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What is an LVDT (linear variable differential transformer)?

A linear variable differential transformer (LVDT) is an absolute measuring device that converts linear displacement into an electrical signal through the principle of mutual induction.

The main components of an LVDT are a transformer and a core. The transformer consists of three coils — a primary and two secondaries (S1 and S2) — wound on a hollow form, which is typically made of glass-reinforced polymer. The coils are arranged such that the primary coil is located between the two secondary coils, which are symmetrical and wound in series but in opposite directions (referred to as series-opposed winding).

The core is made of magnetically permeable material and moves freely inside the bore of the transformer. A non-ferromagnetic shaft, or push rod, is coupled to the core and connects to the object being measured.

When voltage is applied to the primary winding, the resulting flux is coupled to the secondary windings by the core. This induces voltages (often denoted E1 and E2) in each of the secondary windings. The differential voltage in the secondary windings determines the distance moved, and the phase of the voltage indicates the direction of movement.

40756 (1118×410)

Variable Frequency Drive or VFD

VFD is a power electronics based device which converts a basic fixed frequency, fixed voltage sine wave power (line power) to a variable frequency, variable output voltage used to control speed of induction motor(s). It regulates the speed of a three phase induction motor by controlling the frequency and voltage of the power supplied to the motor.

40758 (591×381)

Working of Variable Frequency Drive

A full-wave power diode based solid-state rectifier converts three-phase 50 Hz power from a standard 220, 440 or higher utility supply to either fixed or adjustable DC voltage. The system may include transformers for high voltage system.

Inverter Stage

Power electronic switches such as IGBT, GTO or SCR switch the DC power from rectifier on and off to produce a current or voltage waveform at the required new frequency. Presently most of the voltage source inverters (VSI) use pulse width modulation (PWM) because the current and voltage waveform at output in this scheme is approximately a sine wave. Power Electronic switches such as IGBT; GTO etc. switch DC voltage at high speed, producing a series of short-width pulses of constant amplitude. Output voltage is varied by varying the gain of the inverter. Output frequency is adjusted by changing the number of pulses per half cycle or by varying the period for each time cycle.

The resulting current in an induction motor simulates a sine wave of the desired output frequency. The high speed switching action of a PWM inverter results in less waveform distortion and hence decreases harmonic losses.