What is the purpose of the synchronous motor running without load?

Synchronous condensers are operated with no shaft load when they are only used to regulate the system voltage. Any synchronous machine (either a generator of motor) has the capability of generating or absorbing reactive power [var] when the shaft also has torque and is doing real, active power, work.

How does synchronous motor behave at no load?

Under no load condition, ? is very small and hence ERph is also very small. So current drawn by the motor is also very small on no load which is the case in all the various type of motors. As the load on the synchronous motor increases, there is no change in its speed. … increases but speed remains synchronous.

Why are synchronous motors not started under load?

Synchronous motors are inherently not able to self-start on an AC power source with the utility frequency of 50 or 60 Hz. As stated in Chapter 1, this is because synchronous motors can develop a torque only when running at the synchronous speed.

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What is the effect of load on synchronous motor?

The load on the shaft is increased. The rotor slows down momentarily, as it required some time to take increased power from the line.

When the synchronous motor is made to operate at no load with over excitation it will be termed as?

An over-excited synchronous motor draws leading current. Therefore, if the field of the synchronous motor is overexcited then its power factor will be leading. An over-excited synchronous motor running at no load is known as the synchronous capacitor or synchronous condenser.

Why the speed of synchronous motor is constant?

The rotor of a synchronous motor moves at the same speed as the changing, rotating field in its stator. The rotor’s magnetic field is “synchronized” with the stator’s field. … Their speed is dependent on the number of poles and the supply frequency. They have a constant speed at a given supply frequency.

What happens when motor load increases?

As the load is increased, the motor will slow down, and the torque will increase. Current into the motor will also increase. … The peak torque occurs at relatively high speed for efficient motors. Certain applications use less efficient induction motors which are designed to have maximum torque occur at stall speed.

What is the main disadvantage of synchronous motors?

Disadvantages or Demerits:

Synchronous motors requires dc excitation which must be supplied from external sources. Synchronous motors are inherently not self starting motors and needs some arrangement for its starting and synchronizing. The cost per kW output is generally higher than that of induction motors.

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How do you test a 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.

What is asynchronous speed?

Synchronous motor is a machine whose rotor speed and the speed of the stator magnetic field is equal. Asynchronous motor is a machine whose rotor rotates at the speed less than the synchronous speed.

What happens when excitation of synchronous motor fails?

synchronous generator. synchronous motor. … If the excitation of the generator fails, suddenly there will be no more magnetically locking between rotor and rotating magnetic field of stator.

Does the excitation affect the synchronous motor speed?

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.

What is the power factor of a synchronous motor?

Synchronous motors are designed to operate at unity (1.0) power factor or 0.8 leading power factor. By varying the DC excitation of the motor, the power factor of the motor can be varied widely. Overexcited synchronous motors operate at leading power factor and provide reactive kVAR-like capacitors.

Why is excitation needed?

The excitation system is responsible for supplying the field current to the main rotor. The requirements of an excitation system include reliability under all conditions of service, a simplicity of control, ease of maintenance, stability and fast transient response.

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What is excitation EMF?

The process of generating a magnetic field by means of an electric current is called excitation. Field coils yield the most flexible form of magnetic flux regulation and de-regulation, but at the expense of a flow of electric current.

What is the working principle of synchronous motor?

The principle of operation of a synchronous motor can be understood by considering the stator windings to be connected to a three-phase alternating-current supply. The effect of the stator current is to establish a magnetic field rotating at 120 f/p revolutions per minute for a frequency of f hertz and for p poles.