# Help with inverters (information overload)

Discussion in 'General Electronics Chat' started by Anzhelika, Jan 17, 2015.

1. ### Anzhelika Thread Starter New Member

Jan 17, 2015
2
0
What's up guys and gals?

As a mechanical engineer I've never really found electrical engineering accessible. Most if not all educational material seems to assume I already have an intrinsic understanding of complex circuitry and terminology, which is not the case.
Specifically, I now find myself needing to control a 3-phase AC induction motor, a rather complex application which I consistently keep failing to break down into easily digestible, bite-sized portions.

Without a doubt, you will recognize the primitive 3-phase inverter pictured above. Even such a basic design can do interesting things, but is not suitable for motor control. As such, I started looking into Pure Sine Wave inverters and motor controllers, but information is vague at best. Generally, the motor controller and inverter are seen as one and the same. In order to first fully understand the DC-AC power conversion, I need to look only at the PWM, filtering and other non-controlling components. In essence, I wish to start with the circuit above, and add components one by one to add fucntionality, all the while considering the controlling section a black box to be defined later.

The first additional function that seems logical to me would be to smooth out the PWM-approximated sine-wave into a true sine wave as shown. How would I go about doing that?

2. ### crutschow Expert

Mar 14, 2008
13,475
3,361
For viewing purposes on an oscilloscope you can do that with an RC or LR low-pass filter.
But with a motor load the inductance of the motor generally provides the low-pass filtering function so that the current through the motor is a sine-wave and no further filtering is needed.
If additional filtering is necessary, an inductor in series with each motor winding could be used.
Note that it's the current through the motor that should be a sine-wave, not necessarily the applied voltage.

Apr 5, 2008
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4. ### Anzhelika Thread Starter New Member

Jan 17, 2015
2
0
Ah, I see. Funny thing is I kinda expected that, but then the true sine wave was mentioned so much that I pretty much dismissed that idea.
If the simulated sine-wave frequency is f(sine), then what would f(switch) need to be to ensure a relatively smooth sine? Would 10*f(sine) do it? a 100 times?

I'm getting a 503 but I'll try again later.

Anyway, if filtering is not an issue, then next step would be a way to recover power from the motor as it slows down, that is, regenerative braking. This of course means I need a rectifier. Those are simple in and of themselves, but how would I connect it to the motor/inverter? I obviously can't put it in parellel because it would just drain the power from the inverter.