information courtesy of ALSTOM.

DIRECT-CURRENT MOTORS

1. What are the two basic types of DC motor?

The shunt motor and the series motor. In the shunt motor, the field and armature windings are connected in parallel across the supply. In the series motor, both these windings are connected in series with the supply. The combination of shunt and series field windings result in a compound-wound motor.

2. Why is a commutator essential in a DC motor?

To reverse the currents in the armature coils as they pass from one pole system to the next so as to obtain a steady forward torque. If direct current were to be passed into the armature windings through slip rings, the result would be that after relation through a pole pitch, the current, being in the same direction, would develop a torque in the reverse relation and no continuous motion would be obtainable.

Fig. 56 - Shunt-motor, showing typical connections for windings and control gear.

Fig. 57 - Series motor, showing typical connections for windings and control gear.

3. On what does the speed of a DC motor depend?

The speed at which the armature revolves depends on the number of armature conductors in series and the total magnetic flux of all the poles in the machine. When the line voltage is applied to the armature, certain voltage drops occur due to the passage of the current through the brushes into the commutator, together with further voltage drops due to the circulation of the current around the armature coils.

Finally, with stable conditions, the armature must relate in its magnetic field at such a speed that a back-voltage is generated equal to the applied line voltage, less the voltage drops already mentioned.

4. What are the basic components in DC motor construction?

Fig. 58 - Main components of a DC motor.

  1. The magnetic frame, of cast steel or fabricated from mild-steel plate.
  2. Main field poles. Since the magnetic poles are stationary in space, only one coil is used per pole and each main field pole consists of laminations of electrical sheet steel riveted together. For large machines, the pole tips only may be laminated.
  3. Interpoles of mild steel, with their windings, are employed on all DC motors except the smallest.
  4. Laminated armature core built up on the shaft or rotor hub, with the windings distributed in slots equally spaced around the periphery of the armature.
  5. A commutator built up from thin segments of copper with strips of micanite to insulate the segments from each other. A moulded construction may be employed in very small motors.
  6. Brushes of carbon-copper composition supported in brush boxes and spring loaded.

5. Why are interpoles used?

lnterpoles are fitted between the main poles except in the very smallest machines. As an armature coil passes through the space between the main poles, it undergoes commutation - the two ends of the coil are short-circuited by the brush before current is reversed in the coil.

The dying away of the magnetic flux surrounding the coil conductors and its rise in the opposite direction causes a self-induced voltage in the coil which delays current reversal. This delay leads to bad commutation, indicated by sparking at the trailing edge of the brushes.

Poor commutation can be reduced to some extent by moving the brushes from direction of rotation and by using brushes having higher contact resistance. The delay in current reversal can be corrected by inducing into the shorted coils a voltage in opposition to the self-induced voltage. This is the function of the interpoles.

Fig. 59- Simple 4-pole 12-slot lap armature winding.
Two-layer winding, the lower coil sides being indicated by dotted lines.

Fig. 60 - Simple 4-pole 13-slot two-layer wave armature winding.

Fig. 61 - Relative position of main poles, interpoles, armature coils and brushes.
The polarity of the interpoles is the same as that of the preceding main pole.

Fig. 62 - Armature coil undergoing commutation.

6. How are interpole windings connected?

In series with the armature windings and with a polarity of interpole opposite to that of the main pole following the interpole in the direction of rotation (i.e. the interpole has the same polarity as the preceding main pale against the direction of relation).