Modification in my shearing machine Magnet

[mash98]

Dear guys we have a 50 kg magnet on our shearing machine.The electrical statistics of the same are as under.
- Voltage 400 3phase 50 hz connected in delta.
- Peak current 160 amperes( measured by clamp meter with peak holding option) which is for some very short time i.e for some milliseconds I suppose.After the initial current surge the current reduces to around 5 amperes( holding current).the duty cycle of this magnet is around 10%( it switch ON for 6 -7 times a minute just for 1 sec).
- The wire gauge used is SWG13 while the weight of each coil is roughly 4 kg.Although the holding current seems reseanable but even for this current we have 4-5 coils failure per anum.these failures are always due to some mechanical stuck up in the machine where the plunger of the magnet does not secure its correct position in the magnet and hence the current increses beyond its safe limits and become the cause of coils failures.

Due to the above frequent failures I took intiative and redisgned the coils with DC voltage on the same bobins and magnet arrangements.The purpose behind this redesigning is to have better control when the mechanical system stuck up which is as follows.
- The same three bobins are being used but this time the wire gauge is SWG23 and the weight of each coil is about 6.25 kg as we have enough place in the bobin so I used the max available space.The dc resistance of each coil is 150 ohms. then we connected them in parallel with 360 volts dc supply.the electrical scheme comprises with resistor and timer in such a way that we intially applied the full 360 volts dc for 1 sec when each coil draws 2.4 amperes then after 1 sec as we place 50 ohms resistor in series with the coils and then the current reduces to half. The stroke and power of the magnet seems better than the original design.As we have not yet put the modified magnet in the machine so I would like to have your valuable comments, suggestions and advices about this magnet.

Thanking in anticipation.
regards

 

[edison123]

The magnetic pull is proportional to the amps x turns. Check if the new amp-turns is not less than the old amp-turns.

 

[mash98]

The turn-ampere product is definately higher as iam using a thinner wire with even more weight.

 

[edison123]

Post the old & new numbers.

 

[waross]

On DC, you will find that the amp turns is roughly proportional to the wire size. With a given wire size, doubling the amount of turns doubles the resistance and
halves the current. Net result, same amp turns regardless of the number of turns.
The pull of a magnet is also roughly inversely proportional to the length of the air gap. On an AC coil a long air gap reduces the inductive reactance and allows a heavy pull in current.
DC solenoids typically use a very heavy pull in coil and a high resistance holding coil.
You can probably use the AC coil on DC if you switch in a series resistance to limit the holding current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mash98]

dear edison unfortunately I dont have the number of turns but we can sure approximate that the ampere turn at least remains the same which we could witnessed with the magnetic pull.
The main reason to this change was the fact that in ac the current reduction depends on the position of plunger inside the magnetic coil if due to some mechanical problem it does not come into its correct position the current would remain very high and can burn the coils due to excessive heating while in dc arrangemnet the current limiting is done by putting resistance in series with the coil regardless of plunger positiion.

 

[edison123]

In the absence of amp-turns, I guess the only way is to eat the pudding and see what happens.

I know it's tough to redesign stuff. Please come back here and post the results.

Good luck.

 

[mash98]

dear edison sure I will let you know as soon as I get the results.
regards

 

[mash98]

It has been in opration from the last two weeks and working fine.Infact in the mean time the mechanical team had to adjust their part twice while earlier we usually been informed about the coil damage in such situations.

regards

 

[edison123]

Excellent. Congrats.