vsd squarewave phase angles

DM7.ENG · Apr 26, 2015

Hi there

Recently been looking in to VSD’s and inverter topologies.From looking at the typical vsd schematic consisting of a 6 diode 3 phaserectifier, LC filter and an Inverter I found something that im not entirely sureof. As 3 phase motors have the ability to self-start due to their rotationalmagnetic fields i.e. Each phase displaced by 120 deg how is it that themodulated squarewave leaving the inverter is still capable of starting themotor when there is now only 1 square wave form as opposed to the conventional3 phase sinewaves?

Darkwood · Apr 26, 2015

VSD's strip the mains sign wave down and rebuild it to the requirements of the user they still give a phase rotational cycle but a controlled one, you do not lose the phase shift between fields, its all achieved with rapid firing electronics... your enthusiasm is good but don't dig too deep before you have got a grasp of the basics of VSD's ...too much info can lead to a longer learning curve and confusion.

Besoeker · Apr 27, 2015

DM7.ENG said:
Hi there

Recently been looking in to VSD’s and inverter topologies.From looking at the typical vsd schematic consisting of a 6 diode 3 phaserectifier, LC filter and an Inverter I found something that im not entirely sureof. As 3 phase motors have the ability to self-start due to their rotationalmagnetic fields i.e. Each phase displaced by 120 deg how is it that themodulated squarewave leaving the inverter is still capable of starting themotor when there is now only 1 square wave form as opposed to the conventional3 phase sinewaves?

It isn't just one square wave. It's three pulse width modulated (PWM) waves. These are produced by taking the DC from the rectifier and switching it on and off in a sine wave pattern. This is usually at a frequency of several kilo hertz. Although it is a series of pulses at full DC volts and no volts, the motor inductance smooths out the current to make it a fairly good sine wave.