why speed decreases as field excitation increases in synchronous machine. why the speed of the synchronous motor remains constant on any load. why with the change of excitation of a synchronous motor the magnitude of armature current first decrease and then increase.
Does change in excitation affect the synchronous motor speed?
One of the most important features of a synchronous motor is that by changing the field excitation, it can be made to operate from lagging to leading power factor. Consider a synchronous motor having a fixed supply voltage and driving a constant mechanical load. … hence the power factor cos of the motor changes.
What happens if excitation is changed?
A change in the excitation, thus, affects only the power factor of its output. … This reduces the terminal voltage slightly, so let the excitation of the first alternator be increased so as to bring the terminal voltage back to its original value.
What happens to synchronous motor if only field is excited?
In a Synchronous Motor, a rotating magnetic field is produced in stator by giving 3 phase AC supply. … If AC excitation is given then alternate North and South Pole is created on rotor field the rotor move half cycle clockwise and half cycle anti clockwise and finally stops to rotate.
What factors determine the speed of a synchronous motor?
The frequency of the power supply and the number of poles of the machine determine the synchronous speed. A synchronous motor is one in which the rotor turns at the same speed as the rotating magnetic field in the stator.
Why the speed of synchronous motor is constant?
Why does a synchronous motor rotate at a constant speed? The stator magnetic field rotates at synchronous speed (which depends upon the frequency of supply) and hence the rotor follows it and rotates at synchronous speed as well. The stator is excited from supply which produces a rotating magnetic field in the air gap.
What happens when excitation of synchronous motor fails?
If the excitation of the generator fails, suddenly there will be no more magnetically locking between rotor and rotating magnetic field of stator. But still the governor will supply same mechanical power to the rotor due to this sudden magnetic unlocking; the rotor will be accelerated beyond the synchronous speed.
Why the synchronous motor is not self starting?
Above a certain size, synchronous motors are not self-starting motors. This property is due to the inertia of the rotor; it cannot instantly follow the rotation of the magnetic field of the stator. … Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.
When two alternators are operating in parallel the cause for circulating current is?
The magnitude of internal voltages of both incoming and existing alternator must be identical else a potential difference will be created, causing circulation of currents amongst the two alternators and this is undesirable.
What is excitation current?
Excitation current is the current flowing into the high voltage winding with the low voltage side open. This current should be proportional to the No-load acceptance test but with the difference resultant from the use of test voltages different from nominal values.
What are the advantages of synchronous motor?
The advantages of the synchronous motor are the ease with which the power factor can be controlled and the constant rotational speed of the machine, irrespective of the applied load. Synchronous motors, however, are generally more expensive and a d.c. supply is a necessary feature of the rotor excitation.
Why is excitation always DC?
The primary reason lies in the principle of operation of Synchronous machines. The rotor acts as an electromagnet with a constant flux and gets interlocked with the rotating flux of stator to provide a constant speed rotation. The rotor needs to have a constant flux and that’s why the dc excitation is provided.
How do you test a synchronous motor?
TESTING THE SYNCHRONOUS MOTOR
The rotor can be tested with an ohmmeter for an open winding or a grounded winding. To test the rotor for an open winding, connect one of the ohmmeter leads to each of the slip rings on the rotor shaft, Figure 17–5.
How do you control the speed of a synchronous motor?
Changing the number of poles is not easy, so we do not use that method. However, with the invention of solid-state devices, the frequency of the current fed to the synchronous motor can be varied. We can control the speed of the synchronous motor by changing the frequency of the supply to the motor.
At what speed does synchronous motor run?
The 3-phase 4-pole (per phase) synchronous motor will rotate at 1800 rpm with 60 Hz power or 1500 rpm with 50 Hz power. If the coils are energized one at a time in the sequence φ-1, φ-2, φ-3, the rotor should point to the corresponding poles in turn.
What is synchronous speed?
Synchronous speed is a significant parameter for the rotating magnetic field-type AC motor. It is determined by the frequency and the number of magnetic poles. Synchronous speed No = [rps, revolutions per second] f = Frequency [Hz] p = Number of magnetic poles.