Motors: Electromagnets That Produce Motion
Electric appliance motors vary greatly in size and appearance, ranging from the 8- inch motor in a clothes washer to the inch-wide motor of a timer. But their inner workings are similar: Two electromagnets one a movable magnet called the rotor or armature, one a stationary magnet called the stator or field interact to spin a shaft.
The motor most often found in an appliance is the all-purpose universal motor, so called because it operates on either alternating or direct current AC or DC. Because it is so versatile, the universal motor is also the most mechanically intricate and thus is vulnerable to minor breakdowns. Consequently, universal motors are made to be quickly disassembled for maintainance, testing and repair, and most of their parts are replaceable.
The universal motor is used where a small, fast, medium-strength motor is needed in a hand mixer or a vacuum cleaner, for example. To vary its speed, a universal motor is connected to a multipole switch, which taps selected sections of the motor's windings, sending current only to certain parts of the electromagnet and thus varying the strength of the magnetism produced.
A second type of electric motor relies on induction as its operating principle; this type includes both the most powerful and the smallest of the motors commonly found in appliances the split-phase motor and the shaded-pole motor. Induction motors run on alternating current. Consequently, they are mechanically simple and virtually indestructible; there is little you can do either to test or to repair the motor itself, though you can often test and replace certain of the starting components.
The split-phase motor is found in clothes washers, dryers, and the compressors of refrigerators and air conditioners. Split-phase motors.are so named because they incorporate a separate electromagnet; it contains the start windings, which provide starting torque an initial twist that turns the motor over.
This extra push is usually augmented by a capacitor a battery-like device that stores electricity, then releases it in a surge to start the motor. Capacitors can be tested and replaced. They are rated in microfarads (MFD); a new one should match or exceed the original rating.
After a split-phase motor has reached 80 per cent of its full speed, the start windings and their capacitor must be disengaged; otherwise the motor will run less efficiently. Two kinds of switches are used. One is a centrifugal switch, activated by the spinning of the motor shaft; the other is a relay that senses the drop in the amount of current being drawn as the motor reaches full speed.
Speed, in a split-phase motor, is also governed by a multipole switch; but the switch activates sets of paired windings, one set at a time.
The shaded-pole motor, a much smaller induction motor, is a simple, dependable motor that needs no special starting mechanisms. It is adequate only for light duty, such as powering small fans, and is usually a one-speed device. A shaded- pole motor is easy to test for continuity or resistance in its coils and is inexpensive to replace.
The most common problem in a universal motor is worn brushes, which fail to deliver current to the armature. When a brush has worn to such a point that it is shorter than it is wide, it should be replaced. Also, the brass bars in the commutator may become pitted or corroded. A light sanding with fine-grit sandpaper will correct this problem.
To test the condition of the field windings, set a multitester at RX1K and touch the probes to the field terminals. If the field windings are good, the multitester will read near 0 ohms; if they are defective, the multitester will register near infinity ohms. To test the connection between the armature windings and the commutator bars, touch the probes to pairs of adjacent bars, working around the entire cylinder, A good connection will show as 0 ohms; an infinity reading indicates a broken connection. Finally, check for a short circuit between the shaft and the armature windings: Touch one probe to the shaft and the other to one of the commutator bars; the meter should show no continuity.
The components of a split-phase motor are almost entirely enclosed in its housing. In many cases, an oblong dome fastened to the side of the barrel housing encases the capacitor. Located on the end of the housing opposite the motor shaft are at least three, and often as many as seven, terminals for the sets of windings within the motor.
Disassembled, this motor resembles a universal motor. But the armature-here called the rotor-consists only of an iron cylinder and is not connected to the source of electricity. Instead, current is induced in the rotor by the current in the field windings. In addition, there is a second layer of heavier-gauge windings surrounding the field windings that serves as the start windings for the motor.
The centrifugal switch that on some motors disconnects the start windings consists of two parts. One part is a weighted actuator connected to the motor shaft in front of the rotor. When the motor is off or is just starting, the actuator presses against the other part of the switch a pair of contact arms fastened to the rear wall of the motor housing; the actual contacts are located in the back of the arms. As the rotor starts to spin, centrifugal force swings the weights of the actuator outward, releasing the contact arms and disconnecting the start-windings circuit. If the switch contacts become corroded, clean them with an emery cloth. An internal switch must be tested by someone familiar with wiring diagrams. On some split-phase motors, a relay (/nset) rather than a centrifugal switch disconnects the start wihdings. Test the relay for continuity. To test a start capacitor, remove its housing and discharge the capacitor of stored current by touching the shaft of an insulated screwdriver across its two terminals. Then set a multitester at RX10, and connect it to the capacitor terminals; the meter should momentarily register 0 ohms and then read infinity resistance.
