Hi
I'm hoping someone could head me in the right direction here, I am building a control kit for an Electromagnet used to lift steel off a crane hoist, the existing set-up was destroying the batteries (2 x 12v 110ah in series) due to poor design of a tx 24v charging the batteries unregulated.
I am using a SSR NC to short out a resistor which gives the magnet full power, I have fitted a momentary push button which opens the SSR bringing a 10ohm resistor in series with the magnet thus dropping it field strength.. more info than you probably need, anyway basically what I need is some form of flyback diode to protect the SSR from the voltage spike when power is removed/lost to the magnets.

The electromagnet is 24v DC, it is pulling approx 11amps when powered, I have indulged in a little research but electronics is not my strongest field here but seem to have been directed to a vishay stud diode but I just need a professional take on my scenario to get the sizing correct if indeed I am heading in the right direction to start with.
Other useful info is that their is a few seconds delay on the magnet release when power is disconnected, I am unsure why and it may be something built into the magnet itself .. I would prefer that this delay is not extended so any diode choice need to take this into account if possible.

 

You could try a 25Amp bridge rectifier as they will be quite robust.
Connect the "+" terminal to the +ve supply volts that the electromagnet is connected to, and the "-" terminal to the other end of the coil that goes to the most -ve end of the coil.
Don't connect the "AC" terminals to anything, or just together.
This will give you 2 series pairs of diodes in parallel across the coil. Most of these bridges have a metal case and a handy mounting hole.
The net result is a 50Amp "diode" across your coil.

 

You could try a 25Amp bridge rectifier as they will be quite robust.
Connect the "+" terminal to the +ve supply volts that the electromagnet is connected to, and the "-" terminal to the other end of the coil that goes to the most -ve end of the coil.
Don't connect the "AC" terminals to anything, or just together.
This will give you 2 series pairs of diodes in parallel across the coil. Most of these bridges have a metal case and a handy mounting hole.
The net result is a 50Amp "diode" across your coil.

Cheers for that, never even considered a rectifyer tbh and yes does sound like a solution just seen a 50amp / 1000v for about £3.50 so even better and like you say can bolt it straight down and tap a few spade lugs if it.

 

Let us know how it works out. I reckon it is a good solution all round

 

Your back emf Voltage depends on the Current in the coil and its Inductance, your diode wont need to be passing 11amp, i would be looking at Ultrafast diodes, like Uf54001

 

Other useful info is that their is a few seconds delay on the magnet release when power is disconnected, I am unsure why and it may be something built into the magnet itself .. I would prefer that this delay is not extended so any diode choice need to take this into account if possible.

Any diode across a large magnetic device such as this is going to impart a delay on release, it should normally be less than 1 sec.
Max.

 

It was getting me thinking how a big snubber could be helpful.

 

A resistor in series with the diode will reduce the time for the magnet to release.

 

Thanks for the replies, thinking about it the steel loads will be on ground when magnets are switched off so a small delay is here nor there, just didn't want it to be excessive but the odd second is no real problem.
I gonna try the rectifier route as it has a few other benefits regarding mounting and connection solutions, given these big steel shoes take some bashing it's a bonus to have things tightly bolted down shock resistant.

 

Most lift magnets are fitted with a short reverse polarity to avoid the nuisance of residual attraction retaining the load, but if the load is quite heavy, it may not be a problem.
This is usually a concern on scrap yard cranes etc.
Max.

 

Most lift magnets are fitted with a short reverse polarity to avoid the nuisance of residual attraction retaining the load,

And most of them don't operate on 24V, more like 300V.

 

And most of them don't operate on 24V, more like 300V.

yes, Ohio Locomotive Cranes, 240vdc generator.
Max.

 

yes, Ohio Locomotive Cranes, 240vdc generator.
Max.

My oldest grandson worked for a place called Winkle electric for a year or so, they build and recondition lift magnets, and he was always showing me pictures of the jobs he worked on. I'm beginning to think this thread involves a school project, not an industrial one.

 

These are standalone units, just a big steel box with electromagnetic base plate, they are powered from 2 x 100ah leisure batteries, they are a bit old hat hence they charged the batteries very crudely by plugging in 230v which was stepped down to 24v DC, other than a rectifier to change AC to DC their were no other electronics to control of drop out the charge hence they were ruining the batteries, it was costing them 1k a year per magnet in batteries... I have rebuilt all the control and now have an external smart charger do the actual charging and revamped the controls to give a better operation and more robust to the environment and physical abuse these things get.

 

Your back emf Voltage depends on the Current in the coil and its Inductance, your diode wont need to be passing 11amp, i would be looking at Ultrafast diodes, like Uf54001

A good rule of thumb is the diode needs to be rated at least the same as the coil current, preferably more. The magnetic field is built up by the coil current and when the current is switched off, it collapses generating a back EMF that can instantaneously supply a large current via the diode. A 1Amp diode will most likely pop. This is a case where overkill is a real good idea!
When I was working at a short wave transmitting station, there was an interesting fault. An over voltage spark gap fed to a resistive divider consisting of a couple of 300W plug in resistors. The low volts tap then went to the gate of an SCR that operated a contactor to drop the power off the 30KV DC supply to the 100KW final. Well, the first time it arced over, the SCR triggered and dropped the power. Then after a short delay, the TX came back on air. The station boss was wandering past just as it arced again, and he bent down to hear the arcing noise. A very loud BANG made him jump, much to our amusement!
He then went back upstairs to his office to calm down.
What had happened was the 300W resistors had blown to pieces because the diode across the contactor coil was only a 1Amp one, and the back EMF generated current had reduced it to a short following the first splat. The second arc triggered the SCR into a short so it died and the control volts overload dropped the transmitter off, eventually. After finding all the hot bits of resistor, and replacing the resistors and the now blown SCR and diode with a MUCH bigger one, it all worked again. It is a lesson I have never forgot, along with the boss leaping into the air

 

@dendad
I'm defo good in that sense, the load itself is 11amps max, the rectifier I'm using is putting 2 x 50amps in parallel so more than enough and then some, at a 1000v rated it should cut the mustard and have plenty to spare.

 

i would be looking at Ultrafast diodes, like Uf54001

Ultrafast diodes have no advantage as a snubber in this application.
Such diodes have a fast turn-off, but it's the turn-on time that's of interest here, and a standard diode turns on as fast as an ultrafast type.

 

Most lift magnets are fitted with a short reverse polarity to avoid the nuisance of residual attraction retaining the load,

That possibly could be provided by using a large capacitor as a snubber, which would give a resonant reverse current pulse through the coil.

 

That possibly could be provided by using a large capacitor as a snubber, which would give a resonant reverse current pulse through the coil.

In this case maybe. But might be a little large for a 230v 100amp scrap yard magnet!
Max.

 

Look at the diode 150EBU04.
Spice model:
.model 150ebu04 d is=0.1u n=1.2 rs=1m isr=10u nr=2 bv=400 ibv=40u nbv=50 m=0.2 cjo=300p tt=60n iave=150A vpk=400V mfg=international_rectifier type=silicon
If the supply voltage is 1000 volts, then use a serial connection of three pieces.