DC Motor Control

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

Answer :- The model for Speed control of a DC motor using BJT H-bridge is shwon below.

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.

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.

Question 1:- B. Comment on the armature current shoot-up from the scope results.

Answer :- As given 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. The armature current plot is shown below.

By observing the graph, the motor behaviour is same as given inputs and it changes its direction after 0.5 sec. At the starting of operation in any direction of the motor, the armature current shoots up at the starting and stablises after some time, but it doesn't take much longer. The armature current fluctuation is more while motor is changing the direction rather than at starting.

Question 1:- C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

Answer :- The The Four-Quadrant Chopper DC Drive (DC7) block is shwon as below.

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.

The model follows the simulation as :-

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

Results :-

Comparision of H-bridge Model with Four quadrant chopper DC drive:-

H-bridge Model	Four-quadrant Chopper DC drive
1. Smooth armature current transition	1. Current ripples are found in armature.
2. Simulation type is continuous.	2. Simulation type is discrete.
3. The supply given is 240V DC.	3. The supply given is 380V 50Hz AC supply.
4. No need for AC/DC convertor	4. AC/DC convertor is needed.
5. Max field voltage of the motor used in this model is 300V.	5. Max field voltage of the motor used in this model is 310V.
6. Regeneratitve braking is not possible.	6. Regenerative braking is possible.
7. H-bridge can operate in Forward and reverse only.	7. This four quadrant chopper drive works in four modes such as forward motoring, forward braking,reverse motoring, reverse braking.

[As mentioned in today's live session, Question 2 is skipped. Please consider that while grading]

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

Answer :-

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 :-

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.

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.

Working Principle :-

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.

Advantages :-

1. It has no mechanical commutator and associated problems

2. High efficiency due to the use of permanent magnet rotor

3. High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed.

4. No brushes means no wearing out of brushes which inturns less mainainance.

5. There is no ionizing sparks from the commutator, and electromagnetic interference is also get reduced.

6. Much less noise during operation.

Disadvantages :-

1. These motors are costly

2. Electronic controller required control this motor is expensive

3. Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors

4. Requires complex drive circuitry

5. Need of additional sensors such as postion sensors.

Applications :-

1. Used in Electric and hybrid vehicles.

2. Used in Exhaust fans, computer fans etc.

3. Used in Robotics, CNC machines and Drones etc.