Test the field windings of a shaded-pole motor for continuity or resistance just as you would test a relay or a solenoid. For both tests, touch the multitester's two probes to the terminals, one on each side of the field windings.
The motor most often found in an appliance is the all-purpose universal motor, so called because it operates on either alternating or direct current AC or DC. Because it is so versatile, the universal motor is also the most mechanically intricate and thus is vulnerable to minor breakdowns. Consequently, universal motors are made to be quickly disassembled for maintainance, testing and repair, and most of their parts are replaceable.
The universal motor is used where a small, fast, medium-strength motor is needed in a hand mixer or a vacuum cleaner, for example. To vary its speed, a universal motor is connected to a multipole switch, which taps selected sections of the motor's windings, sending current only to certain parts of the electromagnet and thus varying the strength of the magnetism produced.
A second type of electric motor relies on induction as its operating principle; this type includes both the most powerful and the smallest of the motors commonly found in appliances the split-phase motor and the shaded-pole motor. Induction motors run on alternating current. Consequently, they are mechanically simple and virtually indestructible; there is little you can do either to test or to repair the motor itself, though you can often test and replace certain of the starting components.
The split-phase motor is found in clothes washers, dryers, and the compressors of refrigerators and air conditioners. Split-phase motors.are so named because they incorporate a separate electromagnet; it contains the start windings, which provide starting torque an initial twist that turns the motor over.
This extra push is usually augmented by a capacitor a battery-like device that stores electricity, then releases it in a surge to start the motor. Capacitors can be tested and replaced. They are rated in microfarads (MFD); a new one should match or exceed the original rating.
After a split-phase motor has reached 80 per cent of its full speed, the start windings and their capacitor must be disengaged; otherwise the motor will run less efficiently. Two kinds of switches are used. One is a centrifugal switch, activated by the spinning of the motor shaft; the other is a relay that senses the drop in the amount of current being drawn as the motor reaches full speed.
Speed, in a split-phase motor, is also governed by a multipole switch; but the switch activates sets of paired windings, one set at a time.
The shaded-pole motor, a much smaller induction motor, is a simple, dependable motor that needs no special starting mechanisms. It is adequate only for light duty, such as powering small fans, and is usually a one-speed device. A shaded- pole motor is easy to test for continuity or resistance in its coils and is inexpensive to replace.
Anatomy of a universal motor
Exposed copper windings and two spring-held brushes immediately identify a universal motor. The brushes, actually small bars of carbon, press against opposite sides of the commutator, a movable cylinder of brass bars insulated from each other by strips of mica. If the connecting bolts are removed, the motor can be separated into two sections. The outer section consists of a bundle of copper windings with a hollow core; this is the stationary field, or stator, of the motor. To it are attached the two brushes. The inner section, a second bundle of windings, is the rotating armature; it contains the motor shaft, a cooling impeller and the commutator. The armature 18 windings are magnetized by current flowing through the brushes to the commutator; the field windings are magnetized by current flowing directly from the power source. The interaction of the two magnetized groups of windings makes the motor shaft spin.
To test the condition of the field windings, set a multitester at RX1K and touch the probes to the field terminals. If the field windings are good, the multitester will read near 0 ohms; if they are defective, the multitester will register near infinity ohms. To test the connection between the armature windings and the commutator bars, touch the probes to pairs of adjacent bars, working around the entire cylinder, A good connection will show as 0 ohms; an infinity reading indicates a broken connection. Finally, check for a short circuit between the shaft and the armature windings: Touch one probe to the shaft and the other to one of the commutator bars; the meter should show no continuity.
A split-phase induction motor
The components of a split-phase motor are almost entirely enclosed in its housing. In many cases, an oblong dome fastened to the side of the barrel housing encases the capacitor. Located on the end of the housing opposite the motor shaft are at least three, and often as many as seven, terminals for the sets of windings within the motor.
Disassembled, this motor resembles a universal motor. But the armature-here called the rotor-consists only of an iron cylinder and is not connected to the source of electricity. Instead, current is induced in the rotor by the current in the field windings. In addition, there is a second layer of heavier-gauge windings surrounding the field windings that serves as the start windings for the motor.
A shaded-pole motor
Although shaded-pole motors take a number of shapes, they are usually recognizable by the presence of two pairs of diagonally placed heavy copper wires, called shades, which loop over a block of laminated-iron plates, called a stator. The shades provide the torque needed to start the motor. The stator is indented on one side for the field windings and has a hole to accommodate the rotor and shaft.
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