Week-7 Challenge: DC Motor Control

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.

Ans A:

Simulink model of a speed control of DC motor using BJT H-bridge:

             

Explaination:

• 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.

Pulse Generator:

          

     

The Pulse Generator block generates square wave pulses at regular intervals. The block waveform parameters, Amplitude, Pulse Width, Period, and Phase delay, determine the shape of the output waveform.

In this pulse generator is used to generate the PWM signals, that pulse width modulator is given to four IGBT's to create a pulse. Initially the motor starts in the positive direction with 75% duty cycle.

Scope: 

The scope block shows the motor speed(rpm), armature current(A), and load torque(n-m). And another scope block indicates the IGBT and Diode currents respectivelly.

After running the simulation with 75% duty cycle we getting output as shown in below:

From the above graph we can see that at t=0 the armature current gets shoot up due to increase in the motor speed and load torque at 75% duty cycle. Here motor speed reaches around 900 rpm and load torque of 6 N-m and armature current of near 10A.

Output waveform of IGBT and Diode currents with 75% duty cycle:

The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.

'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

Ans B:

Changing the duty cycle to 30%:

Output waveform of Speed, Armature current and Load torque with 30% duty cycle:

From the above graph we can see that at t=0 the armature current doesn't gets shoot up due to decrease in the motor speed and load torque at 30% duty cycle. Here motor speed reaches around 200 rpm and load torque of 0 N-m and armature current of almost 0A. All these happens because of reduction in dyty cycle.

Hence from this we come to know that by reducing the duty cycle we can control the speed of motor as well as load torque.

Output waveform of IGBT and Diode currents with 30% duty cycle:

The motor starts in the positive direction with a duty cycle of 30%. At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.

'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

Ans C:

Four quadrant chopper (D7) 200 HP DC drive:

Explaination:

• 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 DC drive:

• 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.

Simulation result:

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

Comparision of DC7 block and BJT H-bridge:

DC7 block	BJT H-bridge
Speed control is achieved by changing reference speed.	Speed control is achieved by changing duty cycle of conducting BJT's.
Operates in four quadrant modes.	Operates in two quadrant modes.
Lower switching losses.	Higher switching losses.
Can be used for regenerative braking.	Cannot be used for regenerative braking as only two quadrant operation.
It is used for industrial and commercial applications.	It is used for onlu in robotics.
To change the rotation of motor like forward and reverse, signals are interrupted with one another.	To change the rotation of motor polarity of the current is changed.

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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 )

Ans:

2 Quadrant Chopper:

           

Sometimes a chopper may be required to provide a two quadrant operation by retaining the current direction in both motoring and braking modes. The chopper permits a change in the polarity of terminal voltage keeping the direction of current constant. In above fig S₁ and S₂ are the choppers working as electronic switches and D₁ and D₂ are the feedback diodes. Assuming continuous conduction, both S₁ and S₂ operate simultaneously.

Output wave form:   

                      

When they are ON, the load current I_d increases, with the rate of rise depending upon the back emf and torque of the machine. When they are OFF, the load current is fed to the supply through the diodes D₁ and D₂. The current decreases in this stage. From output wave form it is clear that the load voltage is effectively negative when the diodes conduct. The average value of the output voltage of the chopper V_da is zero for a = 0.5, the output voltage is an ac waveform with average voltage equal to zero. For a > 0.5 the average value of the dc voltage is negative. The chopper works in the regeneration mode. The power flows from the load to the supply and the dc machine operates as a generator. For control ratios a c 0.5, the operation is similar to the inversion of a line commulated converter (α > 90°). For a > 0.5 the chopper has normal operation. The power flows from the supply to the load. This operation of the chopper is similar to rectification of a line commutated converter (α < 90°). For, a = 0.5 the load voltage is zero. The control characteristic of a two quadrant chopper is shown in output wave form.

