Effect of Field Current Changes on a Synchronous Motor

Hello friends, I hope all of you are having fun in your life. In today’s tutorial, we are going to have a look at the effect of field current changes on a synchronous motor. A synchronous motor is an electrical device in which, during operation, the revolution of the rotor is coherent with the rotation of a rotating field of the stator. If the load at the synchronous motor is not varying and its excitation is changing, then the P.F. will vary spontaneously to maintain its active constituent constant under all excitation variations. The value of IA is constant as the load associated with the motor; its power losses do not vary.

In today’s post, we will have a look at the effect of field current variation on motor operation and related factors. So let’s get started with the effect of field current changes on a synchronous motor.

Effect of Field Current Changes on a Synchronous Motor

• To find the effect of field current variation on a synchronous motor, we study the figure given below.

• First, we suppose that the motor is connected with an inductive load (lagging P.F.), so if we increase the field current (IF), its effect is drawn in a given figure.

• We observe that with the increment of field current, internal generated voltage increases, but there is no effect on power provided by the motor.

• The power of the motor will vary only when the torque of the load shaft varies.

• So the field current has zero effect of speed of rotation of a shaft, as the load connected with shaft of motor is constant so active power provided by the motor will also remain constant.

• The terminal voltages (VT) are also constant; these are maintained by the supply connected with the motor.

• The terms Eₐₛₙ and Iₐcosₒ are also constant. With the increment of IF, internal generated voltage also enhances, but their increment is along the line of power. This factor is drawn in the given figure.

• The main point you can note from the figure is that with the increment of internally generated voltage, armature current (IA) first lessens and then starts to increase.

• At a lower value of internally generated voltage, IA is lagging, and the motor is behaving like an inductive load connected with it.

• Its behavior is like a circuit that consists of resistance and an inductor that is using reactive power Q.

• With the increment of IF, IA will become in line with V₀ so the motor behaves like resistive circuitry.

• With the further increment in IF, the IA becomes the leading current, so now the motor behaves like a capacitive load.

• Now it works like circuitry that has resistance and a capacitor, and using a negative value of reactive power (-Q) means the motor is providing reactive power Q to the connected system.

• In the given figure, the graphical representation between the field current and armature current of a synchronous motor is drawn.

• Such a type of arrangement is known as the ‘V curve’ of a synchronous motor; it is so called because its physical structure is like a ‘V.’.

• In the figure there are many V curves constructed at different power values.

• For every curve, the least value of IA exists when P.F. is one, meaning only active power is delivered to the motor.

• But at other points the motor gets or provides reactive power to the system.

• At such a value of field current that is less than the value that provides the least value of armature current, the IA lagging means the motor is using reactive power.

• So we can easily control the reactive power by using a motor or deliver it to a connected system by regulating the IF.

• When the projection of the phasor diagram of internal generated voltage on Vø is less than the magnitude of Vø, now the motor behaves like an inductive load and uses reactive power.

• If the value of IF is lower, the motor is said to be in an under-excited state.

• If the phasor projection internal generated voltage is larger than VØ, now the motor is behaving like a capacitive load and delivering power (Q) to the system.

• As the value of IF is larger, the motor is said to be in the overexcited state. Given the below explains these 2 factors.

So it is the detailed post on the Effect of Field Current Changes on a Synchronous Motor i have explained each and everything related to this topic. If you have any questions, ask in the comments. See you in next tutorial Synchronous Motor Power Correction.

• I also have written some related articles you can also read them are listed here.I
• Introduction to Synchronous Motor
• Synchronous Motor Torque-Speed Characteristic Curve
• Synchronous Motor Starting Method
• What is Synchronous Condenser (Capacitor)
• Synchronous Motor Power Correction. 
• Types of Synchronous Motor
• Effect of Field Current Changes on a Synchronous Motor.