A voltmeter is a measuring instrument which is used to measure voltage across the two terminals in an electrical circuit.A voltmeter has a very high resistance and it is design in such a way that when connected in parallel to circuit for measuring voltage it does not take appreciable current, so that power consumed is small.Voltmeter – Working PrincipleWhen a voltmeter is connected in parallel to a circuit element (load), across which the voltage is being measured. Since the voltmeter has a very high resistance, therefore the combination will have almost same impedance that of the load. As we known, ... Read More

Star (Wye) Connected SystemLet VR, VY and VB represents the three phase voltages while VRY, VYB and VBR represents the line voltages. Assume that the system is balanced, so$$\mathrm{\lvert\:V_{R}\rvert=\lvert\:V_{Y}\rvert=\lvert\:V_{B}\rvert=\lvert\:V_{ph}\rvert}$$From the circuit and phasor diagram of star connected load, it can be observed that the line voltage VRY is a vector difference of VR and VY or the vector sum of VR and –VY, i.e.$$\mathrm{V_{RY}=V_{R}+(-V_{Y})=V_{R}-V_{Y}}$$Applying parallelogram law to obtain the magnitude of this, we get, $$\mathrm{V_{RY}=\sqrt{V_R^2+V_Y^2+2V_RV_{Y}\cos\:60^{\circ}}}$$$$\mathrm{\Rightarrow\:V_{RY}=\sqrt{V_{ph}^2+V_{ph}^2+2V_{ph}^2\cos\:60^{\circ}}=\sqrt{3}V_{ph}}$$Similarly, $$\mathrm{V_{YB}=V_{Y}-V_{B}=\sqrt{3}V_{ph}}$$$$\mathrm{V_{BR}=V_{B}-V_{R}=\sqrt{3}V_{ph}}$$$$\mathrm{\because\:V_{RY}=V_{YB}=V_{BR}=V_{L}=Line\:Voltage}$$$$\mathrm{\therefore\:V_{L}=\sqrt{3}V_{ph}}$$Therefore, in a star connected system, Line Voltage = √3 × Phase VoltageAgain, refer the circuit of star connected system, it can be seen that ... Read More

Depending on the applications and switching methods, the solid state relays (SSRs) are of following types −Instant ON Solid State RelayZero Switching Solid State RelayPeak Switching Solid State RelayAnalog Switching Solid State RelayInstant ON SSRsThe instant ON SSR instantly switches on the load circuit when a sufficient input voltage is applied. It turns off when the input voltage is removed and the load current crosses the next zero. The instant ON SSRs are designed to control the inductive loads. The practical applications are in switching of contactors, magnetic valves, starters etc.Zero Switching SSRsA zero switching SSR switches on when an ... Read More

What is Turn Ratio?The turn ratio of a single phase transformer is defined as the ratio of number of turns in the primary winding to the number of turns in the secondary winding, i.e.$$\mathrm{Turn\:Ratio=\frac{Number\:of\:Primary\:Turns(N_{1})}{Number\:of\:Secondary\:Turns(N_{2})}}$$Since for a transformer, the voltage per turn being equal in both primary and secondary windings, therefore, $$\mathrm{\frac{E_{1}}{N_{1}}=\frac{E_{2}}{N_{2}}}$$$$\mathrm{\Rightarrow\frac{E_{1}}{E_{2}}=\frac{N_{1}}{N_{2}}=Turn\:Ratio}$$Also, if the given transformer is an ideal one, then E1 = V1 and E2 = V2, thus, $$\mathrm{Turn\:Ratio=\frac{N_{1}}{N_{2}}=\frac{E_{1}}{E_{2}}=\frac{V_{1}}{V_{2}}}$$In case of ideal transformer, the input volt-ampere is equal to output volt-ampere, i.e.$$\mathrm{V_{1}I_{1}=V_{2}I_{2}}$$$$\mathrm{\Rightarrow\:\frac{V_{1}}{V_{2}}=\frac{I_{2}}{I_{1}}}$$$$\mathrm{Turn\:Ratio=\frac{N_{1}}{N_{2}}=\frac{E_{1}}{E_{2}}=\frac{V_{1}}{V_{2}}=\frac{I_{2}}{I_{1}}}$$What is Transformation Ratio?The transformation ratio is defined as the ratio of output voltage to the input voltage of the ... Read More

