3 Phase Induction motor control using variable frequency drive(VFD)

Variable Frequency drive or VFD is the most popular and has
found widespread use in industrial and domestic applications because of its ease of implementation. They can be used in control of motors used in conveyors and other machinery. In Industries Variable Frequency Drive (VFD) is an integral part due to its versatility. Internal architecture of any VFD consist of several power electronics circuits (component) and are interfaced to each other which makes this architecture an interesting application of Power Electronics Engineering.

Block Diagram:

VFD Block Diagram
VFD Block Diagram

Variable Frequency Drive (VFD), also known as Variable Speed drive, Micro Drive, or AC drive, is an electronic device that varies the frequency and the speed of AC motors as per the requirements of the loads.

Theory

Sinusoidal Pulse Width Modulation(SPWM) is a scheme of modulation useful for applying Sinusoidal 3 phase voltage to a 3 phase load like 3 phase Induction Motor.

We assume that the 3 phase Inverter circuit is powered by a constant DC voltage source provided by the DC link Capacitor. The output voltage is varied by changing the switching sequence of the IGBT switches as shown in the figure below.

inverter circuit
Inverter circuit

There challenges that are faced are:
• DC link capacitor is the only source of DC voltage that is assumed to be constant
which is a challenge.
• The six pulses generated by the controller should be applied to the IGBT gates by
means of driver circuits and there should be no overlaps causing any sort of short
circuit.

PWM and Sine Wave
PWM and Sine Wave

Sinusoidal pulse width modulation is generation method useful in Inverters. The Inverter produces AC Voltage that is only possible when the IGBT’s are fired by means of some modulation technique. The IGBT gate pulses are generated by means of a control algorithm from a controller and are provided as an input to the gate driver circuit which supplies with the enough current. The pulse width generated in produced using sine table and its
approximated view is a sine wave as shown in the figure. With sinusoidal or sine weighted pulse width modulation, several pulses are produced per half cycle. The pulse at the center are wide while at the ends of the half cycle are narrow. To change the output frequency of the sine wave, the frequency of generated SPWM signal need to be changed.

detailed vfd
Detailed VFD

Level Shifter

level shifter
level shifter

IGBT gate driver circuit is present inside the power module which drives the current capability of the pulses. Pulses generated by  DSP are of 3.3V (CMOS level signal), thus to convert them to 15V, external hex converter IC CD4504 is used.It is also called as level Shifter.

3 phase induction motor
3 phase induction motor

The three phase Induction motor has the following Specifications:
• Power Rating : 0.37 kW or 0.5 HP
• Voltage Rating : 220V
• Current Rating : 2A
• Maximum Speed(N) : 1400RPM

DSP to generate igbt gate pulses.

c2000 dsp
c2000 dsp
  • The F2802x Piccolo family of micro-controllers provides the power of the C28x core coupled with highly integrated control peripherals in low pin-count devices. This family is code-compatible with previous C28x-based code, and also provides a high level of analog integration.
  • An internal voltage regulator allows for single-rail operation. Enhancements have been made to the HRPWM to allow for dual-edge control (frequency modulation).
  • Analog comparators with internal 10-bit references have been added and can be routed directly to control the PWM outputs. The ADC converts from 0 to 3.3-V fixed full-scale range and supports ratio-metric references.
  • The ADC interface has been optimized for low overhead and latency. The use of the DSP is to provide SVM(Space Vector Modulation) equivalent pulses for the Inverter circuit using SVM algorithm embed inside the processor using Code Composer Studio platform.
  • Another uses is to adjust the SVM pulse frequency using analog potentiometer interfaced to the ADC of the C2000 DSP processor

SEE MORE: SINE WAVE INVERTER USING 555

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