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[on:]

Hello there!

We are looking for a solution which is commonly known as a variable-frequency-drive...

There are some hurdles to be overcome, but let's see first what this thing has to do...

The principle idea is in a first step to soft-start a 3-phace AC motor to it's regular RPM.
As a second step, the RPM shall be matched to the supply-frequency, so the motor can then be directly hooked to the power-source and the electronics be shut-off and bypassed by a relay.
The whole thing is to avoid that brute switch-over from star to triangle when starting the motor.

So all-in-all, it's just a soft-start for an electric motor.
There's no need to control the frequency in regular operation.

Motor is around 6kW...

What I was wondering, if anyone already did some brainwork or prototyping of any kind for such an application?
We're a team of aircraft-maintenance-engineer and a software-engineer and are aware of the problems, dangers and solutions.
Re-inventing the wheel sometimes is cool, but when it comes to high-power applications, we rather take the experiences from others into account...

Yes, I know, there are only a few companies that make VFD's, and those are expensive.
Some people say that thus, it's too complicated to make one on your own.
Let's just say this is an attempt in the sense of "Everybody said it's not possible. Then there was that guy who did not know that, and then did it."
So yes, I could buy one. I would have a bunch of functions I do not need, as well as an empty account...
But isn't arduino about doing things on your own?
Let's hear your take on the subject!

Cheers from switzerland,
Dany

[Reply #1 on:]

Can't really help you but a few of questions need to be answered for those who can

1) Supply voltage and frequency.   

2) What is the torque load applied to the motor from 0 RPM to sync speed

3) What type of load is the motor driving:  for example is the load steady (as in a band saw) or pulsing (as in a reciprocating compressor)

4) Is there a large inertia load on the motor: for example does it drive a large flywheel

5) What is the desired acceleration time from 0 to sync

6) What is sync speed (750, 1500, 3000 ?)

Note that the answer to question 2 is really important since it determines the current load on the power semiconductors during the variable frequency phase from 0 to sync

[Reply #2 on:]

Its quite a challenge.  Maybe finding a bargain second-hand VFD would be easier?

Perhaps a forum on electric vehicle drives would be a more fruitful place to ask around - some
people use AC motors for home-brew electric vehicles, 6kW is in that sort of area...

I am guessing a 3-phase PWMable IGBT bridge is the minimal starting point for the
power stage, but you also need some quite sophisticated sensors and control algorithms...

[Reply #3 on:]

We are looking for a solution which is commonly known as a variable-frequency-drive...

Instead of seeking out a VFD design why are you not seeking out a design for what you want the application to perform, a soft start device? They are not all that expensive considering the power level you are talking about. It uses SCRs to provide a ramping current over an adjustable time period to overcome the in rush current you are trying to avoid, often bypassing the SCRs with 'hard contacts' once motor has reached final running speed. No reason to go to the complexity of variable frequency drive as that is a more complex method (requiring a rectifier and inverter sections) that while it can provide soft starting properties, it's real purpose it to allow variable speed control for an otherwise fixed speed AC motor.

Typical 10HP soft starter:

http://www.ebay.com/itm/Soft-Start-10-HP-460-Reduced-Voltage-Electric-Motor-Starter-New-WEG-SSW05-/261094217041?_trksid=p3284.m263&_trkparms=algo%3DSIC%26its%3DI%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D21%26pmod%3D261087527646%26ps%3D54

More info:

http://www.electrobeauce.com/wp-content/uploads/2011/04/WEG-soft-starters-ssw-series-50019077-brochure-english.pdf


While one with proper experience in AC power and 3 phase induction motors might be able to design and build such a starter module from scratch, I suspect that if would cost more in time/material/debugging/testing then a off the self purpose designed module which are almost sold at commodity prices due to competition.

Lefty

[Reply #4 on:]

Why not to use TRIAC, MOC and Arduino to PID control the AC motor??/

[Reply #5 on:]

Why not to use TRIAC, MOC and Arduino to PID control the AC motor??/

Three phase motors don't allow that type of control. The speed of their rotation is locked to the AC frequency.

[Reply #6 on:]

Why not to use TRIAC, MOC and Arduino to PID control the AC motor??/

Three phase motors don't allow that type of control. The speed of their rotation is locked to the AC frequency.

But again the OP wants to control in-rush current to the motor, not to control it's speed. No VFD required to do that, there are cheaper simpler methods as I posted earlier.

Lefty

[Reply #7 on:]

Lefty,

That Input is quite a jewel!

That is quite what we're looking for!

The reason why we had our mind locked into an arduino-based solution is the design of the other accessories.

The motor drives a four-stage air compressor which is used for scuba.
And the accessories like drain-vents etc will be controlled by arduino. (which enables us to open the vents during start-up, so the load is somewhat lessened...)
Of course, one could simply use the switch-over relais that already exist, but honestly, that bang during the switch-over... Don't tell me it's good for any part of the unit...

But if there's a unit that performs the soft-start for us, we can use the arduino to simply supply a digital input to that "external" unit.
I was not aware that such things exist, but then again, it is quite common for some things to be used on a daily-basis on some parts of this world, while the other part thinks about wizardry...

Great, this really simplifies the design!

Cheers from freezing switzerland,
Daniel

[Reply #8 on:]

I was not aware that such things exist, but then again, it is quite common for some things to be used on a daily-basis on some parts of this world, while the other part thinks about wizardry...

Glad to be of help. And it's not just certain world wide location that is slow to gain knowledge of new methods and equipment. Even at the refinery I worked at before retirement, the electrical engineering group that handle the design and construction of high power switchgear was still very slow to adopt new technology like this into their efforts, rather relying on how they always did it before. They tend to favour tried and proven solutions rather then take 'risks' on something they have never used before. Soft-start motor starters should be a no brainer these days but alas thing change pretty slowly in some disciplines.

Lefty

[Reply #9 on:]

Don't know if it is of any help now as this is an old thread, but I have used ebay for vfd's and found the prices not too bad. The last one I bought was around the size you wanted and only cost me $140aus brand new. I know you did not want to buy one based on price, but I've found that this is the easiest solution and most vfd's have some sort of serial port coms which you can interface with arduino.