FAMILIARISATION WITH P.E. COMPONENTS A. SINGLE PHASE PAC USING TRIAC. Object : To study a) The triggering circuit of an A.C. phase angle controller using a triac. b) The performance with a resistive load. c) The performance with an inductive load. Apparatus : 1) Experimental set up. 2) Dual trace oscilloscope 3) M.I. meters: voltmeter ( 0-300V ) - 2nos. Ammeter (0 3 A) - 1 no. 4) 185 ohm rheostat, inductance coil. 5) 0.1 ohm std. resistance. Procedure : a) Study of triggering circuit 1. Connect the 185 ohm rheostat between L 1 and L 2. Set the potentiometer R on the panel to extreme ccw. Position ( R is maximum). Connect the Input to a 230 v ac supply via a DPST switch. 2. Clip GND of oscilloscope probe to point x. The two line terminals are connected to Z and Y. 3. Set the oscilloscope to CHOP mode and observe the load voltage (inverted) in one trace and the gate cathode voltage in the other. Vary the resistance R and observe variation in the phase shift of the trigger pulses. Trace a typical display ( invert the first display if y-invert is available on the oscilloscope.) B. Resistive load. 1.With the same circuit as in A, plot trace xz for several values of the Display angle α. Measure off α from the oscilloscope display. Note also the readings of the meters. Tabulate
VAC IN VAC OUT (measured) VAC OUT (calculated) Switch off power. C. Inductive load. Questions : Connect the inductance across terminals L 1 and L 2. with α maximum, switch on the supply. Observe the load voltage and current waveforms for several values of the delay angle.(gnd to x, line 1 to z and line 2 to L 1 ). Tabulate the readings of the meter and the calculated value of VAC OUT as in the last table. Switch off after observations for minimum values of delay angle have been noted. 1. Design and sketch a triggering circuit for a three phase PAC using triacs. 2. What other power semiconductor devices can be used for control of AC power? 3. Comment on the harmonics in the output voltage and current.
STUDY OF PULSE TRANSFORMER. Object : Apparatus : Procedure : 1. To determine the volt secs of a given pulse transformer. 2. To study the magnetising and de magnetising circuit of a pulse Transformer. 3. To study the network of a three winding pulse transformer. 1. Set up which includes the pulse transformer, its driving amplifier, pulse Generator and an SCR. 2. Oscilloscope ( double- trace). 3. 0 30 V, 2A DC Regulated power supply. A. To determine the volt secs of a given pulse Transformer. The pulse generator is a simple single chip generator of a variable frequency/variable duty ratio signal. The output of this 555 IC is fed to a simple common emitter transistor amplifier which drives the pulse transformer. Plug in the chord of the set-up to a 230V, 50 Hz supply. Study the waveform of the pulse generator at the resistance R 3. Control the resistance R A and R B on the panel and observe how the pulse width and frequency is controlled. Fix it to a low D.R. and a mid frequency Range.
No-Load test: Connect a PSU to the (+)ve and (-)ve power terminals provided, set PSU to about 5V and then switch it on. Observe the waveform at 2,3 terminals of pulse transformer. Observe also at 4,5 terminals. Increase the input pulse width, maintaining the low frequency,till the Trailing edge of the pulse just starts rounding off. Measure t and the v as shown Volt secs = v x t Note the ammeter reading of the PSU. Repeat for v = 2.5, 6, 10 volts. Increase the pulse width slightly. A waveform shown by the discontinuous line appears on the oscilloscope. The transformer is now in saturated condition. Observe the output voltage at points 4,5. Touch the amplifier transistor and note the current supplied by the PSU. Comment on your observation. Note primary waveforms in each case. Specially note the peak negative Voltage and transistor voltage.
B. Demagnetisation Network With the secondary open circuited, connect the following components Across terminals 2,3 (primary) of the pulse trasnsformer. i) Resistance R 1. ii) Resistance and diode in series R 1 & D 2 with anode of D 2 Connected to 3. iii) Diode and zener D 2 & Z with cathode of z at 3 and cathode of Diode at 2. For each case, at 5V supply voltage, note the maximum duty ratio possible without saturation of the transformer, the frequency,the primary voltage waveform, the PSU current and the transistor voltage waveform. In each case,first set the ON time till rounding off of the trailing edge is just absent. Now set the OFF time ( by decreasing it from a high initial value) till the rounding off again is just about to appear. Now connect the terminals 4,5 (secondary)to the circuit shown to the right of it in Fig. 13.This may be done for any of the demagnetising networks. Observe the primary (2,3), secondary(4,5), and gate cathode voltages, Note also the transistor voltage and the PSU current. Short resistance R 2 and repeat the above observations. Comment on your observations. C. Three winding Transformer Remove all demagnetising circuits. Operate the pulse transformer as in A( no load test). Note voltages 1,2 and 2,3.Estimate the turns ratio of the tertiary to the primary. Connect cathode of diode, D 1 to 1 and again find the maximum ON time and minimum off time as in B. Questions : 1. Explain all demagnetising methods. 2. Explain the use of coil 1, 2. 3. Comment on the utility of resistance R2.
Objective: To study D. DRIVE CIRCUIT OF THYRISTOR & BJT. Thyristor gate characteristics. Thyristor static output characteristics. Pulse triggering circuit for thyristors. Switching characteristics of thyristors. Base drive circuit for transistors. Switching characteristics of transistors. Experimental procedure: (i) Thyristor DC triggering. Fig. 1 Make connections as shown in Fig.1. Insert maximum gate resistance (Rg). Put the multimeter in dc current measurement mode( 200ma) Connect channel 1 and chanel 2 of the CRO across the thyristor and across the current sensing resistance respectively. Use a x 10 probe for channel 1 and invert channel 2. With the maximum load resistance inserted in the circuit turn on ac power supply and adjust the variac output to 20 volt peak. With output voltage set to zero turn on the adjustable dc power supply. Set the Coarse adjustment knob in such a position that the firing angle of the thyristor Can be controlled between 0 and 90 degrees with the fine adjustment knob Alone.
Put the CRO in X Y mode with X axis representing the voltage across the Thyristor. Set the firing angle of the thyristor to zero. Note the gate current and the thyristor forward blocking voltage. Also observe the waveform of Thyristor voltage(v AK ), gate voltage(v GK ) and load current waveforms. Increase the firing angle in small steps up to 90 degrees and make the same observations at each step. Thyristor gate characteristics and static output characteristics are obtained from this experiment. Pulse triggering : Trace the pulse triggering circuit shown in Fig-2 and locate the test points. Connect ac and dc power supply to the firing circuit. Set the dc output voltage To 12 volts and observe waveforms at all test points. Disconnect power supplies. Make connections as shown in Fig.3 Connect CRO probe as in the case of DC triggering. Connect ac(220volts) and dc (12 volts) power source to the triggering circuit. From the variac apply around 20 volts (peak)across the thyristor and load resistance and trigger the thyristor at a suitable angle. Trigger the thyristor at the turn on and turn off instants of the thyristor, observe The waveform at expanded scale.( For this part trigger the CRO externally from The signal attp5 of Fig.2 ) Report : Circuit diagram with all component rating. Working principle for each circuit. Thyristor voltage, gate voltage and load current waveform during dc triggering. Thyristor voltage ad load current waveforms during pulse triggering. Plots of i) Forward breakover voltage gate current. ii) Thyristor static output characteristics. iii) Thyristor switching characteristics