nuggetchris
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Switching ON/OFF High Current Load

Sat Jul 14, 2018 1:30 am

Hi! For a project I am designing a system of very high-power electromagnets. Each electromagnet is fed a current of around 75A. To allow them to cool, I alternate between feeding power between one and the other around every 10 seconds or so. The device is controlled by a master raspberry pi or arduino and I am struggling to figure out an effective relay type switching device that won't break the bank. Any help?

MarkDH102
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Re: Switching ON/OFF High Current Load

Sat Jul 14, 2018 6:03 am

I did a commercial job a few years ago working on testing components within the joint strike fighter F35 aircraft.
We used 75A -> 150A CONTACTORS. However, I have just done a quick google search and they are - for you - I think prohibitively expensive.

mosespi
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Re: Switching ON/OFF High Current Load

Sat Jul 14, 2018 8:45 am

Solenoid type switches/relays sold for starters, golf carts, etc.. are fairly common. They can usually do 100-300A. Fairly low cost.. $10-$30 US around here. Ask for one at a local automotive parts store. You can likely drive it with another relay or a big transistor.

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-Moses
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pcmanbob
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Re: Switching ON/OFF High Current Load

Sat Jul 14, 2018 9:11 am

No mention of the operating voltage of these high-power electromagnets, the relay/contactor that you need to switch 75A is also dependent on the voltage.

using a golf cart contactor on mains voltages for example could end in a fire, you need to take both operating voltage and current into consideration when selecting your switching relay/contactor.
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W. H. Heydt
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Re: Switching ON/OFF High Current Load

Sat Jul 14, 2018 4:12 pm

Hmmm.... High current... Lawrence Berkeley Lab used to have to notify the local electric utility before switching on or off the magnet for the 188-inch cyclortron. The room between the poles was about 30 feet across, 6 feet high, and had a constant 37.5 Kgauss feild.... They also had to notify the Field Free Lab, about a mile away, before doing so.

gkaiseril
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Re: Switching ON/OFF High Current Load

Sat Jul 14, 2018 6:08 pm

You might want to be more specific about the amperage (current) and voltages you are expecting to try to control.

High is a relative term. Since the Pi is about a 3 amp device, a 30 amp circuit could be high to you. But for someone working with an electrical panel for a house 200 amp would be normal.

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Z80 Refugee
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Re: Switching ON/OFF High Current Load

Sun Jul 15, 2018 9:14 am

nuggetchris wrote:
Sat Jul 14, 2018 1:30 am
Hi! For a project I am designing a system of very high-power electromagnets. Each electromagnet is fed a current of around 75A.
What voltage? AC or DC?
nuggetchris wrote:
Sat Jul 14, 2018 1:30 am
To allow them to cool, I alternate between feeding power between one and the other around every 10 seconds or so.
That's nuts. Run both magnets at the same time on half the current (or use a magnet that can handle the load continuously).
nuggetchris wrote:
Sat Jul 14, 2018 1:30 am
The device is controlled by a master raspberry pi or arduino and I am struggling to figure out an effective relay type switching device that won't break the bank. Any help?
What do you call "breaking the bank"? How can we advise if you don't specify?
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nuggetchris
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Re: Switching ON/OFF High Current Load

Sun Jul 15, 2018 6:14 pm

Hi, here is some more information:

It is an inductive load of about 75A delivered over 12VDC to the electromagnet.
Switching would occur every 10 seconds or so, with magnetization/demagnetization completing in about 0.5s to 1s.

I would not pay more than about $15 for a single switch - each circuit has four and there are two circuits.

If I were to run each magnet simultaneous at half current I would be accepting half force from the magnets. I need full force supplied by the device at all times. There are two magnets in each device so that one can cool while the other is on, solving overheating yet maintaining a consistent support.

At first I considered the use of starter relays, but found them higher in the cost range and users informed me they aren't designed for continuous load and a high frequency of switching.

Then I considered MOSFETs, which seemed to work well. Still working that out now

pcmanbob
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Re: Switching ON/OFF High Current Load

Sun Jul 15, 2018 7:27 pm

With a 12V DC inductive load of 75A I think you will find you will soon suffer contact wear/failure if you use a mechanical relay, and switch that often.

so transistor switching may be your best option, just make sure you put protection in place to handle the large reverse voltages that could be generated by the collapsing magnetic fields.

It might also be worth including opto isolation between the pi gpio and your switching circuits.
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Re: Switching ON/OFF High Current Load

Sun Jul 15, 2018 10:31 pm

nuggetchris wrote:
Sun Jul 15, 2018 6:14 pm
If I were to run each magnet simultaneous at half current I would be accepting half force from the magnets.
Are you sure about that? Two magnets each contributing half the normal force, when either one of them can do the job on its own...
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nuggetchris
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Re: Switching ON/OFF High Current Load

Sun Jul 15, 2018 11:52 pm

Transistor switching vs MOSFETs, what is the key difference if any?

I am new to circuitry aspects such as these.

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Re: Switching ON/OFF High Current Load

Mon Jul 16, 2018 8:35 am

nuggetchris wrote:
Sun Jul 15, 2018 11:52 pm
Transistor switching vs MOSFETs, what is the key difference if any?

I am new to circuitry aspects such as these.
Transistors are current driven. To gate 75A you would need to provide a base current in the scale of amps. I don't think I have ever seen a 75A transistor. Power electronics uses MOSFETs (which are voltage driven, but power MOSFETs also have large gate capacitance to deal with). You can't just cobble this sort of thing together successfully without a great deal of study, and I'm not prepared to put in that much effort on your behalf. Buy ready made.

75A @ 12V is the sort of power that is used to start a car. That's where to look (and the scale of engineering you need). But I still think this is a fool's errand. You are creating complexity for the sake of complexity, and with complexity comes unreliability.
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pcmanbob
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Re: Switching ON/OFF High Current Load

Mon Jul 16, 2018 9:23 am

The best option might actually be a solid state relay

something like a Fotek-SSR-100DD-Solid-State-Module-100A-DC-DC.

https://www.ebay.co.uk/itm/Fotek-SSR-10 ... c+ssr.TRS0

running it at 75A you might need to mount it on a suitable heat sink.
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Burngate
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Re: Switching ON/OFF High Current Load

Mon Jul 16, 2018 9:28 am

As Z80 Refugee says, bipolar transistors are current-driven.
The ratio of (current ito collector / current into base) is called Hfe, and is quoted on the data sheet for the particular device.
However, as the collector-emitter voltage drops towards zero the Hfe also drops, so you need to put more current into the base.
That means you're going to have to switch a large current into the base!
- and with these sorts of current you want the voltage to be as close to zero as possible, because volts times amps is power, and that's just going to heat everything up towards destruction!
- and don't forget that the base current is not at zero volts - base-emitter voltage will be greater than 0.7v with the attendant power

FETs - Field Effect Transistors are voltage driven.
The higher the voltage between gate and source, the smaller the resistance between Drain and source.
A typical device is IRF7787 http://www.farnell.com/datasheets/2042984.pdf
If you put 10V between gate and source, the drain-source resistance will be (typically) 7mΩ. Raise the gate voltage and the resistance drops further.
At 50A and 7mΩ that's about ⅓W

Another thing to remember - it takes time to switch a device on and off.
Switching it on won't be so bad with an inductive load, but while it's switching it off, both the current and voltage will be significant.
Half way, with the volts at 6V and current at 30A, you're dissapating 180W - you don't want that to happen for too long!

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