AC Electrical Machines: Synchronous Motors
AC Electrical Machines
Synchronous Motors and Generators
AC electrical machines are classified into two main types:
- Synchronous motors and generators: The coil inductor current is supplied by a separate source of direct current (DC).
- Induction motors: The inductor winding current is provided by magnetic induction.
Differences Between AC and DC Machines
The location of the coils and inductors differs depending on the type of machine. In an AC machine, there is no electrical connection between the rotor and the stator. There is no commutator.
AC Synchronous Motors
Internal Stator Structure
The stator is formed by an outer casing and an inner piece shaped like a crown. It has slots where the windings are installed. Three-phase motors (Drehstrommotor) are the most commonly used. The stator windings are isolated into three phases. Each phase winding is connected to a three-phase AC system and placed within the frame so there is a 120-degree electrical gap between them. There are many types of windings, such as a train winding on the interior wall, respecting the 120-degree electrical gap between coils.
Rotor
The rotor consists of a set of copper or aluminum bars arranged in a cylindrical shape, with their ends joined by a metal ring. It has two main parts:
- Coils: These now function as the inducer. The coils are connected longitudinally and between them to create alternating north and south poles.
- Slip Rings: These are two metal rings around one end of the shaft, but they are isolated from the windings. Each of the rotor windings is electrically connected to one of these rings. Brushes make contact with the rings, one with the negative pole and the other with the positive pole, providing DC power to the rotor coils, regardless of its speed.
Operation
Synchronous motors require AC to circulate through the stator windings (induced) and DC to circulate through the rotor windings (inductor). Each current creates a magnetic field. The interaction between these fields causes a rotary movement of the rotor. The DC magnetic field has a constant value, and its direction and orientation are determined by the right-hand rule. In the stator windings, a two-phase or three-phase rotating magnetic field is created. If the two fields are close, they behave like two magnets. As the stator field is rotating, the rotor’s magnetic field and the rotor itself spin, trying to catch up.
Rotation Speed: Synchronous Speed
The magnetic field makes a full turn inside the stator for each cycle of the applied electric power. Bipolar machines consist of one north pole and one south pole. However, real AC machines have their stator coils distributed in a twisted manner due to space limitations, with a 120-degree electrical gap between the coils. Within the stator, four poles create two electromagnets; these machines are called quadripolar. For the rotor to complete one full revolution, the stator poles must rotate half a turn. AC motors work because they are fed at a constant speed by a constant frequency.
Reversing Rotation Direction
To change the direction of rotation, it is enough to change the connection between the stator coils.
Starting a Synchronous Motor (Three Methods)
- Slow-Electric Field of the Stator: Decrease the speed of synchronism to a low enough value for the rotor to lock in.
- Use an External Motor: Accelerate the rotor to synchronous speed using an external motor. Once the rotor reaches synchronous speed, disconnect the external motor.
- Use Damper Windings (Amortisseur Windings): Mount bars on the rotor faces and connect them in a short circuit.
A synchronous motor can be started with damper windings alone. The motor is loaded so that it does not need to bow to get more synchronism speed. It is also more stable when it turns at synchronous speed, as there is no induced voltage in the damper winding bars.