Download 5. Load - St. Aloysius Institute of Technology, Jabalpur

ST. ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR
2015-16
SAIT, JABALPUR
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING
LABORATORY – ELECTRICAL DRIVES (B-312A)
SUBJECT - ELECTRICAL DRIVES
SUBJECT CODE: EX-702
BRANCH – EX
SEMESTER - VII (GRADING)
LIST OF EXPERIMENTS
1. To control the speed of 3-Φ Slip Ring Induction Motor using Static Kramer System.
2. To study the operation of Parallel Inverter.
3. To study the operation of Series Inverter.
4. To study and perform the operation of speed variation of PMDC motor using single
phase bridge converter.
5. To study and perform the operation of full wave half controlled SCR bridge with motor
load- PMDC motor
6. To study Speed Control of DC Motor using Single Phase Half Controlled Rectifier.
7. To study and perform the operation of speed control of Universal Motor in open loop
and closed loop system (P&PI).
8. To study Speed Control of Universal Motor using Single Phase Half Controlled
Rectifier
9. To study of Voltage Commutated Chopper.
DEPARTMENT OF ELCTRICAL & ELCTRONICES
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INDEX
S. NO.
NAME OF EXPERIMENT
PAGE NO.
DATE
DEPARTMENT OF ELCTRICAL & ELCTRONICES
SIGN.
REMARKS
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ST. ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR
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2015-16
EXPERIMENT NO.-1
Aim: - To control the speed of the 3N slip ring Induction motor using the PEC16HV10B trainer
module.
Apparatus Required:i. PEC16HV10B module.
ii. 3 phase slip ring Induction Motor
iii. 3 phase transformer.
.iv Patch chords.
v. Inductor (150mH).
vi. Tachometer.
Theory:In the static Kramer drive, the step power from the rotor of Slip Ring Induction Motor is converted to DC
voltage which is then converted to line frequency and pumped back to the ac source. As the slip power
can flow only in one direction, this system offers a sub-synchronous speed control. As the power flow is
from rotor circuits to supply, static Kramer drive offers a constant torque operation.
Speed Control of 3 Phase Slip Ring Induction Motor
Slip-Ring Induction motors are preferred in applications with the requirement of high starting torque,
high over load capacity, nearly constant speed and low starting current. Applications include driver for
Line shafts, Pumps, Lights, Generators, Winding Machines, Mills etc. This manual discusses about the
static Kramer system of slip power Recovery for speed control of 3N slip Ring induction motor module.
In this system, the slip power is connected to DC by means of a solid state rectifier and by using static
slip power recovery scheme the excess power is again fed back to the supply. The amount of power fed
back depends on the firing angle of the inverter, 3N supply voltage and DC voltage to the inverter.
PRINCIPLE OF THE STATIC KRAMER SYSTEM
A sub synchronous static Kramer drive scheme is shown. It consists of 3N slip ring induction motor, a
rectifier bridge, a filter, an inverter bridge and a transformer. In this scheme of speed control, slip power
is recovered from the rotor of 3N slip ring induction motor and is fed back to the supply line through
rectifier, inverter and transformer. The slip frequency alternating current is first rectified by Diode Bridge
and this unidirectional current is smoothed by a filter circuit after which it is fed to the 3-inverter. This
inverter is line commutated. The slip power returned to the supply depends upon the inverter terminals
and the 3-phase supply voltage. The function of the 3-phase transformer is to match the inverter output
voltage with the 3-phase supply voltage.
The speed control is achieved by varying the firing angle of the inverter. By varying firing angle, the
voltage taken from the supply is reduced and so speed of motor is reduced. The usual range of firing
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angle is for 90° to 160°for a wide speed range. The power flow through the rectifier is unidirectional,
therefore, only sub-synchronous speeds below the normal value are obtainable. Since the slip power is
returned to the supply, the scheme illustrated in Fig. results in a constant torque drive
Circuit Diagram:-
Procedure
1. Switch ON the 3N AC Power Supply to the module.
2. Switch ON the power ON/OFF switch of the module.
3. The pulse release switch should be in OFF condition and the pulse cable should not be Connected.
4. Now check the “Phase Sequence” of the 3N AC i/p at the 3N SCR Bridge firing circuit Test points r, y,
and b.
