Changing ac voltage to dc voltage in mgnetic coil

[mash98]

Dear guys we an electromagnet with ac coil having 400 volts/50 hertz while the same magnet draws one ampere current.This magnet consist of six coils connected in star.The Dc impedence of each coil is 9.5 ohms.Now I intent to connect the above same six coils in series so I will get roughly 57 ohms & apply 60 volts DC so to maintain the same turn-ampere ratio & expecting the same previous results as we were getting with ac voltages scheme.kindly advice with your valuble knowledge.

 

[7anoter4]

Is not so clear what kind of electromagnet it is about, but there are some remarks.
I think the electromagnet is for lifting purpose. So what is interesting is the lifting force.
Lifting force F=B^2*A/2/miuo
Where B is the magnetic flux density, A is cross section area of the iron core and miuo is the air permeability [a constant].
That means the Lifting Force is direct proportional with square flux density and in turn with the ampere-turns.
The 3 phase system exerts 1.5 times more force than single-phase.
If you could get the same ampere-turns in dc in the same 3 phase system of the iron cores you'll get 3 times the single-phase force.
I think the dc resistance was measured in a cold state-20 degres C, let's say.The coils could be 80 degrees C when work so the resistance may atteint 11-12 ohm per coil.
Never the less the force could be 2-2.5 times more than the previous 3 phase arrangement.

 

[roydm]

Something else to consider. With an AC coil the current drops considerably once the armature pulls in and closes the magnetic circuit. This won't happen with DC on the coil. Most large DC coils have an economy resistor to overcome this problem. A contact opens when the armature closed to drop the current to a safe level.
Regards
Roy

 

[waross]

What is the device? It can make a difference.

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

 

[mash98]

Thanks for all of your replies the electromagnet is being used for motor brake. Yesterday I could find time to do the experimenting with the magnet and ultimately successful in meeting the goal.I arranged the coils in such a manner to get around 1 ampere in every coil by applying 24 volts dc intially I was not successful but when I arranged the North and south poles alternatively on successive core faces(six coils) it made an stronger magnet and found suitable for my application.The power which the magnet drawing now is approximately 72 watts while on the earlier ac combination where I did not have the real power but the apparent one which was 690VA.Does somebody know the usual/normally found power factor of an ac magnet?

regards

 

[waross]

Well, reactive current in a transformer is often called magnetizing current. I am not surprised to see such a low power factor on a magnet. The lower the resistance of the winding (and the higher the efficiency) the lower the power factor will be.
Watch for problems with the brake sticking. It may not be an issue with a brake coil but many DC contactors have an air gap built in to the magnetic circuit to reduce the peak flux and aid release. The air gap does not have to be at the moving gap. It may be anywhere in the magnetic circuit.

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

 

[7anoter4]

As waross already stated it is a very low power factor.
I suppose the coil resistance is actually 12 ohms [as 9.5 was at 20 degrees C and for 90 degreesC is 12 ohms.] and in the DC arrangement the coils are connected 2 in series and all 3 in parallel so you get 8 ohms total].
As 1 A will passes through each coil will be 3 A total and the required power will be 8*3^2=72 w.
On 3 phases arrangement the dc resistance per phase will be 2*12 =24 ohms and the phase impedance 400/sqrt(3)/1=230.94 ohms. Then cosFi[PF]=24/230.94=0.104 .So, the required active power will be sqrt(3)*400*1*0.104=72 w.
If we shall neglect the iron losses [may be 1-3%] we'll get the same required power as in dc supply case. The force will be 2-2.5 times more, instead[in dc case, I mean].

 

[roydm]

mash98, It sounds as though you have the problem solved. I assume you only apply the DC while braking so overheating will not be a problem. Is it a disk brake?
Waross you pointed out that it might stick without an air gap, if that's the case would a weak current in the opposite direction help?
Roy

 

[itsmoked]

it might stick without an air gap, if that's the case would a weak current in the opposite direction help?

Certainly! (If sticking is a problem.)




Keith Cress
kcress -

http://www.flaminsystems.com

 

[mash98]

No the power is applied to release the brake,we applied the dc volts on the magnet only when the motor is running,yes it is disk brake.Now the brake has been installed on the motor from the last couple of days and it is working perfect.I have changed the the supply as I have only few failures with dc brake as compared to ac.
7anotor4 you are right about the connection scheme as well as abouut the calculations.

Thanks