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Single-phase motors are used almost exclusively in residential applications and to operate loads that require fractional horsepower motors in industrial and commercial locations. Many of these motors you will recognize from everyday life and may have wondered how they work. Unlike three-phase motors, there are many different types of single-phase motors and they don’t all operate on the same principle.
There are some that operate on the principle of a rotating magnetic field, but others do not. Some single-phase motors are designed to operate at more than one speed. This unit…
Although most of the large motors used in industry are three phase, at times single-phase motors must be used. Single-phase motors are used almost exclusively to operate home appliances such as air conditioners, refrigerators, well pumps, and fans. They are generally designed to operate on 120 volts or 240 volts. They range in size from fractional horsepower to several horsepower, depending on the application.
Previously we stated that there are three basic types of three-phase motors and that all operate on the principle of a rotating magnetic field. Although that's true for three-phase motors, it’s not true for single-phase motors. There are not only many different types of single-phase motors, but they also have different operating principles.
++++ A two-phase alternator produces voltages that are 90 dgr out of phase with each other.
++++A Stator winding of a resistance-start induction-run motor. The start winding contains much smaller wire than the run winding. Start Winding; Run Winding
++++B Stator winding of a capacitor-start capacitor-run motor. The wire size is the same for both start and run windings.
++++The start and run windings are connected in parallel with each other. Applied voltage; Starting current; Running current; Start winding; Run winding 40°
Split-phase motors fall into three general classifications:
1. The resistance-start induction-run motor
2. The capacitor-start induction-run motor
3. The capacitor-start capacitor-run motor
Although all these motors have different operating characteristics, they are similar in construction and use the same operating principle. Split-phase motors receive their name from the manner in which they operate. Like three-phase motors, split-phase motors operate on the principle of a rotating magnetic field.
A rotating magnetic field, however, cannot be produced with only one phase.
Split-phase motors therefore split the current flow through two separate windings to simulate a two-phase power system. A rotating magnetic field can be produced with a two-phase system.
The Two-Phase System:
In some parts of the world, two-phase power is produced. A two-phase sys tem is produced by having an alternator with two sets of coils wound 90 dgr. apart. The voltages of a two-phase system are therefore 90 dgr. out of phase with each other. These two out-of-phase voltages can be used to produce a rotating magnetic field in a manner similar to that of producing a rotating magnetic field with the voltages of a three-phase system. Because there have to be two voltages or currents out of phase with each other to produce a rotating magnetic field, split-phase motors use two separate windings to create a phase difference between the currents in each of these windings. These motors literally split one phase and produce a second phase, hence the name split-phase motor.
Notice the difference in size and position of the two windings of the stator shown. The start winding is made from small wire and placed near the top of the stator core. This causes it to have a higher resistance than the run winding. The start winding is located between the poles of the run winding. The run winding is made with larger wire and placed near the bottom of the core. This gives it higher inductive reactance and less resistance than the start winding. These two windings are connected in parallel with each other.
The stator of a split-phase motor contains two separate windings, the start winding and the run winding. The start winding is made of small wire and is placed near the top of the stator core. The run winding is made of relatively large wire and is placed in the bottom of the stator core. Here are photos of two split-phase stators. The stator is used for a resistance-start induction-run motor or a capacitor-start induction-run motor. The stator is used for a capacitor-start capacitor-run motor. Both stators contain four poles, and the start winding is placed at a 90 dgr. angle from the run winding.
When power is applied to the stator, current flows through both windings. Because the start winding is more resistive, the current flow through it’s more in phase with the applied voltage than the current flow through the run winding. The current flow through the run winding lags the applied voltage due to inductive reactance. These two out-of-phase currents are used to create a rotating magnetic field in the stator. The speed of this rotating magnetic field is called synchronous speed and is determined by the same two factors that determined the synchronous speed for a three phase motor:
1. Number of stator poles per phase
2. Frequency of the applied voltage
++++4 Running current and starting current are 35 degrees to 40 dgr. out of phase with each other.