5. Now check the test points r, y,b and the secondary output of the step down transformer r, y, b are at “In
Phase” using CRO at dual mode. They should be in phase with each other. If they are not at in phase,
change the connection of the transformer and make them in Phase.
6. Now connect the pulse cable.
7. Keep the pot meter at minimum position.
8. Connect the lamp load across the 3N SCR converter output.
9. Switch ON the pulse release switch.
10. Slowly vary the pot meter and verify the converter output.
11. Now switch OFF the pulse release switch and remove the lamp load connection.
12. Now connect the 3N SCR bridge output positive to the negative of the 3N diode bridge Output at the
right side of the module.
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13. Connect the iron core inductor (0mH) tapping to the positive of the 3N diode bridge Output and
connect the 150mH output to the negative of the 3N SCR bridge output.
14. Connect the positive of 3N SCR bridge output to the negative of the 3N diode bridge Output.
15. Now switch ON the stator MCB and the three phase diode bridge MCB at the back side Of the
module.
16. Switch ON the pulse release switch.
17. Vary the pot meter gradually from the minimum position till it reaches 1450rpm (rated Speed).
Note: The motor should be in no-load condition,
18. Check the speed of the 3N slipping motor using tachometer.
Table
S. No
Firing Angle " (deg)
Speed N(rpm)
Result:
Thus the speed of the 3N slip ring Induction motor using the PEC16HV10B trainer module.
Precaution:
1. Using isolation transformer.
2. All the cables and patch chord should be made through 15A wire.
3. Don’t load the motor above 7.5A of rotor current.
4. Don’t vary the pot meter once the motor reaches To view the converter output waveform at the
CRO or the 3N AC input should be Isolated 1450 rpm
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2015-16
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ST. ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR
2015-16
EXPERIMENT- 2
AIM: - To study the operation of Parallel Invertors.
Apparatus Required:1. Single phase Parallel inverter kit (10 V/2 Amp)
2. D.C. Power supply (11V/2 Amp)
3. Firing circuit
4. CRO
5. Load
Theory:Principle of parallel operation of inverter
Balance between generated and consumed real (P) and reactive (Q) power indicates the stable
operation of a power system. Therefore, implementing effective control over P and Q is very
important from the operational and control points of view. The real (P1) and reactive (Q1) power
transferred from the inverter to the common bus or grid can be calculated as described in [1]
and from the following diagram, as shown in Fig. 1; P1 = E1Vg cos1 Zg,1 − V2 g Zg,1 cos g,1
+ E1Vg Zg,1 sin1 sin g,1 (1) Q1 = E1Vg cos1 Zg,1 − V2 g Zg,1 sin g,1 − E1Vg Zg,1 sin1 cos
g,1 (2) Here E1 and Vg represent the inverter output voltage and grid voltage, respectively. For
only real power transfer, Vg and E should have the same amplitude with a phase angle
difference. Different amplitude of voltage with the same phase will give a reactive power
circulation. When both of the magnitude and phase angle differ between the two voltage
sources, it causes real and reactive power flow. Control of frequency dynamically controls the
power angle and hence, the real power flow. As the output impedance of the inverter
CIRCUIT DIAGRAM:-
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Procedure
CRO Settings



Time (X) Axis : 5ms/div
Voltage (Y) Axis : 20V/div
Probe: 1:10 CRO Probe is suitable one.
Initial Settings


Ensure the 230V Power Supply is in proper with Tester.
Ensure the CRO is working properly with probe checking and proper ground line axis.
Experiment Steps








The connections are made as per the circuit diagram given above.
When the load is Rheostat, then it must be in maximum position.
Turn on the SW2 and observe the waveform across between G1&K1 and G2&K2. And ensure it
is like triggering pulse given in model graph.
Turn ON SW1 and observe the waveform across the R load.
Measure the X-axis time interval of ON time of Load Voltage and OFF Time of Load Voltage.
Adjust the value of Firing angle and note down the load voltage and frequency
Repeat the step 5.
