# Making a sine wave out of a trapezoidal wave

#### markzz

Joined May 25, 2016
16
Hello I have a Chinese 3 phase DC controller that outputs a trapezodial wave and I was wondering if there is a way to add an external device to make it a sine wave.

What is that conversion called?
The only term I come across is a type of inverter.

#### Alec_t

Joined Sep 17, 2013
10,361
What is that conversion called?
I doubt there is a special name. What voltage/current is this wave providing?

#### markzz

Joined May 25, 2016
16
Well the battery voltage going into the controller can range between 36V and 100V, the more typical is 48V, 60V or 72V.
I dont know what the voltage would be on the output of the controller, I'd assume its the same.

As for amp's, this is where it gets interesting because I just found out now, that the phase currents are much higher then battery currents.
We are talking about batteries being around 30A to 60A, but the phase amps are much higher, so 150A to 200A.
I believe that would be an issue.

#### markzz

Joined May 25, 2016
16
Well from what I found, I doubt a simple Resistor Capacitor (RC) circuit would be the solution.
I found these quotes from the Quora website

Square wave is a result of superposition of sinusoidal waves of different frequencies. Look up Fourier series. To extract sine wave out of it, you need a low pass filter. Simplest filters can be made out of RC circuits.

You can use a diode wave shaper circuit to convert a square to a sinewave. But you can go slightly more sophisticated by using a low pass filter to pass only the fundamental frequency present in the square wave and filter out all those luckily high frequencied harmonics . This uses your knowledge of the frequency domain signal analysis.

To start off, a capacitor will do the job. Again it depends on the magnitude you want. The Voltage across a capacitor will be exponential in nature. So the charging and discharging should give you some what of a sinosoisald wave. You can control the discharging of a capacitor via a resistor and so can you the charging. This is only a brief overview of how you can do this.
To actually achieve a pure sinosoisald wave, you'd need more electronics in your circuit.

Say your square wave is at frequency f0. From square wave you get all the odd harmonics. Simplest way is to put a band pass filter around f0 (or 3f0, 5f0 ... depending on what you want.)

#### MrAl

Joined Jun 17, 2014
6,485
Hi,

A diode wave shaper works on a triangle wave.

For a trapezoid you'd probably need an LC filter if the frequency is mostly constant. For 100 amps though you're talking some big parts. Dont know if that would be practical.

Another idea is to chop it up into pulses that simulate a sine wave. That requires some hefty transistors too though.

High power is harder to handle because the current is so high usually.

#### Alec_t

Joined Sep 17, 2013
10,361
Chopping it (aka switch-mode), as MrAl says, would be more efficient than any other method. Diode shaping would generate an enormous amount of waste heat.

#### markzz

Joined May 25, 2016
16
OK thanks for the replies.
I will just have to buy a Sinewave controller then for my ebike.

#### Alec_t

Joined Sep 17, 2013
10,361
Surely an ebike will work ok with a trapezoidal waveform?

#### shortbus

Joined Sep 30, 2009
6,979
I will just have to buy a Sinewave controller then for my ebike.
If your Ebike has a BLDC motor the wave form for them from the controller is trapezoidal.

#### markzz

Joined May 25, 2016
16
Yeah its trapezoidal waveform, if thats how its spelled. Makes a lot of noise though, people can hear me coming up behind them and thats not good. I had a wimpy sinewave bldc controller and it was real real silent. Stealth is what I want, but when I see them sell for $150usd its too expensive, when a trap is only$30. I got a schematic from some guru in another forum, I will see what the pcb production places charge to populate a board. I dont mind spending say $60 or up to$75 for a 50-75A controller. And if they are decent, maybe sell a few.

#### Alec_t

Joined Sep 17, 2013
10,361
people can hear me coming up behind them and thats not good.
It's very good from a safety viewpoint. Otherwise people could unknowingly step into your path .