Stepping down 240v to 10v

[breakshift]

Hi, I need to essentially step down the output voltage of a generator:

**broken link removed**

to a lower voltage that can be fed into an NI sbRIO-9631:

NI sbRIO-9631 - Embedded Device with AI, AO, DIO, 1M Gate FPGA - National Instruments

The aim is to measure the power output of the generator and provide a control signal to the engine to keep this power constant with varying load on the generator. By stepping down current and voltage and feeding this into the RIO, the power can be monitored. A current sensor is easy enough, but I'm struggling to find a device that will perform the voltage measuring/down-stepping. This is for use in a UAV, and so the electronics will need to be able to cope with a certain temperature range.

The max output of the gen is 230v, and the max analogue input of the RIO is 10v. So I'd like to get my hands on a device that will do this (need quick delivery to the UK). I've searched rss and Farnell, and other sites, but couldn't find what I need.

Failing that, I'd need to design my own. I was thinking of using a voltage divider circuit. To keep the divider output constant over temperature I could use a negative temperature coefficient thermistor in series with the divider, so that any dR in the divider circuit would be countered by a suitable dR in the thermistor, hence keeping the output constant.

So basically there must be something out there that will do this for me... and if not what do you think about the voltage divider idea?

Thanks,

Mike.

 

[ericgibbs]

hi,
The generator is 230V 50Hz, so you require 230V/6V step down transformer, followed by a simple capacitor filter.

Why have you chosen such a powerful spec for the controller.??

 

[breakshift]

The RIO is to be used as the central electronic controller for the UAV - it will handle all of the electronics. Other features such as autpilot will be added in the future.

I forgot to mention - I need to measure the power without drawing any power away from the gen load. A transformer would draw a significant amount of power... right?

 

[dougy83]

Failing that, I'd need to design my own. I was thinking of using a voltage divider circuit. To keep the divider output constant over temperature I could use a negative temperature coefficient thermistor in series with the divider, so that any dR in the divider circuit would be countered by a suitable dR in the thermistor, hence keeping the output constant.

Hi Mike. If you are referring to a couple of resistors forming a divider, there is no need for any thermistor; provided that you use the same technology of resistor and they're both at the same temperature, the division ratio will always be the same irrespective of temp.

I need to measure the power without drawing any power away from the gen load. A transformer would draw a significant amount of power... right?

A transformer will use a couple of watts; you have a 5.5kW generator, so no, it won't draw a 'significant' amount of power.

 

[ericgibbs]

As already posted the power drain for a 2VA 230v/6v will be insignificant....

BTW: 6Vrms when full wave rectified and smoothed, this will give approx [6v * 1.414] = 8.5Vdc less two diode drops, say about 1.3V, so thats approx 7.2Vdc,,, thats for a 230Vac input.

So at 200Vac with an ideal transformer step ratio of 38.34, they will become 5.2Vac, rectified/smoothed etc.. will give 7.38V - 1.3 = 6.0Vdc

This means that a 30Vac drop in gen output will only be a 1.2Vdc drop,, thats only 0.4Vdc change in DC output for a 10Vac change in gen output.....OK.

 

[Reloadron]

You have a 5 KW generator with built in AVR (Automatic Voltage Regulation). How bad is the AVR that you want to do all this work? Most units including a small 4 KW unit I have actually regulate pretty well additionally I have a whole house backup (18 KW) that also regulates quite well. I can't think of too many applications where the existing AVR would not be adequate.

Ron

 

[breakshift]

As already posted the power drain for a 2VA 230v/6v will be insignificant....

BTW: 6Vrms when full wave rectified and smoothed, this will give approx [6v * 1.414] = 8.5Vdc less two diode drops, say about 1.3V, so thats approx 7.2Vdc,,, thats for a 230Vac input.

So at 200Vac with an ideal transformer step ratio of 38.34, they will become 5.2Vac, rectified/smoothed etc.. will give 7.38V - 1.3 = 6.0Vdc

This means that a 30Vac drop in gen output will only be a 1.2Vdc drop,, thats only 0.4Vdc change in DC output for a 10Vac change in gen output.....OK.

I'm not looking to rectify the signal - actually I want the input to the sbRIO to be as representative of the original voltage as possible.

You have a 5 KW generator with built in AVR (Automatic Voltage Regulation). How bad is the AVR that you want to do all this work? Most units including a small 4 KW unit I have actually regulate pretty well additionally I have a whole house backup (18 KW) that also regulates quite well. I can't think of too many applications where the existing AVR would not be adequate.

Ron

Well we're stripping the engine from the generator, and coupling our own engine to it. We need to keep the gen output constant at some level that will keep the UAV just above stall speed; so this voltage won't be the same voltage that the existing AVR is designed for.

