#7 DC Motor Control

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

Blocks Used in the model:

IGBT: The IGBT block represents a semiconductor device controllable by a gate signal. The IGBT is simulated as a series combination of a resistor, Inductor and a DC voltage source with a switch controlled by a logical signal.

Why IGBT is used here?

The bipolar junction transistor when used for power switching applications is operates as an IGBT. When it is conducting a forward voltage Vf is developed between collector and emittor. Therefore IGBT block is used as an BJT device.

DC Motor:

Here the DC machine block is used as a DC motor. We need to controll the DC motor rotation in such a way that when to spin ACW and When to spin CW.

Diode: This block 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).

Model Explanation:

In the BJT H-Bridge, the speed of the DC motor is controlled by four power electronic switches like Insulated Gate Bipolar Transistor (IGBT) and Diode. The pulse generator with 75% duty cycle is used to give the input control signals to the power electronic switches.

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.

Results:

With the default settings, the obtained results are

The plot shows the current flows through the switches Q3, Intially the current flows through switches Q1& Q4, In the second only the Current flows through Swithces Q2&Q3, so the current flows through the diode in first half and through the IGBT in the second half. We can observe the oscillation of the current in both IGBT and Diode which is not desirable we have to minimize the oscillation of current so that the power electronic devices will operate efficiently.

The second plot shows the speed is positive in the first half and negative in second half which is in accordance with the input signal.

The total armature current is aslo oscillating to a peak value at the initial which is need to be minimized.

The load torque for the torque equation is T=K.w^2 is calculated and plotted against simulation time. During the positive motoring of the DC motor, the torque is positive and in the reverse motoring of the DC motor, the torque is negative.

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

The shoot up in the armature current is controlled by two methods. one we can increase the resistance so that we can controlled the output armature current but adding high resistance to the electrical circuit is not advisable in reality, since it leads to IR losses.

The second method of controlled the output armature current is varying the duty cycle.

when the duty cycle is reduced to 50%, the variation in the armature current is

In the above plot the shoot up in the armature current is reduced.

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

The Chopper is basically a DC-Dc converter which will reduce the input Dc voltage to the suitable voltage of the motor or other applications. The Four quadrant chopper DC drive is a converter which operates in the all the quadrants.

DC7	BJT H-Bridge
A chopper is a device that is capable of producing variable voltage from a more or less, afixed voltage source by chopping or switching the circuits	A H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load
This model contains speed and current controllers in the power converter circuit.	This is model has no controllers in the power converter circuit
Mechanical Input is motor torque Tm	Mechanical input is Load Torque TL
connects to a Ac source with AC to DC converters	Connects to a constant DC source.

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 )

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.

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 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.

The results obtained with the above model i

Brushless DC Motor (BLDC) - Construction, Working & Applications :

Construction of BLDC (Brushless DC Motor)

Brushless DC motors (BLDC) have been a much focused area for numerous motor manufacturers as these motors are increasingly the preferred choice in many applications, especially in the field of motor control technology. BLDC motors are superior to brushed DC motors in many ways, such as ability to operate at high speeds, high efficiency, and better heat dissipation.

They are an indispensable part of modern drive technology, most commonly employed for actuating drives, machine tools, electric propulsion, robotics, computer peripherals and also for electrical power generation. With the development of sensorless technology besides digital control, these motors become so effective in terms of total system cost, size and reliability.

What is a 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.

The armature coils are switched electronically by transistors or silicon controlled rectifiers 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.

Construction of BLDC Motor

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.

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 is 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. For industrial applications and automation systems, 100 V or higher rating motors are used.

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.

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.

Brushless DC Motor Drive

As described above that the electronic controller circuit energizes appropriate motor winding by turning transistor or other solid state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of MOSFET bridge (also called as inverter bridge), electronic controller, hall effect sensor and BLDC motor.

Here, Hall-effect sensors are used for position and speed feedback. The electronic controller can be a microcontroller unit or microprocessor or DSP processor or FPGA unit or any other controller. This controller receives these signals, processes them and sends the control signals to the MOSFET driver circuit.

In addition to the switching for a rated speed of the motor, additional electronic circuitry changes the motor speed based on required application. These speed control units are generally implemented with PID Controllers to have precise control. It is also possible to produce four-quadrant operation from the motor whilst maintaining good efficiency throughout the speed variations using modern drives.

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