Changing ac voltage to dc voltage in mgnetic coil
Changing ac voltage to dc voltage in mgnetic coil
(OP)
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.





RE: Changing ac voltage to dc voltage in mgnetic coil
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.
RE: Changing ac voltage to dc voltage in mgnetic coil
Regards
Roy
RE: Changing ac voltage to dc voltage in mgnetic coil
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Changing ac voltage to dc voltage in mgnetic coil
regards
RE: Changing ac voltage to dc voltage in mgnetic coil
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
RE: Changing ac voltage to dc voltage in mgnetic coil
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].
RE: Changing ac voltage to dc voltage in mgnetic coil
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
RE: Changing ac voltage to dc voltage in mgnetic coil
Certainly! (If sticking is a problem.)
Keith Cress
kcress - http://www.flaminsystems.com
RE: Changing ac voltage to dc voltage in mgnetic coil
7anotor4 you are right about the connection scheme as well as abouut the calculations.
Thanks