Rotating Magnetic Field
The main difference between AC and DC motors is that the magnetic field generated by the stator rotates in the case of AC motors. This is achieved by applying a three-phase AC current to the stator windings. The three phases are 120 degrees apart, which creates a rotating magnetic field.
The rotor of an AC motor is made of a conductor, such as copper or aluminum. When the rotating magnetic field passes through the rotor, it induces an electric current in the rotor. This current creates its own magnetic field, which interacts with the stator’s magnetic field. The interaction of these two magnetic fields causes the rotor to turn.
The speed of an AC motor is determined by the frequency of the AC current and the number of poles in the motor. The frequency is the number of times the AC current changes direction per second. The number of poles is the number of times the rotating magnetic field completes a full rotation.
The synchronous speed of an AC motor is the speed at which the rotating magnetic field would rotate if there was no friction or other losses. The actual speed of the motor is always slightly less than the synchronous speed.
The nameplate of most AC motors lists the actual motor speed rather than the synchronous speed. This is because the actual speed is the speed at which the motor will operate under normal conditions.
- The rotating magnetic field in an AC motor is created by the interaction of the stator windings and the current flowing through them.
- The number of poles in an AC motor determines its synchronous speed. A motor with two poles has a synchronous speed that is twice the frequency of the AC current.
- The actual speed of an AC motor is always slightly less than its synchronous speed due to friction and other losses.
- The nameplate of most AC motors lists the actual motor speed rather than the synchronous speed.
The speed of the rotating magnetic field in an AC motor is determined by the frequency of the power supply and the number of poles in the motor. The frequency is the number of times the AC current changes direction per second, and the number of poles is the number of times the rotating magnetic field completes a full rotation.
The speed of the rotating magnetic field is directly proportional to the frequency and inversely proportional to the number of poles. This means that the higher the frequency, the faster the speed, and the greater the number of poles, the slower the speed.
S =120 f/P
Where:
- S = synchronous speed in rpm
- f = frequency, Hz, of the power supply
- P = number of poles wound in each of the single-phase
Induction Motor
The AC induction motor is the most commonly used motor because it is relatively simple and inexpensive to build. It is also very reliable and efficient. Induction motors are made in both three-phase and single-phase types.
The induction motor gets its name from the fact that no external voltage is applied to its rotor. The rotor windings are short-circuited, so the current in the rotor is induced by the rotating magnetic field of the stator. This interaction of the stator and rotor magnetic fields causes the rotor to turn.
The stator of an induction motor is made up of a series of coils that are arranged in a circle. The coils are connected to a three-phase or single-phase power supply, which creates a rotating magnetic field.
The rotor of an induction motor is made up of a series of metal bars that are connected by shorting rings. When the rotating magnetic field of the stator passes through the rotor, it induces a current in the rotor bars. This current creates its own magnetic field, which interacts with the stator’s magnetic field. The interaction of these two magnetic fields causes the rotor to turn.
The speed of an induction motor is determined by the frequency of the power supply and the number of poles in the motor. The frequency is the number of times the AC current changes direction per second, and the number of poles is the number of times the rotating magnetic field completes a full rotation.
The actual speed of an induction motor is always slightly less than its synchronous speed due to friction and other losses.
A three-phase motor stator winding consists of three separate groups of coils, called phases, which are displaced from each other by 120 electrical degrees. Each phase contains the same number of coils, and they are connected for the same number of poles. The number of poles in a stator winding is always an even number, and it refers to the total number of north and south poles per phase.
The coils in each phase can be connected in a star or delta configuration. In a star connection, the ends of the coils are connected together to form a star, and the beginning of each coil is connected to the power supply. In a delta connection, the ends of the coils are connected together to form a triangle, and the beginning of each coil is connected to the next coil in the sequence.
The connection of the coils in a three-phase motor stator winding determines its voltage rating. A star-connected winding has a lower voltage rating than a delta-connected winding.
Squirrel-Cage Induction Motor
The squirrel-cage rotor is the most common type of rotor used in induction motors. It is made up of a series of metal bars that are short-circuited by end rings. When the stator winding is energized, a rotating magnetic field is produced. This magnetic field induces a current in the rotor bars. The current in the rotor bars creates its own magnetic field, which interacts with the stator’s magnetic field to produce a torque. The torque causes the rotor to turn in the same direction as the rotating magnetic field.
The resistance of the squirrel-cage rotor has an important effect on the operation of the motor. A high-resistance rotor develops a high starting torque at low starting current. A low-resistance rotor develops low slip and high efficiency at full load.
NEMA (National Electrical Manufacturers Association) has standardized three types of squirrel-cage induction motors based on the rotor resistance:
- NEMA Design B: This is the most common type of squirrel-cage induction motor. It has a moderate starting torque and low slip at full load. It is suitable for a broad variety of applications, such as fans and blowers.
- NEMA Design C: This type of squirrel-cage induction motor has a higher rotor resistance than NEMA Design B. This results in a higher starting torque but also a higher slip at full load. It is suitable for applications that require a high starting torque, such as pumps.
- NEMA Design D:Â This type of squirrel-cage induction motor has the highest rotor resistance of the three types. This results in the highest starting torque but also the highest slip at full load. It is suitable for applications with very high inertia starts, such as cranes and hoists.
