Week-7 Challenge: DC Motor Control

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

The Bipolar Junction Transistor (BJT) when used for power switching applications and operates as an IGBT. When it is conducting(BJT operating in the saturated region), a forward voltage V 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 ree-wheeling diodes when Q2 and Q3 are switched off

Simulink model:

The Pulse Generator is used to generate pulses which are fed to the four IGBTs to create Pulse Width Modulation. Here in this model, the simulation is such that the motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). 'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3. Initial the Block Parameters for Pulse Generator are as follows;

At t = 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction. So in the first half cycle, we can see the motor attains a speed of 1000 rpm a load torque of 6 N.m whereas the second half cycle the same values in the negative direction. Here we can see that the armature current of DC motor does get shoot up. To avoid this, we will have to reduce the pulse width from the pulse generator.

MODIFICATION –
We can modify the model by reducing the Pulse width to 40%

Result:

Here we can observe that the Armature current is almost 0 A, that means the armature current doesn’t shoot up at 40% pulse width.Also the motor speed and torque is reduced to 300 rpm and 0.6 N.m respectively due to the reduction in Pulse width. This indicates that, as we reduce the pulse width, the motor speed and torque reduces and can attain zero at a certain value.

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

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.

Simulink model:

After running a simulation we got a graph like as follows.

Based on the above simulation results, we can see that, The plot of duty cycles IGBT 1&4 are exactly opposite to that of duty cycles IGBT 2&3 as the pulse values of IGBT devices 1 and 4
are opposite to those of IGBT devices 2 and 3. The desired armature current is obtained from the average armature voltage. The armature current follows the current reference very well, with fast response time and small ripples.

Comparing the Four-Quadrant Chopper DC Drive (DC7) block with BJT H-bridge –

• In Four Quadrant Chopper DC Drive Block, the speed reference input is used to change the direction of the motor rotation whereas in BJT H-bridge model, the polarity is reversed to change the direction of motor rotation.
• In BJT H-bridge model, the motor can be controlled by changing the pulse width of the pulse generator whereas in Four Quadrant Chopper DC Drive Block, the speed reference values can be changed to control the motor.
• The Four Quadrant Chopper DC Drive operates in four modes wiz. forward motoring, forward regenerative braking, reversing motoring and reverse regenerative braking mode whereas BJT
• H-Bridge model operates only on two modes wiz. forward and reverse.
• PI (Proportional Integral) Controllers are used for speed and current control in DC7 block and it is absent in BJT H-bridge model.
• The Four Quadrant Chopper DC Drive Block uses ramps to avoid current shoot up and smooth working stability. The disturbances and current shoot up is high in H-Bridge model because of switching losses. This is why the overall performance and stability is good in Four Quadrant Chopper DC Drive comparing BJT H-Bridge model.

Result:

Thus the four quadrant chopper and BJT H-Bridge has been compared succesfully.

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Question.No.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 )

Two quadrant chopper is also called as type c chopper. Type C chopper is obtained by connecting type –A and type –B choppers in parallel. We will always get a positive output voltage V as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v will be equal to V . The direction of the load current i will be reversed. The current i will be flowing towards the source and it will be positive regardless the chopper is on or the FD conducts. The load current will be negative if the chopper is or the diode D2 conducts. We can say the chopper and FD operate together as type-A chopper in first quadrant. In the second quadrant, the chopper and D2 will operate together as type –B chopper. The average voltage will be always positive but the average load current might be positive or negative. The power flow may be life the first quadrant operation ie from source to load or from load to source like the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action my cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

Simulink model:

let us take two mosfet blocks and diodes blocks and connect it as shown in circuit diagram. the dc voltage source is connected as shown.

let the voltage of dc voltage source be 24 volts.

let us consider an series RLC branch block and select inductor L . thus block works as a load.

to consider LE load ie motor load , and dc voltage source of 12 volt is also connected in series with inductor.
The pulse generator block is used to create gate pulse.

The not gate is used in the model so that the two mosfet will not get closed at the same time otherwise short circuit will occur .due to use of not gate , when one mosfet is ON the other mosfet will get off and when other mosfet is ON the first mosfet will get off .
The amplitude of pulse generator is kept 10 so that we can clearly see the pulses at scope and let period be 0.02 seconds and let
pulse weidth be 50%

the voltage waveform is not going less than zero . the minimum value of voltage is zero and maximum value is always in the positive direction. however in current waveform , the current increases and decreases. the current may be positive as well as negative and the pulse amplitude is 10 thus from pulse waveform we can see that the pulse amplitude is 10.

Result:

Thus the above graph represents the wave form of two-Quadrant operation of chopper.

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Queston.No.3:Explain in a brief about operation of BLDC motor.

Brushless DC electric motors also known as electronically commutated motors .A brushless DC motor (known as BLDC) is a
permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes electronically
controlled commutation system (commutation is the process of producing rotational torque in the motor by changing phase
currents through it at appropriate times) instead of a mechanically commutation system. BLDC motors are also referred as
trapezoidal permanent magnet motors.

Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as
to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with
permanent magnet rotor and a stator with a sequence of coils. In this motor, permanent magnet (or field poles) rotates and current
carrying conductors are fixed.

The armature coils are switched electronically by transistors or SCR at the correct rotor position in such a way that armature field is in
space quadrature with the rotor field poles. Hence the force acting on the rotor causes it to rotate. Hall sensors or rotary encoders are
most commonly used to sense the position of the rotor and are positioned around the stator. The rotor position feedback from the
sensor helps to determine when to switch the armature current.
This electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more
reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds. The
efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient. There are
wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large
power ranges.
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 commutators .The field inside a brushless motor is switched through an amplifier
which is triggered by the commutating device like an optical encoder.
BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as
single-phase, two-phase, or three-phase motors. However, three-phase BLDC motors with permanent magnet rotor are most
commonly used.
The construction of this motor has many similarities of three phase induction motor as well as conventional DC motor. This motor has
stator and rotor parts as like all other motors.

ADVANTAGES 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

DISADVANTAGES 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 positioningapplications 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.

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