Consider an under-excited (i.e., Ef < V), 3-phase cylindrical rotor synchronous motor driving a mechanical load. The figure shows the per phase phasor diagram of the motor. Since the motor is under-excited, it will be operating at a lagging power factor cos φ.In practice, for a synchronous motor, XS>>Ra, then the armature resistance (Ra) of the motor can be neglected. Since Ra is neglected, the armature copper loss will be zero. Therefore, the mechanical power developed (Pm) by the synchronous motor is equal to the input power (Pi) to the motor.Also, Ra= 0, Er=IaXS, thus the armature current (Ia) lags ... Read More

Hunting in Synchronous MotorHunting is the phenomenon of oscillation of the rotor about its steady state position or equilibrium state in a synchronous motor. Hence, hunting means a momentary fluctuation in the rotor speed of a synchronous motor.In a synchronous motor, when the electromagnetic torque developed is equal and opposite to the load torque, such a condition is known as "condition of equilibrium" or "steady state condition".In the steady-state, the rotor of the synchronous motor runs at synchronous speed, thereby maintaining a constant value of torque angle (δ). If there is a sudden change in the load torque, then the ... Read More

Assume that initially, the rotor of a synchronous motor is stationary. When a pair of rotating stator poles sweeps across the stationary rotor poles at synchronous speed, the stator poles will tend to rotate the rotor in one direction and then in the other direction. As a result, the rotor experiences a torque first in one direction and then in the other. However, the rotor has high inertia and the stator field slides by so fast that the rotor cannot follow it. As a result, the rotor cannot move and hence the resultant starting torque is zero, i.e., a synchronous ... Read More

Assume that the rotor of a synchronous motor is stationary. When a pair of rotating stator poles sweeps across the stationary rotor poles at synchronous speed, the stator poles will tend to rotate the rotor in one direction and then in the other direction. As a result, the rotor experiences a torque first in one direction and then in the other. However, the rotor has high inertia and the stator field slides by so fast that the rotor cannot follow it. As a result, the rotor cannot move and hence the resultant starting torque is zero.But, if the rotor poles ... Read More

A synchronous motor is a doubly-excited system, i.e., it is connected to two electrical systems −A 3-phase supply is connected to the armature winding.A DC supply is connected to the rotor field winding.The figure shows the equivalent circuit for one phase of a three-phase synchronous motorLetV = Terminal voltage per phase applied to the motorEf = Excitation voltageIa = Armature current per phase drawn by the motor from the supplyRa = Effective armature resistance per phaseXS = Synchronous reactance per phase of the motor armature windingZS = Synchronous impedance per phase of the armatureBy applying KVL in the equivalent circuit ... Read More

In induction motors and DC motors, when the mechanical load attached to the shaft of the motor is increased, the speed of the motor decreases. The decrease in the speed reduces the back EMF so that additional current is drawn from the source to carry the increased load at a reduced speed. But, this action cannot take place in a synchronous motor, since it runs at a constant speed (i.e., synchronous speed) at all loads.For a synchronous motor, the armature current per phase is given by, $$\mathrm{I_{a}=\frac{V-E_{f}}{Z_{s}}=\frac{E_{r}}{Z_{s}}\:\:\:\:\:\:...(1)}$$When we apply the mechanical load to a synchronous motor, the rotor field poles ... Read More

One of the most important characteristics of a synchronous motor is that, by changing the field excitation of the motor, its power factor can be made both lagging and leading. The change in the power factor of the synchronous motor with the change in excitation can be explained with the help of its phasor diagram.Consider a synchronous motor having a constant supply voltage and driving a constant mechanical load. The input power to the motor is given by, $$\mathrm{P_{i}=\frac{VE_{f}}{X_{s}}Sinδ=3V\:I_{a}\:Cos\varphi}$$Since V and XS are constant for a given synchronous motor, thus for constant power output, $$\mathrm{E_{f}\:Sinδ=Constant}$$$$\mathrm{I_{a}\:Cos\varphi=Constant}$$When the field excitation (Ef) of ... Read More

A motor in general is an electrical machine that converts electrical energy into mechanical energy. Electric motors can either be DC Motors or AC Motors, depending on the type of power supply that is supplied as its input.AC motors are further classified into two types−Asynchronous or Induction MotorSynchronous MotorA synchronous motor always runs at synchronous speed, while an induction motor runs at a speed less than the synchronous speed.Read through this article to find out more about synchronous motors and induction motors and how they are different from each other.What is a Synchronous Motor?A synchronous motor is a type of ... Read More

Units of Synchronizing Power Coefficient (𝑷𝐬𝐲𝐧)Generally, the synchronizing power coefficient is expressed in Watts per electrical radian, i.e., $$\mathrm{𝑃_{syn} =\frac{𝑉 𝐸_{𝑓}}{𝑋_{𝑠}}cos\:𝛿 \:\:Watts/electrical\:radian …(1)}$$$$\mathrm{∵ \:𝜋\:radians = 180\:degrees}$$$$\mathrm{\Rightarrow\:1\:radian =\frac{180}{𝜋}\:degrees}$$$$\mathrm{∵ \:𝑃_{syn}=\frac{𝑑𝑃}{𝑑𝛿}\:\:Watts/ \left(\frac{180}{𝜋}\:degrees \right)}$$$$\mathrm{\Rightarrow\:𝑃_{syn}=\left( \frac{𝑑𝑃}{𝑑𝛿}\right)\left(\frac{𝜋}{180}\right)\:\:Watt/electrical\:degree …(2)}$$If p is the total number of pole pairs in the machine, then$$\mathrm{𝜃_{electrical} = 𝑝 \cdot 𝜃_{mechanical}}$$Therefore, the synchronizing power coefficient per mechanical radian is given by, $$\mathrm{𝑃_{syn} = 𝑝 \cdot\left( \frac{𝑑𝑃}{𝑑𝛿}\right)\:\:Watts/mech. radian …(3)}$$And, the synchronizing power coefficient per mechanical degree is given by, $$\mathrm{𝑃_{syn} =\left( \frac{𝑑𝑃}{𝑑𝛿}\right)\left(\frac{𝑝\:𝜋}{180}\right)\:Watts/mech.degree …(4)}$$Significance of Synchronizing Power CoefficientThe synchronizing power coefficient ($𝑃_{syn}$) is the measure of the stiffness of the electromagnetic coupling between the stator ... Read More

In a salient-pole rotor synchronous machine, the air-gap is highly non-uniform. Consider a synchronous machine having a 2-pole salient-pole rotor rotating in the anti-clockwise direction within a 2-pole stator, as shown in Figure-1.In Figure-1, the axis shown along the axis of the rotor is known as direct axis or d-axis and the axis perpendicular to the d-axis is called quadrature axis or q-axis. It can be seen that the two small air-gaps are involved in the path of d-axis flux ($𝜑_{𝑑}$), thus the reluctance of the path is minimum. The q-axis flux ($𝜑_{𝑞}$) path has two large air-gaps and it ... Read More