The rotor lags behind the synchronous speed and does not spin at the same rate. This slip is essential for the motor to rotate. If the rotor and the field had no relative motion, there would be no voltage induced in the rotor. The rotor has voltage and current induced in it because it slips relative to the stator’s rotating magnetic field. The slip is the difference between the speed of the rotating magnetic field and the rotor in an induction motor, and it is calculated as a percentage of the synchronous speed as follows:
Percent slip = (Synchronous speed – Actual speed) × 100 / Synchronous speed
The slip is higher when the load is higher and it is needed to generate useful torque. A typical slip for a 60 Hz, three-phase motor is 2 or 3 percent.
An induction motor’s loading is similar to a transformer’s loading, because both involve changing flux linkages between a primary (stator) winding and a secondary (rotor) winding.
The current at no-load is small and similar to the magnetizing current in a transformer.
The no-load current consists of a magnetizing component that creates the rotating flux and a small active component that covers the rotor’s windage and friction losses and the stator’s iron losses.
When the induction motor is loaded, the rotor’s current creates a flux that opposes and, therefore, reduces the stator’s flux.
This causes more current to flow in the stator windings, just like an increase in the current in the transformer’s secondary leads to a corresponding increase in the primary current.
A consequent pole motor is a single-winding motor that can operate at two speeds. The low speed is always one-half of the high speed. The stator windings of a consequent pole motor are arranged so that the number of poles can be changed by reversing some of the coil currents.
The speed of a consequent pole motor is determined by the connection of the stator windings. In the series delta connection, the windings are connected in series, which results in a low speed. In the parallel wye connection, the windings are connected in parallel, which results in a high speed.
The torque rating of a consequent pole motor is the same at both speeds. The horsepower rating of a consequent pole motor can be the same or different at both speeds, depending on the connection of the stator windings.
Here is a diagram of a dual-speed three-phase squirrel-cage single-winding motor with six stator leads brought out:
[Diagram of a dual-speed three-phase squirrel-cage single-winding motor with six stator leads brought out]
Single-speed AC induction motors are often supplied with multiple external leads to allow for different voltage ratings in fixed-frequency applications. The multiple leads can be configured in a variety of ways, such as series-parallel, wye-delta, or a combination of the two.
- Series-parallel connections are used to increase the voltage rating of the motor. In this configuration, the stator windings are connected in series, which increases the impedance of the motor. This results in a lower current draw and a higher voltage rating.
- Wye-delta connections are used to change the number of poles in the motor. In this configuration, the stator windings are connected in wye or delta, which changes the number of magnetic poles. This results in a change in the synchronous speed of the motor.
The specific configuration of the multiple leads will be determined by the application requirements. For example, a motor that needs to operate at two different voltages would use a series-parallel connection. A motor that needs to operate at two different speeds would use a wye-delta connection.
It is important to note that not all single-speed AC induction motors have multiple external leads. Some motors are only designed for a single voltage or speed rating. It is important to check the motor nameplate to determine the specific configuration of the leads.
Wound-Rotor Induction Motor
A wound-rotor induction motor is a type of induction motor that has a three-phase winding on the rotor. The rotor winding is connected to slip rings, which allow external resistance to be connected to the rotor circuit. This can be used to control the starting torque and speed of the motor.
When the motor is started, the external resistance is high, which limits the current flow in the rotor windings. This results in a high starting torque. As the motor accelerates, the external resistance is gradually reduced, which allows the motor to reach its full speed.
Wound-rotor induction motors are often used in applications where a high starting torque is required, such as hoists and cranes. They can also be used for variable-speed applications, such as fans and pumps.
Here are some of the advantages of wound-rotor induction motors:
- They can provide high starting torque.
- They can be used for variable-speed applications.
- They are more efficient than squirrel-cage induction motors.
Here are some of the disadvantages of wound-rotor induction motors:
- They are more expensive than squirrel-cage induction motors.
- They require external resistors, which can be damaged if the motor is overloaded.
- They are not as rugged as squirrel-cage induction motors.
Three-Phase Synchronous Motor
A synchronous motor is a type of electric motor that rotates at a constant speed, in synchronism with the frequency of the electric power supply. The synchronous motor is different from an induction motor, which rotates at a speed slightly less than the synchronous speed due to slip.
The synchronous motor has a rotating magnetic field produced by the stator windings, just like an induction motor. However, the rotor of a synchronous motor also has a permanent magnet or an electromagnet that is excited by a DC current. This creates a second magnetic field in the rotor, which interacts with the stator’s magnetic field to produce torque.
The synchronous motor is not self-starting. In order to start the motor, the rotor must be brought up to near synchronous speed before the DC current is applied to the rotor. This is typically done by using a squirrel-cage winding on the rotor, which allows the motor to start as an induction motor.
Once the motor is started, the DC current is applied to the rotor, and the motor will begin to rotate at the synchronous speed. The synchronous motor is a very efficient motor, and it is often used in applications where a constant speed is required, such as generators and clocks.
Here are some of the advantages of synchronous motors:
- They have a high efficiency.
- They can provide a constant speed.
- They are not affected by load changes.
Here are some of the disadvantages of synchronous motors:
- They are not self-starting.
- They require a DC source for excitation.
- They are more expensive than induction motors.