Repeat the experiment for different values of α and note down Vo.
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Table
S.No
Result
F output
(Hz)
Scr off time
(ms)
Above experiment are done and results are verified
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EXPERIMENT-3
AIM: - To study the operation of series Invertors.
Apparatus Required:- 1. Single phase series inverter kit (10 V/2 Amp)
2. D.C. Power supply (11V/2 Amp)
3. Firing circuit
4. CRO
5. Load
Theory
Basic Series Inverter Circuit
Fig.1 shows the circuit of a series inverter. The commutating elements L and C are such that R,
L and C form an under damped circuit. The capacitor has an initial voltage E c, Thyristorised
Th1 is turned on first by an external pulse. Since Th 1 is already forward biased (due to dc
voltage V), Th1 starts conducting and a current I flows in the circuit through Th 1, C, L and load.
Because of under damped nature of the circuit the current is not constant but has the wave
shapes as shown in fig.2. It rises to maximum value and then decreases to zero. When the
current is at its peak value, the voltage across capacitor is nearly equal to supply voltage V.
After this the current starts decreasing but the voltage across the capacitor continues to
increase as it is still getting charged.
When the current becomes zero the voltage across capacitor is maintained at V+E c. The
voltage across L is zero. The time interval ab must be more than time toff of the thyristor.
This is necessary to ensure that the stored charges in Th1 are reduced to zero so that at point b
Th1 is in completely off state. At point b when Th 2 is turned on by an external gate pulse the
anode of Th2 is positive (with respect to cathode) due to charge on capacitor
Th2 starts conducting. The capacitor discharges and the current I flows through the circuit in
the direction opposite to that in the start. The current reaches its negative peak value and then
decreases to zero at point c when Th2 is turned off. The above sequence of operation is
repeated in the next cycle when Th1 is turned on. The frequency of the output voltage is –
Circuit diagram
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Procedure
1.
2.
3.
4.
Connecting are made as show n in the circuit diagram.
Select values of c =
,L=
Set input voltage to 5 volts.
Apply trigger voltage; observe corresponding output voltage (ac voltage and wave
forms) at load terminal.
5. Note down the voltage & frequency of output wave form. 6) The o/p ac voltage is
almost equal to the two times of the dc i/p voltage
OBSERVATION: Note down the value of voltage and frequency. And draw the waveforms.
S.N.
VOLTAGE (VS)
CURRENT(IS)
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RESULT: The waveforms are determined. The o/p ac voltage is almost equal to the two times of the dc
i/p voltage
PRECAUTIONS: 1) All the connection should be tight...
2) The electrical current should not flow the circuit for long time, otherwise its temperature will
increase and the result will be affected.
3) It should be care that the values of the components of the circuit is does not exceed to their
ratings (maximum value).
4) Before the circuit connection it should be check out working condition of the entire
Component.
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EXPERIMENT-4
AIM: - To study and perform the operation of speed variation of PMDC motor using single
phase bridge convertor.
Apparatus Required:1 .Power thyristors
2. Rheostat
3. CRO
4. Transformer (1-phase) 230V/24V
5. Connection wires
Theory:Speed Control of DC Series Motor
Speed control of dc series motor can be done either by armature control or by field control.
Armature Control of DC Series Motor
Speed adjustment of dc series motor by armature control may be done by any one of the
methods that follow,
1. Armature resistance control method: This is the most common method employed. Here the
controlling resistance is connected directly in series with the supply to the motor as shown in
the fig. diagram
The power loss in the control resistance of dc series motor can be neglected because this
control method is utilized for a large portion of time for reducing the speed under light load
condition. This method of speed control is most economical for constant torque. This method of
speed control is employed for dc series motor driving cranes, hoists, trains etc.
2. Shunted armature control: The combination of a rheostat shunting the armature and a
voltage applied to the armature is varies by varying series rheostat R 1. The exciting current can
be varied by varying the armature shunting resistance R2. This method of speed control is not
economical due to considerable power losses in speed resistances.
Here speed control is obtained over wide range but below normal speed. Rheostat in series
with the armature is involved in this method of speed control. The
3. Armature terminal voltage control: The speed control of dc series motor can be accomplished
by supplying the power to the motor from a separate variable voltage supply. This method
involves high cost so it rarely used.
