Basic theoretical knowledge of VFD 2

- Jan 16, 2019-

Current Source Inverter (CSI) Inverter

A current source inverter, also known as a current feed inverter, behaves like a constant current generator, producing an almost square wave current. Due to its simplicity, regenerability, reliability and low cost, current source inverters are used instead of variable source inverters for large VFDs of approximately 200 horsepower.

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Although more robust and reliable than VSI, CSI has a poor power factor at low speeds and is not suitable for multi-motor operation. Another disadvantage of CSI is the "cogging", the jerk start/stop motion or the pulse axis during operation. A closer look at the CSI output voltage waveform shows how this happens.

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Pulse width modulation (PWM) frequency converter

This type of VFD creates a unique method of variable voltage that provides a name for the PWM VFD. Pulse Width Modulation or PWM VFD provides a constant voltage Hertz ratio, wireless path notch and very stable current input to the motor.

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The pulse width modulation converter section uses a diode bridge to rectify the AC power source instead of the SCR bridge. Since the input to the inverter section is constant, the inverter controls the voltage and frequency. The switching mode of the semiconductor is used to convert DC power to AC output. These semiconductors may be transistors, GTO or SCR.

Basically, the main advantages of using pulse width modulation instead of voltage or current inverters are high efficiency, constant power factor, regardless of speed and no cogging problems.

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Other advantages include wide speed range, open circuit protection and multi-motor operation.

In terms of shortcomings, PWM requires additional hardware to achieve line regenerative capabilities, and the device is more complex, with some PWM units producing significant audible noise.

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