Duty Cycle

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

Power Electronics:

• Power electronics involves the study of electronic circuits intended to control the flow of electrical energy. These circuits handle power flow at levels much higher than the individual device ratings. The power electronic circuit contains switches, lossless energy storage elements, and magnetic transformers. A circuit built from ideal switches will be lossless. As a result, switches are the main components of power converters.
• Ideally, when a switch is on, it has zero voltage drop and will carry any current imposed on it. When a switch is off, it blocks the flow of current regardless of the voltage across it. Device power, the product of the switch voltage and current, is identically zero at all times. The switch controls energy flow with no loss. In addition, reliability is also high.
• If the power electronics circuit is to provide energy of a high power level, as in motor drives or in DC or AC UPS, then the efficiency of the energy conversion is of greatest interest. As all the energy is processed through the electronic circuit, one cannot afford large losses in this process. As a matter of principle, resistors are excluded from such circuits. Special power electronics devices (power insulated-gate bipolar transistors [IGBTs], power Metal Oxide Silicon Field Effect Transistors [MOSFETs], diodes, thyristors, etc. are used as switches to carry large currents and support in high-voltage applications.

Efficiency:

1. Power electronics deals with the conversion and control of electrical energy with the help of power semiconductor devices that operate in a switching mode, and, therefore, the efficiency of power electronic apparatus may approach as high as 98–99%. This technology uses electronic components such as inverters and transformers to convert the electricity from the wall outlet into the right voltage and current to power each of handy or handheld devices. It is also used more widely to distribute electricity throughout the power grid to connect renewable energy systems and to charge up electric vehicles. Electrical energy has already become our lifeline so we want to get a regulated form of this energy which in itself is not usable until it is converted into a tangible form of energy such as motion, light, sound, heat, etc. In order to regulate these forms of energy, an effective way is to regulate the electrical energy itself and this forms the content of the subject power electronics. Power electronics is the study of switching electronic circuits in order to control the flow of electrical energy, its generation, transportation, and applications. Power Electronics is the technology behind switching power supplies, power converters, power inverters, motor drives, and motor soft starters. Power electronics is an enabling technology, its development, together with internal developments, such as wide bandgap semiconductors, will be driven externally by applications in the future.
2. Power electronics is very important in modern high-efficiency energy processing systems, such as HVDC, SVC, flexible ac transmission system (FACTS) for active and reactive power flow control, uninterruptible power supply (UPS), and industrial process control with variable-frequency drives for improving productivity and quality of products in modern automated factories. Variable-frequency drives are now used extensively in pumps and compressor drives, paper and textile mill drives, subway and locomotive propulsion, electric and hybrid vehicles, elevators, metal rolling and textile mills, home appliances, machine tools, and robotics, variable-speed air conditioners, wind generation systems, ship propulsion. Power electronics plays a very important role in solving or mitigating our global warming problem, which is a very serious concern in our society.

• High reliability of power electronic converter system.
• Long lite and less maintenance due to the absence of any moving parts.
• Flexibility in operation
• Fast dynamic response compared to electromechanical converter system.
• Small Size and less weight, thus low installation cost

In practice, there is some amount of power dissipation in the power electronics circuit which is in the form of losses. Mainly there are two types of losses in power electronics, which are switching losses and conduction losses.

1. Switching Losses - occur when the device is transitioning from the blocking state to the conducting state and vice-versa. This interval is characterized by a significant voltage across its terminals and a significant current through it. During this, some energy is dissipated from the device.
2. Conduction Losses - occur when the device is in full conduction. The current in the device is whatever is required by the circuit and the voltage at its terminals is the voltage drop due to the device itself. These losses are in direct relationship with the duty cycle.

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

Here we will use the Buck Converter Model in MATLAB to simulate the nature of the duty cycle control signal. The Buck Converter Model is as follows,

A buck converter is a DC/DC power converter that steps down the voltage from its input (source) to its output (load). In continuous conduction mode (current through the inductor never falls to zero), the theoretical transfer function of the buck converter is:

where  is the duty cycle.

In this example, the converter is feeding an RC load from a 200 V source and the PWM frequency is set to 10 kHz.

Simulation:

The parameters for the duty cycle is as follows,

So the initial value is set as 0.25 and the final value is set as 0.75. After running the simulation for 0.008 seconds, we get the following result,

Here we can see that for the 0.25 duty cycle, the voltage reaches up to 60 V and for the 0.75 duty cycle, it reaches up to 160 V. The Mosfet current lfet  and diode current is ls  generates duty cycle as per the initial and final values which were mentioned to maintain the output voltage regulation properly. We can see the variation in pulses of the duty cycle which is different in Mosfet and diode to obtain the desired output result.

Now we will change the input parameters as follows,

Here we will change only the initial value to 0.5 and the rest of the parameters will be the same. The simulation results obtained are as follows,

Here we can observe the change in the graph for the initial parameter. At 0.5 duty cycle, we get the output voltage as 120 V, and also the nature of the graph for 0.5 duty cycle is quite increased compared to 0.25 duty cycle. This states that if the duty cycle pulses increases, the voltage increases with current, increasing the nature of the pulses.

So this is how a duty cycle control signal can be generated for a simple power converter circuit.

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

VFD method for induction motor in EV application:

• Speed regulation with VFD control can be achieved only 2-3% of rated motor frequency, so these methods are not suitable for applications where precise speed control is required, so the precision control in EV applications cannot be achieved.
• In VFD control, the current and flux are coupled so current variation affects the flux value.
• This method has low efficiency and thus it can reduce vehicle efficiency and so this method is not used in induction motor in EV application.