Week-7 Challenge: DC Motor Control

AIM : To control the DC motor using various power electronics.

OBJECTIVE :

1. To run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’ and change the model such that armature current doesn’t shoot up when motor changes direction from forward to reverse.

2. To compare The Four-Quadrant Chopper DC Drive (DC7) block with H-bridge model.

3. To make a suitable EV model using DC7 block and plot the result.

4. To explain operation of BLDC motor.

SOLUTION :

1. To run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’ and change the model such that armature current doesn’t shoot up when motor changes direction from forward to reverse.

A.) The example shows the 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 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.

Motor details:

Rated voltage = 240V

Power = 5 HP

Rated speed = 1750 RPM

Motor starts in positive direction with duty cycle of 75%.

At 0.5 sec of t the armature voltage is suddenly reversed and motor runs in reverse direction

By running the example we got the plot as below

From above plots the following points are notified:

i. Plot 1 shows the Speed with respect to time, Speed is proportional to applied torque. Speed reached upto 1000RPM.

ii. Plot 2 shows the Armature current at every instant of time. At first instant the current is raised suddenly upto 40A then it became mostly constant for some time and it became negative when motor runs in reverse direction.

iii. Plot 3 shows the Load torque. The torque is around 5-6 N-m. As soon as the motor is reversed the direction of torque is also reversed with almost same value in negative side.

From the theory of motor,

Eb=Back EMF

V=Rated Voltage

R=Armature resistance

I=Armature current

Eb= V - I *R

I = (V - Eb)/R

At starting of motor, Speed is zero, Eb also remains zero, also armature resistance is very low which is less than one ohm. Then I = (V-0)/R

V is rated voltage that can be in order of around 230 V like that.

So, I = 230/R, which means the current will very higher than usual. For suppose R=0.25 ohms then,

I = 230 / 0.25

= 920 A

So the current is very high, which damages the motor

Armature current shootup is more at start which is unstable. Even when the motor runs in negative direction another spike is occured

To overcome this problem duty ratio can become a solution.

Actual duty ratio is 75%, if we change the duty ratio to 30%

then the result is

From above plot the explanation is as below:

i. As duty cycle is changed from 75% to 30%, Armature current shootup is reduced completely.

ii. But No load torque also fall 5-6 N-m to around 1 N-m

iii. No load Speed also changed from 900-1000 RPM to 150-200 RPM.

• Using of duty ratio reduction makes the reduction of armature current spikes but the torque and speed also reduces.

2. To compare The Four-Quadrant Chopper DC Drive (DC7) block with H-bridge model.

A.)

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.

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.

This example shows the DC7 four-quadrant chopper DC drive during speed regulation.

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.

FOUR QUADRANT CHOPPER WORKING:

A four quadrant chopper is a chopper which can operated in all the four quadrants. The power can flow either from source to load or load to source in this chopper. In first quadrant, a Class-E chopper acts as a Step-down chopper whereas in second quadrant it behaves as a Step up chopper. This type of chopper is also known as Class-E or Type-E 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. In the above circuit diagram, the choppers are numbered CH1 to CH4. For first quadrant operation CH1 is made ON, for second quadrant operation CH2 is made ON and so on.

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.

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.

For third quadrant operation, CH1 is kept off, CH2 is kept ON and CH3 is operated. When CH3 is ON, load gets connected to source and hence load voltage is equal to source voltage. But carefully observe that the polarity of load voltage vo is opposite to what shown in the circuit diagram. Hence, v_o is assumed negative. Let us now see what is the status of load current i_o. It may be seen that i_o is flowing in the direction opposite to shown in the circuit diagram and hence negative.

Now, when CH3 is turned OFF, the negative load current free wheels through the CH2 and D4. In this manner, v_o and i_o both are negative. Hence, the chopper operates in third quadrant.

Fourth Quadrant Operation:

To obtain fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be reversed in this case too like third quadrant operation.

When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON. When CH4 is made OFF, current is fed back to the source through diodes D2, D3. Here load voltage is negative but the load current is always positive. This leads to chopper operation in fourth quadrant. Here, power is fed back to the source from load and chopper acts as a step-up chopper.

