Week-7 Challenge: DC Motor Control

AIM :-

• To explain the MATLAB model Speed control of a DC motor using BJT H-bridge. To comment on the armature current shoot up from the results and to compare the H-bridge with Four-quadrant chopper dc drive.
• To develop a 2-quadrant chopper and explain the working of the same with relevant results.
• Explain the operation of BLDC motor.

1. SPEED CONTROL OF A DC MOTOR USING BJT H-BRIDGE :-

A transistor is a semi conductor device that is used to switch or amplify electronic signals and electrical power. The transistor consists two PN diode connected back to back. It has three terminals namely emitter, base and collector. The base is the middle section which is made up of thin layers. The right part of the diode is called emitter diode and the left part is called collector-base diode.  The emitter based junction of the transistor is connected to forward biased and the collector-base junction is connected in reverse bias which offers a high resistance.

Bipolar Junction Transistors (BJT): A Bipolar Junction Transistor (also known as a BJT or BJT Transistor) is a three-terminal semiconductor device consisting of two p-n junctions which are able to amplify or magnify a signal. It is a current controlled device. The three terminals of the BJT are the base, the collector and the emitter. A BJT is a type of transistor that uses both electrons and holes as charge carriers.

The above simulink model is speed control of a DC motor using the BJT H-bridge. An H-bridge is a simple circuit that controls a DC motor backward or forward. Generally the transistors are controlled using Arduino. In the simulink model the transistors Q1, Q2, Q3 and Q4 use IGBT blocks.

It is simulated as a series of resistor Ron, inductor Lon and DC voltage source Vf in series with a switch controlled by a logic singal g. The IGBT turns on when the collector-emitter (CE) voltage is positive and greater than Vf and a positive signal is applied at the gate input. It turns off when the CE voltage is positive but no signal is applied at the gate. The IGBT will remain in off state if the CE voltage is negative.

When IJBT Q1 and Q4 are switched on, a positive voltage is applied and the motor runs in forward direction. When the IJBTs Q2 and Q3 are switched on, a negative voltage is applied which runs the motor in the reverse direction. 

DC machine block is used to simulate a permanent magnet DC motor which comes with a preset of configurations which in this case are,

• 5HP motor power.
• 240V rated voltage.
• 1750rpm rated speed.
• 300V field voltage

The simulation is run and the following plots are obtained.

The simulation is run for 1s. For 0 to 0.5s the motor runs in forward direction. From 0 to 0.1s the torque increases and maintains a steady 6 N-m till 0.5s. When the direction of the motor is reversed negative toque of the same magnitude.

From the curves of the motor speed and load torque we can see that they are proportional to each other in forward and reverse direction.

The second plot shows the armature current. It can be observed that there is a sudden spike or a shoot up of armature current in both the forward and reverse directions at the 0.2s and 0.52s marks respectively. This shootup can be rectified by adjusting the duty cycle.

The current duty cycle is 75%, by lowering the duty cycle the shootup can be rectified.

At 50% duty cycle,

Here the shoot up when the motor is running in forward direction has been eliminated but a small amount of shoot up is observed in the reverse direction. This can be eliminated by further reducing the duty cycle.

At 30% duty cycle,

By reducing the duty cycle further to 30%, the shoot up of the armature current in both forward and reverse direction of the DC motor's operation has been completely eliminated.

2. DEVELOP AND EXPLAIN A 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.

 We will always get a positive output voltage Vo as the freewheeling diode FD is present across the load. When the chopper with the freewheeling diode starts conducting and the output voltage Vo will be equal to Vs. The direction of the load current Io will be reversed. The current Io 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.

Below is the simulink example of a 2 quadrant chopper.

The model consists of three subsystems which are the control, gate driver and signals systems. The input signal is provided using a step block. 

The gate driver block is connected to the gate control port of the two-quadrant chopper block. A DC voltage source is connected to the postive terminal of the 1 terminal and the negative terminal is connected to an electrical reference (grounding). A solver configuration block is connected to the negative port of the DC source and reference.

Io and Vo are current and voltage sensors which is connected to an inductor.

Here a MOSFET is used at default along with it's default parameters. The simulation is run for 1s and the following plot is obtained.

It is observed that when the chopper 1 is in effect the current and voltage increases. Once it crosses the threshold, the second quadrant's chopper turns on and the current and voltage decreases.

3. EXPLAIN THE OPERATION OF A BLDC MOTOR :-

A brushless DC motor (BLDC) is an electronically commutated DC motor which does not have brushes. The controller provides pulses of current to the motor windings which control the speed and torque of the synchronous motor.

These types of motors are highly efficient in producing a large amount of torque over a vast speed range. In brushless motors, permanent magnets rotate around a fixed armature and overcome the problem of connecting current to the armature. Commutation (commutation is the process of producing rotational torque in the motor by changing phase currents through it at appropriate times) with electronics has a large scope of capabilities and flexibility. They are known for smooth operation and holding torque when stationary.

Brushless DC motor has only two basic parts: rotor and the stator. The rotor is the rotating part and has rotor magnets whereas stator is the stationary part and contains stator windings. In BLDC permanent magnets are attached in the rotor and move the electromagnets to the stator. The high power transistors are used to activate electromagnets for the shaft turns. The controller performs power distribution by using a solid-state circuit.

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.

Components used in BLDC motors are,

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 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.

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.

 The advantages of BLDC motor are,

• It has no mechanical commutator.
• High efficiency due to permanent magnet rotor.
• High speed of operation and low noise due to absence of brushes.
• Smaller and lighter than brushed DC and AC induction motors.
• Long life as no commutator inspection is required.
• Higher dynamic response due to low inertia and carrying windings in the stator.
• Less electromagnetic interference.

The disadvantages of BLDC motor are,

• 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.

CONCLUSION :-

• The MATLAB model Speed control of a DC motor using BJT H-bridge was analysed and the process to eliminate current shoot up was addressed.
• A 2-quadrant chopper model was analysed and explained.
• The operation of a BLDC motor was explained in brief.