Single -phase induction motors are widely used in homes and businesses because most appliances and utilities operate on single-phase AC power. However, single-phase induction motors are larger and less efficient than three-phase motors for the same horsepower rating.
Single-phase induction motors are not self-starting, unlike three-phase motors. This is because the stator of a single-phase induction motor produces only a pulsating, stationary field when the rotor is not turning. To start a single-phase induction motor, an auxiliary means of starting is required, such as a capacitor or centrifugal switch.
Once a single-phase induction motor is running, it develops a rotating magnetic field. This rotating field induces currents in the rotor, which cause the rotor to turn.
Here is a simplified explanation of how a single-phase induction motor works:
- When the motor is first turned on, the stator produces a pulsating, stationary magnetic field.
- This pulsating field induces currents in the rotor.
- The currents in the rotor create a magnetic field that interacts with the stator field.
- The interaction between the stator and rotor fields causes the rotor to start turning.
- Once the rotor is turning, it develops its own rotating magnetic field.
- The rotating magnetic field of the rotor induces currents in the stator.
- The currents in the stator create a magnetic field that interacts with the rotor field.
- The interaction between the stator and rotor fields keeps the rotor turning.
Single-phase induction motors are used in a wide variety of applications, including:
- Fans
- Blowers
- Pumps
- Compressors
- Refrigerators
- Washing machines
- Dryers
- Air conditioners
- Vacuum cleaners
- Power tools
Single-phase induction motors are a reliable and efficient way to convert electrical energy into mechanical energy. They are also relatively inexpensive and easy to maintain.
Split-Phase Motor
A single-phase split-phase induction motor is equipped with a squirrel-cage rotor that is similar to that of a three-phase motor. To create a rotating magnetic field, the single-phase current is divided into two windings: the main running winding and an auxiliary starting winding, which is positioned 90 electrical degrees apart from the running winding in the stator. A switch, which can be centrifugally or electrically operated, is connected in series with the starting winding to disconnect it when the starting speed reaches about 75 percent of full-load speed.
Phase displacement in a capacitor start motor is achieved by the difference in inductive reactance between the start and run windings, as well as their physical displacement in the stator. The start winding has fewer turns of smaller-diameter wire, which gives it a lower inductive reactance than the run winding. The run winding has many turns of larger-diameter wire, which gives it a higher inductive reactance. The start winding is also wound on the top of the stator slots, while the run winding is wound on the bottom of the stator slots. This physical displacement also contributes to the phase difference between the two windings.
When the motor is first turned on, the capacitor is connected in series with the start winding. This creates a circuit with a high inductive reactance and a low capacitive reactance. The inductive reactance of the start winding creates a lagging current, while the capacitive reactance of the capacitor creates a leading current. The difference in phase between the current in the start winding and the current in the run winding creates a rotating magnetic field. This rotating magnetic field starts the rotor turning.
Once the rotor is turning, the capacitor is disconnected from the circuit. The start winding is no longer needed, and the motor continues to run on the run winding.
The phase displacement between the start and run windings is essential for the operation of a capacitor start motor. Without the phase displacement, the rotating magnetic field would not be created and the motor would not start.
The way in which the two windings of a split-phase motor produce a rotating magnetic field is illustrated in Figure 5-50 and can be summarized as follows.
- When AC line voltage is applied, the current in the starting winding leads the current in the running winding by approximately 45 electrical degrees.
- Since the magnetism produced by these currents follows the same wave pattern, the two sine waves can be thought of as the waveforms of the electromagnetism produced by the two windings.
- As the alternations in current (and magnetism) continue, the position of the north and south poles changes in what appears to be a clockwise rotation.
- At the same time the rotating field cuts the squirrel-cage conductors of the rotor and induces a current in them.
- This current creates magnetic poles in the rotor, which interact with the poles of the stator rotating magnetic field to produce motor torque.
- A split-phase motor has two windings: a start winding and a run winding. The start winding has fewer turns of smaller-diameter wire than the run winding. This gives the start winding a lower inductive reactance and a higher resistance than the run winding.
