Simulation of Motor / Controller

I would like to use Simulink to model the motor and control system. Under the SimPowerSystems toolbar, there is a list of useful blocks.

The DC machine requires the following information:

• Armature Resistance = 1.67 ohms
  
• Armature Inductance
• Field Resistance = 12.02 ohms
  
• Field Inductance
• Field-Armature mutual inductance
• Total Inertia
• Viscous friction coefficient
• Coulomb friction torque
• Initial Speed
  

Current Controller devices

Cafe Electric, LLC makes a controller called the "Zilla" which is more than capable of driving the motor on the snowmobile. The controller costs ~$2500, which is probably more than we'd like to spend.

Alltrax: www.alltraxinc.com makes motor controllers for just about every application.

Question: Is this a shunt or a series application? This would be good to know.

This site: http://partsonsale.com/evparts.html contains a list of EV parts for sale and includes thyristor sets, etc...

"Build your own Electric Vehicle" http://books.google.com/books?id=XcFbEX-de4kC&lpg=PP1&dq=electric%20vehicle%20motor%20control&pg=PP1

"Electric Motors and Control Techniques" http://books.google.com/books?id=99xxCjXFbnEC&lpg=PP1&dq=electric%20vehicle%20motor%20control&pg=PP1

"Electric Motors and Drives" http://books.google.com/books?id=gbIDM60AvGAC&lpg=PP1&dq=electric%20vehicle%20motor%20control&pg=PP1

DC Machines

DC machines are classified under two catagories:

• Permanent Magnet machines (typically under 5hp)
• Electromagnetic macines (larger scale)

The machine that the Electric Snowmobile will use is of the electromagnetic machine type, where a current is applied to an electromagnetic, called the field current. The field current is what induces the flux in the DC machine.

Electromagnetic DC machines have 4 different sorts of excitation:

• separate excitation
• self-excited shunt
• self-excited series
• compound

Another book: Electrical Machines and Power Electronics by R.E. Steven also discusses chopper control for DC machines and discusses the back EMF effect on the control system.

Thursday reading/thoughts

Went to see Adam Perkins today about an inductance bridge / digital oscilloscope. He had neither one of them. The digital o-scopes are guarded by professors and kept in certain labs. Richard Messner's communications lab has a few that could be used.

THE THYRISTOR:

• 3 Terminal Device

1. Anode - touches p-type
   
2. Cathode - touches n-type
   
3. Gate

• 2 pn junctions (2 BJTs that share the innermost p and n type layers)
  

Thyristors:
ON state - Thyristor behaves like a pin diode and is able to carry very high currents with an extremely low voltage drop
Flow of current cannot be interrupted at any arbitrary instant. Current commutation must be used to revert back to blocking state.
The thyristor can be forced to switch off by reversing the direction of current flow (reversing voltage)

A special type of thyristor, called a Gate Turn Off (GTO) thyristor, can stop the load current at any time by applying a negative current to the gate. This will allow the thyristor to act as a normal on/off switch. A pulse width modulator could be used to supply the current to the gate, allowing for turn on/turn off for given periods of time depending on how much power is requested to be delivered to the motor.

The current requirement to turn off the GTO thyristor is a minimum value proportional to the load current through a ratio known was the turn-off gain. GTO thyristors must be equipped with protection circuitry. GCT thyristors are similar to GTOs but are gate commutated and do not need as much protection circuitry.

Two such examples of protection circuitry are the dv/dt snubber and di/dt snubber, which reduces the back emf stress on the power electronics.

Principles of Chopper Control (PWM Torque control)

The preferred scheme for Chopper Control is called Constant-Frequency. The chopping frequency is kept constant the on time (t_on) is varied. The amount of time that t_on is on divided by the chopping period is the duty cycle. The duty cycle for our application will most likely run from 25% - 90% depending on how much current needs to be delivered to the motor for proper torque.

To make the drive reversible, power needs to be able to flow both directions to the motor. To achieve this, a Two-quadrant type B chopper needs to be implemented.


Commutation needs to be researched and understood for chopper control. The controller needs some way to make the current flowing through the thyristor to equal zero for a certain period of time. This can be done with either load commutation or forced commutation.

In Load Commutation, the current flowing through becomes zero or is transferred to another device (besides the thyristor)
In Forced Commutation, there is a force that is forcing the current to become zero.

