If DC field excitation of a synchronous motor is such that back EMF Eb is greater than applied voltage V, then the motor is said to be over-excited. 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.
What will be the field power factor of a synchronous motor in excited mode?
CASE 3: If the field current is more than the normal field current, motor is said to be over excited. … Conclusion: An overexcited synchronous motor operate at leading power factor, under-excited synchronous motor operate at lagging power factor and normal excited synchronous motor operate at unity power factor.
How an over excited synchronous machine improves power factor?
The great feature of Synchronous Motor is its ability to operate at leading power factor when over-excited. A Synchronous Motor can be made to operate at unity and leading power factor by just increasing its excitation voltage i.e. by increasing the field current.
What is the effect of under excitation and over excitation on the synchronous motor?
The over-excitation limiter protects the generator against over-temperature caused by high field current for long time. Under-excitation limiter prevents the decrease of generator field current to the values lower than the stability margin or the thermal limit of end side of the stator core.
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.
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 field current in synchronous motor?
The field current may be supplied from an externally controlled rectifier through slip rings, or, in larger motors, it may be provided by a shaft-mounted rectifier with a rotating transformer or generator. A synchronous motor with only a field winding carrying a direct current would not be self-starting.
How can the power factor of a synchronous motor be adjusted?
The fact that the field poles of a synchronous motor are magnetically in line with the stator flux allows dc current on the field to replace ac magnetizing current on the stator. Since power factor is determined by the magnetizing current, the power factor is adjustable by changing the field current.
How power factor is improved?
What can I do to improve power factor? You can improve power factor by adding power factor correction capacitors to your plant distribution system. When apparent power (kVA) is greater than working power (kW), the utility must supply the excess reactive current plus the working current .
What is synchronous speed?
: a definite speed for an alternating-current machine that is dependent on the frequency of the supply circuit because the rotating member passes one pair of poles for each alternation of the alternating current.
Does the excitation affect the power factor?
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 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.
What is normal excitation in synchronous motor?
A synchronous motor is said to have normal excitation when its Eb = V. If field excitation is such that Eb < V, the motor is said to be under-excited. In both these conditions, it has a lagging power factor as shown in Fig. 38.12.
What is the power factor of induction motor?
The power factor of induction motors varies with load, typically from around 0.85 or 0.90 at full load to as low as about 0.20 at no-load. At no load, an induction motor draws a large magnetizing current and a small active component to meet the no-load losses.
What is synchronous speed and it’s formula?
The synchronous speed of an AC motor is determined by the frequency of the source and the number of poles. The RPM is calculated by multiplying the frequency times 60 and dividing by the number of pairs of poles. … Actual speed of the induction motor will be less than the synchronous speed.
Can power factor of a synchronous motor be controlled?
This error signal is evaluated by FLC to obtain a control signal for the synchronous motor excitation circuit since the power factor of a synchronous motor can be controlled and held at a desired value by adjusting the excitation current. … Therefore, operating power factor of T&D systems must be corrected.