What is DC motor starter? How to perform modelling of DC motor starter in MATLAB Simulink? Which blocks are required ? How to verify the modelled starter?
The starting current of DC motor is very high because armature resistance is very low and there is no back emf to limit the starting current. Approximately the starting current of DC motor is 10 times higher than its rated current. Such high current can damage the motor internal parts. Therefore it is necessary to limit the starting current of DC motor using starter.
Requirement for DC motor starter
The starting current of DC motor can be reduced using just a resistor. However that resistor can be the cause of I2R losses as well as it will reduce the torque capability of the motor. Therefore starter is only required during starting when there is no back emf. Once the motor gains speed and back emf is developed, starter needs to be removed. It is recommended that starter should not be removed suddenly rather it should be detached in steps so that transition from maximum starting resistance to no external resistance is smooth. According to the rule of thumb, starting resistance should be removed in three steps. All the requirements to design and select appropriate values of starting resistors can be given in following article
Consider 240V, 5HP, 16.2A 1220rpm DC motor. Since the rated current is 16.2A, the maximum allowable current is 32.4A. The starter should be designed according to these current ratings.
The DC motor should be connected as shown in above figure. 240V should be applied to both armature and field windings. Since the armature current needs to be controlled, the starter should be connected with the armature in series. Speed of motor (w) multiplied with 0.2287 is given as a feedback to the motor. Scopes are connected at different points to analyze different output values. The most important among them is the armature current scope which will show the performance of our designed starter.
To calculate the values of resistor for three point starter Consider
Rtotoal= Total resistance of starter and armature
Ea= Back emf developed by motor
VT= terminal voltage=240V
Imin= Irated= 16.2A
Imax= 2xIrated = 34.2 A
Ra = armature resistance
Rototal1= Rtotal – R1
Rtotal2= Rtotal1-R2 = Rtotal –(R1 + R2)
R1, R2, R3 = three resistors of motor starters
Find the voltage drops for and the resistance of each segment of starter resistor and calculate the values of each step of starting resistor using my article how to design three point DC motor starter.
R1= 3.87 Ω
R2= 1.80 Ω
The resistor R1 should be removed first and R3 should be removed last.
It can be seen that three resistor are connected in series. With each resistor, a switch (breaker) is connected in its parallel. In the case the breaker is open; the current will pass through the resistor. If a breaker is closed, the resistor will be bypassed and the circuit will not see the resistor. Each breaker is controlled by the output of the comparator block. Comparator compares each the value of armature current with the constant value written on it.
Ideally, each comparator should have the value of 16.25. However it is required that R1 should be removed first, therefore its threshold value is taken slightly above 16.25 that is 16.30. Next R2 should be removed so its threshold value is less than R1 and greater than R3. Resistor R3 should be removed last so its threshold value is taken least.
Initially all three breaker are open and all three resistors are included in the circuit. The motor will start with a high current 32.4A and will reduce slowly as the motor gains speed. Once the speed reaches the value of 16.3, the breaker across R1 will close as a result resistor R1 will be bypassed. Motor current will shoot again to maximum allowable current that is 32.4A.
The motor current starts declining since the back emf is increasing due to rise in its speed. When the current reaches 16.27A, breaker across R2 will close and resistor R2 will be bypassed. Similarly R3 is also removed from the circuit.
First, the starter is not connected to the system and the motor is started. The result is shown in the figure.
It can be seen that starting current is very high that is 400A. It is more than 10 time of the rated current.
Now when the starter is connected with the motor.
It can be seen that starting current is limited to nearly 32. Once the armature current reaches near the rated value, one resistor is removed and current shoots back. Three peaks shows that three resistors are removed and since the size of each peak is same, it means our calculations are correct.
Please post your questions in comments.