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Field Control of DC Series Motor
The speed of dc motor can be controlled by this method by any one of the following ways –
1. Field diverter method: This method uses a diverter. Here the field flux can be reduced by
shunting a portion of motor current around the series field. Lesser the diverter resistance less is
the field current, less flux therefore more speed. This method gives speed above normal and
the method is used in electric drives in which speed should rise sharply as soon as load is
decreased.
2. Tapped Field control: This is another method of increasing the speed by reducing the flux
and it is done by lowering number of turns of field winding through which current flows. In this
method a number of tapping from field winding are brought outside. This method is employed in
electric traction.
CIRCUIT DIAGRAM:-
As in PMDC motor the field is produced by permanent magnet, there is no need of drawing field
coils in the equivalent circuit of permanent magnet dc motor. The supply voltage to the
armature will have armature resistance drop and rest of the supply voltage is countered by back
emf of the motor. Hence voltage equation of the motor is given by,
Where I, is armature current and R is armature resistance of the motor. Eb is the back emf and
V is the supply voltage.
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Procedure
1) Connecting are made as show n in the circuit diagram.
2) Select values of c =, L =
3) Set input voltage to 5 volts.
4) Apply trigger voltage, observe corresponding output voltage (ac voltage and wave forms) at
load terminal.
5) Note down the voltage & frequency of output wave form. 6) The o/p ac voltage is almost
equal to the two times of the dc i/p voltage
Advantages of Permanent Magnet DC Motor or PMDC Motor
PMDC motor has some advantages over other types of dc motors. They are:
1.
2.
3.
4.
No need of field excitation arrangement.
No input power in consumed for excitation which improves efficiency of dc motor.
No field coil hence space for field coil is saved which reduces the overall size of the motor.
Cheaper and economical for fractional kW rated applications.
Disadvantages of Permanent Magnet DC Motor or PMDC Motor
1. In this case, the armature reaction of DC motor cannot be compensated hence the
magnetic strength of the field may get weak due to demagnetizing effect armature reaction.
2. There is also a chance of getting the poles permanently demagnetized (partial) due to
excessive armature current during starting, reversal and overloading condition of the motor.
3. Another major disadvantage of PMDC motor is that, the field in the air gap is fixed and
limited and it cannot be controlled externally. Therefore, very efficient speed control of DC
motor in this type of motor is difficult.
Applications of Permanent Magnet DC Motor or PMDC Motor
PMDC motor is extensively used where small dc motors are required and also very effective
control is not required, such as in automobiles starter, toys, wipers, washers, hot blowers, air
conditioners, computer disc drives and in many more.
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EXPERIMENT-5
AIM: - To study and perform the operation of full wave half controlled SCR Bridge with motor
load PMDC motor.
Apparatus required: Trainer kit,
Patch cards,
Multi meter,
Connection wires
Theory:-
The single phase fully controlled rectifier allows conversion of single phase AC into DC. Normally this is
used in various applications such as battery charging, speed control of DC motors and front end of UPS
(Uninterruptible Power Supply) and SMPS (Switched Mode Power Supply). All four devices used are
thyristors. The turn-on instants of these devices are dependent on the firing signals that are given. Turnoff happens when the current through the device reaches zero and it is reverse biased at least for duration
equal to the turn-off time of the device specified in the data sheet. When an uncontrolled (diode)
converter is to be simulated, all 4 devices should be fired at a delay angle of 0⁰. When a semi-converter is
to be simulated, the lower two devices can be fired at 0⁰ and 180⁰ respectively and the upper two devices
are fired at α and 180⁰+α.
Normally in a fully controlled converter, the current transfer takes place from T1 to T2 instantaneously
without any time delay. But when a source inductance is present, the stored energy in Ls has to be
expended before the current transfer or commutation takes place from T1 to T2. Because of this, T1
and to T2 will conduct simultaneously from α to α+u (where u is the overlap angle) causing short
circuiting of the DC load during the overlap period u. This is known as commutation overlap.