The operation of a four quadrant chopper or Class-E chopper is summarized in the figure below.

Working of BJT H-bridge:

The circuit given here is of a simple H bridge motor driver circuit using easily available components. H Bridge is a very effective method for driving motors and it finds a lot of applications in many electronic projects especially in robotics.

The circuit shown here is a typical four transistor H Bridge. The diodes D1 to D4 provide a safer path for the back emf from the motor to dissipate and thus it protects the corresponding bipolar transistors from damage. Resistors R1 to R4 limit the base current of the corresponding transistors. Working of this circuit is very easy to understand. When terminal D is grounded and A is pulled to +Vcc, transistors Q1 and Q4 will be on and current passes through the motor from left to right. When terminal B is grounded and C is pulled to +Vcc, transistors Q3 and Q2 will be on and current passes through the motor from right to right making the motor to rotate in the opposite direction.

Comparison between Four Quadrant DC chopper and BJT H- bridge drives :

• Regenerative braking is possible in Four quadrant DC chopper but in H bridge Regenerative braking is not possible but dynamic braking is possible
• In Four quadrant chopper current values are chopped which are not continuous. In H bridge current is continuous.
• In Four quadrant chopper to change direction motor current signals are interrupted with one another. In H bridge to change the direction of motor the polarity of current is changed.

3. To make a suitable EV model using DC7 block and plot the result.

A.)

The figure shows the EV model using a DC7 four quadrant chopper DC drive block. The operation of DC7 block is already explained in previous question. To make the suitable EV model the following changes are made.

i. For an Electric Vehicle, the energy source is either battery or fuel cells. So consider DC voltage source as input to the motor.

ii. Replace the Speed reference block with drive cycle

iii. Modify load torque block by subsystem with standard equation of

T = 24.7 + 0.0051 W^2

This takes speed as input from Motor and calculates load torque.

By running the model, the plots are

above figure is current measurement from the battery source.

i. We can see the duty ratio plot of different igbt's

ii. Speed values depend on the input duty cycles.

iii. The duty cycles of IGBT 1-4 and IGBT 2-3 are opposite to each other. So 1-4 are used for positive cycles and 2-3 are used for negative cycles.

iv. Duty ratios will control the voltage and armature current.

4. To explain operation of BLDC motor.

A.)

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:

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.

Brushless motor does not have any current carrying commutators. The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.

The layout of a DC brushless motor can vary depending on whether it is in “Out runner” style or “Inrunner” style.

• Outrunner – The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.

• In runner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out runner style, but is capable of spinning at very high rpm.

Working: BLDC motor works on the principle similar to that of a brushed DC motor. 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 the BLDC motor, the current carrying conductor is stationary and the permanent magnet is moving.

When the stator coils get a supply from 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.

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 which causes the motor to rotate.

Advantages:

• Less overall maintenance due to absence of brushes
• Reduced size with far superior thermal characteristics
• Higher speed range and lower electric noise generation.
• It has no mechanical commutator and associated problems
• High efficiency and high output power to size ratio due to the use of permanent magnet rotor
• High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed
• Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors.
• Long life as no inspection and maintenance is required for commutator system
• Higher dynamic response due to low inertia and carrying windings in the stator
• Less electromagnetic interference
• Low noise due to absence of brushes

Applications:

Brushless DC motors (BLDC) use for a wide variety of application requirements such as varying loads, constant loads and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipments etc.

• Computer hard drives and DVD/CD players
• Electric vehicles, hybrid vehicles, and electric bicycles
• Industrial robots, CNC machine tools, and simple belt driven systems
• Washing machines, compressors and dryers
• Fans, pumps and blowers.

CONCLUSION :

1. MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’ is simulated under given conditions and results are plotted.

2. The Four-Quadrant Chopper DC Drive (DC7) block and H-bridge model are explained by comparing both of them.

3. An EV model is designed using DC7 block and result is plotted.

4. The operation of BLDC motor is explained.