Applied voltage; Starting current; Running current 40°
++++5 A centrifugal switch is used to disconnect the start winding from the circuit. Centrifugal switch; Run winding; Start winding
Resistance-Start Induction-Run Motors
The resistance-start induction-run motor receives its name from the fact that the out-of-phase condition between start and run winding current is caused by the start winding being more resistive than the run winding. The amount of starting torque produced by a split-phase motor is determined by three factors:
1. The strength of the magnetic field of the stator
2. The strength of the magnetic field of the rotor
3. The phase angle difference between current in the start winding and current in the run winding (Maximum torque is produced when these two currents are 90 dgr out of phase with each other.)
Although these two currents are out of phase with each other, they are not 90 out of phase. The run winding is more inductive than the start winding, but it does have some resistance, which prevents the current from being 90 out of phase with the voltage. The start winding is more resistive than the run winding, but it does have some inductive reactance, preventing the current from being in phase with the applied voltage. Therefore, a phase angle difference of 35 degrees to 40 dgr. is produced between these two currents, resulting in a rather poor starting torque.
++++ The centrifugal switch is closed when the rotor is not turning.
++++ The contact opens when the rotor reaches about 75% of rated speed.
Spring-loaded weight; Closed contacts; Fiber washer; Spring-loaded weight; Fiber washer; Open contacts
Disconnecting the Start Winding:
A stator rotating magnetic field is necessary only to start the rotor turning. Once the rotor has accelerated to approximately 75% of rated speed, the start winding can be disconnected from the circuit and the motor will continue to operate with only the run winding energized. Motors that are not hermetically sealed (most refrigeration and air-conditioning compressors are hermetically sealed) use a centrifugal switch to disconnect the start windings from the circuit. The contacts of the centrifugal switch are connected in series with the start winding. The centrifugal switch contains a set of spring-loaded weights. When the shaft is not turning, the springs hold a fiber washer in contact with the movable contact of the switch. The fiber washer causes the movable contact to complete a circuit with a stationary contact.
When the rotor accelerates to about 75% of rated speed, centrifugal force causes the weights to overcome the force of the springs. The fiber washer retracts and permits the contacts to open and disconnect the start winding from the circuit. The start winding of this type motor is intended to be energized only during the period of time that the motor is actually starting. If the start winding is not disconnected, it will be damaged by excessive current flow.
++++Hot-wire relay connection.
M—Motor Start capacitor; Spring metal; Start-winding contact; Overload contact; Resistive wire; Spring; L2 L1
++++Hot-wire type of starting relay.
++++ Current type of starting relay.
++++11 Current relay connection. Thermostat; Start contact; Current relay coil
++++ Solid-state starting relay.
++++ Solid-state starting relay connection. S M Starting relay; Thermostat
Resistance-start induction-run and capacitor-start induction-run motors are sometimes hermetically sealed, such as with air-conditioning and refrigeration compressors. When these motors are hermetically sealed, a centrifugal switch cannot be used to disconnect the start winding. Some device that can be mounted externally must be used to disconnect the start windings from the circuit. Starting relays are used to perform this function. There are three basic types of starting relays used with the resistance-start and capacitor-start motors:
1. Hot-wire relay
2. Current relay
3. Solid-state starting relay
The hot-wire relay functions as both a starting relay and an overload relay.
In the circuit shown, it’s assumed that a thermostat controls the operation of the motor. When the thermostat closes, current flows through a resistive wire and two normally closed contacts connected to the start and run windings of the motor. The high starting current of the motor rapidly heats the resistive wire, causing it to expand. The expansion of the wire causes the spring-loaded start winding contact to open and disconnect the start winding from the circuit, reducing motor current. If the motor is not overloaded, the resistive wire never becomes hot enough to cause the overload contact to open and the motor continues to run. If the motor should become overloaded, however, the resistive wire expands enough to open the overload contact and disconnect the motor from the line. A hot-wire starting relay is shown.
