ECS649U / ECS790P - Electrical Machines and Systems

Coursework Examination 3

MATLAB/Simulink experiment for PWM Control of a Brushed DC Motor

This is an open-book exam, which should be completed in approximately 2 hours.

Deadline: 19/03/2022

You MUST submit your answers within the deadline from the time the coursework being released.

Instructions:

You can refer to textbooks, notes and online materials to facilitate your working, but               normal  referencing  and  plagiarism  rules  apply,  and  you  must  cite  any  sources  used  (if applicable). If any case of plagiarism and copying from other students, or from any other resources have been detected by the teaching assistant or the module organiser, ‘0’ mark will be allocated to the student without the organisation of any video viva.

Multiple submissions are not permitted, so be sure that you check your submission before uploading it.

The  use  of  Calculators  and  MATLAB  software  (any  version)  are  permitted  for  this examination.

Report:

Students are expected to answer Questions and Discussions (specified by blue colour) in the submitted report. You must upload a single PDF document containing your solutions in the QM+ portal under “Assessment Submission” .

Students are usually supposed to simulate multiple separate models in one course work. Please include all your Simulink/Simscape models and MATLAB codes in the report in the PDF document.

Marking:

The mark for this coursework is 20% of the total final mark for this module. The final mark will be provided after a video viva which will be arranged after 19/03/2022 and published in the QM+ page.

Coursework Objectives:

•   Basics of DC electrical machines control in MATLAB Simulink

•    Brushed DC motor control using PWM signals

•   Investigating the equivalent circuit model of brushed DC motors

Brushed  DC  motors  have  widely  used  applications  ranging  from  toys  to  push-button adjustable car seats. Brushed DC (BDC) motors are inexpensive, easy to drive, and are readily available in all sizes and shapes. PWM is the standard way to control the torque of a brushed DC motor.

All BDC motors are made of the  same basic components: a  stator, rotor, brushes and a commutator. The following reminds you of their role in a DC machine.

Stator: The stator generates a stationary magnetic field that surrounds the rotor. This field is generated by either permanent magnets or electromagnetic windings.

Rotor: The rotor, also called the armature in DC machine, is made up of one or more windings. When the windings are energized, they produce a magnetic field. The magnetic poles of this rotor field will be attracted to the opposite poles generated by the stator, causing the rotor to turn. As the motor turns, the windings are constantly being energized in a different sequence so that the magnetic poles generated by the rotor do not overrun the poles generated in the stator. This switching of the field in the rotor windings is called commutation.

Brushes  and  Commutator:  Unlike  other  electric  motor  types  (i.e.,  brushless  DC,  AC induction), BDC motors do not require a controller to switch current in the motor windings. Instead, the commutation of the windings of a BDC motor is carried out mechanically. A segmented copper sleeve, called a commutator, resides on the axle of a BDC motor. As the motor turns, carbon brushes slide over the commutator, coming in contact with different segments of the commutator. The segments are attached to different rotor windings; therefore, a dynamic magnetic field is generated inside the motor when a voltage is applied across the brushes of the motor.

Question 1: PWM Control of DC motor using RL model:

The requirement is to create the simulation model of a PWM controller for a resistive and inductive model connected to a DC voltage source as shown on Figure 1. Note that we are using an ‘ideal switch’ to represent the semiconductor switch; hence, finite switching times, switching loss and conduction loss are not modelled here.

Fig 1. PWM control of an RL load

Question 1a – Determine Average Power dissipated in the load resistance for the duty cycle values of 0.2, 0.4, and 0.8 of the PWM generator.

Question 1b – Verify the obtained power dissipation value by hand calculations. Solve the problem with duty cycle of 0.4 (no need to repeat the hand calculation for all the duty cycle values)

Question 2: PWM Control of a Brushed DC Motor

We  can represent the motor  as  series  connection  of resistance,  inductance  and the back electromotive force (EMF) voltage source, as shown in Fig.3.

Fig 3. Equivalent circuit of a Brushed DC motor

back EMF is a voltage that appears in the opposite direction to current flow as a result of the motor’s coils moving relative to a magnetic field. The back EMF is directly related to the speed of the motor, so knowing the value of back EMF allows us to calculate the speed of that motor.

  = 

Back EMF always acts to reduce the changing magnetic field through the coils by generating a voltage which opposes the supply voltage, thus reducing the current.

A Company is designing a new electric trolley and has asked your Consultancy Firm to provide technical support. They have decided to base their design on a PWM controlled Brushed DC motor. The required circuit components are listed in table below:

Develop a suitable Simulink model for this system. Then using the model and supporting calculations answer the following questions.