Week-7 Challenge: DC Motor Control

Aim:

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

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

Transistors:

Transistors are used for Amplifying and switching electrical signal in order to maintain required time delay. Transistors are current controlling device. There are several types in transistors, all these are classified according to their use in the circuit.

Transistors are made of two PN diodes which are connected with each other. Current can be flow form collector to Emitter or vice versa it depends on arrangement of diods and their connection.

Types:

BJT (Bipolar Junction Transistor): this Transistor are used to handle high voltage and current in the circuits. These are three terminal semiconductors. Terminals are Collector, Emitter, base.

MOSFET (Metal Oxide Semiconductor Field Emission Transistor):

Mosfet is used where frequency has to be maintained or high. Mosfet can handle high frequency of Switching, we can see Mosfet in common in every circuit. Mosfet has three terminal they are Gate, discharge, source.

IGBT (Insulated Gate Bipolar Transistor):

This is combination of BJT and Mosfet where it can in take more Load compared to both and it can work at high switching frequency condition, work same as BJT.

The below shown mathematical model is a Speed control of a DC motor using BJT H-Bridge

in this model the BJT (Bipolar Junction Transistor) H-bridge with 4 quadrant Chopper Type-E is Used.

In this above source voltage of 240V is considered, but the voltage has to be varied according to speed of the Motor, this operation is performed by 4 quadrant Chopper Type-E, which is connected with the motor in above Mathematical Model.

At switching condition Bipolar Junction Transistor is acts like IGBT. H-bride BJT is taken in order to maintain the Torque and Speed Characteristics of the DC Motor.

The control signals which are generated by Comparator, these signals are generated by Analyzing the position of Acceleration pedal and break pedal.

The Switching operation is performed by the BJT semiconductor according to the received control signals. In this model GOTO and FROM blocks are used for sending and receiving the control signals.

In this model t = 0.5 s is taken as Armature Voltage Control. At 0.5 sec the dc motor undergoes in Regenerative braking mode by simply changing the polarity of suppling current, because of changing polarity of the Suppling Current the negative Torque Develops in the motor undergoes in to Forward breaking mode (Regenerative Mode).

Results:

By examine the below results we can say that the DC motor first operated in the 1^st Quadrant Forward Motoring Mode and then it undergoes in to Forward breaking and Reverse Motoring Mode.

In this we considered 75% of duty cycle, where the current Flows in the Circuit 75% of total time.

Below Result Shows that the Motor Runs at Top speed of +1000RPM during Forward breaking mode, To Runs at 1000Rpm the motor utilizes only 180v from 240v.

Duty cycle:

Average Voltage               =            Input Voltage * (T_on/Total time)

                                             =             240 * (0.75/1) = 180 v

B . In this we considered 50% of duty cycle, where the current Flows in the Circuit 50% of total time.

Below Result Shows that the Motor Runs at Top speed of +480RPM during Forward breaking mode, To Run at 480Rpm the motor utilizes only 120v from 240v.

Duty cycle:

Average Voltage               =            Input Voltage * (T_on/Total time)

                                             =             240 * (0.50/1) = 120 v

By examine the below results we can say that the DC motor first operated in the 1^st Quadrant Forward Motoring Mode and then it undergoes in to Forward breaking and Reverse Motoring Mode. The maximum negative torque obtained is as same as Torque obtained in Forward Motoring condition.

the direction of current supply to the DC motor is get reversed after 0.5s, because of change in polarity the negative torque is generated in the Motor which leads run engine in reverse direction after 0.5s.

In this we considered 25% of duty cycle, where the current Flows in the Circuit 25% of total time.

Below Result Shows that the Motor Runs at Top speed of +100RPM during Forward breaking mode, To Run at 100Rpm the motor utilizes only 60v from 240v.

Duty cycle:

Average Voltage               =            Input Voltage * (T_on/Total time)

                                             =             240 * (0.25/1) = 60 v

By examine the below results we can say that the DC motor first operated in the 1^st Quadrant Forward Motoring Mode and then it undergoes in to Forward breaking. In this vehicle does not generate any negative torque as above results.

the direction of current supply to the DC motor is get reversed after 0.5s, because of change in polarity the negative torque is generated in the Motor which leads run engine in reverse direction after 0.5s.

C- below shown is DC Drive with 4 quadrant chopper also known as Type-E chopper.

