Challenge

Week-7 Challenge: DC Motor Control

RRamesh Rajkumar K24 May 2022

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 and torque of the synchronous 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

The advantages of a BLDC motor are:

Brushless motors are more efficient as its velocity is determined by the frequency at which current is supplied, not the voltage.
As brushes are absent, the mechanical energy loss due to friction is less which enhanced efficiency.
BLDC motor can operate at high-speed under any condition.
There is no sparking and much less noise during operation.
More electromagnets could be used on the stator for more precise control.

The disadvantages of a BLDC motor are:

BLDC motor cost more than a brushed DC motor.
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.

QUESTION:-1

CASE1:-

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

The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between collector and emitter (in the range of 1 V). Therefore, the IGBT block can be used to model the BJT device.

The IGBT block does not simulate the gate current controlling the BJT or IGBT. The switch is controlled by a Simulink® signal (1/0). The DC motor uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to the square of speed). The armature means voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.

The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models). Two transistors are switched simultaneously:

Q1 and Q4 or Q2 and Q3.

When Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off.

When Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.

RESULT:-

Case 2 :

‘Speed control of a DC motor using BJT H-bridge’ – 5HP 240V 1750RPM

Field Voltage = 150

Torque TL Mechanical input

CASE 3:-Refer to help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with H-bridge model.

The Four-Quadrant Chopper DC Drive (DC7) block

The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper drive for DC motors. This drive features closed-loop speed control with the four-quadrant operation.

The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, four switching devices are required, which increases the complexity of the drive system.

The Four-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives

Speed Controller (DC)
Regulation Switch
Current Controller (DC)
Chopper

H-bridge model

The H-Bridge block represents an H-bridge motor driver. The block has the following two Simulation mode options:

PWM — The H-Bridge block output is a controlled voltage that depends on the input signal at the PWM port. If the input signal has a value greater than the Enable threshold voltage parameter value, the H-Bridge block output is on and has a value equal to the value of the Output voltage amplitude parameter. If it has a value less than the Enable threshold voltage parameter value, the block maintains the load circuit using one of the following three Freewheeling mode options:

One semiconductor switch and one freewheeling diode
Two freewheeling diodes
Two semiconductor switches and one freewheeling diode

Averaged — This mode has two Load current characteristics options:

Smoothed
Unsmoothed or discontinuous

Comparison:-

S.No	Four-Quadrant Chopper DC Drive (DC7) block	H-bridge model
1.	Generally used in Automotive for controlling DC motor speed and direction.	It is generally used in Robotics & Electro-mechanical devices for controlling the DC motor speed and direction.
2.	Regenerative braking is possible in DC7	Dynamic braking is only possible.
3.	It can operate in all 4 quadrants of DC motors.	It can only operate only in forward and reverse directions.
4.	Only DC-DC converters high switching frequency is possible.	It shows the simulation value in accurate mode.
5.	It has a wider range of torque and output power.	It has a narrow torque and output power.

2.Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - multi qudrant model:-

Block diagram

OUTPUT:-

Scope shows the motor speed, armature current and load torque

As we change the Duty cycle percentage speed, amarture current, load torque varies as per the percentaage varies in the duty cycle

If we change the Duty cycle the diode current and IGBT current is changes

Explanation:-

The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with four-quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived. This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

WORKING OF FOUR QUADRANT CHOPPER:

The circuit of a four quadrant chopper or class-E chopper basically consists of four semiconductor switches CH1 to CH4 and four diodes D1 to D4. The four diodes are connected in anti-parallel. The circuit diagram of this type of chopper is shown below.

First Quadrant Operation:

For first quadrant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 are ON simultaneously, the load gets directly connected to the source and hence the output voltage becomes equal to the source voltage. This essentially means that v_o = v_s. It may be noted that the load current flows from source to load as shown by the direction of i_o.

When CH1 is switched OFF, the load current free wheels through CH4 and D2. During this period, the load voltage and current remains positive.

Thus, both the output voltage v_s and load current i_o are positive and hence, the operation of chopper is in first quadrant. It may be noted that, Class-E chopper operates as a step-down chopper in this case.

Second Quadrant Operation:

To obtain second quadrant operation, CH2 is operated while keeping the CH1, CH3 & CH4 OFF. When CH2 is ON, the DC source in the load drives current through CH2, D4, E and L. Inductor L stores energy during the On period of CH2.

When CH2 is turned OFF, current is fed back to the source through D1, D4. It should be noted at this point that (E+Ldi/dt) is more than the source voltage Vs. As load voltage Vo is positive and Io is negative, it is second quadrant operation of chopper. Since, the current is fed back to the source, this simply means that load is transferring power to the source. Kindly read step up chopper for detailed analysis and better understanding.

For second quadrant operation, load must contain emf E as shown in the circuit diagram. In second quadrant, configuration operates as a step-up chopper.

Third Quadrant Operation:

To obtain third quadrant operation, both the load voltage and load current should be negative. The current and voltage are assumed positive if their direction matches with what shown in the circuit diagram. If the direction is opposite to what shown in the circuit diagram, it is considered negative. One important thing to notice is that the polarity of emf E in load must be reversed to have third quadrant operation. Circuit diagram of Class-E chopper for third quadrant operation is shown below.

COMPARISON OF 4 QUADRANT CHOPPER AND BJT H BRIDGE:

1) the 4 quadrant chopper works in all four modes like

forward motoring
forward breaking
reverse motoring
reverse breaking

but the BJT h bridge model works on only two modes

forward motoring
reverse motoring

2)to change the direction of rotation of motor from forward to reverse and vice versa , the current signals get transited from one to other switch while in h bridge model, the direction of rotation of motor is changed by changing the polarity of current.

3)the 4 quadrant chopper have high switching frequency

while h bridge model have low switching frequency

4)the armature current in 4 quadrant chopper have a little spike at transition from one to other switch because of high swithing frequency while h bridge model have high spike of current.

5) the 4 quadrant chopper is generally used in industrial application while h bridge model is used in robotics.

GRAPHICAL COMPERSSION:-

2-quadrant chopper

As we change the Duty cycle percentage speed, amarture current, load torque varies as per the percentaage varies in the duty cycle

If we change the Duty cycle the diode current and IGBT current is changes

CONCULUSION:-

1) MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge is studied and the the armature current shoot-up is studied and ‘The Four-Quadrant Chopper DC Drive (DC7) block is also studied and Compare it with the H-bridge model

2) 2-quadrant chopper using simulink model is made and the various results obatined is studied.

3)operation of BLDC motor is studied.

Week-7 Challenge: DC Motor Control