Restricting current flow?

[hotponyshoes]

I'm trying to plug several devices into a generator.

Generator has a 20a breaker.

I've made up a box with 2 sockets, one socket is monitored by a current sensing relay. The relay controls a contactor which supplies power to the 2nd socket.

This works fine, 16a of load is available on the 2nd socket. If a load of more than 4a is detected on the first socket the 2nd socket cuts out. So (not tested it yet!) the generator should never see more than 16a of load and never trip out.

I'm expecting this to work as I want. All the devices I will plug into the 2nd socket are basic things like an electric hotplate. If power is removed it just comes back on when power is restored.

But, I'm thinking of improvements.
For example, I have a portable induction hob,
If power is removed from this I need to use the dial on the hob to turn it off and back on again. It won't just come back on as it was when power is lost.

I'm wondering if, rather than disconnect the 2nd socket completely I can just limit the power draw to 4a somehow?
And, if it is safe (for the equipment) to do that?

I'm also wondering about some method of monitoring the load on both sockets and only disconnect load as required?
Stuff like the hotplate has a simple on/off thermostat, so it's drawing 13a until it gets to temperature then nothing. Then turning itself on/off as required.

I am thinking of a box with 6 sockets that I could somehow arrange in order of priority,
Ie, stuff that always needs power (card machine) as #1,
hand-wash station that only needs power when you turn the water on as #2
Induction hob that needs power but could maybe be current limited at times as #3
Hotplate that can be completely turned off on #4 and so on..

Any bright ideas ?

 

[Robotstar5]

Not a simple solution, CT's (current transformers) on each socket supply feeding the readings into a PLC where the program decides which socket to turn on/off via contactors.

 

[MetalMonkey]

Any bright ideas ?

Buy a bigger generator...


The induction hob not coming on when power is restored is probably a good safety measure to be honest.
For your card machine you could buy a cheap UPS, then it wouldn't matter if the power dipped off for a few minutes.

 

[hotponyshoes]

Buy a bigger generator...


The induction hob not coming on when power is restored is probably a good safety measure to be honest.
For your card machine you could buy a cheap UPS, then it wouldn't matter if the power dipped off for a few minutes.

It's not a big deal to flick the induction hob off and on again, certainly easier than walking over to the generator to reset the breaker.

I have got a bigger generator
But, its unnecessarily big for some events.

The 20a one is about the biggest I can handle on my own, I can push it up a ramp into the van and drag it across grass.
Anything bigger and I'm using the winch to load it and paying staff to arrive earlier and help me set up on site.

 

[Anguz]

or another smaller generator ?
cheaper in the long run than extra staff?

 

[sako243]

Short answer is you can't "limit"* current, it is drawn. The device decides how much current to pull and it's up to your supply to handle it.

Thinking outside the box what about a battery pack in line with the generator. They're coming down in price quite a bit and have the benefit of being able to supply large amounts of current. Basically a buffer between your generator and the loads that can handle any sudden demand and then the generator can catch up with the charging. Depending on how much you want to spend the smaller events might able to be powered completely off the battery pack.

* you can limit it by switching in a resistor bit but that just wastes power. We quite often do that on big power supplies to limit the inrush, basically a big resistor to limit how quickly the caps can charge up, once they're charged then the resistor is switched out of the circuit.

 

[hotponyshoes]

Short answer is you can't "limit"* current, it is drawn. The device decides how much current to pull and it's up to your supply to handle it.

Thinking outside the box what about a battery pack in line with the generator. They're coming down in price quite a bit and have the benefit of being able to supply large amounts of current. Basically a buffer between your generator and the loads that can handle any sudden demand and then the generator can catch up with the charging. Depending on how much you want to spend the smaller events might able to be powered completely off the battery pack.

* you can limit it by switching in a resistor bit but that just wastes power. We quite often do that on big power supplies to limit the inrush, basically a big resistor to limit how quickly the caps can charge up, once they're charged then the resistor is switched out of the circuit.

A battery pack might be a simple solution,
In theory, at an event we would use the smaller 20a generator, we would run enough kit to be pulling 30a+

In practice,
The kit is turned on in stages so we usually run fine on 20a. It's only the hand-wash that pulls us over. That trips the gen most times its used (unless i turn off other stuff before using it) and the box i have just made should take care of that.