• Voltage commutation, a charged capacitor momentarily reverse biases.
• Current commutation, a current pulse is made to flow in the reverse direction making the net current equal zero.

This type of controller is ideal for use with DC motor require continuous motor current that needs to be varied. It is costlier than other control methods, however, because there are more components necessary for the commutation circuitry.

In order to reduce ripple from supply current, a capacitor can be placed in shunt with the chopper circuit. An LC filter can also be used to reduce the size of the capacitor and has an additional function of providing transient isolation between supply and load during a short circuit.

The chopper circuit will supply the current to the armature. The field current will be fixed in this application. The controller will measure the speed of the motor and compare it to a control setting which will generate the proper signaling.

A microcontroller will control the circuit, thyristors will provide the power electronics needed for switching and passive components capable of handling high power will provide filtering/chopper circuitry.

1. http://www.freepatentsonline.com/3845379.html
2. http://books.google.com/books?id=s0k9kGs5bHYC&lpg=PA290&ots=kB9BqiJTMG&dq=chopper%20circuit%20forklift&pg=PA290
3. http://books.google.com/books?id=4-Kkj53fWTIC&lpg=PA210&ots=959jgndlmD&dq=chopper%20circuit%20forklift&pg=PA210
4. http://books.google.com/books?id=WwXi9LI5W1sC&lpg=RA2-PA249&ots=K5z0SAJOfM&dq=chopper%20circuit%20forklift&pg=RA2-PA249
5. http://www.lgep.supelec.fr/uploads/images/lgep/Mocosem/Cocodi/Pages_perso/gremy/Hebron2009/Practice4_DC_Motor_Chopper_Control.pdf
   

Library: TK2851 is a good place to look for motor control books.

Measure Inductance of Armature

Set up circuit to apply a small voltage to the armature nodes. Place a small sense resistor (of about 1/10 an ohm) to the circuit. Measure the voltage across the sense resistor to get a feel for the motor's time constant.

Remember, Time Constant = L/R

The time constant is how long it will take to get to 63% of final value.

The other way to measure inductance is with an inductance bridge. See Adam Perkins for this.

Thyristor DC Drives

DC motors are much easier to control in a variable-speed application. Thyristor-controlled DC drives are capable of controlling high-power applications.

Thyristor Drives used because:

• Eliminates electrical time lag of the field and armature
• Operation is simple and reliable
• Minimal Maintenance
• Operating efficiency above 95%
• Small in size and weight

Disadvantages:

• Higher ripple content of the converter output adds to motor heating and commutation problems. (Might need a reactor in the armature circuit to smooth ripple current)
• The overload capability is relatively low.
• Complex control circuitry needed to achieve regeneration.

Thyristors can be placed in series and parallel for higher voltages and current ratings.

Different control schemes for DC motors:

• Phase Control
• Integral Cycle Control
• Chopper Control (Pulse Width Modulation)

PWM control requires special thyristors because they switch very fast to help eliminate current ripple. Auxiliary circuits are needed to switch the thyristors.

Control system needs to incorporate either digital or analog feedback control. Phase-locked loops are sometimes employed to provide precise speed control with very low speed regulation. Microcontrollers can be employed.

Other ways of implementing variable speed dc drives:
- Resistance control. Essentially a potentiometer. Very inefficient.
- Motor-Generator. Three machines involved. Losses in each machine = inefficient and slow response.

Pulse Width Modulation has efficiency ratings of 95% or better. This is ideal for use in battery-operated vehicles where energy savings is a top consideration when deciding upon a motor controller. PWM can also be used with regenerative braking systems. It is possible that the EV: Snowmobile could incorporate regenerative braking.

Field/Armature Resistance

Resistances:
Armature: 1.67 ohms
Field: 12.02 ohms

Today, I looked over my ECE 618 Junior Laboratory II - Lab 7 report from the Spring semester of 2009. A future step to determine the parameters of the Motor/Generator is going to be to measure the resistance of the motor and generator portion of the aircraft DC generator.

The field current of a DC motor is the current that is used to create the magnetic flux inside the stator of a DC motor. The armature current is the current in the rotor of a DC motor. The basic Electromagnetic principle of the Lorentz-Force law is the guiding principle behind DC motors. A current (electrical power) is passed through a magnetic field (created from the field current) to create torque (mechanical power).

