Week-10 Challenge: Duty Cycle

Question 1:

Why power electronics circuits are efficient? In practice, which types of losses occur in power electronics circuits?    

Answer:

Power converters are used to process and  control  flow of electric energy by supplying Voltage & Current in a form optimally suited for load.

Power converters generally use switching technique for efficient power conversion. Conversion of power from one level through switching to a desired level may be produced by temporary storage of input energy and then releasing energy to load at desired energy level. Energy storage may be in magnetic field storage components (inductors, transformer etc) or electric field storage (capacitors).

Switching mode power conversion is more efficient than linear conversion since unwanted power is dissipated as heat in linear converters. Transistors capable to switch at high frequency also produce a high efficiency in power converters. Very low resistance in converter circuit components also adds overall efficiency of the converters.

Even though power converters are highly efficient, there is a small amount of power loss that is due to conduction and switching losses.

Conduction losses that depend on load:

• Resistance when the transistor switch is conducting.
• Diode forward voltage drop
• Inductor winding resistance
• Capacitor equivalent series resistance

 Switching losses:

• Voltage-Ampere overlap loss
• Frequency  loss
• Reverse latency02 loss
• Losses due driving MOSFET gate and controller consumption.
• Transistor leakage current losses, and controller standby consumption.

Question 2:

 Using a suitable MATLAB model, show how duty cycle control signal can be generated for a simple power converter circuit? 

Answer:

Taking Boost Converter Model:

Boost Converter is a DC-DC converter which steps up the voltage from input (Source) to the output (Load).

The output Voltage produced from the output of boost converter is given by the equation:

                                                               V(out) = V(in)/(1 - D)

where,

         V(out) - Output Voltage

         V(in)   - Input Voltage

         D         - Duty Cycle (Eg.: D = 0.5,  for 50% duty cycle)

The model is supplied with a 240V Supply and is connected to a Load of 15`ohm` resistance with PWM frequency set to 20kHz. The control signal to the gate is generated by using a PWM Generator at specified duty cycle. 

Simulation Result:

The duty cycle (D) is set to 3 values and simulation is done

Simulation is performed at D = 0.25,0.5 & 0.75 producing V(out) = 320V, 480V & 960V respectively.

Simulation results explains that with the change in duty cycle, the output produced from the Boost converter can be changed. Hence Output voltage produced by the converter is said to be controlled with the signal from PWM generator at specified duty cycle. 

Question 3:

Make a comparison of square wave inverter output voltage with sine modulated one. 

Answer:

The main purpose of an inverter is to convert the DC electricity into AC electricity.  either Square wave AC current or Sine wave AC current.

The Sine wave inverter output is very similar to the AC current, while the Square wave inverter produces a switch from ON to OFF state rapidly .

In a sine wave, the voltage rises and falls smoothly with a smoothly changing phase angle and also changes its polarity instantly when it crosses 0 Volts.

In a square wave, the voltage rises and falls abruptly. Thus, any device that uses a control circuitry that senses the phase (for voltage / speed control) or instantaneous zero voltage crossing (for timing control) will not work properly from a voltage that has a square  wave-form.

The high frequency harmonic content in a square wave produces enhanced radio interference, higher heating effect in motors and produces overloading .

There is also one other type of inverter known as modified square wave inverter which has a similar wave pattern to square wave but with some stepup/down within  pulses in one direction.

This modification reduces the heating and harmonics/ distortion caused by square wave and is cheaper than sine wave inverter.

But with advancing technologies, sine wave inverters are less expensive and are the most commonly used one as it produces much similar wave form to that of supply wave form and is accepteble by wide range of applicaions.

Question 4:

Explain the difference between VFD and Vector control drive. Can we use VFD method for induction motor in EV application?

Answer:

• V/f control produce a constant strength of magnetic field regardless of speed ensuring constant torque production. Vector control controls speed/torque by controlling stator current vectors similar to DC motor control by transforming the 3-phase system to a 2 coordinate time invariant system( R Y B to d q transformation)
• V/f control which is a scalar control method is based on varying the supply frequency and voltage simultaneously ,i.e., flux and current cannot be controlled independently whereas is vector control method, flux producing current and torque producing current can be controlled independently.
• V/f control method is sufficient in controlling steady-state conditions ,hence instantaneous torque control cannot be addressed by scalar control methods whereas vector control is appropriate for dynamic conditions instantaneous torque control  be achieved
• V/f control doesn’t control load speed or position, i.e., no feedback is used, while vector control uses sensors to get feedback for dynamic control .

Using SCALAR (V/f) control in EV application:

Scalar control is a low cost and simple method, which keeps the air gap flux constant with simultaneous variation of voltage and frequency.

Some of the limitation of this control method is :

• Stator Voltage drop Compensation: At very low frequency, the supply voltage should be boosted to overcome stator resistance voltage drop
• Scalar control of Induction Motor produces very low accuracy in case of Electric Vehicle Application since V/F control method might produce decreased overall efficiency
• Flux control & Voltage control cannot be controlled independently since flux is coupled with current of an Induction Motor.

Despite having limitations , A scalar control along with Closed loop providing a feed-back of motor speed/torque using a  PID controller or a fuzzy logic speed controller Improves the system response reduces limitation to  steady state operation,  higher performance , smooth speed response and higher efficiency all together producing a highly accurate control method for EV application.