Week-7 Challenge: DC Motor Control

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

in the circuit the input voltage is generated by pulse generator and the amount of supply and duty cycle is aasign in the pulse generator.

DUTY CYCLE = ON/(ON+OFF)

PULSE WIDTH = ON+OFF

We maintain the pulse width to 70%

H-Bridge is consist of 4 BJT-DIOD pair. Two transistor fire symultaniously Q1 AND Q4 OR Q2 AND Q3.

Switch block is connected to pulse generator through armature voltage controle block.

when Q1 AND Q4 get fired then positive voltage is applied accros the motor and diode D2 AND D3 act as a freewheeling diode.and Q2 AND Q3 get switch off.

when Q2 AND Q3 get fired then negative voltage is applied accros the motor and diode D1 AND D4 act as a freewheeling diode.and Q1 AND Q4 get switch off.

SWITCH:

Q1 - Q4 =FORWORD MOTORING

Q2 - Q3 =REVERSE MOTERING

This circuit is called H-BRIDGE , it is an arrangement of transistor that allows a circuit full controll over standerd DC MOTOR.

Que 1B) Comment on the armature current shoot-up from the scope results.

OUTPUT RESULT:

In the given waveform scope shows the diode current and IGBT current passing through D3 AND Q3.

During starting stage the IGBT CURRENT IS OFF AND DIODE current is in on stage for 0.5 sec of half cycle.

on next cycle after 0.5 sec DIODE current is off and IGBT current is remains ON.

the speed and armature current is obtain by DC motor and load torque is obtain by load torque is proportional to square of speed.

there is littel shoot up at starting.speed increase for a certain period of time at upto that time current and torque also remains positive.

after 0.5 sec there is sharpe decrease in speed,simultaniously current and torque start decreasing

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

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.

A motor operates in two modes – Motoring and Braking. A motor drive capable of operating in both directions of rotation and of producing both motoring and regeneration is called a Four Quadrant variable speed drive.

In motoring mode, the machine works as a motor and converts the electrical energy into mechanical energy, supporting its motion. In braking mode, the machine works as a generator and converts mechanical energy into electrical energy and as a result, it opposes the motion. The Motor can work in both, forward and reverse directions, i.e., in motoring and braking operations.

The product of angular speed and torque is equal to the power developed by a motor. For the multi-quadrant operation of drives, the following conventions about the signs of torque and speed are used. When the motor is rotated in the forward direction the speed of the motor is considered positive. The drives which operate only in one direction, forward speed will be their normal speed.

In loads involving up and down motions, the speed of the motor which causes upward motion is considered to be in forward motion. For reversible drives, forward speed is chosen arbitrarily. The rotation in the opposite direction gives reverse speed which is denoted by a negative sign.

The rate of change of speed positively in the forward direction or the torque which provides acceleration is known as Positive motor torque. In the case of retardation, the motor torque is considered negative. Load torque is opposite to the positive motor torque in the direction.

The figure below shows the four-quadrant operation of drives:

In the I quadrant power developed is positive and the machine is working as a motor supplying mechanical energy. The I (first) quadrant operation is called Forward Motoring. II (second) quadrant operation is known as Braking. In this quadrant, the direction of rotation is positive, and the torque is negative, and thus, the machine operates as a generator developing a negative torque, which opposes the motion.

The kinetic energy of the rotating parts is available as electrical energy which may be supplied back to the mains. In dynamic braking, the energy is dissipated in the resistance. The III (third) quadrant operation is known as the reverse motoring. The motor works, in the reverse direction. Both the speed and the torque have negative values while the power is positive.

In the IV (fourth) quadrant, the torque is positive, and the speed is negative. This quadrant corresponds to the braking in the reverse motoring mode.

‘The Four-Quadrant Chopper DC Drive (DC7) block’

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.

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

The Two-Quadrant Chopper block represents a two-quadrant controlled chopper for converting a fixed DC input to a variable DC output. The block contains two switching devices. Options for the type of switching devices are:

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

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 BLDC MOTOR:

In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or Armature winding 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 Commutator. 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 PRINCIPAL OF BLDC MOTOR:

BLDC motor works on the principle similar to that of a shaded 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.

ADVANTAGE OF BLDC MOTOR:

• 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

LIMITATION OF BLDC MOTOR:

• These motors are costly
• Electronic controller required control this motor is expensive
• Requires complex drive circuitry
• Need of additional sensors

APPLICATION OF BLDC MOTOR:

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.