Week-7 Challenge: DC Motor Control

OBJECTIVE :

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 article - Multiquadrant operation of motor )

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

SOLUTIONS :

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.

1. A)

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.

When the pulse width is 75, the following output is obtained.

The IGBT current is given for a duration of 0 to 1 sec and the diode current is given for a duration of 0 to 0.5 sec.

During this period, the armature current shows a slight transient.

For a duty cycle of 50%, the following output is obtained.

As the duty cycle decreases, the diode current falls below 10A and the motor speed also decreases to less than 500 rpm.

So as the duty cycle reduces, the motor speed also reduces. In order to achieve higher rpm, the duty cycle should also be high.

B)

The sudden peaks can be reduced by reducing the duty cycle. In the above case, we reduced the duty cycle to 50%. Now we will reduce it to 30% and observe the results.

We can observe that the sudden change in the output is completely reduced. Hence it is proved that as the duty cycle reduces, the sudden peak in the output is also minimised.

C)

Detailed Explanationn :

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

Before starting the simulation, set the initial bus voltage to 515 V via the GUI block ('Initial States Setting' button and 'Cbus' variable).

Start the simulation. You can observe the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope. The current and speed references are also shown.

The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.

The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.

The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.

At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.

At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.

At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

Comparison of Four-Quadrant Chopper DC Drive with H bridge model

FOUR-QUADRANTT	H-BRIDGE
Regenerative braking is possible	Regenerative braking is not possible but dynamic braking can be achieved
To change direction of motor, current signals are interrupted with one another	To change direction of motor, only the polarity of the current is changed
The current values are chopped and hence are not continuous	The current values are continuous
Uses PI controller for both speed and current control	It does not have a PI controller
Can operate in all four modes	Can operate only in two modes
Operates at high power levels	Operates at moderate power levels

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 buck-boost converter controlled by two PI regulators.

The buck-boost converter is fed by a 630 V DC bus obtained by rectification of a 460 V AC 60 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 DC6 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 two IGBT devices (Pulse Width Modulation).For proper system behaviour, the two IGBT devices have opposite instantaneous pulse values.

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.

• Before starting the simulation, set the initial bus voltage to 630 V via the GUI block ('Initial States Setting' button and 'Cbus' variable).

• Start the simulation. You can observe the motor armature voltage and current, the two IGBT pulses and the motor speed on the scope. The current and speed references are also shown.

• The speed reference is set at 400 rpm at t = 0 s. Initial load torque is 814 N.m.

• Observe that the motor speed follows the reference ramp accurately (+250 rpm/s) and reaches steady state around t = 2 s. The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.

• At t = 2.1 s, the load torque passes from 814 N.m to 100 N.m. The motor speed recovers fast and is back at 400 rpm at t = 2.75 s. The current reference lowers to about 40 A to generate a smaller electromagnetic torque, the load torque being reduced. As observed before, the armature current follows its reference perfectly.

• At t = 2.75 s, the speed reference jumps down to 100 rpm. In order for the motor to decelerate following the negative speed ramp, the armature current reverses down to -160 A to generate a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase. The braking chopper limits the voltage value.

• At t = 3.4 s, the motor speed reaches 100 rpm and the current reverses back to 40 A.

At t = 4 s, the speed stabilizes around its reference.

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

The below animation given is taken as reference from google images is for easy understanding

A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor) or synchronous DC motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic closed loop controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors

The construction of a brushless motor system is typically like a Permanent Magnet Synchronous Motor (PMSM), but can also be a Switched Reluctance Motor, or an Induction (Asynchronous) Motor.

They may also use neodymium magnets and be out-runners (the stator is surrounded by the rotor), in-runners (the rotor is surrounded by the stator), or axial (the rotor and stator are flat and parallel)

BLDC Motor also known as Brushless DC Motor is a motor-operated without brushes to overcome certain difficulties.

The BLDC operates on the same law as of a regular type. This type requires some coils to be energized in order to make the rotor rotate.

• 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 “In-runner” 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 Principle :

• BLDC motor works on the principle like that of aBrushed DC motor.
• The Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. Because 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 aligns with stator poles which causes the motor to rotate.
• In brushless DC motors, an electronic servo system replaces the mechanical commutator contacts.
• An electronic sensor detects the angle of the rotor and controls semiconductor switches such as transistors which switch current through the windings, either reversing the direction of the current or, in some motors turning it off, at the correct angle so the electromagnets create torque in one direction. The elimination of the sliding contact allows brushless motors to have less friction and longer life; their working life is only limited by the lifetime of their bearings.

The coils placed in the stator slots are named as A, B, and C. When the coil A is energized by a DC supply, it gets excited and attracts the rotor permanent magnet.

Similarly, when the coil B is energized rotor permanent magnet is attracted this continues when the coil C is also energized. After the coil C is energized, the coil A is again energized but with negative polarity. This continues with the remaining coils as well with negative polarity.

Four Quadrant torque ripple-free operation of BLDC Motor

This cycle continues alternately and the rotor continuously rotates. But there is a problem of developing the low power output.

This can be overcome by connecting the free ends of two coils together I,e A and B. This produces greater power output as the coils energize double the field and pull the rotor permanent magnets along with them.

This way a greater output can be achieved.The problem with the BLDC type is that which coil to energize first. It can be known by finding the position of the rotor coils. This position can be known by using a controller. The controller helps in energizing the coil after receiving the position from the sensor.

Merits/Advantages of BLDC :

• Highly reliable
• Smoother operation
• High power output
• Can be used for higher speed applications
• No brush is required
• No sparking
• Light in weight
• Less noise

Limitations/ Disadvantages :

• Complex operation
• Requires maintenance
• A controller is required
• Costly
• Sensors are required

Applications of BLDC Motors :

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 equipment 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.

Conclusionss:

1. A. MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge was explained.Armature current shoot-up from the scope results were described and refering to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’ the H- bridge model was compared
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)
3. Operation of BLDC motor was explained in short

Links and references :

Grive Link

https://drive.google.com/drive/folders/1JlIz6LanQIri8MoDxANblTsQ7BwRisaQ

Refrence :

1. Google
2. wikipedia
3. mathworks documentation
4. article - Multiquadrant operation of motor
5. MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge