CHOPPER CONTROLLED DC DRIVEINSTRUCTED BY: Dr. BuddhikaNAME: D.K.PathiranaINDEX NO: 080332PGROUP: DG7DATE OF PER : 16/03/2012DATE OF SUB : 30/04/2012OBSERVATION SHEETNAME: D.K.PathiranaINDEX NO: 080332PGROUP: DG7DATE OF PER: 16/03/2012DATE OF SUB: 30/04/2012INSTRUCTED BY : Dr. BuddhikaObservations(b) Measurement of chopper performance with non back emf loadSpeed-InputVL (V)IL (A)Potentiometer Setting07.60.30.02117.60.30.02129.60.360.042316.60.620.085423.60.90.155291.120.196371.440.267451.720.308532.020.369612.320.4410622.380.45Chopper input DC voltage = 131.8V(c) Measurement of DC motor speed control characteristicsSpeed-InputSpeed (rpm)Potentiometer Setting0208120021933351452657066932711068135291590101616Armature Voltage and Armature Current waveforms corresponding to step 7Field Voltage and Field Current waveforms corresponding to step 7(d) Study of control circuit operation Gate waveform of TH2Saw tooth waveformComparator control input waveformComparator output waveformGate waveform of TH1CALCULATIONS(b) Measurement of chopper performance with non back emf loadVL = . VdcHere Vdc = 131.8VSpeed-InputCalculated VL (V)Practical VL (V)Potentiometer Setting00.0212.767.610.0212.767.620.0425.539.630.08511.2016.640.1519.7723.650.1925.042960.2634.263770.3039.544580.3647.455390.4457.9961100.4559.3162Discussion(1) Explain why there is a voltage at the load even when the speed-input potentiometer at minimum.In the TH1-ON time period, a voltage is applied to the motor terminal because the armature is connected to the source. During this period an increasing current flows through the armature. When the TH1-OFF, motor current freewheels through diode named as Df. During this period the energy stored in the armature inductance flows through the freewheeling diode and a zero voltage appears across the armature. And also the motor current is decreases. If the current decreases to zero value before the next TH1-ON state, discontinuous current conduction occurs. During this period back emf of the armature winding due to the field flux will appear across the armature. It is the reason for a voltage to be appeared across the armature even at the speed potentiometer is at minimum.Why the speed-input potentiometer, in the first few steps did not respond to the motor speed, how the problem can possibly be eliminated?In the first few steps, the on-period of TH1 is very small. This result in a small average voltage applied across the motor. Therefore the average power of the motor becomes small. This results in lower speeds. Even if there is a small increase in the speed it is not fairly reflected when the speed level is low. However after the few first steps of the potentiometer settings a fair speed variation can be identified. This is due to the increased on-period and hence increased average power. The motor is operating according to the PWM output given by TH1. This problem can be eliminated by setting the signal range set potentiometer input to a lower value at the beginning.(3) State why field current waveform contains less ripples compared with the field voltage waveform.From the observation it can be seen that the field current wave form is almost constant and has less ripples while the voltage possesses huge ripples. The field winding has a high inductance and this high inductance will not allow sudden rapid changes in the field current. So it shows a fairly constant (average) value. But this is not the case in the field voltage waveform.(4) Comment on the shape of armature voltage and current waveforms of part-(C)The output voltage consists of several harmonics some times up to 9th harmonic. This is a alternating square wave pulse with frequency 1/T(T is the turn-on time of TH1). Having being a square wave the field voltage consists of a considerable amount of harmonic contents. But in the current waveform has near sinusoidal shape causing very low harmonic content. This reason causes the current waveform to be smoother than the voltage waveform. As shown in the observations the field voltage waveform looks like more to a square waveform and the field current to a sinusoidal form.(5) Comment on the control circuit waveforms in part (D)The reference for the observation of the control circuit waveforms is the pulse generator output.Gate waveform of TH2 (CT3) is a square wave pulse with a slight distortion. Thyristor 2 is used to turn off the thyristor 1. Triggering TH2 applies a reverse blocking voltage across TH1.Sow tooth waveform (CT4) allows setting the duty factor. CT5 is the output waveform, which is given out with respect to VR5 setting. Changing the VR5 we could set the duty factor as we desired. Comparator outputs a compensating triggering signal (CT6) to TH1. The gate waveform of thyristor TH1 is a square wave pulse as expected and is fires at the falling edge of CT6. CT7 is the actual pulse appearing across the gate, which allows firing the main thyristor.(6) The functions of potentiometers VR1, VR2, VR3, VR4 and VR5In dc drives, speed can be sensed without a tachometer when field current is held constant. Use is made of the fact that the back emf is directly proportional to speed when the flux is held constant. The back emf is measured by deducting from motor terminal voltage a signal equal to its armature resistance drop. Accuracy of measurement is affected by difficulty in sensing armature current accurately due to the presence of ripple, variation of flux due to field supply disturbances and variation of temperature of field armature windings etc. The use of potentiometer VR1 is to amplify the armature voltage feed back.The amplified armature voltage feedback is compared with the set-speed input (the desired output) and the generated error signal is sent through the potentiometer VR2 by varying as desired.Armature current fed back is used to compensate for the armature resistance drop and for stability reasons. Therefore the purpose of potentiometer VR3 is to amplify the armature current feed back. VR2 and VR4 can eliminate the faulty responses that occur due to the transient conditions. VR5 potentiometer adjusts the signal range.(7) Practical applications of chopper drivesIn chopper-controlled dc drives, energy is received from dc sources, such as batteries or overhead trolley wires. This is useful in battery-driven vehicles, particularly forklift trucks and in traction systems (which means electric trains and trams). Besides, choppers are used in areas where controllability of speed is necessary. For example trolley lines, production lines, milling machines and robotics etc.One important feature of chopper control is that regenerative braking can be carried out up to very low speeds even when the drive is fed from a fixed voltage dc source.