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INDUSTRIAL CONTROL HANDBOOK - 2.6 CONSTRUCTION OF ELECTRIC MOTORS

Electric motors, being devices to convert electricity to rotary motion, must all be constructed alike to a certain extent. Figure 2-15 shows the construction and some of the terminology used in describing the construction of various types of electric motors.

All electric motors have a central rotating section called a rotor or an armature. A rotor usually does not require connection to the power supply. It may contain conductors, permanent magnets, or alloy metals selected for their magnetic properties. Some rotors have copper windings that are connected to the current supply through slip rings. An armature is similar to a rotor except that it includes windings of copper wire to which electric current is supplied via a commutator.

Slip rings are simple electric connections. The inner rings of the slip ring pair can rotate inside the stationary outer rings without loss of the electrical connection. The slip rings do not affect the direction of current flow through the rotor. A commutator, on the other hand, is intended to control the direction that current flows through the armature. A motor with an armature also includes carbon brushes, mounted in the housing, that conduct electricity to the portions of the rotating commutator that they are bearing on at that time. We will see why control of current direction in the armature is important later, in the section on theory of operation of electric motors. Some motors have neither slip rings nor commutators.

The rotor or armature is supported on bearings in the housing of the motor. Radial roller element bearings are common, and these may require periodic lubrication on larger motors. Many very small motors now come with oil-filled bronze bushings instead of roller bearings. Some heavy duty motors come with thrust bearings as well as radial bearings.

The stationary housing of the motor (hence stator) provides the magnetic field essential to the operation of the motor. The field may be provided by permanent magnets or by electromagnets. If the field is electromagnetic, the field strength can be varied. In some motors, the orientation of the magnetic field can be controlled. Later sections explain the effect of the field.

All electric motors require electrical power to operate. Some motors need only to be connected to 120 Volt, 60 Hz AC. Others require only simple DC power supplies. More sophisticated power supplies, often called motor controllers, are needed if:

  • The motor is large, and it requires gradual starting or stopping to avoid damaging surges of current.
  • The motor torque, speed, or shaft position must be controlled.

Figure 2-15 Construction of an electric motor.

Figure 2-15 Construction of an electric motor. (Photograph by permission, Pittman, A Division of Penn Engineering & Manufacturing Corp., Harleysville, Pennsylvania.)

Motor controllers, usually purchased with the motor, may convert AC to DC. They may allow control of DC power, or of AC frequency, or may simply consist of switches that open and close in a timed sequence.

In an automated system, a command signal to the motor controller can be used to specify the motor speed or shaft position. The command signal may be an analog DC signal from a PLC (programmable controller) or robot controller in the same workcell as the motor, or from a cell controller in the larger Flexible Manufacturing System. In a Computer Integrated Manufacturing (CIM) system, the command signal might originate with the plant host computer and arrive as a digital value via a Local Area Network (LAN). The motor controller would require a communications interface to receive LAN messages.

GO TO NEXT PAGE: INDUSTRIAL CONTROL HANDBOOK - 2.7 THEORY OF OPERATION OF ELECTRIC MOTORS

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