The magnetic reluctance (S) is defined as the opposition offered by the magnetic circuit to the magnetic flux (ΦMagnetic Reluctance FormulaCase 1 – When physical dimensions of the magnetic circuit are knownThe reluctance of a magnetic circuit depends upon its length (l), cross-sectional area (a) and permeability (μ) of the material. Thus, for a magnetic circuit (as shown in the figure), The reluctance is directly proportional to the mean length of the magnetic circuit, i.e.$$\mathrm{Magnetic\:reluctance, S\varpropto\:l\:\:\:\:....(1)}$$The reluctance is inversely proportional to the cross-sectional area of the mag. circuit.$$\mathrm{Magnetic\:reluctance, S\varpropto\:\frac{1}{a}\:\:\:\:....(2)}$$The reluctance also depends upon the nature of material that makes up ... Read More

The torque-speed characteristics of a 3-phase induction motor is defined as the curve plotted between torque developed and rotational speed of the motor. It gives the information about variation in the motor torque with the change in its speed.As the torque of three-phase induction depends upon its speed but the relationship between them cannot be expressed by a simple equation. Therefore, we use the torque-speed curve to express the relationship between them.Refer the torque-speed curve, it reveals the following facts −If the full-load torque is τ, then the starting torque or locked rotor torque is 1.5 times of τ and ... Read More

A three phase induction motor has a stator and a rotor. The stator carries a 3-phase winding called as stator winding while the rotor carries a short circuited winding called as rotor winding. The stator winding is fed from 3-phase supply and the rotor winding derives its voltage and power from the stator winding through electromagnetic induction. Therefore, the working principle of a 3-phase induction motor is fundamentally based on electromagnetic induction.Consider a portion of a three phase induction motor (see the figure). Therefore, the working of a three phase induction motor can be explained as follows −When the stator ... Read More

There are two types of three phase induction motors viz. squirrel cage motor and slip ring motor. Both have different methods of starting, let see each of these one by one.Squirrel Cage Induction Motor Starting MethodsThe following four methods are available for starting squirrel cage motors −Direct On-Line (D.O.L.) StartingAs the name suggests, the induction motor is started by connecting it directly to three-phase supply. In this method, the motor draws a high starting current (about 4 to 7 times of the rated current) and at low power factor. Therefore, DOL starting is suitable for relatively small motors (up to ... Read More

Electric PowerElectric power is defined as the rate of doing work in an electric circuit. In other words, the electrical power is the energy expanded per unit time in an electric circuit, i.e.Electric Power, $$\mathrm{p=\frac{Work\:Done}{Time}\:\:\:\:...(1)}$$The electrical power is measured in watts (W).Three Phase Electric PowerAs the single phase instantaneous power (for lagging load) is given by, $$\mathrm{p=\frac{V_{m}I_{m}}{2}\cos\varphi-\frac{V_{m}I_{m}}{2}\cos(2\omega\:t-\varphi)}$$If the RMS values of voltage and currents are V and I respectively, then$$\mathrm{p=VI\cos\varphi-VI\cos(2\omega\:t-\varphi)\:\:\:\:...(2)}$$Now, consider a balanced three phase system, then the above expression can be applied for each phase of the three phases of the three-phase system. Therefore, the instantaneous power in three phases ... Read More

A solid state relay circuit has following main components −Input Circuit ComponentsControl Circuit ComponentsOutput Circuit ComponentsInput Circuit ComponentsThe input circuit of solid state relay (SSR) is part to which the control circuit is connected and the control voltage is applied to this circuit. The input circuit of SSR performs the same function as the coil of an electromechanical relay. The input control voltage of a solid state relay is controlled by means of mechanical switches, transistors, logic gates, low-voltage signals etc. The input circuit is activated by applying an input control voltage to the SSR that being higher than the ... Read More