- When AC line voltage is applied to the motor, the current in the start winding leads the current in the run winding by approximately 45 electrical degrees. This is because the start winding has a lower inductive reactance than the run winding.
- The difference in phase between the current in the start winding and the current in the run winding creates a rotating magnetic field. This rotating magnetic field cuts the squirrel-cage conductors of the rotor and induces a current in them.
- The current in the rotor creates magnetic poles in the rotor. These poles interact with the poles of the stator rotating magnetic field to produce motor torque.
If the centrifugal switch fails to operate properly, it can cause problems. If the switch fails to close when the motor stops, the starting winding circuit will be open. As a result, when the motor circuit is again energized, the motor will not turn but will simply produce a low humming sound. The starting winding is typically designed for operation across line voltage for only a short interval during starting. If the centrifugal switch fails to open within a few seconds of starting, it may cause the starting winding to char or burn out.
Split-phase induction motors are the simplest and most common type of single-phase motor. Their simple design makes them typically less expensive than other single-phase motor types. Split-phase motors are considered to have low or moderate starting torque. Typical sizes range up to about ½ horsepower. Reversing the leads to either the start or run windings, but not to both, changes the direction of rotation of a split-phase motor. Popular applications of split-phase motors include fans, blowers, office machines, and tools such as small saws or drill presses where the load is applied after the motor has obtained its operating speed.
Dual-voltage split-phase motors have leads that allow external connection for different line voltages. A NEMA-standard single-phase motor with dual-voltage run windings is shown in Figure 5-52. When the motor is operated at low voltage, the two run windings and the start winding are all connected in parallel. For high-voltage operation, the two run windings connect in series and the start winding is connected in parallel with one of the run windings.
Split-Phase Capacitor Motor
The capacitor-start motor is a modified split-phase motor that uses a capacitor to increase the starting torque and reduce the starting current. The capacitor creates a phase shift of approximately 80 degrees between the start and run windings, which is substantially higher than the 45 degrees of a split-phase motor. This results in more than double the starting torque with one-third less starting current. Like the split-phase motor, the capacitor start motor also has a starting mechanism that disconnects the start winding and capacitor when the motor reaches about 75 percent of rated speed.
In simpler terms, the capacitor-start motor is a more powerful split-phase motor that is better suited for applications where a high starting torque is required, such as compressors, pumps, and refrigerators. It is also a good choice for applications where the motor is frequently started and stopped, such as air conditioners and washing machines.
The capacitor-start motor is a popular choice for a wide variety of applications because it is relatively inexpensive, easy to maintain, and reliable. It is also a good choice for applications where energy efficiency is a concern, as it uses less starting current than a split-phase motor.
The capacitor-start motor is more expensive than a split-phase motor because of the cost of the capacitor. However, it is more versatile because of its higher starting torque and lower starting current. The capacitor is only in the circuit for a few seconds at the start of the motor, and its purpose is to improve the starting torque, not the power factor.
A faulty capacitor can cause problems with the motor. A short-circuited capacitor will cause too much current to flow through the start winding, while an open capacitor will prevent the motor from starting.
Dual-speed capacitor-start motors have two sets of start and run windings, which allows them to operate at two different speeds. For low-speed operation, the eight-pole set of windings is connected to the source. For high-speed operation, the six-pole set of windings is connected to the source.
The permanent-capacitor motor is a type of single-phase motor that does not have a centrifugal switch or a capacitor solely for starting. Instead, it has a run-type capacitor that is permanently connected in series with the start winding. This makes the start winding an auxiliary winding once the motor reaches running speed. However, since the run capacitor must be designed for continuous use, it cannot provide the starting boost of the capacitor-start motor. Typical starting torques for permanent-capacitor motors are low, ranging from 30 to 150 percent of rated load, so these motors are not suitable for hard-to-start applications.
