Anzhelika

Joined Jan 17, 2015
2
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?

crutschow

Joined Mar 14, 2008
24,093
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.

Anzhelika

Joined Jan 17, 2015
2
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.
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?

Hello,

The following page of the EDUCYPEDIA will give you links with information on SPWM: