Week-7 Challenge: DC Motor Control

Week-7 Challenge: DC Motor Control

1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
2. Comment on the armature current shoot-up from the scope results.
3. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
4. Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor)
5. Explain in a brief about operation of BLDC motor.

Answer: -
1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’

Fistly we can go to matlab and click help and search the ‘Speed control of a DC motor using BJT H-bridge’ and open the model we can see

In these model we can see the speed control of a DC Motor using BJT-Bridge.
we can see the simple BJT H-Brigde

In the BJT H-Brigde are four diode used D1,D2,D3 and D4 also four Bipola Junction transister (BJT) Q1,Q2,Q3 & Q4 are used for swiching the motor
in this BJT & diode arangment are H type that means this is a BJT H-Brigde.

The BJTs when used for power switching applications, operates as an IGBT. When it is conducting, a forward voltage Vf is developed between collector and emitter. Therefore, the IGBT block can be used to model the BJT device.

now when Q1 and Q4 are closed, a positive voltage is applied to the motor, then current will flow from +Ve – Q1 – MOTOR – Q4 – Gnd. So motor will rotate in Forward direction.
Open Q1 and Q4 to stop the motor and diodes D2-D3 operate as free-wheeling diodes.
When Q2 and Q3 are closed, a negative voltage is applied to the motor and current will flow from +Ve – Q2 – MOTOR – Q3 – Gnd. So motor gets reverse supply and it will rotate in reverse direction. Diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.

If we can run the model and see the result

1. Comment on the armature current shoot-up from the scope results

We can see the below result
armature current of DC Motor is given,
Ia=(V-Eb)/Ra
where,

Ia=Armature current
Ra=Armature Resistance
Eb=Back Emf
V=Supply voltage
starting time Back Emf (Eb)=0 & current are more.

Starting of the motor initial condition armature current are increasing, D1 & D4 are closed and motor we be run in Forword direction from 0 to 0.5 Sec.

After 0.5Sec open the D1 & D4, and Closed D2 & D3 at that time armature current are shoot-up and motor speed rapidly reducing and motor run reverse direction and gives negative value. In this result are show the pulse width of 50%.

1. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model

the 200HP DC motor is separately excited with a constant 150 V DC field voltage source. The armature voltage is provided an IGBT converter controlled by two PI regulators. The converter is fed by a 515 VDC bus obtained by rectification of a 380 VAC 50Hz voltage source. In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and DC7 block.

In the Four-quadrant operation: -

2. Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor)

Chopper: -

A dc-dc converter, also known as dc chopper, is a static device which is used to variable dc voltage from a constant dc voltage source. Choppers are widely used in trolley cars, battery-operated vehicle, traction motor control, control of large numbers of dc motor. They also used as dc voltage regulators. Chopper is two type 1. Step-down chopper 2. step-up chopper.

We can see the 2- quadrant chopper
see the simple circuit

When chopper CH1 is switched ON, source Vs directly gets connected to the load and hence, load voltage Vo is equal to source voltage. The direction of load current is from source to load.
When CH1 is switched OFF, the free-wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load. Therefore, the output voltage Vo becomes zero. However, the io continues to die down through the FD and L in the same direction as shown in the circuit diagram. Thus, the average output voltage Vo and current Io are positive and hence the operation of the chopper is in the first quadrant.

When chopper CH2 is switched ON, load DC source E drives current through CH2 and load. The direction of this current io will be opposite and hence is assumed negative. Output voltage Vo is zero during this time. When CH2 is made OFF, diode D2 gets forward biased and hence the current into the source from the load. The output voltage is Vs in this time as the load is connected to the source through D2 during the OFF time of chopper CH2. Thus, the load current is always negative i.e. operation of the chopper is within the second quadrant.

If we can run these models and see the output

we can observe that, the load voltage is always positive but the load current may be positive or negative. Therefore, power may flow from source to load or load to source.

3. Explain in a brief about operation of BLDC motor.

A brushless DC motor (also known as a BLDC motor or BL motor) is an electronically commuted DC motor which does not have brushes. The controller provides pulses of current to the motor windings which control the speed and torque of the synchronise motor.

These types of motors are highly efficient in producing a large amount of torque over a vast speed range. In brushless motors, permanent magnets rotate around a fixed armature and overcome the problem of connecting current to the armature. Commutation with electronics has a large scope of capabilities and flexibility. They are known for smooth operation and holding torque when stationary.

Working of BLDC Motor
Before explaining the working of a brushless DC motor, it is better to understand the function of a brushed motor. In brushes motors, there are permanent magnets on the outside and a spinning armature which contains electromagnet is inside. These electromagnets create a magnetic field in the armature when the power is switched on and help to rotate the armature.

The brushes change the polarity of the pole to keep the rotation on of the armature. The basic working principle for the brushed DC motor and for brushless DC motor are same i.e. internal shaft position feedback.

Brushless DC motor has only two basic parts: rotor and the stator. The rotor is the rotating part and has rotor magnets whereas stator is the stationary part and contains stator windings. In BLDC permanent magnets are attached in the rotor and move the electromagnets to the stator. The high power transistors are used to activate electromagnets for the shaft turns. The controller performs power distribution by using a solid-state circuit.

Types of Brushless DC Motors

Basically, BLDC are of two types, one is outer rotor motor and other is inner rotor motor. The basic difference between the two is only in designing, their working principles are same.

Inner Rotor Design
In an inner rotor design, the rotor is located in the centre of the motor and the stator winding surround the rotor. As the rotor is located in the core, rotor magnets do not insulate heat inside and heat get dissipated easily. Due to this reason, inner rotor designed motor produces a large amount of torque and validly used.

Outer Rotor Design

In outer rotor design, the rotor surrounds the winding which is located in the core of the motor. The magnets in the rotor trap the heat of the motor inside and do not allow to dissipate from the motor. Such type of designed motor operates at lower rated current and has low cogging torque.

Advantages of Brushless DC Motor

1. Brushless motors are more efficient as its velocity is determined by the frequency at which current is supplied, not the voltage.
2. As brushes are absent, the mechanical energy loss due to friction is less which enhanced efficiency.
3. BLDC motor can operate at high-speed under any condition.
4. There is no sparking and much less noise during operation.
5. More electromagnets could be used on the stator for more precise control.
6. BLDC motors accelerate and decelerate easily as they are having low rotor inertia.
7. It is high performance motor that provides large torque per cubic inch over a vast sped rang.
8. BLDC motors do not have brushes which make it more reliable, high life expectancies, and maintenance free operation.
9. There is no ionizing sparks from the commutator, and electromagnetic interference is also get reduced.
10. Such motors cooled by conduction and no air flow are required for inside cooling.

Disadvantages of Brushless DC Motors

1. BLDC motor cost more than a brushed DC motor.
2. The limited high power could be supplied to BLDC motor, otherwise, too much heat weakens the magnets and the insulation of winding may get damaged.