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This lab is particularly concentrated of the principles of dc motor operation and to understand the difference in behaviour between a shunt dc motor and series motor. In this particular experiment, we are going to using a dc motor that works as both shunt and series field winding to measure the variation of speed with the torque that the motor is producing and effect of the field current on the speed. This experiment has been performed using a laboratory system from Feedback Instruments that connects the machines to a dynamometer that can give varying loads to the motor and has computer based instrumentation system to measure currents(i) and voltages(v).
Background Theory :
Power engineering deals with the transmission, generation, and distribution of electric power. It also works as the electrical devices connected to such systems including transformers, generators and motors. In fact It is called as a network of interconnected networks. This network converts different forms of energy to electrical energy. Power engineering consists of three main subsystems: the generation subsystem, the transmission subsystem and the distribution subsystem.
DC Shunt Motor :
DC Shunt Motor is a motor which uses DC supply. It has got an inductor connected with the armature in parallel connection. One of the main characteristic of a DC shunt motor give it very good speed regulation. And it is known as a constant speed motor, even though the speed does slightly decrease as load is increased. This shunt motor is used in industrial and automotive applications where precise control of speed and torque are required. In the graph below, typical shunt motor speed-torque relationship is shown:
Figure : Torque vs Speed Graph
A Shunt DC motor has decrease in torque when we increase the speed. The decreasing torque vs. speed is caused by the armature resistance, voltage drop and armature reaction. At a value of speed near 2.5 times the rated speed, armature reaction becomes very excessive. It causes a rapid decrease in field flux and a rapid decline in torque until it reaches a stable condition.
In addition with in the shunt motor, the field and the armature are connected in parallel connection across the dc supply.
Figure: Shunt Motor
Here, the difference between V and E divided by the resistance of the armature is called the armature current.
Mathematically, the torque is proportional to the product of the armature current and the flux.
Input Power, Pin = Vf If + Va Ia
Output Power, Pout = 2 Pi TN/60
Efficiency = Pout / Pin
= (2piNT/60) / (Vf If + Va Ia) x 100%
Figure : Series Motor
This kind of motor develops a large amount of turning force which is called torque. It happens from a stable position. Because of this characteristic, the series dc motor can be used to operate small electrical appliances, portable electrical tools. It has another characteristic that the speed varies widely between no-load and full-load, series motors canâ€™t be used where a relatively constant speed is required under conditions of varying load.
One of the major disadvantage of a series motor is that the speed of a series motor with no load connected to it increases to the point where it can damage the motor. In this case, either the bearings are damaged or the windings fly out of the slots in the armature. There is a danger to both personnel and equipment. Before we turn it on, some load must always be connected to a series motor.
1. Shunt Motor Experiment :
At first, we have to set up a circuit as it is shown in the diagram below:
Figure : Shunt Motor connection
Motor System :
Now we have to set up the connections carefully on the control frame and motor system.
After that we have to set the variable output voltage to 0% and set the demand voltage to positive. Then set the on position of the circuit breaker and turn on the button of dynamometer power.
Now we have to start the software on PC with voltmeters V1 and V2 and ammeters I1 and I2. Use a DC setting of 500V, 10A. Increase the variable output voltage control until the motor speed is about 3200rpm with maximum voltage of 220V.
Then we have to note down the readings of armature volts (V1), Field volts (V2), Armature current (I1), Torque in Nm and speed in rpm in a table. The results are like below :
The table for the torque, input power, output power and efficiency is given below:
Input Power (W)
Torque vs. Speed
It is noticeable from the graph that as we increase the torque, the speed also decreases but at a certain value of torque, the speed increases and then it decreases rapidly.
2. Torque vs. Armature current
From the shape of the armature current vs. torque graph, it can be noticed that as the torque is increased, the armature current also increases. As per theory, a shunt wound motor has a high- resistance field winding connected in parallel with the armature. It responds to increased load by trying to maintain its speed and this leads to an increase in armature current.
Torque vs. Efficiency
The graph above shows that as the torque increases, the efficiency also increases and after a certain torque value the efficiency almost becomes constant.
Factors that will decrease the efficiency less than 100% are stray losses, mechanical losses, core losses, and brush drop losses. The efficiency increases with the increase of the torque and there will be more mechanical losses.
The effect of field current:
We should remove the link shown as a dotted line in the circuit diagram below. Set the circuit breaker to the on position and press the dynamometer power button.
Then increase the variable output voltage control until the motor speed is about 3200rpm with a maximum voltage of 220V and note the readings of field current(I2), torque in Nm and speed in rpm in a table which is shown below:
Now connect the resistors so that R1, R4 and R7 are on. Again set the circuit breaker to the on position and press the dynamometer power on button and note the readings of field current(I2), torque in Nm and speed in rpm in a table which is shown below:
Speed vs. Torque graph
Graph 1 resistance= 182 ohms
Graph 2 resistance =316ohms
2. Series Motor Experiment
Set up the circuit as shown in the diagram below:
Then set the variable output voltage to 0% and demand voltage to positive. After rechecking all the connections of the circuit, set the circuit breaker to the on position and turn on the dynamometer power button. On the computer, remove the V2 and I2 instruments.
As the motor runs very fast at low load, so recording a large set of results is
Now increase the variable output voltage until the voltage is 160V and slowly reduce the supply voltage until the speed is 4950rpm. Ensure that the torque control is at a minimum.
Note down the readings of Motor volts (V1), Motor current (I1), torque in Nm and speed in rpm in a table which is as follows:
When we zero the voltage, turn the voltage to zero and open the circuit breaker. Then we should calculate the input power, output power and efficiency for each torque value in a table which is as follows:
Speed vs. Torque
Armature current vs. Torque
Efficiency vs. Torque
At this moment, we should notice the speed variation with voltage. After that reset the voltage and torque controls to the lowest setting and set the circuit breaker to the on position. Then we should slowly increase the supply voltage until the speed is 4950 rpm and note down the readings of Motor volts (V1) and speed in rpm for each voltage in a form of table which is as follows:
Speed vs. Applied motor voltage