Week-7 Challenge: DC Motor Control

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

This example shows the simulation of an H-bridge used to generate a chopped voltage and to control the speed of a DC motor.

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 pre set model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed). The armature mean 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.

An H-Bridge is a circuit that allows voltages to be applied across a load in either direction. Electric current flows from the source to ground, and many components need to be oriented according to the direction of current to work as expected. An H-Bridge is a circuit built to change the direction of the voltage and thus the current flowing to a load.

An H-Bridge is made up of four switches: two in series, and two in parallel, with the load placed in between the switches. In this configuration the circuit takes an “H” shape.

In order to change the direction of the voltage supplied, the H-Bridge controls the switches that deliver power to the load (S1). Looking at the diagram, if we close S1 and S4 while leaving the rest open, the voltage will be applied from left to right across the motor. If S2 and S3 are closed instead and the others open, the voltage will be applied from right to left.

When switches S1 and S4 are switched on, motor runs in clockwise direction.

When S2 and S3 are switched on, motor runs in anticlockwise direction.

In actual practice these switches are replaced by transistors. Nowadays H Bridge ICs such as L293D and L293 are available.

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

ans -

This model open in matlab by using given link and open and run the simulation.

Set the duty cycle 75% -

Selecting in duty cycle 75% and run the model after see the scope of armature current its shown the speed is increases in almost 1000 rpm in 0.5 sec.

In Armature Current at time of starting suddenly increase when speed is reach a constant value the armature current its flow in very small value. After that the speed is reduced at breaking action take place. Armature current in shoot-up to negative direction. After that the breaking speed is gos their constant value current flow its very slightly .

Set duty cycle 50%-

The duty cycle set the 50% the armature current is flow in very sligtly in shown the scope

When speed is reduced the armature shoot -up its very small.At time of breaking is almost zero. That means the armature current shoot - up its control by applying duty cycles.

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

ans -

The Four-Quadrant Chopper DC Drive (DC7) block –

Four Quadrant Operation of any drives or DC Motor means that the machine operates in four quadrants. They are Forward Braking, Forward motoring, Reverse motoring and Reverse braking.

The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source. The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz 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 the DC7 block.

The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed. The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system. The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation). For proper system behaviour, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3. This generates the average armature voltage needed to obtain the desired armature current. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

The Four-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives/Fundamental Drive Blocks library:

1 - Speed Controller (DC)

2 - Regulation Switch

3 - Current Controller (DC)

4 - Chopper

Compared to other DC drives, the main advantage of using the four quadrant chopper DC drive is that it can operate in all 4 quadrants. Each quadrant in its characteristics represents a different mode of operation.

Compare DC7 drive and H-bridge Drive -

Parameter DC-H Bridge Drive DC7 Drive

No Quadrant Operation Two Four

Switching frequency More Less

Complexity More Less

Ripple in Armature current Less More

The motor can provide motoring and breaking operations in both forward and reverse directions. Therefore, a DC motor can operate in both directions of rotation and of producing both motoring and breaking, hence it is known as four quadrant operation of DC motor.

The power developed by the motor is given by the product of angular speed and the torque. If the developed power by motor is positive, then the mode of operation is called motoring and when the developed power is negative, the mode of operation is called breaking.

step to open model -

DC 7 block open to refer the help in matlab and open the DC 7 drive block.

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 )

ans -

Type -C chopper or Two-quadrant type-A 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 Vs . 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 –

Steps -

1 - we required to build the model plus gen block , powergui,mosfet block , diode block , not block - rectangular , current measurement block , voltage measurment block, dc volatage source , l branch load, scope.

2 - this block are get form sumilink library

3 - connect this block

4 - this block connectd and build the model in 2-quadrant chopper.

5 - set the various value and run it.

Output waveform –

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

ans -

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 of Brushless DC motor –

In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature windings 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.

Working Principle and Operation of BLDC Motor –

BLDC motor works on the principle similar to that of a conventional DC motor, i.e., 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 case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.

When the stator coils are electrically switched by a supply 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.

Consider the figure below in which motor stator is excited based on different switching states. 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 causing motor to rotate.

Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction.

Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.

Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.