Week-7 Challenge: DC Motor Control

QUESTION1

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

in above circuit input voltage is generated from the pulse generator.where the amount of supply and duty cycle is assigned in the pulse generator.then switch block is connected to pulse generator through an armature voltage control block.the H-bridge consists of 4BJT diode piars two transistors are switched simeltaneously Q1andQ4,or Q2and Q3.when Q1and Q4 are fired a positive voltage is applied to 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 motor and diodes D1-D4 operates as free wheeling diodes when Q2 and Q3 are switched off.

Q1 and Q4 -forward motoring

Q2 and Q3 -reverse motoring

dc motor is connected to a battery and the the dc motors mechanical input is intiated as torque. torque for mechanical input of dc motor also switched by goto and from the output of dc motor is connected with bus selector whereas the speed is plotted in the scope .

the pulse width was set at 75 in pulse generator:

plot1:

plot2:

in the graph shows that when the switch Q3 is off on the starting stage of current flow and diode curent is on that stage.during the 0.5 esc of time D3 is on that are given according to the pulse generator and the after the time period of 0.5 sec D3 switch is off and IGBT current is acted after 0.5 sec and Q3 switch remains on the speed and armature current is obtained by dc motor and load torque is calculated by torque proportional to the square of speed.the speed increased in a certain period of time and at that time armature current is positive .after the time period 0.5 sec which is given in pulse generator sudden shoot up acted and speed gets decreased.

pulse width at 50:

In this starting current is generated as diode current for time period 0.5 sec,then IGBT current is acted while diode current is stopped Q3 is off on the starting stage of current flow and diode current is on at that stage.there will be little variation as shown in graph at the starting point ofcurrent but the complete reduction of shoot up is not held here.

time period of forwarding and reverse action is maintained as same as the previous duty cycle.

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

Basic operational voltage equation of a DC motor is given as
E = E_b + I_aR_a and hence, I_a = (E - E_b) / R_a
Now, when the motor is at rest, obviously, the back emf E_b = 0. Hence, armature current at the moment of starting can be given as I_a = E / R_a. In practical DC machines, armature resistance is basically very low, generally about 0.5 Ω. Therefore, a large current flows through the armature during starting. This current is large enough to damage the armature circuit.

The armature current spikes at each duration of switching when the armature voltage is reversed.

This is due to the transient switching between the two states of positive and negative voltage applied to the motor instantenously. This switching causes the input-surge current or inrush currents. The maximum instantaneous input current drawn by an electrical device when first turned on is known as Inrush current.

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.

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 :

BJT H-Bridge drive	four quadrant chopper
is used for changing the direction of motor	here the motor can be oprated in 4 signals are different modes
regenerative braking cannot be available	regenarative braking can be achieved
the source is of fixed dc supply	the source is an AC supply which can be rectified to dc varying supply
the armature current shoots up during the phase transition	the armature current doesn't shoot up to the vehicle of BJT H-bridge
signals are contineous	signals are not contineous and they are discrete
less control of vehicle	better control of vehicle

Q2.Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

As shown in circuit diagram current flows in two directions across the load inductor from source to load and from load to source

souce to load:

whenever CH1 is ON current flows in forward direction.when CH1 is off ,the remaining current in inductor flows through freewheeling diode by changing its inductor polarity in same direction thus 1st quadrant operation is done until vL>E

load to source:

when CH2 is ON current flows in opposite direction thorough CH2 when E>vL when CH2 is off current flows back to battery source through D2.

2-quadrant chopper simulink model:

pulse generator parameters:

DC voltage parameters:

scope:

the voltage wave form is not going less the minimum value of voltage is zero and maximum value is always in the positive direction.however in current wave form the current increase and slightly decreases.pulse wave also increasing and then decresing

Q3.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 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.

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 Principle of Brushless DC motor

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 of Brushless DC 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

Limitations of Brushless DC motor

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

Applications of Brushless DC 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.