We will get 1 or 2 trips from the remaining kit when multiple things decide to reheat at the same time so I'd like to eliminate that as well if possible.

A battery system that could provide extra power when required and then charge of the surplus power would be good. If it could provide an extra 20a (for 3-4 minutes max) it would donthe job fine.

Although I think it would need to work on the basis of:
If 18a was being drawn the battery would need to only pull 2a for charging.
If 28a was being drawn the battery would need to supplement the generator with the extra 8a.

Maybe one of the solar type inverters would control that out-of-the-box?

 

[Kram]

How much power is the handwash, does it heat the water or just pump it?

If its heating, then perhaps a different idea of heating a small tank whilst the current demand is low.

For my car, I have a 16L tank and a USB powered shower pump which is great after doing messy tree work, particually in summer where I'll be melting. I hate driving with messy hands and getting the car dirty.

Card reader, power that from an lithium inverter.

 

[minimutly]

The (old) solution would be to use a current meter connected to a series of trip amps, set at suitable thresholds. A more modern solution would be a serial output from the current meter to a little plc, with the switching threshold set in code - will need some googling to get a cheap solution...
Second hand current meters (diaries or similar) should be available, little plc's are cheap as chips.

 

[hotponyshoes]

How much power is the handwash, does it heat the water or just pump it?

If its heating, then perhaps a different idea of heating a small tank whilst the current demand is low.

For my car, I have a 16L tank and a USB powered shower pump which is great after doing messy tree work, particually in summer where I'll be melting. I hate driving with messy hands and getting the car dirty.

Card reader, power that from an lithium inverter.

Hand wash is a 3kw model. Ideal I would like to upgrade it anyway as its a bit pathetic (but it meets the environmental health regulations for food service). I can upgrade it to a 5kw unit easily or replace it completely.

Free-standing mobile unit, You fill a tank with cold water. 2 momentary switches you press with your knees.
Pressing either switch gives you a 30second run of either hot or cold water.
It needs a permanent live supply for the control circuit but that draws very little power.
If the cold button is pressed it just runs a pump for 30 secs.
The hot button pumps the same but through an inline 'instant' heater element.
There are a couple of solonides that control which way the water flows.

As you can imagine, it's not a huge flow or particularly hot.
But it is compact, easy to clean and easy to use.

It's basically this hand-wash that is the main reason for our tripping issues, upgrading that might be a decent idea..
Rather than diverting extra power into a battery I could divert it into a hot water storage tank..

I'd imagine it would be simpler enough to remove the instant heater (that draws most of the power) and replumb it to pump from either a hot or cold tank.

Just need some way to sense 'excess' power and divert it

 

[hotponyshoes]

Actually, diverting excess power might not be that hard?

If I used a current monitoring relay I cold set it to trigger at 16a.
The output of the relay controlling a 4a heater element (if I can find a suitable heater)

So if all the other kit is using less than 16a the heater comes on.

A nicer solution would be to divert all excess current to the heater, so use a 20a heater, if all the other kit is using 10a the heater gets 10a as well.
Can't figure that one out though.

 

[Kram]

Yes an SCR unit will let you adjust power but you would need one controlled by micro controller or smart thingy.

 

[daleyd]

Yes an SCR unit will let you adjust power but you would need one controlled by micro controller or smart thingy.

You can adjust current through a heating element (it’s actually the voltage AND current) using a thyristor in phase angle control, but you would need this inline with only the heating elements, as the other electrical bits in the corcuit (eg relay coils, any devices with ic’s in etc) wouldn’t appreciate the reduced voltage or chopped up waveform.

 

[hotponyshoes]

You can adjust current through a heating element (it’s actually the voltage AND current) using a thyristor in phase angle control, but you would need this inline with only the heating elements, as the other electrical bits in the corcuit (eg relay coils, any devices with ic’s in etc) wouldn’t appreciate the reduced voltage or chopped up waveform.

So, i could I make a water tank with a heating element and fed it a 16a plug. Thermal cutout/thermal switch mounted in the tank/element to disconnect everything once it hits temp.

How would I then go about putting a 16a socket into the distribution system that fed the heater with all the excess power available?

Looking at the data sheets for some current sensing relays makes me think I might be able to use the 'window' function.
Ie, if the load is between 16a and 20a then the heater socket is live.

