Week-7 Challenge: DC Motor Control

*AIM:-

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

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

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

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

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

*SOLUTION:-

Q1. <A> Explain ‘Speed control of a DC motor using BJT H-bridge’.

Model:-

pulse generator:

Here in the model, four switches are used thus four pulse signals are generated accordingly.

Working OF the H-bridge model:-

• Here H-bridge DC chopper circuit is designed to control the speed of the motor.
• IGBT acts as BJT in power-switching devices.
• Four IGBT switches are used for inverting the DC voltage 0 to 240 v supply. The current flows from the collector(C) to the emitter(E).
• The gate is controlled by the pulses(0 and 1) according to the armature voltage of the motor given by the pulse generator.

• This image shows the voltage value given from initial to final during the step time. According to the pulses on and off the voltage supplied to the IGBTs.
• Here four switches are used Q1, Q2, Q3, and Q4. Pulses P1, P2, P3, and P4 are connected with corresponding switches.
• In the first state, Q1 and Q4 are closed by giving 1V to IGBT, and current from the battery passes through Q1 and Q4. Diode D2 and D3 are used as free wheeling diodes in this state. In this case, Forward motoring and forward breaking are held.
• In the Second state, Q2 and Q3 are closed by giving 1V to IGBTs, current from the battery passes through Q2 and Q3. Diode D1 and D4 are used as free wheeling diodes in this state this case reverse motoring and reverse braking are held. The current direction changes its direction in this state.
• Here free-wheeling diodes are used for a safety purpose of this inverter circuit as when the flow of current on low values protects the circuit and machine by feedbacking the current or in high voltage fluctuations voltage gets split as protection to the motor and circuit components.
• In motor operation, armature terminals are connected with a 240V along the inverter circuit and field windings are connected with a 300 V DC supply to act as an electromagnet for the motor.The load torque is given as mechanical input to the motor for its reference.
• Thus through these magnetic fields generated, variable current carrying conductors in rotor armature produces forces according to the fleming left-hand rule principle. Thus the motor starts to rotate.
• Here run this demo model and study the results from scopes.

*RESULT:-

From the resulting scope, we are getting,

Speed of motor:

• Initially increases to 0 to nearly 900 rpm up to 0.5 secs(Forward motoring action takes place).
• Then speed reduces to 0 rpm(Forward dynamic braking action takes place).
• Then speed increases from 0 to -900 rpm at up to 1 sec( Reverse motoring action takes place).
• Then reverse dynamic breaking occurs(Not shown in scope).

Load torque of motor:

• Initially torque increase from 0 to 0.5 secs in forwarding motoring action.
• Then torque reduces to 0 Nm in forwarding dynamic braking mode.
• Then torque increases in the reverse direction in reverse motoring action up to 1 sec.
• Again torque reduces to zero. (Not shown in scope).

<B> Armature current shoots up in the motor.

Here the initial current shoot-up is due to a sudden increase in voltage which inurns regulated by the duty cycle. In this case, the motor is driven in 75% of the duty cycle.(i.e) 0.75*240= 180 V.

We can change the duty cycle to 50%(120 V) and see the results as,

Sudden shoot up canceled in starting condition but slight shoot up after 0.5 sec in reverse motoring condition.

Again changing the duty cycle to say 35%, we get the result as,

The shoot up of current get dimlished on both the ends.

*CONCLUSION:-

Thus to reduce the shoot up armature currents we have to change the duty cycle gradually from 0 to 100%, that inturnthe s increase voltage gradually and speed also changes accordingly.

<C> Comparing ‘The Four-Quadrant Chopper DC Drive (DC7) the Block’ with H bridge model:

Block:

>Connections:-

>Inputs and outputs:

Armature voltage, speed reference, and motor torque references are given as inputs.

Motor, convertor, controller, and motor angular speed get as outputs.

*WORKING:-

• Here Four Igbt switches are used.
• Here pulses are given from the PI current controller, which controls the pulses by comparing armature current measurement and current reference.
• Current reference can be generated by using the regulation switch in that torque reference and PI speed controller pulses, which are internally compared by the speed of the motor and reference speed given as input.
• Mainly speed is controlled by the current controller.
• In this case, Four quadrant operations are performed.
• Forward motoring mode, forward regenerative breaking mode: Q1 and Q4 are on and Q2 and Q3 are off.
• Reverse motoring mode, reverse regenerative breaking mode: Q2 and Q3 are on and Q1 and Q4 are off.

>Comparisons:-

• In the DC7 chopper model speed is controlled by varying current pulses from the PI current controller and in the H-bridge model speed is controlled by varying duty cycle voltages.
• Here regenerative breaking in occurring while in the H-bridge model breaking is occurred by changing polarity of the current flow, dynamic breaking occurs in it.
• In four quadrant DC chopper model smoothing inductance is used for the smooth increase and decrease in current flow which in turns controlling the motor speed smoothly, while in
• H-bridge is not present.
• Here the operation is discreet and in the H-bridge model, it is continuous.
• Hence 4 quadrant DC choppers are used mostly in electric vehicles.

