Energy Content and Electromagnetic Force in Capacitors

Energy Contents / Force Manifestations

Figure C1-26. Magnetic force action, F, between conductors with a current flow, I. B = magnetic flux density.

If

• the conductor length l is expressed in m,
• the current I is expressed in A and
• the distance a is expressed in m,

the force per meter between the conductors will be

………………………[C1-21]

According to the formula (C1-1) Q = C x V (As). If this expression is derived we obtain dQ/dt = I = C x dV/dt (A). Pulse loads are not unusual, especially in conditions with high voltage gradients, and thus high charging currents also occur which might cause appreciable magnetic fields between close lead patterns, for example.

C 1.4.3   Force action in electrostatic fields

Capacitors are typical examples of applications where electrostatic fields are applied. These fields can generate significant mechanical forces­. If we know the electrode distance d (m) it’s easy to determine the electric field strength E (V/m). Then we can outline the force per unit area, i.e. the pressure that the electrodes exert on the dielectric.

……………………………….. [C1-22]

Example. Suppose we have an oil impregnated paper capacitor with r = 5 and the dielectric = 15 m (0.6 mils) which is loaded with 250VAC. Then the instantaneous maximum pressure will be

0.1 kp/cm² !

If we instead calculate on a 35 V solid tantalum capacitor with a typical and approximate dielectric thickness of 0.2 mm (0.008 mils) the formula gives at 30 V DC a pressure of

2 N/mm² !

It is difficult to determine how much the dielectric is influenced by such forces, especially when the electrodes have such complex configurations. Electrostatic action of such forces here is of vital importance.

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