The most widespread forms of electromagnets of this type are electromagnets, presented on fig 1.3. As it follows from pictures, in examined cases there are two identical basic air-gap, in this connection a full electromagnetic force F is determined by a formula
F = 2 F 0 = 2∙5,1 ∙B 02∙ S 0 / μ0,
Fig. 1.3, a where В 0 − induction in a basic air-gap, Wb / сm 2; S 0 − equivalence cross-section ofeach of basic gaps, сm 2.
So, MMF, being on both gaps, determined so:
(w∙I)0 = φ (w∙I)П = 2∙δ0∙ B 0 / μ0,
where φ − coefficient, taking into account of MMF drop in steel and non-working gaps.
Induction В 0 with a glance of possible in exploitation lowering of MMF (w∙I)п = χ∙ w∙I, where χ ≤ 1, equals to: B 0 = μ0∙φ∙χ∙ w∙I / 2δ0
Permissible MMF w∙I of electromagnet coils is determined coming from its operation mode, terms of heating and presence of two coils, having a cooling surface 2 Scl.
a) Continuous running duty of Fig. 1.3, b electromagnet
For this mode next correlations, similar to got before, are correct. Resistance of one coil:
R = 10-4∙ρ∙π∙(1 + n)∙ dc∙w / 2 Sm
where w − total number of loops of both coils; Sm – cross-section of wire metal, equals to:
Sm = 2 fap∙m∙n∙ dc 2 / w.
So, general losses in resistance of electromagnet are equal to:
P = 2 R∙I 2 = 10-4∙ρ∙π ∙ (1 + n)∙ w 2 ∙I 2 / fap∙m∙n∙dc
On the other hand, Р is determined from correlation
Θper = P / 2 h∙Scl = P / [2 h∙ (Sex + α∙ Sin)].
Substituted Р and Scl from (1.5) into the formula Θ per, we will define the value of MMF of electromagnet:
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w∙I = 2√[104∙ fap∙m 2 ∙n∙ (1 + 2 n + α)∙ h∙ Θper∙ dc 3 / ρ(1 + n)]
the value of electromagnetic force
F = 8∙104∙μ0∙φ2∙ε2∙χ2∙ fap ∙τ2∙ m 2∙ n ∙(1+ 2 n + α)∙ h∙ Θper∙ dc 5/ [ρ(1 + n) δ02],
and key size of a core
dс = 5√{[103∙ρ∙(1 + n)∙ F∙ δ02 / [φ2∙ε2∙χ2∙ fap ∙τ2∙ m 2∙ n ∙(1+ 2 n + α)∙ h∙ Θper]}
Designating, as well as before,
C 1 = [2∙103∙ρ(1+ n)] / [φ2∙χ2 ∙fap∙ τ2 ∙m 2 ∙n∙ (1+2 n +α)∙ h∙ Θ per ], we will get accordingly:
F = 2ε2∙ dc 5 / (C 1∙ δ02) (1.42)
and dc = 5 √ [ C 1∙ F ∙δ02 / 2ε2] (1.43)
Transformation of the last formula gives dependence
F / δ03 = 2ε2∙χ5 / C 1, (1.44)
facilitating, as it was explained before, determination of dc = χ∙δ0. Thus under F they understand full force of electromagnet. In this case we determine:
1) MMF of coils
w∙I = (9∙103∙ dc / φ∙χ∙τ)∙√(dc / C 1) (1.45)
2) cross-section of wire
Sm = [2.82∙ρ∙(1 + n)∙ dc 2 / (φ∙χ∙τ∙ U)]∙√(dc / C 1) (1.46)
3) number of coil loops
w = U∙√ [103∙ fap∙n / ρ∙(1 + n)∙(1 + 2 n + α)∙ h∙ Θ per ∙ dc)] = C 2 ∙U∙√ (C 1 /dc) (1.47)
4) induction in a working air-gap
B 0 = (0,396∙10-4√ F) / (τ∙ε∙ dc)
approximately by a formula
B 0 ≈ (4∙10-5 / τ∙ 5√ C 1)∙√(F 3/ δ04)
b) Recursive short-time mode
Conclusions, similar to given above, determine next correlations for the electromagnets of this type:
F = 2 p 2 cr∙ε 2 ∙d 5 c / (C 1 ∙ δ02) (1.48) dc = 5 √ [ C 1 ∙F∙ δ02 / (2 p 2 cr∙ε 2)] (1.49)
F / δ03 = 2 p 2 cr∙ε 2χ5 / C 1(1.50)
w∙I = (9∙103∙ pcr ∙ dc / φ∙χ∙τ)∙√(dc / C 1) (1.51)
Sm = [2.82∙ρ∙(1 + n)∙ pcr ∙ dc 2 / (φ∙χ∙τ∙ U)]∙√(dc / C 1) (1.52)
w = (U/ pcr) ∙√ {103∙ fap∙n / [ρ∙(1 + n)∙(1 + 2 n + α)∙ h∙ Θ per ∙ dc)]}(1.53)
The coefficient of overcurrent pcr can be defined under the set size of relative duty ratio DR %: pcr = √100 / DR %.
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c) Short-time duty
In this mode, ignoring heat emission from a surface, they accept, that all radiated heat in a coil goes to heating of active material. For electromagnets with externalforward armature travel and two coils it is got a like § 1.1:
permissible current density in the cross-section of wire
j = I / Sm = w∙I / (2 fap∙m∙n∙d 2 c),
MMF of electromagnet
w∙I = 2 fap∙m∙n∙j∙d 2 c,
induction in a basic air-gap
B 0 = μ0 ∙ φ∙χ ∙ w∙I / 2δ0,
full electromagnetic force
F = 5,1π∙μ0 ∙ φ2∙χ2 ∙j 2 ∙f 2 ap∙m 2 ∙n 2 ∙ε 2 ∙τ 2 ∙d 6 c / δ02.
If to enter denotation
C3 = √[2∙103∙ρ / c∙ γ m ∙Θper∙ φ2∙χ2 ∙f 2 ap∙m 2 ∙n 2 ∙τ 2] ≈ 0,14 / (φ∙χ ∙fap∙m∙n∙τ), (1.54)
that we will get
F = 2 ε 2 ∙d 6 c / C 23 ∙ δ02∙ ton.. (1.55)
The key size of examined electromagnetic core in short-time duty is determined by a formula
dc = 3 √ [(C 3∙ δ0 / 1,41∙ε)∙√(F∙ton) (1.56)
Dependence of F = F (χ) is determined by correlation
√ F / δ02 = 1,41∙ε∙ x 3 / (C 3∙√ ton) (1.57)
from which it is determined by a method, considered before:
dc = χ∙ δ0
and accordingly cross-section of wire metal
Sm = 2π ∙ ρ∙(1 + n)∙ fap∙m∙n∙d 3 c / (U∙ √ ton)
and number of loops of two coils
w = U∙ C4∙√ ton / dc ≈ U∙ √ ton / (π ∙ ρ∙(1 + n)∙ dc) (1.58)
Induction in a working air-gap is calculated under the found values of dc, χ, ε or approximately by a formula:
B 0 ≈ (3,8∙10-5 / τ∙ 3√ C 3)∙ 3√[ F / (δ0∙√ ton)]