Function:

Chopper is a power Electronic Device which is used to convert high DC voltage in to Low DC Voltage and Vice-Versa. This conversion of voltages is used to change the speed of DC- Motor.

Choppers are connected with the semiconductor devices which performs switching operation to maintain required time delays.

By using 4 quadrant chopper/Type-E chopper vehicle can achieve different motoring and breaking conditions by simply supping the current through it.

Conditions:

Forward Motoring mode:

In this condition speed and torque of the DC motor is in positive phase, current flows from IGBT 1 to IGBT 4.

Forward Braking mode:

In this condition speed is in Positive phase and Torque is in Negative phase, current Flows from IGBT 2 and diode 4.

Reverse Motoring mode:

In this condition speed and torque of the DC motor is in negative phase, current flows from IGBT 3 to IGBT .

Reverse Braking mode:

In this condition speed is in negative phase and torque of the DC motor is in positive phase, current flows from IGBT 4 to diode 2.

In this mathematical model 300V Alternate Current is converted by using Full wave rectifier and supplied this converted Direct current to 200 HP DC Motor to Run.

Four quadrant chopper is used to convert voltages in order to make the motor run at required speeds. Here H-Bridge IGBT (Insulated Gate Bipolar transistor) are used to supply the current to DC motor.

From the above result we can say that the Step-up chopper is used to increase the output voltage and that increased voltage supplied to DC motor to Run.

We see that the starting armature is 500v which is supplied to the motor to run in forward motoring condition. At that condition current flows through IGBT1 to IGBT4, which is shown in duty cycle layout in above scope results. And that 500V of charge is supplied to motor around in 0.1 to 0.2 sec, which is shown as small wave form in above results.

 As we see after some period the Suppling charge is reduced to around 450V, and this change in charge shows effect in speed of the motor.

At the same time current from IGBT3 to IGBT 2 is supplied in order to obtain required speed without any deflection.

We see that the current supply through IGBT 1 to 4 is getting reduced and IGBT 3 to 2 is getting increased after 4 sec where the negative torque is generated which leads motor to run opposite direction, thus the speed of the motor is recorded as -1200RPM.

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

The above shown mathematical model is 2 quadrant chopper which is used to convert the Suppling DC-voltage in to required voltage.

Blocks used:

Pulse generator: to Generate the pulses at required duty cycle

Current measurement: To measure the current

Voltage measurement: To measure the voltage

Mosfet: Transistor used for switching operations according to signals received from the analog or digital controler

Diode: used to allow the current with a condition

Series RLC branch

Power GUI: to make the whole mathematical model works continuously

NOT gate: to switch the pulses

Scope: is to view the obtained results

In above mathematical we have used not gate, which makes transistor on/off one after another.

Duty cycle:

Average Voltage               =            Input Voltage * (T_on/Total time)

                                             =             100 * (0.50/1) = 50 v

Form the above graph we can see that the voltage suppled to the DC motor is about 50V for t=0.5 sec and remaining 0.5 sec the Mosfet 1 remains in off mode. At that time the vehicle is in Forward motoring condition.

The Mosfet 2 is activated after 1 sec and remains on till 0.5 sec at that the polarity of the current is changes, Negative torque starts generating inside the DC motor, its leads to stop the vehicle.

3- BLDC Motor (Brushless Direct Current Motor):

As conventional DC motors BLDC Motor does not have any Brushes to supply current to the Armature of the Motor, because of that we no need to worry about wear and tear of the brushes.

BLDC motors are classified in to two types according to the placements of the Rotor.

OUT Runner BLDC: The Rotor is located on top of the stator

IN Runner BLDC: The Stator is located above the Rotor.

In this BLDC permanent magnet is used as a Rotor, the working of BLDC is same as a conventional DC motor with some modifications.

When none of the coil is energized rotor remains stationary, supplying current in to winding generates magnetic field and the rotor starts rotating inside the field.

In BLDC motor the rotor has to rotate according to the switching condition of stator so we can call BLDC motor in to Synchronous DC motor. Even though we call BLDC in to DC motor we need to switch the polarity of current in the coil winding (Stator), this can be done by using 6 Step inverter.

By using 6 step inverter the poles are energized in a predefined sequence, this sequence is observed by using some sensors mainly HALL- Sensor.