Changing mains frequency for motor control

[vik1501]

Hi,

I want to control 0.5HP motor by changing the frequency of voltage

(230V). Is it possible :?: Any idea :idea:

 

[Optikon]

Hi,

I want to control 0.5HP motor by changing the frequency of voltage

(230V). Is it possible :?: Any idea :idea:

Yes. design a freq control drive. Similar to inverter but with freq. control.

 

[mechie]

Inverter speed controller

Yes it is possible - commercial inverter speed controllers do exist.

The general idea is to rectify the incoming mains, smooth it and then chop it at the required frequency. This sounds simple - the first bit is done in most computer power supplies (SMPSUs) but the chopper may need some care as a square wave could upset some motors due to the high frequency harmonics causing magnetic saturation of the motor and generating heat/noise/R.F interference.

You need to be driving a mains-synchronous motor -- a cheap universal motor as used in electric drills won't respond very well to this form of control.
Also you need to consider heat dissipation - if the motor has a shaft-driven fan then it could be bad voodoo to run it slowly as the fan will not shift enough air to keep the windings cool.

 

[stevez]

Yes, if it's an AC motor that is dependent on frequency for RPM then changing the frequency will change the RPM.

I hesitate to use the term synchronous to describe frequency dependency because to me that mean the RPM is locked in to the line frequency. Most AC motors will run at the synchronous RPM minus some small amount of slip. An example of this in the US - a 1750 RPM motor means that it's an 1800 RPM motor that looses 50 RPM to slip. Larger industrial motors are made to be locked in to line frequency and are expensive - though they have many advantages. They do make small universal motors that run on AC. I do not know if these respond to changes in frequency but I suspect that they don't.

Worth noting - the HP/torque will drop off as the RPM is reduced. You must be sure that the torque demand by the load can be met by the motor at reduced RPM conditions. Also note that some motors cannot be run at low RPM for very long or they'll overheat.

 

[Styx]

Yes, if it's an AC motor that is dependent on frequency for RPM then changing the frequency will change the RPM.

I hesitate to use the term synchronous to describe frequency dependency because to me that mean the RPM is locked in to the line frequency. Most AC motors will run at the synchronous RPM minus some small amount of slip. An example of this in the US - a 1750 RPM motor means that it's an 1800 RPM motor that looses 50 RPM to slip. Larger industrial motors are made to be locked in to line frequency and are expensive - though they have many advantages. They do make small universal motors that run on AC. I do not know if these respond to changes in frequency but I suspect that they don't.

Worth noting - the HP/torque will drop off as the RPM is reduced. You must be sure that the torque demand by the load can be met by the motor at reduced RPM conditions. Also note that some motors cannot be run at low RPM for very long or they'll overheat.

That is true for induction machines, But he hasn't stated what kind of AC machine he is dealing with. Variable freqency inverters are alot mroe common for variable speed syncrnous drives

 

[vik1501]

Thanks to all of you who came up with gr8 answers.

But how do I maintain the a/c output. I don't want square wave as an output. :roll:

 

[Styx]

Thanks to all of you who came up with gr8 answers.

But how do I maintain the a/c output. I don't want square wave as an output. :roll:

What you need to do is have a switching frequency many orders higher that the maximum AC frequency you want (say 10kHz switching frequency).

Then via varying the PWM duty you can synthisis a sinewave output, since the load seen by the inverter is effectively an inductor it will smooth out the square wave PWM and make it resemble the sinewave voltage you want

 

[vik1501]

Thanks Styx !!

Can somebody suggest a good starting point for this R & D....may be a circuit or web site .....book etc.

 

[vik1501]

I have started working on this project thanks to a gr8 article I found on net


**broken link removed**

Anybody interested in seeing this project through is most welcome ! Keep in touch !!

Bye

 

[spuffock]

look at www.tinaja.com the stuff about magic sinewaves. Its a good place to start you thinking. I built a 3 phase inverter based on this and it works a dream. Frequency low enough to use IGBT's as well.

 

[vik1501]

Spuffock your attention plz

Thanks spuffock

I visited the link only to find a lot of links there ! Before I go thro' each of them can u plz answer my following queries.

1. Whats the power o/p of your 3 phase inverter you designed.

2. Can magic sinewaves method be used to produce single phase inverter.

3. Do we need special components for that.

4. Did u use any uC ? Which make ? 'Cause I am familier with only 8051

5. How much time you took to develop this ?

Bye & thanks again !

 

[spuffock]

1.It depends on the output devices, I used IGBT's. I was running about 4KW.
2. Yes. Less trouble than 3 phase.
3. Not really. Use FETs or bipolars if you like
4. I used a PIC16C84. Why not use the 8051?
5. Difficult to say. A month or so, on and off!

 

[vik1501]

Yeah 'Magic Sine waves' is gr8 stuff indeed Spuffock !

I tried to download fourier.ps, zeroharm.ps, sn1\210cat.ps, hack73.ps, magic210.ps sncat420.ps from the link, and still trying ! I want to read more on fourier analysis of the words give there for generating magic sine waves. If u have can you mail me !!


Or plz suggest any other reading material. The theory is slowly sinking in my head !!!

Thanks & Regards,

------------------------
I now understand it better !! Plz comment on the following

1. I intend to use H-bridge. This means using 4 igbt's. isn't it ?
2. The load is induction motor. But I intend to use isolation transformer to isolate the motor supply from output of H-bridge. Is this ok ?
3. Which 'word' did u use ?
4. What about IGBT driver. I want to provide opto-isolation between driver and microcontroller circuit.

Regards,