Found 1006 Articles for Electronics & Electrical

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

Conditions Required for Paralleling AlternatorsIn order to connect an alternator in parallel with another alternator or an alternator to the infinite busbars, the following conditions are met −The phase sequence of the voltages of the incoming alternator should be the same as that of the busbars.The incoming alternator voltages must be in phase with the busbar voltages.The terminal voltage of the incoming alternator must be the same as the busbar voltage.The frequency of the generated voltage of the incoming alternator must be equal to frequency of the busbar voltage.Synchronization by a SynchroscopeA stationary alternator must not be connected to live ... Read More

A set of three synchronizing lamps can be used to check the conditions for paralleling the incoming machine with other machines. The dark lamp method along with a voltmeter used for synchronising is shown in figure. This method is used for synchronizing low-power machines.In this method, one lamp is connected between corresponding phases while the other two lamps are cross-connected between the other two phases, i.e., R1 is connected to R2, Y1 to B2 and B1 to Y2 as shown in the figure.Now, the prime mover of the incoming machine is started and the alternator is brought up to near ... Read More

Short circuits at the terminals of the unloaded alternators or synchronous generators are very rare. They generally occur due to insulation failure or accidental damage on some part of the power system supplied by the generator. Therefore, it is important to deal with the case of a 3-phase alternator delivering power to a load or to an infinite bus.If a short circuit occurs across the armature terminals of the alternator, the short circuit armature current will pass through a sub-transient period, a transient period and finally will settle down to a steady-state condition. Also, when the short circuit occurs, the ... Read More

A sudden 3-phase short-circuit at the armature terminals of a synchronous machine is used to analyse the transient phenomenon. This is the most severe transient condition that can occur in a synchronous generator. It is assumed that the machine is to be initially unloaded and to continue operating at synchronous speed after short-circuit occurs.The machine being producing normal voltage under no-load condition and their instantaneous values are given by, $$\mathrm{𝑒_{𝑅} = 𝐸_{𝑚}\:sin\:𝜔𝑡}$$$$\mathrm{𝑒_{𝑌} = 𝐸_{𝑚}\:sin(𝜔𝑡 − 120°)}$$$$\mathrm{𝑒_{𝐵} = 𝐸_{𝑚}\:sin(𝜔𝑡 + 120°)}$$Since the machine is initially unloaded, the only pre-disturbance current in the machine is the field current. When the rotor rotates, ... Read More

The prime-mover governor characteristic is a graph plotted between the speed of the prime-mover (or generator frequency) and the active power. A typical prime-mover governor characteristic is shown in the figure.The prime mover characteristic usually drawn as a straight line, but the actual characteristic has a slight curve.For the successful parallel operation of the alternators or synchronous generators, the load-speed characteristics of the prime movers should be drooping, i.e., the speed of the prime mover should decrease slightly with increasing loads. The drooping characteristic provides inherent stability of the operation of an alternator when paralleled with the other alternators. The ... Read More