AC Machines: Synchronous and Induction Motors Explained
AC Machines: Types and Differences with DC
AC machines encompass synchronous motors and generators, where the inductor winding’s current requirement is provided by a DC source independent of the machine. The induction machine is primarily used as a motor, and the inductor winding current is supplied through magnetic induction.
Key differences between AC and DC machines include:
- The location of the induction and induced coils differs depending on the type of machine.
- In AC machines, there is no electrical connection between the rotor and the stator.
- AC machines do not have a commutator.
Synchronous Motors: Principle of Operation
Synchronous AC motors require a DC current to flow through the rotor windings and an AC current to circulate through the stator windings. Each current creates a magnetic field with distinct characteristics. The interaction of these fields causes the rotor to rotate.
In the rotor, the DC current creates a constant magnetic field. The direction of this field is determined by the right-hand rule. A two-pole stator winding creates a rotating magnetic field. When these two magnetic fields are in proximity, they interact. As the stator field rotates, the rotor’s magnetic field, and consequently the rotor itself, rotates, attempting to align with the stator field.
Starting Procedures for Synchronous Motors
There are three primary methods for starting synchronous motors:
- Reduce the speed of the stator’s magnetic field: This involves decreasing the synchronous speed until it is low enough for the rotor to engage. The speed is then gradually increased to accelerate the rotor to the desired synchronous speed.
- Use an external motor: An external motor accelerates the rotor to synchronous speed.
- Utilize damper windings: This is the most common method. It involves mounting a cage on the rotor faces and connecting them in short-circuit.
Induction Motors: Principle of Operation
In an induction motor with a squirrel-cage rotor, a three-phase current is applied to the stator, creating a rotating magnetic field. This field passes over the rotor bars, inducing a voltage. This voltage creates a current in the rotor bars, generating a magnetic field. The interaction between the stator and rotor magnetic fields produces a torque that rotates the rotor in the same direction as the stator field.
Displacement
Displacement is defined as the relative velocity between the rotor’s rotation and the stator’s magnetic field, expressed as a percentage.
Starting Methods for Induction Motors
- Direct connection: The stator is directly connected to the power grid. This method is suitable for low-power motors.
- Resistance in series with the stator circuit: A series resistor is connected to each stator phase. The motor accelerates progressively, and the resistors are eventually disconnected.
- Autotransformers: The stator is connected to the power grid using an autotransformer.
- Star-delta starting: This is the least frequently used method.
Alternator: Principle of Operation
An alternator is a machine that receives mechanical energy to rotate its shaft and a DC excitation current through the rotor winding. This generates an alternating current in the stator, where the armature winding is located.
A prime mover rotates the alternator’s rotor. Simultaneously, the rotor is energized by a DC current, creating a magnetic field whose direction and sense are determined by the right-hand rule. This rotating magnetic field, driven by the shaft’s rotation, induces an alternating current in the stator windings.