This electrical setup is used in POWER ELECTROINCS LAB to study the No load characteristics of a DC SHUNT motor. A shunt motor is connected in the same way as a shunt generator. The field windings are connected in parallel (shunt) with the armature windings. Once you adjust the speed of a dc shunt motor, the speed remains relatively constant even under changing load conditions. One reason for this is that the field flux remains constant. The DC shunt motor has two windings, namely Armature winding and Field winding. In order to activate the motor, both Field and armature windings have to be given sufficient power to enable them to start under controlled condition. Since the consumption of current by the armature is large compared to field current, when the motor is at rest, it is necessary to excite the armature gradually to enable the motor to start properly. It means, the field winding must provide the starting current, and eventually the current flowing through the field winding can be minimized, once the rated speed is achieved. Hence this leads to a conclusion that the armature current has to be provided in a controlled atmosphere starting from rest to rated speed. This needs further discussion for considering load conditions. There are several methods adopted for this purpose. The speed control is achieved by varying the firing angle of the thyristor, which is controlling the armature winding. In this experimental setup, the DC Motor is mechanically coupled to a Tachogenerator, to provide necessary feedback for the measurement of speed as set by the speed control potentiometer. This feedback signal is further processed through conditioning circuits and a suitable signal is provided to the triggering controller. The trigger circuit is further processed through pulse transformer, before applying as Gate signal to the Thyristors. This trainer has necessary power supplies, signal conditioners, feedback circuits, pulse triggering circuits, power driver, Ramp generator for smooth starting of drive, necessary current amplifiers and current limiting circuit, SCR firing controller, suitable pulse transformer for isolation and Opto-Isolator circuits. The scope of this trainer is limited to the extent of observing various waveforms at different points along the whole circuit flow, till the DC motor using an external 15MhZ oscilloscope. Specifications:
There are two options of Motors are available. You must choose the correct option before placing your purchase order. The options are Option ?1, BENN make DC Motor with the following specifications will be supplied
HP =1; RPM = 1500; Excitation voltage: 180V DC, Current handling: 5.1A;Class: B Option ?2, the DC Motor with the following specifications will be supplied.
HP =0.25;RPM = 1500;Excitation voltage: 220V DC, Current handling: 1A;Class: B The following are common specifications for the rest of the equipment:
? Tachogenerator : It provides 33V DC / 1000 RPM
? Speed : 1500RPM
? Input voltage to the system : 220 V AC / Single phase
? Speed Indicator : An external Digital meter is supplied to indicate RPM
? Speed variation : 0 to 1500 RPM continuously variable. The Thyristor used works on CLIP ASSEMBLY technology with
a) An average ON-State current of 10A @ 180? conduction angle.
b) Critical rate of rise of on-state current 50 A/ ?sec.
c) Peak gate current with 4A.
d) Average gate power dissipation 1 Watt.
e) Maximum peak reverse gate voltage 5 V.
f) Threshold voltage 0.77V.
g) Average gate power dissipation 1Watt. The following test points are available for observations using an Oscilloscope.
? Tachogenerator feedback voltage.
? Signal and summing amplifier.
? Signal conditioner output before the UJT firing circuit
? Relaxation oscillator output
? Pulse transformer output
? Anode and Cathode points of Thyristors for ARMATURE control
? Anode and Cathode points of FIELD windings Using the above trainer, the user can experiment and study
1. Plot the graph of Speed Vs Armature/ Field voltages.
2. Speed Vs No load characteristics. This is a table type model. However the motor assembly, need to be mechanically fastened to the table top with Nut and Screw arrangement for stability of operation at high speeds.The entire system is supplied with functional block diagrams printed on the front panel for easy identification of various blocks of the controller. Various test points are provided on the front panel for observations of signals at various points of the circuit. An external 15MhZ, dual beam Oscilloscope is required to observe the waveforms. |
