Week-7 Challenge: DC Motor Control

1A) Speed Control of DC Motor -

- This model consists of the following blocks ,

DC motor

Pulse Generator

Armature Voltage control

IGBT Transistor

Diodes

- This model mainly gives the output of armature current , load torque , speed and also currents of IGBT and diode .

- The working of this model is that motor rotates in the forward direction and then in the reverse direction .

- To decrease the peak change in the current we need to change the input signal generator duty cycle to decrease the sudden shoot in the current .

- There are four IGBT Transistors used in this model .

- The specification of DC motor is 5HP,240V,1750 RPM .

- The pulse width is considered to be 75% .

- The results after running the model for 10s is give below .

IGBT Current and Diode Current -

Armature Current , Speed and Load torque -

1B) The shoot in the armature current is shown below

When the speed starts decreasing means rotor starts rotating in reverse direction there is there is sudden change in the polarity of the armature current . It goes in negative direction .

1C) The Four-Quadrant Chopper DC Drive (DC7) -

Working of Four-quadrant chopper DC drive is describe below-

First Quadrant – Forward motoring mode
For first quadrant operation, thyristor S4 is kept on, thyristor S3 is kept off and thyristor switch S1 is operated. With S1, S4 ON, armature voltage Va = Vs and armature control Ia begins flow. Here both Va and Ia are positive giving first quadrant operation, when S1 is turned off, positive current freewheels through S4, D2. In this manner, Va, Ia can be controlled in this first quadrant, and operation gives forward motoring mode.

Second Quadrant – Forward braking mode
Here thyristor S2 is operated and S1 , S3 and S4 are kept off. With S4 on, reverse or negative current flows through La , S2, D4 and Eb. During the operation time of S2, the armature inductance ‘La’ stores energy during the time S2 is on. When S2 is turned off,current is fed back to source through diodes D1 , D4 . Note that here ( E+L(di/dt)) is more than the source voltage Vs .As the Vs is positive and Ia is negative, it is a second quadrant operation gives forward braking mode. In that power is fed back from armature to source.

Third Quadrant – Reverse motoring mode
For third quadrant operation, thyristor S1 is kept off, S2 is kept on and S3 is operated, polarity of armature back emf Eb must be reversed for this quadrant operation. With thyristor S3 is on, armature gets connected to source V. So that both Va , Ia are negative, leading to third quadrant operation. When S3 is turned off,negative current free wheels through S2,D4. In this manner only Va and Ia can be controlled in the third quadrant.

Fourth Quadrant – Reverse Braking mode
Here thyristor S4 is operated and other devices kept off, back emf Eb must have its polarity reversed as in third quadrant operation. With S4 on, positive current flows through S4,D2,La and Eb (armature). Armature inductance La stores energy during the time S4 is on. When S4 is turned off, current is fed back to source through diodes D2

H-bridge -

The working of H-bridge is explained below .

The basic operating mode of an H-bridge is fairly simple: if Q1 and Q4 are turned on, the left lead of the motor will be connected to the power supply, while the right lead is connected to ground. Current starts flowing through the motor which energizes the motor in (let’s say) the forward direction and the motor shaft starts spinning.

If Q2 and Q3 are turned on, the reverse will happen, the motor gets energized in the reverse direction, and the shaft will start spinning backwards.

In a bridge, you should never ever close both Q1 and Q2 (or Q3 and Q4) at the same time. If you did that, you just have created a really low-resistance path between power and GND, effectively short-circuiting your power supply. This condition is called ‘shoot-through’ and is an almost guaranteed way to quickly destroy your bridge, or something else in your circuit.

3) BLDC Motor -

Brushless DC electric motors also known as electronically commutated motors (ECMs, EC motors). Primary efficiency is a most importent feature for BLDC motors. Because the rotor is the sole bearer of the magnets and it doesn't require any power. i.e. no connections, no commutator and no brushes. In place of these, the motor employs control circuitry. To detect where the rotor is at certain times, BLDC motors employ along with controllers, rotary encoders or a Hall sensor.

Construction -

In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature current are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.

The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit. A BLDC Motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor revolve at the same frequency.

Brushless motor does not have any current carrying commutator . The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.

The layout of a DC brushless motor can vary depending on whether it is in “Out runner” style or “Inrunner” style.

• Outrunner – The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.

• In runner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out runner style, but is capable of spinning at very high rpm .

Working Principle of Brushless DC motor -

BLDC motor works on the principle similar to that of a brushed dc motor . The Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In the BLDC motor, the current carrying conductor is stationary and the permanent magnet is moving.

When the stator coils get a supply from source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between 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 align with stator poles which causes the motor to rotate.

Advantages of Brushless DC motor -

• Less overall maintenance due to absence of brushes
• Reduced size with far superior thermal characteristics
• Higher speed range and lower electric noise generation.
• It has no mechanical commutator and associated problems
• High efficiency and high output power to size ratio due to the use of permanent magnet rotor
• High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed
• Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors.
• Long life as no inspection and maintenance is required for commutator system
• Higher dynamic response due to low inertia and carrying windings in the stator
• Less electromagnetic interference
• Low noise due to absence of brushes

Limitations of Brushless DC motor -

• These motors are costly
• Electronic controller required control this motor is expensive
• Requires complex drive circuitry
• Need of additional sensors

Applications of Brushless DC motor -

• Computer hard drives and DVD/CD players
• Electric vehicles, hybrid vehicles, and electric bicycles
• Industrial robots, CNC machine tools, and simple belt driven systems
• Washing machines, compressors and dryers
• Fans, pumps and blowers.

2) Two-Quadrant Chopper -

The model of two-quadrant is shown below ,

Result -

After simulating the model for 1s by keeping the following parameter in Pulse generator ,

As we see in the above result that voltage is not going to negative .The minimum value of Voltage is Zero and maximum value is 24V and current is increasing and decreasing .