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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
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.
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
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.
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.
I know how to make an inverter but controlling the frequency is beyond my scope of knowledge.
**broken link removed**
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?
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.
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.
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?
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
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?
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?
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!).
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?
You take the 240Vac and rectify it to give you a 336Vdc. Then with a H-bridge and suitable current-control (or just go for open-loop voltage control) you PWM the relevant FET's to produce a sinewave fundement which you require of 216Vac
you wount get 240Vac out due to volt drops, modulation depth.... so keep that in mind
Here's another thought. Get a 230V 3-phase VFD control for fractional HP and use just one phase plus a ground. I don't know if that is legal in the UK. In the US, our mains have a white wire ("common") that is grouded at the service entrance. We can't use the ground (green), but we can use the white wire in the configuration described. One can get FHP VFD's that run on 230V for about $100USD. I checked the UK eBay and didn't see any listed. Maybe they are called something else in the UK.
John
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!).
Of course you can get variable frequency oscillators but he wanted to vary the frequency at set time periods which is possible using a VCO and another oscillator but it's probably more convenient to use a PIC.
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