Two-wire remote control unit for speed control of DC hunt motors
ABSTRACT
Electrical switchyards and High Voltage substations are risky places to work with and particularly during a rainy season. However, to work with the system, it’s a required urgent as a part of continuity of power supply.
This paper started with an intention to control, rectify the faults with, and operate our electrical apparatus from a place what can be treated as risk less.
We have taken DC shunt motor for study and the same can be extended for other applications. Here we have studied Characteristics & Performance of DC shunt motor and a remote control operation on to those characteristics.
MATLAB was used as a study for results and as verified with the results the proposed control unit will be very much helpful to reduce risky jobs and to make those into a sophisticated and comfortable, and charming task.
1. D.C. SHUNT MOTOR
An electric motor is a machine which converts electric energy into mechanical energy. Its action is based on the principle that when a current carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming’s left hand rule and whose magnitude is given by
F= BIL Newtons
The field excitation is constant in this type of motor. The back emf is given by
Eb = V - IaRa = Kn φn
From which speed in r.p.m. N= (V - IaRa) / Knφ where Ra is resistance in armature circuit including brushes and additional resistance, if any for speed control, speed constant.
Kn= (Z * P) / (60 * A)
N = ( V - [IR / Kφ] ) / Knφn
Again if demagnetization ampere-turns are neglected, φ is constant and speed Vs torque characteristic is linear.
No = V / Ka φ
The speed of a DC shunt motor is given by N= (V - IaRa) / Knφ
From the above equation, there are three methods of speed control.
a. By varying the resistance in the armature circuit.
b. By varying the excitation current and hence, the field flux of the motor.
c. By varying the voltage applied to the armature terminals.
2.TWO-WIRE REMOTE CONTROL UNIT
Master control unit
The unit comprises six ICs (including a 5V regulator) IC 7805 (6) is used to stabilize the rectified output developed from the AC mains using step-down transformer XI, followed by a bridge rectifier and smoothing capacitor C1. Timer IC1 is wired as an stable multivibrator with a measured time period of 850 ms (‘on’ period of 100 ms and ‘off’ period of 750 ms) with component values as shown in fig. 1
Waveform 1 in fig. 1 shows IC1’s output. Diode D5 is used to limit the ‘on’ period of IC1. Reset pin 4 of IC1 is controlled by the latch circuit comprising NAND gates N1 through N3. Since N3 output normally goes low on switching on of the master control unit, IC1 is initially inactive (inhibited).
The latch output at pin 3 of gate N2 is initially high, while the output at pin 6 of gate N3 is low. Thus green LED1 is on, while red LED2 is off. Binary counter 7493 (IC3) is reset due to logic ‘1’ voltage at its reset pin 2 and 3. IC1 is also reset due to logic ‘0’ voltage at its reset pin 4.
IC3 is a 4-stage binary counter with three flip-flops forming a 3-bit binary counter. The remaining flip-flop can be used independently, and it is triggered at the trailing edge of the last count of the 3-bit counter stage, which makes pin 12 (Q0) go high for a brief period. The counter flip-flops in transitions of the clock. To synchronize the circuit operation, the clock input to IC3 is inverted by NAND gate N4.
