Week-7 Challenge: DC Motor Control

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

H-bridge:

A H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards. The name is derived from its common schematic diagram representation, with four switching elements configured as the branches of a letter "H" and the load connected as the cross-bar.

The term H-bridge is derived from the typical graphical representation of such a circuit. An H-bridge is built with four switches (solid-state or mechanical). When the switches Q1 and Q4 (according to the first figure) are closed (and Q2 and Q3 are open) a positive voltage is applied across the motor. By opening Q1 and Q4 switches and closing Q2 and Q3 switches, this voltage is reversed, allowing reverse operation of the motor.

Using the nomenclature above, the switches Q1 and Q2 should never be closed at the same time, as this would cause a short circuit on the input voltage source. The same applies to the switches Q3 and Q4. This condition is known as shoot-through.

Description:

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.

A DC motor operates in two modes – motoring and breaking

• In motoring operation, it converts electrical energy into the mechanical energy, which assists its motion.

• In breaking operation, it converts mechanical energy into electrical energy, thus works as a generator and opposes its motion.

The motor can provide motoring and breaking operations in both forward and reverse directions. Therefore, a DC motor can operate in both directions of rotation and of producing both motoring and breaking, hence it is known as four quadrant operation of DC motor.

The power developed by the motor is given by the product of angular speed and the torque. If the developed power by motor is positive, then the mode of operation is called motoring and when the developed power is negative, the mode of operation is called breaking.

For the four quadrant operation of the motor, the sign conventions about the signs of speed and torque are given as follows −

• The speed of the motor is considered positive, when it is rotating in forward direction.

• In motor drives involving up and down motions, the speed of the motor in upward direction is considered to be forward motion.

• The torque is considered positive, when it produces acceleration.

• The torque is assigned negative sign, if it produces retardation.

Four Quadrant Operation

The four quadrant operation of the motor can be described as follows −

Quadrant I

In the first quadrant, the developed power by the motor is positive, thus the motor operates in motoring mode and converts electrical energy into mechanical energy. Therefore, the operation of the motor in first quadrant is called forward motoring.

Quadrant II

In the second quadrant, the direction of the rotation is positive i.e. speed is positive and the torque is negative. Hence, the developed power is negative and the machine operates as a generator which opposes its motion. The kinetic energy of the moving parts of the machines is converted into electrical energy. Therefore, the operation of motor in second quadrant is called forward breaking.

Quadrant III

In the third quadrant, the motor speed and torque both are negative, hence the power is positive. Therefore, the operation of the motor in the third quadrant is known as reverse motoring.

Quadrant IV

In the fourth quadrant, the motor speed is negative and the developed torque is positive, hence the motor power is negative which is corresponding to the breaking operation. Therefore, the operating in fourth quadrant is known as reverse breaking.

Simulation

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.

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

With a duty cycle of 75% (mean DC voltage of 180V):

In the first half cycle, we can see that the motor attains a speed of 1000 RPM and a load torque of 6 Nm. Whereas, for the second half cycle the same values in the negative direction. Here we can see that the armature current of DC motor shoot up.

With a duty cycle of 50% (mean DC voltage of 120V):

For 50% duty cycle, in the first half cycle, we can see that motor speed reduced to approx 420 RPM and a load torque of 1.3 Nm respectively. Whereas, for the second half cycle the same values in the negative direction. Here the armature current of DC motor does not shoot up at 50%.

Observation:

It is observed that, by reducing the duty cycle, the motor speed and torque reduces and can attain zero whereas when we increase the value of pulse width gradually the speed of motor and torque also increases gradually to its maximum value.

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

Description

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.

Simulation

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

Start the simulation. You 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.

Based on the above simulation:

• The plot of duty cycles IGBT 1&4 are exactly opposite to that of duty cycles IGBT 2&3 as the pulse width 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.

Comparison of Four Quadrant Chopper DC Drive(DC7) with BJT H bridge

• In four quadrant chopper DC drive model, the speed reference input is used to change the direction of the motor rotation whereas in BJT H-bridge model, the polarity is reversed across the terminals of the motor with respect to the battery, to change the direction of the motor rotation.

• In four quadrant chopper, the speed reference values is changed to control the speed of the motor whereas, in BJT H-bridge the speed of the motor is controlled by varying the with of the pulse in pulse generator.

• The four quadrant chopper DC drive operates in four mode i.e, forward motoring, forward regenerative breaking, reverse motoring and reverse regenerative braking whereas, BJT H- bridge only operates in two modes i.e, forward and reverse.

• PI (Proportional) controllers are used for speed and current control in DC7 and it is absent in BJT H- bridge.

• The four quadrant chopper uses ramps to avoid current shoot up and smooth working and stability.

• The disturbances and current shoot up is low in DC7 whereas for BJT the shoot up is high because of switching losses.

• Due to the high switching frequency in the four quadrant chopper (DC-DC converters), a low armature current is obtained.

• This why the overall performance and stability is good in Four Quadrant Chopper DC Drive as compared to BJT H- bridge model.

• Only disadvantages is due to its complexity as it consist of four switching devices

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

Quadrant I

In the first quadrant, the developed power by the motor is positive, thus the motor operates in motoring mode and converts electrical energy into mechanical energy. Therefore, the operation of the motor in first quadrant is called forward motoring.

Quadrant II

In the second quadrant, the direction of the rotation is positive i.e. speed is positive and the torque is negative. Hence, the developed power is negative and the machine operates as a generator which opposes its motion. The kinetic energy of the moving parts of the machines is converted into electrical energy. Therefore, the operation of motor in second quadrant is called forward breaking.

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.

Two quadrant chopper using simulink:

In this model, two IGBT and two diode are used and connected across 24 V DC voltage source. Motor load is assumed to be LE load and same was connected as shown in the circuit. Load Current and Load voltage are measured and connected to the scope. A pulse generator is used for gate triggering.

Scope

From the above graph it can be observed that the voltage is always positive but the current is both positive and negative.

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

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 s ends 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