Week-7 Challenge: DC Motor Control

Q:-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) 

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.

Initially, we run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’ with all it’s default values and different pulse width (duty cycle) 75%, 55%, and 45%.

From this we get following three plot results-

using 75% pulse width:-

Q3 and D3 currents graph

scope graph:

using 55% pulse width:-

Q3 and D3 currents graph

scope graph:

using 45% pulse width:-

Q3 and D3 currents graph

 

scope graph: 

In the above-attached results, I observed that the 

• From the plot, we can see that the currents flow through in the IGBT And free wheeling diodes in the positive half cycle and negative half cycle.
• we can see that the speed has been increasing gradually in the +ve half-cycle period at that time the Q1 and Q4 are switched ON and D2 and D3 will act as free wheeling diodes are also conducted.
• In the -ve half-cycle period at that time, the Q2 and Q3 are switched ON and D1 and D4 will acts as free wheeling diodes also conduct and the -ve supply is supplied to the DC Motor and the speed gradually decreases after +half cycle and motor runs in negative speed.
• the current plot has initially very high value and then becomes steady and after that shows the same path in the -ve direction due to regenerative.
• The load torque graph seems like the speed graph.

In these three plots, the value of this current shoot-up is decreasing and the plot has an almost smooth current flow.

To get a steadier and smoother current flow we need to decrease and control the duty cycle.

To change the pulse width (duty cycle):

• Double click on the pulse generator
• the block parameter of the window is a display
• change the pulse width value and click on Apply and Click OK Button. 

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B)

This shoot-up happens because of the switching in the circuit that causes heat losses during switching. To avoid these losses, we use HEAT SINK material to dissipate the heat. From this, the motor can change its direction of rotation easily and very smoothly.

C)

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 behavior, 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.

 Results:

H-BRIDGE :

An H-bridge is a simple circuit that lets you control a DC motor to go backward or forward.

If you close switches 1 and 4, you have a +ve terminal is connected to the left side of the motor and a -ve terminal to the other side. And the motor will start spinning in Forwarding direction.

 

If you instead close switch 2 and 3, you have the +ve terminal is connected to the right side and the -ve terminal connected to the left side. And the motor spins in the reversing direction.

 

the circuit diagram of the h- bridge is shown below

 

Comparison:

Four-Quadrant Chopper DC Drive	H-bridge model
To change the motor direction the polarity is changed	To change the motor direction the current signal has to be interrupted.
It has broader torque and power output ranges	It uses a single pulse generator which gives narrow torque and power output range.
Up to 400v can be achieved.	Up to 350v can be achieved.
In this the current has chopped values.	In this the current has continuous values.
Regenerative braking is possible	Regenerative braking is not possible

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Q:-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:

CIRCUIT DIAGRAM FOR TWO QUADRANT CHOPPER:

A two-quadrant chopper is also called a 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 V₀ as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v₀ will be equal to V_s The direction of the load current i₀ will be reversed. The current i₀ will be flowing towards the source and it will be positive regardless the chopper is on or the FD conducting. 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 positiv 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.

TWO QUADRANT CHOPPER MODEL USING SIMULINK:

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 loa

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.

SCOPE RESULT:

the voltage waveform is not going less than zero . the minimum value of voltage is zero an 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.

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

ANS:

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.

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 s

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.

witch 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.

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.

Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. In order to form an even number of poles, each of these windings i distributed over the stator periphery.

The stator must be chosen with the correct rating of the voltage depending on the power supply capability. For robotics, automotive and small actuating applications, 48 V or less voltage BLDC motors are preferred.

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 th application requirement. In order to achieve maximum torque in the motor, the flux densit of the material should be high. A proper magnetic material for the rotor is needed to produce required magnetic field density.

ferrite magnets are inexpensive, however they have a low flux density for a given volume. Rare earth alloy magnets are commonly used for new designs. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and Boron (NdFeB). The rotor can be constructed with different core configurations such as the circular core with permanent magnet on the periphery, circular core with rectangular magnets, etc.

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

BLDC motor has several advantages over conventional DC motors and some of these are

• 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 Brushless 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 Brushless DC Motors (BLDC)

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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