Week-7 Challenge: DC Motor Control

Simulation

The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.

'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

The block has two values as a display of the output. One is the current output for the IGBTand the diode current and the other is the scope for the values of the DC motor. The values of the DC motor include-

1. Speed (in RPM)

2. Armature Current (in AMPS)

3. Load Torque (in N-m)

The output values are as follows:-

1. IGBT Current

2. Diode Current

The values of the SCOPE are as follows-

In Reverse motoring switch S2 and S3 are fired a negative voltage is applied to the motor ,so that the motor rotates in antilock-wise rotation the torque produced & Armature current are negative value and diodes D1 and D4 operates as free wheeling diodes when the switch S2 and S3 are switched off.

In forward motoring switch S1 and S4 are in closed condition fired a positive voltage is applied to the motor which rotates in clock wise rotation the torque produced. Armature current are positive value thus power is positive and diodes D2 and D3 operates as free wheeling diodes when the switch S1 and S4 are switched off.

2. Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results.

Solution:

CIRCUIT DIAGRAM FOR TWO QUADRANT CHOPPER:

Two quadrant chopper is also called as type c chopper. Type C chopper is obtained by connecting type –A and type –B choppers in parallel. We will always get a positive output voltage V₀ as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v₀ will be equal to V_s . The direction of the load current i₀ will be reversed. The current i₀ will be flowing towards the source and it will be positive regardless the chopper is on or the FD conducts. The load current will be negative if the chopper is or the diode D2 conducts. We can say the chopper and FD operate together as type-A chopper in first quadrant. In the second quadrant, the chopper and D2 will operate together as type –B chopper.

The average voltage will be always positive but the average load current might be positive or negative. The power flow may be life the first quadrant operation ie from source to load or from load to source like the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action my cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

SIMULINK MODEL FOR TWO QUADRANT CHOPPER :

let us take two mosfet blocks and diodes blocks and connect it as shown in circuit diagram.

The dc voltage source is connected as shown.

let the voltage of dc voltage source be 24 volts.

let us consider an series RLC branch block and select inductor L . thus block works as a load.

To consider LE load ie motor load , and dc voltage source of 12 volt is also connected in series with inductor.

The pulse generator block is used to create gate pulse.

The not gate is used in the model so that the two mosfet will not get closed at the same time otherwise short circuit will occur .due to use of not gate , when one mosfet is ON the other mosfet will get off and when other mosfet is ON the first mosfet will get off .

The amplitude of pulse generator is kept 10 so that we can clearly see the pulses at scope and let period be 0.02 seconds and let pulse weidth be 50%

SCOPE RESULT:

The pulse amplitude is 10 thus from pulse waveform we can see that the pulse amplitude is 10.

3. Explain in a brief, the operation of the BLDC motor.

BLDC- Brushless DC motor

DC motor which does not have brushes and controller provides current pulses to the motor winding which controls the torque and speed of the motor is BLDC.

In brushless motors, the problem of connecting current to the armature is overcome by permanent magnets rotating around a fixed armature. Over a vast speed range, these type of motors is very efficient in producing large amounts of torque.

Working principle:

There are only two basic parts in the BLDC motor they are stator and rotor. The stator is the resting part and contains windings the rotor is the rotating part and contains magnets. In BLDC permanent magnets are attached to the rotor and move the electromagnets to the stator.

To activate electromagnets for the shaft turns high power transistors are used. Using a solid-state circuit the controller performs power distribution

When the stator coils get a supply from the source, it becomes an electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generates an AC voltage waveform with a trapezoidal shape. Due to the force of interaction between the electromagnet stator and permanent magnet rotor, the rotor continues to rotate.

With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles aligns with stator poles which causes the motor to rotate.

In two-quadrant chopper drive both motoring mode and regenerative mode (two quadrant dc motor drive).

Types of BLDC

1. Inner rotor design
   
2. Outer rotor design
   

Advantages of BLDC:

1. More efficient because the velocity of BLDC is determined by the frequency at which the current is applied and does not depend on voltage.
2. Mechanical energy losses because of friction are less due to the absence of brushes.
3. Can operate at high speed under any condition.
4. Less noise and no sparking under any condition
5. For much precise control, more electromagnets can be used

Disadvantages of BLDC:

1. Cost is higher compared to brushed dc motor
2. Only limited power could be supplied to the motor because the high amount of heat weakens the magnets and insulation may get damaged.