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GOALS:
- List applications of DC motors.
- Describe the electrical characteristics of DC motors.
- Describe the field structure of a DC motor.
- Change the direction of rotation of a DC motor.
- Identify the series and shunt fields and the armature winding with an ohmmeter.
- Connect motor leads to form a series, shunt, or compound motor.
- Describe the difference between a differential and a cumulative compound motor.
Application
DC motors are used in applications where variable speed and strong torque are required. They are used for cranes and hoists when loads must be started slowly and accelerated quickly. DC motors are also used in printing presses, steel mills, pipe forming mills, and many other industrial applications where speed control is important.
Speed Control
The speed of a DC motor can be controlled by applying variable voltage to the armature or field. When full voltage is applied to both the armature and the field, the motor operates at its base or normal speed. When full voltage is applied to the field and reduced voltage is applied to the armature, the motor operates below normal speed. When full voltage is applied to the armature and reduced voltage is applied to the field, the motor operates above normal speed.
Motor Construction
The essential parts of a DC motor are the armature, field windings, brushes, and frame (Ill. 1).
The Armature
The armature is the rotating part of the motor. It is constructed from an iron cylinder that has slots cut into it. Wire is wound through the slots to form the windings. The ends of the windings are connected to the commutator, which consists of insulated cop per bars and is mounted on the same shaft as the windings. The windings and commutator together form the armature.
Carbon brushes, which press against the commutator segment, supply power to the armature from the DC power line. The commutator is a mechanical switch that forces current to flow through the armature windings in the same direction. This enables the polarity of the magnetic field produced in the armature to remain constant as it turns.
Armature resistance is kept low, generally less than 1 ohm. This is because the speed regulation of the motor is proportional to the armature resistance. The lower the armature resistance, the better the speed regulation will be. Where the brush leads extend out of the motor at the terminal box, they are labeled A1 and A2.
Field Windings
There are two types of field windings used in DC motors: series and shunt. The series field is made with a few turns of large wire. It has a low resistance and is designed to be connected in series with the armature.
The terminal markings, S1 and S2, identify the series field windings.
The shunt field winding is made with many turns of small wire. It has a high resistance and is designed to be connected in parallel with the armature. Since the shunt field is connected in parallel with the armature, line volt age is connected across it. The current through the shunt field is, therefore, limited by its resistance. The terminal markings for the shunt field are F1 and F2.
Ill. 1 DC motor, field structure, and armature assembly.
Ill. 2 DC motor connections.
Ill. 3 Series and shunt field windings are wound.
Identifying Windings
The windings of a DC motor can be identified with an ohmmeter. The shunt field winding can be identified by the fact that it has a high resistance as compared to the other two windings. The series field and armature windings have a very low resistance. They can be identified, however, by turning the motor shaft. When the ohmmeter is connected to the series field and the motor shaft is turned, the ohmmeter reading won't be affected. When the ohmmeter is connected to the armature winding and the motor shaft is turned, the reading will become erratic as the brushes make and break contact with different commutator segments.
Types of DC Motors
There are three basic types of DC motors: the series, the shunt, and the compound. The type of motor used is determined by the requirements of the load. The series motor, for example, can produce very high starting torque, but its speed regulation is poor. The only thing that limits the speed of a series motor is the amount of load connected to it. A very common application of a series motor is the starter motor used on automobiles.
Shunt and compound motors are used in applications where speed control is essential.
Ill. 2 shows the basic connections for series, shunt, and compound motors. Notice that the series motor contains only the series field connected in series with the armature. The shunt motor contains only the shunt field connected parallel to the armature. A rheostat is shown connected in series with the shunt field to pro vide above normal speed control.
The compound motor has both series and shunt field windings. Each pole piece in the motor will have both windings wound on it (Ill. 3). There are different ways of connecting compound motors. For instance, a motor can be connected as a long shunt compound or as a short shunt compound (Ill. 4). When a long shunt connection is made, the shunt field is connected parallel to both the armature and the series field. When a short shunt connection is made, the shunt field is connected parallel to the armature, but in series with the series field.
Compound motors can also be connected as cumulative or differential. When a motor is connected as a cumulative compound, the shunt and series fields are connected in such a manner that as current flows through the windings they aid each other in the production of magnetism (Ill. 5). When the motor is connected as a differential compound, the shunt and series field windings are connected in such a manner that as current flows through them they oppose each other in the production of magnetism (Ill. 6).
Ill. 4 Compound motor connections.
Ill. 5 Cumulative compound connection.
Ill. 6 Differential compound connection.
Ill. 7 Armature rotates in a clockwise direction.
Direction of Rotation
The direction of rotation of the armature is determined by the relationship of the polarity of the magnetic field of the armature to the polarity of the magnetic field of the pole pieces. Ill. 7 shows a motor connected in such a manner that the armature will rotate in a clockwise direction due to the attraction and repulsion of magnetic fields. If the input lines to the motor are re versed, the magnetic polarity of both the pole pieces and the armature will be reversed and the motor will continue to operate in the same direction (Ill. 8).
To reverse the direction of rotation of the armature, the magnetic polarity of the armature and the field must be changed in relation to each other. In Ill. 9, the armature leads have been changed, but the field leads have not. Notice that the attraction and repulsion of the magnetic fields now cause the armature to turn in a counterclockwise direction.
When the direction of rotation of a series or shunt motor is to be changed, either the field or the armature leads can be reversed. Many small DC shunt motors are reversed by reversing the connection of the shunt field leads. This is done because the current flow through the shunt field is much lower than the current flow through the armature. This permits a small switch, instead of a large solenoid switch, to be used as a reversing switch. Ill. 10 shows a double-pole, double throw (DPDT) switch used as a reversing switch. Power is connected to the common terminals of the switch and the stationary terminals are cross connected.
When a compound motor is to be reversed, only the armature leads are changed. If the motor is reversed by changing the shunt field leads, the motor will be changed from a cumulative compound motor to a differential compound motor. If this happens, the motor speed will drop sharply when load is added to the motor.
Ill. 11 shows a reversing circuit using magnetic contactors to change the direction of current flow through the armature. Notice that the direction of current flow through the series and shunt fields remains the same whether the F contacts or the R contacts are closed.
Ill. 8 changing input lines won't reverse the direction of rotation.
Ill. 9 When the armature leads are reversed, the direction of rotation
is changed.
Ill. 10 Double-pole, double-throw switch used to reverse the direction
of rotation of a shunt motor.
Ill. 11 Contactors reverse the direction of current flow through the
armature.
Standard Connections
When DC motors are wound, the terminal leads are marked in a standard manner. This permits the direction of rotation to be determined when the motor windings are connected. The direction of rotation is determined by facing the commutator end of the motor, which is generally located on the rear of the motor, but not always. Ill. 12 shows the standard connections for a series motor, Ill. 13 shows the standard connections for a shunt motor, and Ill. 14 shows the standard connections for a cumulative com pound motor.
Ill. 12 Standard connections for series motors.
Ill. 13 Standard connections for shunt motors.
Ill. 14 Standard connections for compound motors.
QUIZ:
1. How can a DC motor be made to operate below its normal speed?
2. Name the three basic types of DC motors.
3. Explain the physical difference between series field windings and shunt field windings.
4. The speed regulation of a DC motor is proportional to what?
5. What connection is made to form a long shunt compound motor?
6. Explain the difference between the connection of a cumulative compound and a differential compound motor.
7. How is the direction of rotation of a DC motor changed?
8. Why is it important to reverse only the armature leads when changing the rotation of a compound motor?