Synchronous Speed Under Overload Condition?

What Happens When You Give More Load?

Synchronous Machines

A synchronous machine is one in which the rotor speed is synchronized with the frequency of the supply current. This synchronization means that the rotational speed of the rotor (in RPM) is directly related to the supply frequency (in Hz) and the number of poles of the machine.

Declaration of Synchronous Operation

The defining characteristic of a synchronous motor lies in its ability to keep the rotor speed synchronized with the stator’s rotating magnetic field, governed by the frequency of the electronic commutation. This synchrony is maintained up to the motor’s load limit, beyond which the motor may stall or shut down, but it will not operate asynchronously.

Synchronous machines, such as synchronous motors and generators, operate with the rotor locked in step with the rotating magnetic field of the stator. Here's how we declare it:

1. Constant Speed: Synchronous motors run at a constant speed determined by the supply frequency and number of poles. This is their defining characteristic.

2. Excitation: Synchronous machines have a field winding on the rotor, which is supplied with DC current to create a constant magnetic field. This DC supply is known as the excitation.

3. Synchronism: The rotor's magnetic field interacts with the stator's rotating magnetic field. The rotor must spin at the same speed as the rotating field for effective power conversion, ensuring synchronization.

4. Load Handling: When the load on a synchronous generator increases, the prime mover must supply more mechanical power to keep the rotor spinning at the synchronous speed. If the load on a synchronous motor increases, the motor draws more electrical power to maintain synchronous speed.

What is Synchronous?

In electrical engineering, 

"synchronous" refers to the state where two or more processes or signals are coordinated in time. For synchronous machines:

- Synchronous Generators: The generated electrical frequency is directly tied to the mechanical rotational speed.

- Synchronous Motors: The rotor spins in exact synchronization with the supply current's rotating magnetic field, maintaining constant speed under varying load conditions as long as the load does not exceed the motor's capability to stay in synchronism.

When you increase the load on an electrical machine, such as a generator or motor, several things can happen:

1. Increased Power Demand: The machine has to supply more power to meet the load demand.

2. Voltage Drop: In generators, an increased load can cause a drop in terminal voltage if the excitation is not adjusted accordingly.

3. Increased Current: The current through the machine's windings increases to supply the additional power.

4. Temperature Rise: Higher current can lead to increased losses (I²R losses), causing the machine to heat up.

5. Mechanical Effects: For motors, increased load means more mechanical torque is required, which can affect the speed and stability.