When the thyristor converter has a resistive load, both the DC current and voltage will be in phase. As
soon as the positive half-cycle ends, the current through thyristors T1 and T11 will be reaching zero this
causes the voltage pulses to be spreading from firing angle α to π as shown. Both the current after which
they would be reverse biased due to the supply voltage embarking into negative half-cycle. in the DC link
(Idc) and the current in the source (Is) will be discontinuous on completion of this unit you will under
frequency pulse train can be used after validating the same with line synchronized SCR triggering.
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Circuit diagram
Procedure
1) Connect the circuit as per wiring schedule given bellow. The arrangement ensures SCR full wave fully
controlled bridge receiving firing pulses from ‘Ramp firing Section’ of INV/CON Panel.
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2) Put ON the rectifier with 100W lamp load.
3) Observe the wave forms per the diagram.
4) Keep 2p6w switch on LSPT card at 4’Th position.
Conclusion
High frequency getting turns on the SCR repeated, and fires it at appropriate time when Anodes Cathode
is forward biased current is ready to pass overcoming the inductance effect.
The arrangement also helps in spreading the current though out the
SCR junction area at a faster rate and avoids hot spot formation and subsequent damage of SCR.
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EXPERIMENT-6
AIM: - To study speed control of DC Motor using single phase half controlled Rectifier PMDC
motor.
Apparatus Required:Trainer kit,
Patch cards,
Multi meter,
Connection wires
Theory:The single phase half controlled rectifier allows conversion of single phase AC into DC. In this we understand how
controlled half wave SCR based converter functions and also its associated waveforms.TH1 conducts only when
its anode is positive w.r.t & a synchronized gate pulse is applied to it.
During positive half cycle of input AC (as anode is positive w.r.t cathode) if gate pulse is given at an angle α (ref
wave form fig) then SCR will conduct for π- α.During negative half cycle of input this SCR is reverse biased so it
will not conduct even gate pulse is applied.
We can vary the conduction period of an SCR by varying the firing angle. The angle between starting of AC
waveform and firing instance is known as firing angle where as the angle between firing instance and end of the
respective half cycle is known as conduction angle. if firing angle increases then conduction angle decreases &
vice versa.
V0 =1/2π 0∫∏ VmSinwtd (wt) = Vm/2π (1+cos α)
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Procedure:For this configuration only one SCR is required. Choose any one SCR from con/Lnv panel.
The Lamp load is available on top board .For input AC use 100W lamp as load.
Make the connections as per wiring sequence given above to form half wave-controlled converter.
All connection should be done using patch chords provided.
The firing pulse selector switch should be kept on 4th position for getting required gate pulse.
By doing so you are selecting UJT relaxation oscillator based SCR firing. Now put on AC mains.
Connect CRO across load & observe change in conduction angle by varying the conduction angle control pot on
LSPT panel. Take the reading as per following table. Use 300V DC meter from top board for this purpose
CIRCUIT DIAGRAM:-
Table
Output DC Half wave control.
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ST. ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR
Sr
Firing Angle
Peak
Average
voltage(Vm) voltage Vdc
2015-16
On 300V DC
meter
Conclus
ion
SCR can
be used
as
controlled half wave Rectifier. It can be fired at any point in half conduction cycle. Thus
controlled DC voltage
EXPERIMENT-7
AIM: - To study speed control of Universal motor in open loop & close loop( P&PI).
Apparatus Required:Trainer kit,
Patch cards,
Multi meter,
Connection wires
Theory:The Universal Motor is a phase angle triac controller having all the necessary functions for universal motor speed
control in washing machines. It operates in closed loop configuration and provides two ramp possibilities. The
TDA 1085C triggers a triac accordingly to the speed regulation requirements. Motor speed is digitally sensed by a
tachogenerator and then converted into an analog voltage.
The speed set is externally fixed and is applied to the internal linear regulation input after having been submitted to
programmable acceleration ramps. The overall result consists in a full motor speed range with two acceleration
ramps which allow efficient washing machine control (Distribute function). Additionally, the TDA 1085C protects
the whole system against AC line stop or variations, over current in the motor and tachogenerator failure.
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Procedure:Thyrister actuator panel EMT9 is used to control DC power & same is supplied to universal motor.
