Microcontroller Based DC Drive

 Experiment :Micro controller Based DC Drive 

Aim: 

To study the operation of Microcontroller based DC drive System

Learning Objectives:

• To study speed variations of a DC motor

• To study open loop and closed loop controls.

• To study Micro computer based dc drive
  

Apparatus:

• Experimental kit

• DSO/CRO (Unearthed)

• Multimeter

• Patch cords and DSO probes

 Pre Test

Theory

Microcomputer control of DC drive

 The analog control scheme for a converter fed dc motor drive can be implementedbby hardwired electronics. An analog scheme has several disadvantages: nonlinearity of speed sensor, temprature dependency,drift and offset.Once a control circuit is build to meet certain performance criteria,it may require major changes in the hardwired logic circuits to meet other performance requirements.

 A microcomputer reduces the size and cost of hardwired electronics,improving reliablity and control performance. This control scheme is implemented in software and flexible to change the control stratergy to meet different performance charactersistics or to add extra control features.

A microcomputer can also perform desirable functions:

• ON and OFF of the mains power supply
• Start and stop of the drive
• Speed control
• Current control
• Monitoring the control variables 
• Initiating protection and trip circuit 

 

                          Experimental Setup

 

This Drive is consists of a Single phase Controlled Semi converter and a permanent magnet motor. The Semi converter is fed by 110 V. The firing angle of a single phase Semi converter is changed to control the ac output voltage and thus the speed of a permanent magnet motor. The firing pulses to the SCR’s can be controlled by microcontroller circuit. Microcontroller based Control unit consists of Keyboard, ARM 7. From the keyboard we can Enter Different parameters to control the operation of AC drive. They include Run, Stop, Firing angle Open lope and closed loop controls. The microcontroller reads these parameters and generates required control signals for Single phase Controlled Semi converter.

Procedure

Open Loop Drive:

 

1.      Connect two pin FEMALE connector from the kit to transformer 230 V connected MALE connector on panel.

2.      Connect two pin MALE connector from the kit to 110 V FEMALE connector of transformer on the fixture.

3.      Then connect motor +ve & -ve terminals to motor points shown on the panel.

4.      Switch ON mains supply.

5.      For open loop control press “F1” given on keyboard on the panel.

6.      Default 3000 RPM speed is given can be seen on the LCD display. To change the default speed press “DEL” key and then enter RPM count using keyboard.

7.      Then press “ENTER”. On display set speeds is shown & for further action press “ENTER” again. Motor will start and the actual speed will displayed on LCD display.

8.      To stop the motor press “F4” & then press “ENTER”. Motor will stop.

9.      Tabulate the results.

10.  Draw the graph of Speed Vs Armature Voltage.

 

Closed Loop Operation:

 

1.      Press “F2”, default speed is 3000 RPM. To change the speed, follow the same procedure as above.

2.      Then press “ENTER”. On display set speed is shown. Then again press “ENTER”. Motor will start running. In few seconds motor will run at the set speed. Speed of motor is displayed on the panel. Miner variation in RPM is due the optical sensors & disc misalignment due to minor variations in whole positions in the disc. Set speed remains fixed within + 2% of set speed.

 

Note:  To know the firing angle & set speed at any time, press any key on the keyboard. Measure actual voltage (given to the motor) across motor terminals on the panel using multi-meter. Vact is the attenuated voltage pointed of motor voltage.

Vact = Vmotor /100   

 

Observation Table

Open Loop

Sr. No.	Firing Angle α (in µs)	Firing Angle α (in Degree)	Vout (V)	Speed (rpm)

Closed Loop

Sr.No.	Firing Angle α  (in µs)	Firing Angle α  (in Degree)	Vout (V)	Speed (rpm)