  2-Quadrant chopper Simulink model:

• Mosfet:

          

MOSFET and internal diode in parallel with a series RC snubber circuit. When a gate signal is applied the MOSFET conducts and acts as a resistance (Ron) in both directions. If the gate signal falls to zero when current is negative, current is transferred to the antiparallel diode.

• Diode:

           

Implements a diode in parallel with a series RC snubber circuit. In on-state the Diode model has an internal resistance (Ron) and inductance (Lon). For most applications the internal inductance should be set to zero. The Diode impedance is infinite in off-state mode.

• DC voltage source:

               

The DC Voltage Source block implements an ideal DC voltage source. The positive terminal is represented by a plus sign on one port. You can modify the voltage at any time during the simulation.

• Pulse generator:

              

Time-based is recommended for use with a variable step solver, while Sample-based is recommended for use with a fixed step solver or within a discrete portion of a model using a variable step solver.

• Not:

              

Logical operators. For a single input, operators are applied across the input vector. For multiple inputs, operators are applied across the inputs.

• Current measurement:

             

Ideal current measurement. The Output signal parameter is disabled when the block is not used in a phasor simulation. The phasor simulation is activated by a Powergui block placed in the model.

• Voltage measurement:

              

 

Ideal voltage measurement. The Output signal parameter is disabled when the block is not used in a phasor simulation. The phasor simulation is activated by a Powergui block placed in the model.

• Inductor:

       

Implements a series branch of RLC elements.

Use the 'Branch type' parameter to add or remove elements from the branch. Here we added inductor parameter only.

• Scope:

        

The real-time Scope block acquires data in chunks of size Number of samples from the real-time application that is executing on the target computer.

• Powergui:

      

The powergui block allows you to choose one of these methods to solve your circuit:

- Continuous, which uses a variable-step solver from Simulink

- Discretization of the electrical system for a solution at fixed time steps

- Continuous or discrete phasor solution

Output of Simulink model:

From the above graph we can observe that the pulse generation graph is in yellow colour, and it's value was 15 because we gave amplitude of 15 in pulse generation block set.

And red colour indicates the voltage measurement which is going below the 0 value, due to the minimum value of voltage is zero.

And blue colour wave indicates the current measurement, which goes positive and negative current value because of change in the operation of choppers.

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

Ans:

Brushless DC Motor (BLDC) :

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.

Construction of BLDC Motor:

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.

Stator:

                   

Stator of a BLDC motor made up of stacked steel laminations to carry the windings. These windings are placed in slots which are axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta. However, most BLDC motors have three phase star connected stator.

Rotor:

              

BLDC motor incorporates a permanent magnet in the rotor. The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate south and north poles depending on the application requirement. In order to achieve maximum torque in the motor, the flux density of the material should be high. A proper magnetic material for the rotor is needed to produce required magnetic field density.

Hall Sensors:

Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energizing a particular stator winding, acknowledgment of rotor position is necessary. So the Hall Effect sensor embedded in stator senses the rotor position.

Most BLDC motors incorporate three Hall sensors which are embedded into the stator. Each sensor generates Low and High signals whenever the rotor poles pass near to it. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s response.

Working Principle and Operation of BLDC Motor:

          

• BLDC motor works on the principle similar to that of a conventional DC motor, i.e., 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 case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves. 
• When the stator coils are electrically switched by a supply 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.
• Consider the figure below in which motor stator is excited based on different switching states. 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 causing motor to rotate.
• Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction.
• Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.
• Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.

Advantages of BLDC Motor:

• It has no mechanical commutator and associated problems
• High efficiency 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
• Quite operation (or low noise) due to absence of brushes

Disadvantages of BLDC Motor:

• These motors are costly
• Electronic controller required control this motor is expensive
• Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors
• Requires complex drive circuitry
• Need of additional sensors

Applications of BLDC Motor:

Brushless DC Motors (BLDC) are used 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. Some specific applications of BLDC motors are

• 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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