Or use a few current sensing relays in series (more than one monitoring the same cable?)
Over 20a disconnects the heater.
Under 16a triggers a thyristor to allow 4a to the heater.
Under 12a triggers a 2nd thyristor in parrallel with the first to allow 8a to the heater.

Bear in mind I've no idea how a thyristor works so that might not be possible!

 

[daleyd]

So, i could I make a water tank with a heating element and fed it a 16a plug. Thermal cutout/thermal switch mounted in the tank/element to disconnect everything once it hits temp.

How would I then go about putting a 16a socket into the distribution system that fed the heater with all the excess power available?

Looking at the data sheets for some current sensing relays makes me think I might be able to use the 'window' function.
Ie, if the load is between 16a and 20a then the heater socket is live.

Or use a few current sensing relays in series (more than one monitoring the same cable?)
Over 20a disconnects the heater.
Under 16a triggers a thyristor to allow 4a to the heater.
Under 12a triggers a 2nd thyristor in parrallel with the first to allow 8a to the heater.

Bear in mind I've no idea how a thyristor works so that might not be possible!

For phase angle control you usually have an analogue signal doing the control - so if you had a variable 0-10v signal (for example) and wanted 50% current you would send it 5V (it’s not a completely linear relationship as it chops the waveform but it’s a near enough approximation for illustration).

You would need some kind of either plc/arduino/controller to measure in the total current from a ct and then do some calculation to send a variable analogue signal out to the thyristor to make the current up to your max allowed value. It’s not that difficult to do, just some basic maths or a simple PID loop to send the correct output.

We have some big thyristor systems in work that have a dedicated connection between them to share the load out. One is dedicated as a master and controls all the others that are daisy chained out, you set the max power you want out of the entire “system” and the master decides how best to share out the load. It’s typically used for similar situations like the one you describe, but on a bigger scale - there’s one in Runcorn for a furnace that makes insulation, the incoming supply is rated for 1000A but if every thyristor was on 100% it would pull double that.

 

[daleyd]

In your example a ct would measure the total current. Your plc/controller would measure this input and you would give it a setpoint for the max current you wanted.

If the current on the CT was below the setpoint then you would increase the analogue signal to the thyristor, until you approached the threshold then your analogue signal would level out.

As the current on the ct went above your setpoint then you would decrease the signal to the thyristor, to reduce the current.

 

[hotponyshoes]

For phase angle control you usually have an analogue signal doing the control - so if you had a variable 0-10v signal (for example) and wanted 50% current you would send it 5V (it’s not a completely linear relationship as it chops the waveform but it’s a near enough approximation for illustration).

You would need some kind of either plc/arduino/controller to measure in the total current from a ct and then do some calculation to send a variable analogue signal out to the thyristor to make the current up to your max allowed value. It’s not that difficult to do, just some basic maths or a simple PID loop to send the correct output.

We have some big thyristor systems in work that have a dedicated connection between them to share the load out. One is dedicated as a master and controls all the others that are daisy chained out, you set the max power you want out of the entire “system” and the master decides how best to share out the load. It’s typically used for similar situations like the one you describe, but on a bigger scale - there’s one in Runcorn for a furnace that makes insulation, the incoming supply is rated for 1000A but if every thyristor was on 100% it would pull double that.

So, potentially I could make a box with a dedicated socket for a 20a heater element.
And a pass-through for all the other equipment.
Monitor the downstream load and allow the heater to use anything from 0a to 20a?

 

[daleyd]

So, potentially I could make a box with a dedicated socket for a 20a heater element.
And a pass-through for all the other equipment.
Monitor the downstream load and allow the heater to use anything from 0a to 20a?

Yeah pretty much. You can use something like a little 1/8 din controller, doesn’t need to be anything fancy. Or you could make it as fancy as you wanted.

The principle is to just measure the current going out as a total and then let the thyristor make up the rest.

Edit - more accurately you want to measure the total current coming IN.

 

[daleyd]

The important bit is to have something that controls on phase angle, if working in burst mode (eg on/off) the current won’t be controlled, so wouldn’t be suitable for what you wanted.

 

[daleyd]

I might have some suitable bits if you want to have a try? I’ll have a look round tomorrow….