Okay, so a transformer won't draw too much power. I've had a brief look on rs (need to go now...) and the data sheets for the transformers specify 'input voltage range 207v - 253v (230v +-10%). I would have thought that a transformer would only have a max input voltage... not a minimum too. I need it to work all the way from zero to ~230v.

 

[Nigel Goodwin]

Okay, so a transformer won't draw too much power. I've had a brief look on rs (need to go now...) and the data sheets for the transformers specify 'input voltage range 207v - 253v (230v +-10%). I would have thought that a transformer would only have a max input voltage... not a minimum too. I need it to work all the way from zero to ~230v.

It will be perfectly fine for your application, the design range doesn't really matter.

 

[Reloadron]

Well we're stripping the engine from the generator, and coupling our own engine to it. We need to keep the gen output constant at some level that will keep the UAV just above stall speed; so this voltage won't be the same voltage that the existing AVR is designed for.

OK, I can understand that. I may be way off base with this but you have a 50 Hz generator which tells me it is likely a 2 pole generator designed to run at 3,000 RPM. The generator will produce its rated output voltage, output current, and output frequency of 50 Hz. when turning at 3,000 RPM. Here in the US it would be 60 Hz and 3,600 RPM. Anyway to maintain output voltage and frequency of 50 Hz. you need a rotational speed of 3,000 RPM. Now I guess if the output voltage, current and frequency don't need to be 220 VAC @ 50 Hz. then a lower speed shaft is fine.

Do I have that right for this type generator?

If that is true you really only need a tachometer pickup and use the speed to maintain 3,000 RPM rotational speed of the generator. Unless I am wrong and missing something here.

Ron

 

[Diver300]

You could just use a 3 V transformer. Connect one side of the secondary to +5 V and measure the other side. It will vary over the 0 - 10 V range at 50 Hz. If your A2D is sampled sufficiently quickly you can find the peak voltage reasonably easily.

Transformers are reasonably linear. Their minimum voltage is just the minimum at which they produce the nominal output. If you halve the input voltage, the output voltage will halve.

 

[tcmtech]

So what are you powering on your UAV that needs a 5 KW 230 volt AC power source?

 

[breakshift]

Reloadron - we don't want to be running the generator at 3000rpm. At this moment we don't know what rpm we want, but we can be sure it will be lower than 3000rpm. So running it at a lower rpm will produce less power/voltage. A group member is sorting the rpm-sensing side of it, I just need to measure the electrical output power and feed it into the sbRIO. Does that explain it? I'm not sure what you're not understanding

How can I work out how much power a transformer will draw? If the gen is rated at 5kW, and max voltage is 230v, then max current would be 5000/(230/root2) = 31A.

The power is for driving the propellor.

 

[tcmtech]

So the engine drives a generator that drives electronics that drive a electric motor that spins the prop?

Sounds like a unnecessarily complicated, inefficient, and heavy design to do a simple task that a engine and maybe a simple gearbox can do directly.

 

[Reloadron]

Reloadron - we don't want to be running the generator at 3000rpm. At this moment we don't know what rpm we want, but we can be sure it will be lower than 3000rpm. So running it at a lower rpm will produce less power/voltage. A group member is sorting the rpm-sensing side of it, I just need to measure the electrical output power and feed it into the sbRIO. Does that explain it? I'm not sure what you're not understanding

I get what you are doing, I was merely pointing out the generator freq out would be other than 50 Hz. If that matters not then I would look to transducers built and designed for the testing you want to perform. Transducers from someone like CR Magnetics to allow accurate measuring and recording of voltage, current and power by your acquisition system as you I assume apply loads to your generator. As to using a small transformer, the power consumed by the transformer would likely be negligible.

As to the entire process it does seem a long way to get where you want to go but best of luck with it.

Ron

 

[Diver300]

You should also be aware that you could saturate a 240 V / 50 Hz transformer if the voltage is ever more than 240 / 50 times the frequency. So if there is a voltage regulator on the generator that means that it aims for 240 V whatever speed it is running, it could achieve 240 V at 30 Hz. Similarly, it could get to 100 V at 10 Hz. It is worth measuring as some generators will go the other way, in that if the frequency reduces a bit, the voltage reduces a lot. If it is like that, the 240 V / 50 Hz transformer is fine.

100 V at 10 Hz will kill or overheat any 240 V / 50 Hz transformer, and the output will no longer be linearly related to the input voltage.

If you use a 415 V / 50 Hz transformer that would give you safety margin. However, the 415 V transformers that I have found are much bigger ones than you would want and have a larger output voltage than you want. Two 240 V ones with the primaries in series and the secondaries in parallel would be cheaper and smaller.

If you do use that solution, make sure that you connect the windings in phase.