1. Make the connection as per wiring sequence provided.
2. Do not connect motor terminal to respective sockets.
3. Use 230V/100W lamp load, to check proper control (smooth o/p) voltage regulation.
4. Switch off the trainer ensures some load is applied on motor.
5. Before switch ON the trainer ensure some load is applied on motor
6. Keep SP pot on EMT9 at minimum position
7. Now switch ON the trainer ensure some load EMT9 to get I/P to motor
CIRCUIT DIAGRAM:-
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Table
Open loop, Close loops (P&PI)
Sr
I/P motor (V)
1A
SPEED (OPEN)
(p)
(PI)
Conclus
ion
1) Do not run universal motor without some load as speed.
2) Speed drops in open loop considerably.
3) Speed drops somewhat in close loop (p)
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4) Speed is well regulated under close loop(PI)
EXPERIMENT-8
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AIM: - To study speed control of Universal Motor using single phase half controlled Rectifier
PMDC motor.
Apparatus Required:i) Universal kit
ii) CRO
iii) Batch cards motor
iv) Universal motor
Theory:a) Firing Circuit: This unit generates line synchronized 2 pulse transformer isolated trigger pulses. These
trigger pulses can be used to trigger.
(i) Single phase AC phase control using SCR’s ( Anti parallel SCR’s)
(ii) Single phase AC phase control using triac.
(iii) Single phase half wave rectifier (single SCR)
(iv) Single phase Full wave rectifier (Two SCR’s)
(v) Single phase half controlled bridge rectifier (Two SCR’s & Two diodes) power circuits. The firing
circuit is based on zero crossing detector, ramp generator, op-amp comparator and amplifier / pulse
transformer isolation method. Front Panel Details: 1. Power: Mains switch for firing circuit with built in
indicator 2. Firing angle: Potentiometer to vary the firing angle from 180o to 0o 3. SCR / Triac: Selection
switches for trigger O/P 1 for SCR/Triac 4. OFF/ON: Switch for trigger O/Ps with soft start feature. 5.
Trigger O/Ps: T1 / TR: T2: Trigger O/P for SCR2 Power Circuit: The power circuit consists of 2 SCR’s, 3
diodes and a Triac. The power devices are mounted on suitable heat sink for power dissipation. The
snubber circuit is connected for dv/dt protection. A fuse is also provided in series with the devices for
short circuit or over current protection. In the input side a 59 MCB is provided to switch ON/OFF the
supply to the power circuit. A voltmeter and an ammeter is provided to measure the Input / Output
voltage and current. Front Panel Details: 1. AC input: Terminals to connect AC input 2. AC output : AC
supply terminals after the MCB to be connected to power circuit. 3. MCB: A 6A / 2 pole MCB for ON/OFF
the AC supply to the power circuit 4. T1 & T2 : SCR’s 16 Amps / 600 volts 5. D3 & D4 : Diodes – 16amps /
600V 6. Dm: Freewheeling diode 7. TR : Triac – 10 amps / 600 volts 8. Voltmeter: 3 ½ Digit digital AC/DC
Voltmeter to measure input / output voltage 9. Ammeter : 3 ½ Digit digital AC/DC Ammeter to measure
current
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Procedure:1) Make the inter connections in the power circuit as given is the circuit diagram.
2) Switch ON the firing circuit and observe the trigger outputs.
3) Make sure that the firing pulses are proper before connecting to the power circuit.
4) Then connect the trigger output from firing circuit to corresponding SCR’s / Triac.
5) In the power circuit initially set the AC input to 30 volts. Switch ON and MCB. Switch ON the Trigger outputs
switch. Select the SCR / Triac selection switch and observe the output wave forms across ‘R’ load by varying the
firing angle potentiometer.
6) If the output wave form is proper then you can connect the motor & increase the input voltage to rated value 0230V gradually. Vary the firing angle and note down output voltage and speed of the motor.
Note:
1) If you are not getting the output after all proper connections interchange AC output terminals, after switch
OFF the MCB. This is just to synchronize the power circuit with firing circuit.
