GICHURA ROBERT F17/ 10663/ 2006 UNIVERSITY OF NAIROBI DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING 3RD YEAR 2ND SEMESTER LABS. MACHINES LAB. STARTING, SPEED CONTROL AND REVERSAL OF A D.C SHUNT MOTOR
EXPERIMENT: STARTING, SPEED CONTROL AND REVERSAL OF A D.C SHUNT MOTOR
Objective: To observe the effect of starting a shunt motor with line, field and armature resistance, to observe the effect of reversing armature and field connections and to examine the construction of the motor starter.
Apparatus: Mawdsley’s Ltd. D.C shunt motor; 1hp, 110V, 9.0A, 1500rpm R = 2Ω D.C motor starter Field regulator Tachometer Rheostats Voltmeters Ammeters
Theory: The speed of a motor is given by the relation; N = (V – IaRa)/ZΦ = K (V – IaRa)/Φ r.p.s Where, K → constant Ra → armature resistance It is therefore evident that the speed of the motor can be controlled by varying the following parameters; The applied voltage, V The flux per pole, Φ The armature resistance, Ra Speed control of shunt motors i.) Flux control method: The speed is inversely proportional to the flux and hence the speed can be increased by flux decrement and the vice-versa. Mechanism The flux of a d.c motor can be varied by varying the shunt current, Ish with the aid of a shunt field rheostat. The shunt field rheostat carries only a small current since the shunt current is relatively small. As a result, the I2R loss small and a small rheostat can be used, making this method a quite efficient one. In non-interpolar machines the speed can be increased by this method in the ratio 2:1. Any further weakening of the flux adversely affects the communication and puts hence, a limit to the maximum speed obtainable using this method. For machines fitted with inter-poles, a ratio of maximum to minimum speed of 6:1 is normally employed. a.) Rheostat control method This method is normally used where the required speeds are below the no-load speed. Since the supply voltage is normally constant the voltage across the armature is varied by use of a rheostat (controller resistance), inserted in series with the armature circuit as shown below;
When the controller resistance is increased, the potential difference across the armature is decreased, thereby decreasing the armature speed. For a load of constant torque, the speed is approximately proportional to the potential difference across the armature. The following graph sketch portrays fall in speed of d.c motor with increase in armature resistance. ii.)
Voltage control method: a. Multiple voltage control In this method the shunt field of the motor is connected permanently to a fixed exciting voltage, but the armature is supplied with different voltages by means of suitable switchgears. The armature speed is approximately proportional to the voltages. The intermediate speeds are obtainable by adjusting the shunt field regulator
b. Ward – Leonard system: This method is normally used where unusually wide and very sensitive speed control is required e.g. electric excavators, elevators and the main drives in steel mills and blooming and paper mills
Procedure: Starting with line resistance The circuit was connected up as shown below:
The resistance was reduced gradually until the motor started. The current at which this motion just started was noted and observations made on the ammeter as the motor speed up. The resistance was eventually reduced to zero to have the motor running at full speed. The voltage across the armature resistance was observed. The effect of varying the line resistance on the motor speed was noted. Readings were taken and tabulated for the voltage across the armature, line current and the speed. Starting with armature resistance The circuit was connected as shown below;
The resistance was reduced gradually until the motor started. The current at which this motion just started was noted and observations made on the ammeter as the motor speed up. The resistance was eventually reduced to zero to have the motor running at full speed. The effect of changing the field regulator resistance was observed as well. The direction of rotation was also noted. The field current was kept constant and the armature voltage varied in steps, noting the voltage and speed at each step. The measurements were taken and tabulated as shown below; Reversal of Motor The supply connections were reversed and the motor started. The new direction of rotation was noted. The field connections were then reversed and the motor started. The new direction of rotation was also noted. The armature connections were finally reversed, the motor started and the direction of rotation noted.
Results: 1. Motor started at a current of 2.1 A The speed of the motor increased with decrease in line resistance Voltage across the armature at full speed was 112 V Table1: Variation of the other experimental parameters V [V] 112 90 Ia [A] 0.55 0.55 If [A] 0.45 0.4 IL [A] 1 1 N [rpm] 1470 1350
62 0.6 0.25 0.9 1150
96 0.6 0.4 1 1400
A plot of speed against the generated emf (E = V – IaRa) was drawn. 2. Motor started at the following currents; If = 1A, Ia = 0.3A Reduction of the field regulator resistance was found to result in an increase in the speed of the motor. Direction of rotation was clockwise when observed onto the direction of taking the speed measurement.
Table2: Speed, voltage characteristics with constant field current Voltage [V] 108 102 100 96 90 73 66 59 N [rpm] 110 104 100 97 85 75 65 570 Table3: Speed, field 0 0 0 0 0 0 0 current characteristics with constant voltage Field current, If [A] 0.3 0.5 0.6 0.7 0.8 1.0 N [rpm] 150 130 123 115 110 970 0 0 0 0 0 Graphs of speed, N against V and speed against I were plotted. Table4: Reversal of motor; observations Reversal of: Direction of rotation Supply connections Clockwise still Field connection No rotation Armature connection Anti-clockwise
Discussion: From the results obtained in Table1, the following table was deduced in aid to give a relationship between speed and the generated emf, E: Table5: Speed, generated emf characteristics Speed, N [rpm] 1470 135 115 1400 0 0 E = V – IaRa [V] 110. 88.9 60.8 94.8 9 The theoretical speed was earlier stated to linearly increase with generated emf by the relation; N = (V – IaRa)/ZΦ = K (V – IaRa)/Φ r.p.s. from the graph the speed is seen to increase linearly with generated emf as well, which marries both the theoretical and practical results. It is also seen from graph of Table3 that the speed decreases with increase in field current. This is because an increase in controller resistance is accompanied by a decrease in the potential difference across the armature, thereby decreasing the armature speed.
Conclusion: The effect of starting a shunt motor with line, field and armature resistance was observed. The effect of reversing armature and field connections was also studied and it was found that the direction of rotation of the motor changed only when both the armature and field connections were interchanged but not one of them. The speed of rotation was found to decrease proportionally with increase in field current which is the theoretical expectation. In other part of the experiment the speed was found to increase proportionally with increase in the terminal voltage. The plot of speed against generated e.m.f indicated a non-linear increase in speed at higher values of induced e.m.f. The experiment was successfully conducted within experimental errors to meet the stated objectives.
References: 1. University of Nairobi “Machines lab. Manual”, Dept. of Electrical and Electronics Engineering, 2005.
2. G. R. Slemon, Electrical Machines and Drives, Addison-Wesley, 1992. 3. R. H. Engelman and W. H. Middendorf, Eds., Handbook of Electric Motors, New York: Marcel Dekker, 1995.