The current relay also operates by sensing the amount of current flow in the circuit. This type of relay operates on the principle of a magnetic field in stead of expanding metal. The current relay contains a coil with a few turns of large wire and a set of normally open contacts. The coil of the relay is connected in series with the run winding of the motor, and the contacts are connected in series with the start winding. When the thermostat contact closes, power is applied to the run winding of the motor.
Because the start winding is open, the motor cannot start, causing a high cur rent to flow in the run winding circuit. This high current flow produces a strong magnetic field in the coil of the relay, causing the normally open contacts to close and connect the start winding to the circuit. When the motor starts, the run-winding current is greatly reduced, permitting the start contacts to reopen and disconnect the start winding from the circuit.
The solid-state starting relay performs the same basic function as the current relay and in many cases is replacing both the current relay and the centrifugal switch. The solid-state starting relay is generally more reliable and less expensive than the current relay or the centrifugal switch. The solid-state starting relay is actually an electronic component known as a thermistor. A thermistor is a device that exhibits a change of resistance with a change of temperature.
This particular thermistor has a positive coefficient of temperature, which means that when its temperature increases, its resistance increases also. The schematic diagram illustrates the connection of the solid-state starting relay.
The thermistor is connected in series with the start winding of the motor. When the motor is not in operation, the thermistor is at a low temperature and its resistance is low, typically 3 or 4 ohms. When the thermostat contact closes, cur rent flows to both the run and start windings of the motor. The current flowing through the thermistor causes an increase in temperature. This increased temperature causes the resistance of the thermistor to suddenly change to a high value of several thousand ohms. The change of temperature is so sudden that it has the effect of opening a set of contacts. Although the start winding is never completely disconnected from the power-line, the amount of current flow though it’s very small, typically 0.03 to 0.05 amperes, and does not affect the operation of the motor. This small amount of leakage current maintains the temperature of the thermistor and prevents it from returning to a low value of resistance. After the motor is disconnected from the powerline, a cooldown time of two to three minutes should be allowed to permit the thermistor to return to a low resistance before the motor is restarted.
++++14 Squirrel-cage rotor used in a split-phase motor.
Relationship of Stator and Rotor Fields:
The split-phase motor contains a squirrel-cage rotor very similar to those used with three-phase squirrel-cage motors. When power is connected to the stator windings, the rotating magnetic field induces a voltage into the bars of the squirrel-cage rotor. The induced voltage causes current to flow in the rotor, and a magnetic field is produced around the rotor bars. The magnetic field of the rotor is attracted to the stator field, and the rotor begins to turn in the direction of the rotating magnetic field. After the centrifugal switch opens, only the run winding induces voltage into the rotor. This induced volt age is in phase with the stator current. The inductive reactance of the rotor is high, causing the rotor current to be almost 90 dgr. out of phase with the induced voltage. This causes the pulsating magnetic field of the rotor to lag the pulsating magnetic field of the stator by 90 dgr.. Magnetic poles, located midway between the stator poles, are created in the rotor. These two pulsating magnetic fields produce a rotating magnetic field of their own, and the rotor continues to rotate.
++++15 A rotating magnetic field is produced by the stator and rotor flux.
++++16 An AC electrolytic capacitor is connected in series with the start winding.
Run winding Start winding Centrifugal switch AC electrolytic capacitor
++++17 Run-winding current and start-winding current are 90 dgr. out of phase with each other. Applied voltage; Running current Starting current 90°
Direction of Rotation:
The direction of rotation for the motor is determined by the direction of rotation of the rotating magnetic field created by the run and start windings when the motor is first started. The direction of motor rotation can be changed by reversing the connection of either the start winding or the run winding, but not both. If the start winding is disconnected, the motor can be operated in either direction by manually turning the rotor shaft in the desired direction of rotation.
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