Synchronous Machine Functioning and Applications
Synchronous Machine: Functioning and Applications
Functioning
Rotor (Inductor)
The rotor, fed with DC, creates a magnetic field. In a salient pole machine, this field is considered sinusoidal. Rotating the rotor creates a revolving sinusoidal field.
Stator (Armature)
The stator has a distribution similar to an induction machine. The rotating magnetic field induces a voltage in each stator phase. These voltages are displaced, forming a balanced three-phase system.
Load and Field Interaction
When a balanced load is connected, the stator currents create a rotating field matching the rotor’s speed. The total field is a sinusoidal field rotating at the rotor’s geometric rotation speed.
Voltage and Excitation
The induced stator voltage is proportional to the field’s intensity (created by the rotor) and the rotor’s speed. Increasing the excitation voltage increases the induced voltage.
Equivalent Circuit and Power
The synchronous impedance (Zs) represents the stator’s resistance and reactance. Power consumption in the rotor represents Joule losses. The rotor is modeled as a DC source (exciter) feeding a coil (inductor and resistor).
The synchronous machine can absorb or deliver active power and can also provide or absorb reactive power.
Testing
DC Test on Stator
Applying DC voltage to the stator windings allows measurement of stator resistance.
Open-Circuit Test
Driving the rotor at synchronous speed and varying the rotor field current allows determination of the induced voltage (E) as a function of excitation.
Short-Circuit Test
Rotating the rotor at synchronous speed with a shorted stator allows measurement of synchronous reactance (Xs).
Starting and Applications
To start the motor, the rotor’s DC voltage is initially off. The rotor is brought to near synchronous speed (e.g., by an auxiliary motor or cage winding). Then, the DC source is connected.
A synchronous motor maintains constant speed unless the load torque exceeds the maximum allowable value, causing loss of synchronism.
Varying the rotor’s excitation controls reactive power consumption or delivery. Synchronous motors can consume negative reactive power, improving power factor.
These motors are useful for constant-speed applications, such as radar antennas.
Connecting Synchronous Generators in Parallel
Connecting synchronous generators in parallel requires matching frequency, voltage, and phase sequence.
A synchroscope verifies frequency matching. Careful synchronization is crucial to avoid accidents due to high power levels.
Connected generators should share the load equally. Power sharing in a network is a complex process.
Regulation of synchronous generators ensures balance between power consumption and production. Torque and excitation control maintain system stability and meet reactive power demands.