Experiment No.:24
Aim of the Experiment:
Torque-Speed characteristics of Separately excited DC Motor.
Objective:
Measurement of different parameters experimentally such as speed, torque, current of DC motor drive.
Apparatus Required:
| Sl. No. | Name | Specification | Quantity |
| 01 | High Voltage Thyristor Control Trainer Kit | 02 nos. | |
| 02 | DC Motor with spring load | Arm. 180V, 2A Field. 180V, 0.45A 1500 rpm | 01 nos. |
| 03 | Patch Cord | PVC Insulated copper | As per requirements. |
| 04 | Tachometer | Digital Type | 01 nos. |
Circuit Diagram:
Theory:
In case of separately excited DC Motor the supply is given separately to field and armature winding’s. The main distinguishing fact in these types of DC motors is that the armature current does not flow through the field winding’s, as the field winding energizes from a separate external source of DC current as shown in figure. So, the coils are electrically isolated from each other.
The speed of DC motor is given by
We know that the speed of a dc motor is proportional to back emf / flux i.e., Eb / φ. When load is increased back emf Eb and φ flux decrease due to armature resistance drop and armature reaction respectively. However, back emf decreases more than φ so that the speed of the motor slightly decreases with load.
Following methods are used to control the speed: –
● Field Control Method – When using the field control method for DC motors, the field is weakened to increase the speed, or it can be strengthened to reduce the motor’s speed. Attaining speeds that are above the rated speed can be achieved by providing variable resistance in series to the field circuit, varying the reluctance of the magnetic circuit, or by varying the applied voltage of the motor to the field circuit with constant voltage being supplied to the armature circuit.
Armature Control Method – With armature control the voltage is varied using several methods. One way is by implementing armature resistance, which involves connecting a variable resistance in series to the circuit of the armature. Once resistance has been increased, the current flow through the circuit is reduced and the armature voltage drop is less than the line voltage. This in turn reduces the motor speed in proportion to the voltage that’s being applied. The armature resistance control method is used in applications that require speed variation for shorter periods of time, not continuously.
Other methods of armature control are armature voltage control and shunt resistance control.
In modern technologies there is much advancement in Power electronics devices. Hence the speed of a separately excited DC motor can also be controlled by varying armature voltage and field voltage with the help of a single/three phase full wave converter circuit. As per the circuit diagram, six thyristors are used namely T1, T2, T3, T4, T5 & T6.
At the terminal of armature and field winding we get controlled dc voltage, with this controlled dc voltage we can control our speed.
Procedures:
Sub Panel in HV Thyristor Control trainer Kit
1. EMT-1: Input Three Phase DOL Starter Panel.
2. EMT-20: Integrated AC (Three Phase) Measurement Panel.
3. PE4A: Three Phase SCR Firing/Synchronizing Panel.
4. PE4B: 6 SCR/Diode Power Panel.
5. EMT6B: DC Voltmeter & DC Ammeter Panel.
6. EMT7: Lamp Load Panel.
In High Voltage Thyristor Control Trainer Kit no. 01(converter_01)
- From EMT-1 panel R, Y, B output terminals, connect to EMT-20 panel R, Y, B input terminals points.
- From EMT-20 panel R, Y, B output terminals, connect to PE4A panel R, Y, B input terminals and connect output terminals from EMT-1 panel to PE4A panel input N terminal.
- From PE4A panel output terminals R, Y, B points connect to PE4B panel 1,7,13 terminals point respectively.
- In PE4B panel make three phase full wave converter by connecting 3/4 to 9/10 to 15/16 get output + ve terminal, 19/20 to 25/26 to 31/32 get output – ve terminal, and 21/22 to 2, 27/28 to 8, 33/34 to 14.
- From PE4B output + ve terminal connect to EMT6B panel Voltmeter terminal at 1, from voltmeter terminal 2 to ammeter terminal at 5, from ammeter terminal 6 to EMT7 panel at terminal 1.
- In the EMT7 panel connect 3 to 5, 7 to 9 terminals.
- Connect from EMT 7B – ve terminal to EMT7B voltmeter terminal 3 and from terminal 4 to EMT7 terminal 11.
- Now make sure that all the terminals are connected properly.
- Switch on the supply of the EMT1 panel, short the terminal 5 to 6, and push the start button.
- In PE4A panel select function to converter mode and by varying 3 phase firing angle control knobs check the output voltage.
- Remove +ve (1) and –ve (11) terminals from EMT7 and connect to DC machine Armature winding at terminal 1 & 3 respectively.
- Repeat all the procedures from Sl. No 1 to 11 in High Voltage Thyristor Control Trainer Kit no. 02.
- In HV Thyristor Control Trainer Kit no. 02(converter_02), Remove +ve (1) and –ve (11) terminals from EMT7 and connect to DC machine Shunt Field winding at terminal 5 & 7 respectively.
- For Field Control Method, fix the input voltage at rated armature voltage of the dc machine and by varying the firing angle of converter_02 to control the input voltage to field winding of the dc machine.
- For Armature Control Method fix the input voltage at rated field winding voltage of the dc machine and by varying the firing angle of converter_01 to control the input voltage to armature winding of the dc machine.
- Now apply the load at motor shaft and calculate Torque applied to the motor by the formulae
- Torque in N-m = W kg*9.81* r in meter
Where, W kg = (W1-W2) kg & r = radius of pulley in meter
Torque in N-m= W kg*9.81*0.0302
Torque in N-m= W kg*0.296(torque constant=0.296)
- By varying the firing angle of both converter and load in the motor you can plot torque speed characteristics of separately excited DC motors.
- Now take readings as per the observation table.
- Plot the graph according to observation table
- (i) curve between output voltage (V) and No-Load Speed.
- (ii) curve between Load Torque (T) and Speed when the motor is loaded.
Observation Table:With No-Load
| Sl. No. | Output Voltage, V (in Volts) | No Load Speed (in RPM) | W1 (in kg) | W2 (in kg) | W=W1-W2 (in kg) | Torque (in N-m) | Field Current (If) in Amp. | Armature Current (Ia) in Amp. |
| 1 | ||||||||
| 2 | ||||||||
| 3 | ||||||||
| 4 | ||||||||
| 5 | ||||||||
| 6 | ||||||||
| 7 | ||||||||
| 8 |
Observation Table:With Load
| Sl. No. | Output Voltage, V (in Volts) | No Load Speed (in RPM) | W1 (in kg) | W2 (in kg) | W=W1-W2 (in kg) | Torque (in N-m) | Field Current (If) in Amp. | Armature Current (Ia) in Amp. |
| 1 | ||||||||
| 2 | ||||||||
| 3 | ||||||||
| 4 | ||||||||
| 5 | ||||||||
| 6 | ||||||||
| 7 | ||||||||
| 8 |
Plot the graph Torque vs Speed.
Conclusion: Written by students.