Q2. Two-quadrant chopper operation using Simulink .

Objective:

To create a model for control of DC motor using two quadrant type C model DC chopper.

Chopper model:

Where,

Vs=DC voltage source.

CH1= Power electronic switch 1.

CH2= Power electronic switch 2.

D2,FD = Free wheeling diodes.

L=Inductor load.

E Back emf from the load.

Mode of operation:

• Here either one should be ON at a time, either CH1 or CH2.
• If CH1 is ON the current flows from source +ve to load(here DC motor), and passes through L and E to the -ve terminal of the source.(1st quadrant)
• If CH1 is OFF the, FD diode conducts and completes the circuit (L-E-FD-L) as stored energy in inductor L. The voltage across is also positive voltage, and the current direction also positive.
• Now If CH2 is ON, the Current flow from E(Load current) completes the circuit(L-CH2-E).
• If CH2 gets OFF, the current passes through D2 towards the source and completes the circuit(E-L-D2-+ve source- -ve source-E).
• In this stage, regenerative breaking occurs. (2nd quadrant).
• This cycle continues again up to stop time.

>Model:-

*WORKING:-

Pulses for the two IGBT switches are given by using a pulse generator and NOT logical operation for alternate switch ON/OFF.

Here DC machine is used and load torque is given as input to the motor.

The motor parameters are given to scope for a result.

The duty cycle is given as 65%. For simplicity, the period is set as 1 sec.

Run the model and study the results.

*RESULT:-

The result scope shows,

Speed increases in the first quadrant as forwarding motoring action. The current is in a positive direction.

Speed reduces at OFF time, So current gets set back to source as regenerative braking mode when CH2 opens up.

Torque changes according to the speed crossing zero and produces negative torque while in the second quadrant.

Thus results get justified.

Note: Model file attached outside.

Q3. operations of BLDC motor.

BLDC motor named as brushless dc motor has high efficiency than other types of motors. This motor is also known to be a synchronous motor.

*CONSTRUCTION:-

Most of the motors are having brushes in their rotor as it transfers current into the rotor circuit. But here different rotor is placed with a permanent magnet.

The stator coil is wounded with coils, they are stationary but produce variable DC voltages for rotating the permanent magnet as changing the magnetic field of stator poles continuously.

The controlling of the motor is by changing the magnitude and direction of current flowing through the stator windings.

A BLDC motor with three coils on the stator will have six electrical wires (two to each coil) extending from these coils. In most implementations, three of these wires will be connected internally, with the three

remaining wires extending from the motor body. Wiring in the BLDC motor case is more complicated than simply connecting the power cell’s positive and negative terminals. we will look more closely at how

these motors work in the second session of this series.

>Operation of BLDC motor:

We can consider three winding terminals coming out as U, V, W, and their other end are connected in a single point as shown in the below figure. The angle shift between two coils makes 120 degrees.

The current moves from U to W making magnetizing the coils attract the rotor consisting of a permanent magnet-making moment as shown above to align with the resultant arrow(larger arrow).

North pole of the magnet towards the arrow mark of the resultant arrow.

Rotation is maintained by continually switching the flux so that the permanent magnet is constantly chasing the rotating magnetic field induced by the coils. In other words, energizing of U, V, and W must be

continually switched so that the resultant flux keeps moving, producing a rotating field that continually pulls on the rotor magnet.

The above movement is achieved by a continuous change in the current direction over two poles of the stator at a single time. But this is slightly not continuous in their rotation.

This makes motor vibration and noise will be produced.

So, to attain smooth continuous rotation of the rotor we have to control the current flow through all three poles at the same time.

As shown below we can achieve continuous change in current flow in all stator poles at the same time in a 120-degree phase shift.

As we have seen, we drive BLDC motors by continually changing the directionality of the flux produced by the coils. The permanent magnets on the rotor continually chase the shifting rotating magnetic field,

causing the rotor to turn.

This is the working of the BLDC motor.

>Advantages of BLDC motor:

• One big advantage is efficiency, as these motors can control continuously at maximum rotational force (torque).
• Even small BLDC motors can deliver considerable power.
• BLDC motors can be controlled, using feedback mechanisms, to deliver precisely the desired torque and rotation speed.
• Reduce energy consumption and heat generation.
• BLDC motors also offer high durability and low electric noise generation.
• The sparks will not be produced as brushed motors between brushes and commutator.

>Control of BLDC motor:

By using the inverter circuitry to also adjust the voltage into each coil, we can control the magnitude of the current.

A typical way to adjust the voltage is with pulse width modulation (PWM). We alter the voltage by lengthening or reducing the pulse ON time ( “Duty cycle”: the ON time

expressed as a ratio of the ON+OFF switching interval). Increasing the duty cycle has the same effect as raising the voltage, and reducing the duty cycle has the same effect as lowering the current.

PWM can be implemented using MPUs equipped with dedicated PWM hardware. While 120-degree conduction control only requires two-phase voltage control.

sinusoidal control uses three-phase voltage control and is considerably more complicated as appropriate inverter circuitry is therefore essential for driving BLDC motors.