DUTY CYCLE

DUTY CYCLE:

OBJECTIVE:

• To explain why the power electronics circuits are efficient and the losses occur in the power electronic circuit.
• Using a suitable Matlab model show how duty cycle control signal can be generated for a simple power converter circuit.
• To Make a comparison of square wave inverter output voltage with sine modulated one.
• To explain the difference between VFD and Vector control drive and to explain whether we can use VFD method for induction motor in EV application.

1)POWER ELECTRONICS:

• Electronics applied to the conversion and control of electric power.
• Electronics signals controlling the electric power in the circuit is called Power Electronics.
• To process and control the flow of electric energy by supplying voltages and currents in a form that is optimally suited for user loads.
• The conversion is performed with the help of semiconductor switching devices such as
• The conversion is performed with semiconductor switching devices such as diodes, thyristors, and power transistors such as the power MOSFET and IGBT.
• In contrast to electronic systems concerned with the transmission and processing of signals and data, in power electronics, substantial amounts of electric energy are processed.
• The power range is typically from tens of watts to several hundred watts.
• In industry, a common application is the variable speed drive(VSD) that is used to control an induction motor.
• The power range of VSDs start from a few hundred watts and ended at tens of megawatts.
• Switching Technique is adopted from power electronics to control the voltage and current for an electric motor.
• This avoids the high amount of losses by replacing the resistor with a power module/switch.
• These power modules are operated at the high frequency where the rated battery voltage (Vbat) can be reduced to the desired voltage by on-off the modules.
• The duty cycle is varied in order to achieve the desired voltage.
• The on-off switching frequency is referred to in terms of duty ratio.
• Duty Ratio(d)=`(ON Time)/(ON+OFF Time)`
• Vavg=Vbat*d

POWER MODULES/SWITCHES:

• When it comes to EV application, the controlling of the speed of the motor mainly depends on the controlling of the voltage and current flowing to the motor.
• In order to achieve this controllers/Inverters are used to adjust the voltage in the coil and to control the magnitude of the current.

EFFICIENCY:

• Since power electronics use semiconductor devices that operate in switching mode its efficiency is higher.
• These devices don't have any mechanical moving parts and therefore, the losses are very minimal and therefore their efficiency may approach as high as 98-99%.

LOSSES IN POWER ELECTRONICS CIRCUIT:

There are two main losses in the power dissipation of the circuits,

• Switching loss
• Conduction loss

SWITCHING LOSS:

• Switching loss occurs when the device is transitioning from the blocking state to the conduction state and vice-versa.
• This interval is characterized by a significant voltage across its terminals and a significant current through it.
• The energy dissipated in each transition needs to be multiplied with the frequency to obtain the switching losses.

CONDUCTION LOSS:

• Conduction loss occurs 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)Duty cycle control signal:

The Buck converter model in Matlab is used to simulate the nature of the duty cycle control signal.

• A Buck converter is a DC/DC power converter which 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 :

d=Vout/Vin

where d is the duty cycle.

The parameters for the duty cycle as follows:

CASE-1:

d=0.3:

In the constant block, the input is given as 0.3 and the output voltage is viewed through the scope block.

CASE-2:

d=0.5:

In the constant block, the input is changed to 0.5 and the output voltage is viewed through the scope block.

CASE-3:

d=0.7:

In the constant block, the input is changed to 0.7 and the output voltage is viewed through the scope block.

RESULT:

• From the above graph, it is observed that while increasing the duty cycle the output voltage also increases correspondingly.
• This is how the duty cycle signals are generated and varied according to the required output voltage.

3)Comparison of square wave inverter output voltage with sine modulated one.

• The inverter is used to convert the DC electricity to AC electricity.
• An Inverter converts DC current to either square wave AC current or Sinewave AC current.
• Sine wave inverters are more efficient than square wave inverters when it comes to the conversion of DC to AC.
• This ensures that the power loss is minimized due to the greater efficiency.

ADVANTAGES:

• The sine wave minimises the risk of damage to your sensitive appliances compared to square wave and there is very little harmonic distortion in a sine wave, essential for expensive equipment.
• Devices that use control circuits, which sense the phase for speed/voltage control, or devices that sense instantaneous zero voltage crossing (for timing control) must have a sine wave supply.
• From the figure, it is evident that the sine wave changes polarity instantaneously and smoothly when it crosses zero volts.
• The rise and fall of voltage are smooth as compared to the shape of the square wave.
• Due to this feature, the inductive loads(motors or microwaves) run quieter,cooler and faster.
• Sine wave inverters are used to support household appliances such as refrigerators, computers, laptops etc.. whereas square wave inverters are usually used to support motors alone.
• Square wave inverters are less reliable and also unsafe to use for appliances whereas sine wave inverters are highly safe to use.
• Square wave inverters produce a very loud noise when used compared to sine wave inverters which produce normal sound only.
• Practically, the square wave can also be modified further using more sophisticated inverters to produce a sine wave.
• To produce a sine wave output, high-frequency inverters are used.
• These inverters use the pulse width modification method by switching currents at high frequency, and for variable periods of time.
• Using high-frequency gradual changes in pulse width is generated.
• The pulse width modulation methods have resulted in very efficient inverters.

4)Difference between VFD and Vector control drive:

Variable Frequency Drive(VFD)	Vector Control Drive
Variable Frequency Drive(VFD) control method work by optimizing the motor flux and keeping the strength of the magnetic field constant, which ensures constant torque generation.	Vector control drive controls the speed or torque of an AC motor by controlling the stator current space vectors, similar to the DC control methods by transforming a three-phase system that depends on time and speed to a two-coordinate time-invariant system
VFD control method varies both the voltage and frequency of power to the motor in order to maintain a fixed, constant ratio between the two, so the strength of the magnetic field is constant, regardless of motor speed. Here flux and current cannot be controlled independently.	The stator current in an AC motor consisting of the magnetising component of the current and the torque producing component is controlled independently each with its own PI controller.
VFD control is typically implemented without feedback.	Vector control method uses sensors to get feedback for dynamic control.
Instantaneous torque control cannot be achieved by this method.	Due to the dynamic response, instantaneous torque control can be achieved.

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 the vehicle efficiency and so this method is not used in induction motor in EV application.