Magnetic circuit breakers have DC threshold currents.  When magnetic devices are operated at their threshold levels, the trip or detection times are also “long” times.  Action on the onset of mechanical movement in a magnetic circuit breaker, however, is more abrupt than in a thermal circuit breaker.

Mechanical movement of the thermal element is evident up to the detection threshold value in a thermal breaker.  In a magnetic breaker, there is the possibility of no mechanical action, (i.e. no movement whatsoever) until the threshold current is exceeded.

From the equation of motion (4.1) we have that there will be a net accelerating torque on the armature at angular position θ = 0 whenever

where we have assumed that the latch mechanism is not engaged until θ>˭θ_latch.  We will term the torque value ƴθ₀, the breakaway torque T_B.  If we now plot the load torque – the restraining spring plus latch mechanism – as a function of armature angular position, we obtain a load torque locus such as that shown in Figure 4.5.

Note that we have approximated the load torque of the latch as a constant average torque, <T_latch>, over the operating range of latch mechanism.  We also show the load torque diminishing (dashed line) after the latch release angle θ_th.  In truth, we care little as to the actual behavior of armature after tripping, and therefore the true locus after this point is of no concern.

From Equations 4.9 and 4.11,

we see that the driving torque of magnetic attraction T_m is given by

The current needed to produce a torque of magnetic attraction equal to the breakaway torque will be termed the threshold current I_th, and is defined by the expression

Equation 4.12, the equation for the magnetic torque, can then finally be written in a compact form as

We can now plot, for a given value of frame reluctance fraction m and various values of constant coil current i, the value of magnetic drive torque T_m as a function of the armature angle θ.  These curves are shown, along with the load torque curve T_L, in Figure 4.6.

The difference between the drive torque T_m and the Load torque T_L at any value of armature angle θ, is the acceleration torque.  Once the threshold current I_th has been exceeded, the net acceleration torque T_m – T_L is seen to increase as the armature approaches its closed position.  This is a positive feedback effect, contributing to the “fast” characteristic behavior of magnetic circuit breakers.

In some magnetic breakers, the positive feedback effect is diminished somewhat by magnetic saturation of the armature iron.  As the armature gets closer to its closed position, the gap reluctance becomes smaller.  Thus, the total reluctance of the armature-gap path also becomes smaller, which for constant coil current, induces higher levels of armature-gap flux.  If this level of flux approaches the saturation level of the armature, the effective armature reluctance R_ca begins to rise, and thus the frame reluctance fraction m begins to rise as well.

The effect of a rising value of m, due to armature saturation for a constant coil current i, is shown in Figure 4.7. 

Here, curves of drive torque T_m at constant coil current i, but varying values of frame reluctance fraction m, are plotted as functions of the armature angle θ.  If armature saturation occurs as θ approaches θ_g, the actual trajectory of T_m would be along a curve, such as the one shown as T_m (sat).  The net accelerating torque over the operating range of θ is thus seen to diminish, if armature saturation is present.

We have defined the threshold current as that coil current which induces a value of magnetic torque of attraction equal to the armature breakaway torque.  There are situations, however, where this definition is inadequate.  For example consider the drive torque – load torque case shown in Figure 4.8.

Here, an impressed coil current is of sufficient magnitude such that

                                              T_m (θ = 0) > T_L (θ = 0)

so the armature would start to move.  But, at an armature angle value θ_A, less than the latch angle θ_th, the drive torque curve crosses over the load torque curve.  This cross over point, point A, is a stable operating point (net accelerating torque for θ<θ_A and net decelerating torque for θ>θ_A), and thus motion will stop at A.  The “trip” threshold drive torque curve for this situation is shown by the dashed line in Figure 4.8.

Situations such as the one shown in Figure 4.8 should be avoided in the design of magnetic circuit breakers.  Breakers designed as in Figure 4.8 will always exhibit a certain value of armature overshoot, beyond angle θ_A, due to the armature inertia.  If this overshoot is large enough, the latch mechanism could be tripped inadvertently.  Rather than depend on the uncertain impact behavior between an overshooting armature and the latch mechanism, the design of a magnetic circuit breaker trip-threshold should be based on the behavior indicated in Figure 4.6.  Namely, no armature movement should occur until the coil current exceeds the threshold value.  We can insure this type of response simply by requiring that the drive torque curve never cross-over the load torque curve at any value of θ, including θ>θ_latch.