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I would like to float this idea of using a good (audio) power amplifier in the inverter stage of a sub 1KW VFD. How does this idea sound ?

On the plus side, there is far less switching noise, since we can synthesize the pure sine from an MCU and feed that to the power amplifier directly.

On the minus side, power dissipation would be much more than using a class D design?

All expert/intermediate/novice views are welcome. use a good audio power amplifier in the inverter portion of a sub 1KW VFD?

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    \$\begingroup\$ Did you actually calculate the power dissipation? I doubt it is going to be a good idea anywhere... \$\endgroup\$ – PlasmaHH Feb 16 '16 at 11:08
  • \$\begingroup\$ It's a common technique at circa 10W power levels, e.g. for audiophile turntables. So it works. But I can't imagine any good reason to waste that much power at the kw level. \$\endgroup\$ – user_1818839 Feb 16 '16 at 12:31
  • \$\begingroup\$ @PlasmaHH What you point out is a HUGE issue in this kind of a design. What if we could use harvest the heat for electricity and use it for cooling the assembly. \$\endgroup\$ – Vishal Feb 16 '16 at 14:49
  • \$\begingroup\$ What if a class H amplifier is used, where the DC link value is variable, and based on power required by the reference. Instead of 6 diodes, we have six SCRs which are also controlled by the VFD. \$\endgroup\$ – Vishal Feb 16 '16 at 15:32
  • \$\begingroup\$ You would need 6 GTO devices, and associated circuitry, or 6 power transistors. You can use SCR's, but force-commutating them off is pretty nasty. You would need to force commutate them, since you can't actually drop your variable DC bus to zero in a three phase system. \$\endgroup\$ – R Drast Feb 16 '16 at 17:48
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A linear inverter will work perfectly fine but don't expect anywhere near any real efficiency OR power density.

We have a linear 5kVA PSU at work we use when we need Clean supply (to rule out other issues) or when testing some motors.

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  • \$\begingroup\$ Thanks for the info. Infact one of the use is a bench supply where I need 3 phase clean supply (simulate a DG set) at different frequencies (50Hz and 60Hz and +/- 20% at each level). The idea that we could make a clean inverter output (devoid of all the power quality issues) is so riveting. We as a community ought to work on this !! \$\endgroup\$ – Vishal Feb 16 '16 at 14:53
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If you simulate the system, you will see that the phase shift created by the motor severely distorts the sinewave and thereby creates harmonics. However, for some purely resistive load there does not seem to be a problem. Clearly, electric motors are not purely resistive load.

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Motors may create serious phase shift in its current which your AB class audio amplifier may not handle so well. What would happen is possibly over heating of output transistors.

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