Hi all
A simple question is it possible to change the frequency of a voltage i.e 240v @ 40Hz or does changing the hertz effect the voltage?

john

 

Yes, it's done all of the time for running 3-phase (and other) motors. Check out "variable frequency drives." You can probably just Google on VFD. John

 

Of course it's possible.

You need to provide more information though.

Are you talking about a generator? If you reduce the speed you will reduce the frequencyand voltage but you can increase the voltage again by increasing the feild current to compensate.

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Im trying to slowdown the speed of an aquarium pump to make a wavw system,
They are 240vAC, I connected one to a light dimmer switch just to run some test when i turned the switch the voltage dropped and the pumped stopped so i think the only way i can do this is to adjust the frequency, I would like to be able to control the frequency with a PIC if it possible

 

do you know what type of machine is used?
is it a direct-on-line machine or does it have electronics with it?

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Machine? if you mean what pump then they are seio pumps they simply plug straight into the mains with no other electronics, they do sell controller for these pumps which reduce the speed of them but here too expensive and dont do what i want them to.

I think there an inductance motor?

 

The pump motor is a synchronous AC motor, you've discovered tha hard way that a lamp dimmer won't work, you need to alter the frequency to change the speed.

To change the frequency an AC power supply you need to convert it to DC (with a rectifier and smoothing capacitor) then use an inverter to convert it back to AC at the desired frequency. Another problem is that the motor's core will probably saturate as it won't be desigened to work at lower frequencies so your best bet is to build a modifiedsinewave inverter and reduce the duty cycle with frequency. This is normally done by keeping the on time the same but increasing the gaps between pulses.

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yup a CFVF converter needed
They arn't that hard just tend to be a bit distructive if you get things wrong

do you know the VA of the machine?

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Thanks guys

I know how to make an inverter but controlling the frequency is beyond my scope of knowledge.

http://www.uoguelph.ca/~antoon/circ/555dcac.html

I found this diagram am i right in thinking that the 555 timer controls the pulse to the inverter if so could i replace the 555 with a pic to vary the pulses when i need to?

 

Don't say that!

The inverter I'm talking about doesn't convert 12V to 230VAC but 325VDC to 230VAC. It's just an oscillator with an h-bridge on the output and no transformer.

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I think i may not have explained myself properly I need to raise and lower the speed of the motor every minute or so, wouldn't an oscillator and h-bridge give a fixed frquency?

 

I have designed and built prototype inverters with an H bridge that switch 12VDC for 12VAC aquarium pumps. The submersible pumps that I analyzed are are synchronous and sensitive to frequency and voltage and won't work out of a range of 10% or so for both when driven by a square wave. We also had to get the pump coil windings custom built at the factory in China for 10.5VAC so they won't make rattling noises. The pumps are used for liquid cooling of gaming computers.

Are 240V aquarium pumps legal where you are? Are they approved by UL, CSA, or whatever applies in your country?

Bob

 

Yes, an oscillator would give a fixed frequency.

You could easilly do what you said using a microcontroller though.

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???????????????????????????????

Why?, a variable frequency oscillator is an EXTREMELY simple device, certainly no need to use a micro-controller (apart from it's usually a good idea!).

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just to see if ive got this right

I turn the 240VAC into DC and feed this to the h-bridge i then use the Pic/oscillator to control which side of the h-bridge turns on giving me 240VAC at the frequency i desire ?

What transistors would i need to use for a 240v h-bridge?