The purpose of this project/study will be to design a motor controller that can be used as a torque controller for this motor. The idea of a Pulse Width Modulation electric drive controller seems to be one that is both practical and affordable. Pulse Width Modulation is a voltage/current control scheme that uses bursts (called pulses) of varying length to control the average voltage/current over a certain period of time. The overall time from the start of one pulse to the start of another is called the period of the pulse. The time from the start of one pulse to the end of the same pulse is called the pulse width. The duty cycle is:

[(Pulse Width)/(Period)]*100%

A graphical representation of 3 differing pulse width modulation conditions are shown below:

As can be seen, the top-most situation is one where the pulse widths are short in duration (low duty cycle) and the average voltage is very low. The middle situation is one of a longer duty cycle and the bottom is one of almost 100% duty cycle. These pulses switch in the magnitude of 10kHz+. The motor controller for this project will need to switch a large amount of current and be able to handle the back emf associated with switching such a large current off.

To control the speed of a DC motor, a voltage source will be switched on and off quickly enough so that the motor will run at some speed part way between zero and full speed. The modulation is how the width of the pulses varies in response to the input from a switch (rotary switch).

The duty cycle is the major variable in any PWM control system.

Some researching was done and some articles were found that may be of help:

A Torque Controller Suitable for Electric Vehicles - IEEE Transactions on Industrial Electronics, Vol. 44, No. 1 February 2007

Analysis on Modeling and Simulink of DC Motor and its Driving System Used for Wheeled Mobile Robot - Proceedings of World Academy of Science, Engineering, and Technology Volume 26, December 2007

DC Motor Speed Control Methods Using MATLAB/Simulink and their integration into Undergraduate Electric Machinery Courses - Department of Electrical and Electronics Engineering, Nigde University, Nigde 51100, Turkey

An Overview in Power Electronics in Electric Vehicles - IEEE Transactions on Industrial Electronics, Vol. 44, No. 1 February 2007

DC Motor Speed Control using PWM Method - Air Force Academy of Basov, Romania

Low-Cost PWM Speed Controller for an Electric Mini-Baja Type Vehicle - J. of the Braz. Soc. of Mech. Sci. & Eng., January-March 2007, Vol XXIX, No. 1

Application of PWM speed control - JMC, the Fan Company

Electronics Circuits Reference Archive, PWM speed control - 4QD-TEC

Topics still to be researched:

• What types of Electric Vehicles currently use PWM? Golf Carts, Forklifts, electric tractors
• What sort of power electronics are needed for a PWM controller capable of controlling a high current, high back emf motor?
• What sort of frequency is ideal for a PWM controller for this application?
• What is the Armature Winding Resistance?
• What is the electrical time constant of the motor?
• What is the inductance of the armature windings?

I'd really like to model the motor/controller and am making that my goal for this class. I'd like to use simulink modeling to achieve this.

A little about DC generators

Simple direct current (DC) generators contain an armature (or rotor), a commutator, brushes, and a field winding.

A basic DC generator has four basic parts: (1) a magnetic field; (2) a
single conductor, or loop;(3) a commutator; and (4) brushes. The
magnetic field may be supplied by either a permanent magnet or
an electromagnet. For now, we will use a permanent magnet to
describe a basic DC generator.

In a direct current generator, the commutator's job is to change the alternating current (AC), which flows into its armature, into direct current. To put it another way, commutators keep the current flowing in one direction instead of back and forth. They accomplish this task by keeping the polarity of the brushes stationed on the outside of the generator positive. The commutator is made up of copper segments, with a pair (of segments) for every armature coil being insulated from all the others.

The stationary brushes, which are graphite connectors on the generator, form contact with opposite parts of the commutator. As the armature coil turns, it cuts across the magnetic field, and current is induced. At the first half turn of the armature coil (clockwise direction), the contacts between communicator and brushes are reversed. The first brush now contacts the opposite segment that it was touching during the first half turn, while the second brush contacts the segment opposite the one it touched during the first half turn. By doing this, the brushes keep current going on one direction, and deliver it to and from its destination.

(source: http://micro.magnet.fsu.edu/electromag/java/generator/dc.html)

Introduction To Generators And Motors