Permanent-capacitor motors are considered to be the most reliable of the single-phase motors because no starting switch is needed. The run and auxiliary windings are identical in this type of motor, allowing for the motor to be reversed by switching the capacitor from one winding to the other. Single-phase motors run in the direction in which they are started, so whichever winding has the capacitor connected to it will control the direction. Permanent split-capacitor motors have a wide variety of applications that include fans, blowers with low starting torque needs, and intermittent cycling uses such as adjusting mechanisms, gate operators, and garage door openers, many of which also need instant reversing. Since the capacitor is used at all times, it also provides an improvement in motor power factor.
The capacitor-start/capacitor-run (CSCR) motor is a type of single-phase motor that uses two capacitors to improve its starting torque and running performance. The two capacitors are connected in parallel at startup to provide a large amount of capacitance, which creates a strong rotating magnetic field. Once the motor is up to speed, the start capacitor is disconnected from the circuit and the run capacitor remains connected. The run capacitor helps to improve the motor’s power factor and efficiency.
Here is a simplified explanation of the CSCR motor:
- It uses two capacitors: a start capacitor and a run capacitor.
- The start capacitor is connected in parallel with the run capacitor at startup to provide a large amount of capacitance and starting torque.
- Once the motor is up to speed, the start capacitor is disconnected from the circuit and the run capacitor remains connected.
- The run capacitor helps to improve the motor’s power factor and efficiency.
- The start capacitor is typically an electrolytic type, while the run capacitor is an oil-filled type.
Capacitor-start/capacitor-run (CSCR) motors are a type of single-phase motor that offers several advantages over other types of single-phase motors, including:
- Lower full-load currents
- Higher efficiency
- Lower operating temperature
- Longer lifespan
CSCR motors are more expensive than other types of single-phase motors, but this is due to the additional cost of the capacitors and starting switch. However, the benefits of CSCR motors often outweigh the costs, making them a good choice for a wide range of applications, including:
- Woodworking machinery
- Air compressors
- High-pressure water pumps
- Vacuum pumps
- Other high-torque applications
Shaded-Pole Motor
Shaded-pole motors are a type of single-phase motor that is simpler and less expensive than other types of single-phase motors. They have only one main winding and no start winding or switch. Shaded-pole motors produce a low starting torque, but they are reliable and efficient. Shaded-pole motors start by using a copper loop around a small portion of each motor pole. This copper loop delays the phase of magnetic flux in that part of the pole enough to provide a rotating field.
The direction of rotation of a shaded-pole motor is not normally reversible, but some shaded-pole motors are wound with two main windings that can reverse the direction of the field. Shaded-pole motors are not susceptible to slip because the current in the stator is not controlled by a countervoltage determined by rotor speed. This means that the speed of a shaded-pole motor can be controlled by varying the voltage or by using a multitap winding.
Shaded-pole motors are commonly used in applications where a low starting torque is acceptable, such as fans, blowers, and small pumps. They are also used in applications where speed control is important, such as record players and tape recorders.
Advantages:
- Simple and inexpensive
- Reliable and efficient
- Speed can be controlled by varying the voltage or using a multitap winding
Disadvantages:
- Low starting torque
- Direction of rotation is not normally reversible
Universal Motor
The universal motor is a type of electric motor that can be operated on either direct current (DC) or single-phase alternating current (AC). It is constructed like a series-type DC motor, with a wound series field on the stator and a wound armature on the rotor. The armature and field coils are connected in series.
Universal motors are commonly used in household appliances and portable hand tools because they can easily exceed one revolution per cycle of the main current. This makes them useful for applications where high-speed operation is desired, such as blenders, vacuum cleaners, and hair dryers.
The speed and direction of rotation of a universal motor can be controlled by varying the voltage applied to the motor and by reversing the current flow through the armature with respect to the series field.
Here is a simplified explanation of the universal motor:
- It can be operated on either DC or AC power.
- It has a high starting torque and can easily exceed one revolution per cycle of the main current.
- It is commonly used in household appliances and portable hand tools.
- Its speed and direction of rotation can be controlled by varying the voltage applied to the motor and by reversing the current flow through the armature with respect to the series field.