CIRCUIT DIAGRAM:-
DEPARTMENT OF ELCTRICAL & ELCTRONICES
Page 29
ST. ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR
2015-16
Table
Sr
Input Voltage Vin Firing Angle
Output Voltage –
V0
Output
Current
I0
Speed
RPM
Result:
Thus the
speed
control of
Universal
motor is
performed
by varying
armature
voltage
through
phase
controlled
converter
Conclusion
1) Do not run universal motor without some load as speed.
2) Speed drops in open loop considerably.
3) Speed drops somewhat in close loop (p)
4) Speed is well regulated under close loop(PI)
DEPARTMENT OF ELCTRICAL & ELCTRONICES
Page 30
ST. ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR
2015-16
EXPERIMENT-9
AIM: - To study of voltage Commutated Chopper..
Apparatus Required:i) Trainer kit
ii) CRO
iii) Batch cards motor
iv) Chopper
Theory:The junctions of any semiconductor exhibit some unavoidable capacitance. A changing
voltage impressed on this junction capacitance results in a current, I = C dv/dt. If this
current is sufficiently large a regenerative action may occur causing the SCR to switch to
the on state.
This regenerative action is similar to that which occurs when gate current is injected. The
critical rate of rise of off-state voltage is defined as the maximum value of rate of rise of
forward voltage which may cause switching from the off-state to the on-state. Since dv/dt
turn-on is non-destructive, this phenomenon creates no problem in applications in which
DEPARTMENT OF ELCTRICAL & ELCTRONICES
Page 31
ST. ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR
2015-16
occasional false turn-on does not result in a harmful affect at the load. Heater application
is one such case. However, at large currents where dv/dt turn-on is accompanied by partial
turn-on of the device area a high di/dt occurs which then may be destructive.
The majority of inverter applications, however, would result in circuit malfunction due to
dv/dt turn-on. One solution to this problem is to reduce the dv/dt imposed by the circuit to
a value less than the critical dv/dt of the SCR being used. This is accomplished by the use
of a circuit similar to those in Figure 3.8 to suppress excessive rate of rise of anode
voltage. Z represents load impedance and circuit impedance. Variations of the basic circuit
are also shown where the section of the network shown replaces the SCR and the R-C
basic snubber. Since circuit impedances are not usually well defined for a particular
application, the values of R and C are often determined by experimental optimization.
A technique can be used to simplify snubber circuit design by the use of nomographs
which enable the circuit designer to select an optimized R-C snubber for a particular set of
circuit operating conditions. Another solution to the dv/dt turn-on problem is to use an
SCR with higher dv/dt turn-on problem is to use an SCR with higher dv/dt capability. This
can be done by selecting an SCR designed specially for high dv/dt applications, as
indicated by the specification sheet. Emitter shorting is a manufacturing technique used to
accomplish high dv/dt capability
Procedure:1) Make the inter connections in the power circuit as given is the circuit diagram.
2) Switch ON the firing circuit and observe the trigger outputs.
3) Make sure that the firing pulses are proper before connecting to the power circuit.
4) Then connect the trigger output from firing circuit to corresponding SCR’s / Triac.
5) If the output wave form is proper then you can connect the motor & increase the input
voltage to rated value 0-230V gradually. Vary the firing angle and note down output
voltage and speed of the motor.
Note:
DEPARTMENT OF ELCTRICAL & ELCTRONICES
Page 32
ST. ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR
2015-16
6) If you are not getting the output after all proper connections interchange AC output
terminals, after switch OFF the MCB. This is just to synchronize the power circuit with
firing circuit.
CIRCUIT DIAGRAM:-
Table
Sr
Input Voltage Vin Firing Angle
Output Voltage –
V0
Output
Current
I0
DEPARTMENT OF ELCTRICAL & ELCTRONICES
Speed
RPM
Page 33
ST. ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR
2015-16
Result:
Above
experimen
t are done
and
results are
verified
Conclusion
1) Do not run universal motor without some load as speed.
2) Speed drops in open loop considerably.
3) Speed drops somewhat in close loop (p)
4) Speed is well regulated under close loop (PI)
DEPARTMENT OF ELCTRICAL & ELCTRONICES
Page 34