Three-Coil Permanent Magnet Actuator: How to Make the Opening Speed Fast First and Slow Later - Blog

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Core Principle

For a three-coil permanent magnet actuator to achieve an opening speed that is "fast first and slow later," the core is to coordinate the electromagnetic force, reaction force, and buffering in stages along the stroke, rather than keeping the opening coil under strong excitation throughout. In the initial stage of opening, a relatively high just-after-separation speed is required to shorten the arcing time; in the final stage, the speed must be limited to reduce impact and rebound. The mechanical characteristics of a circuit breaker themselves have different requirements for "just-after-separation speed" and "end opening speed," and this is the basis for staged control.

How to implement segmented control?

The first stage relies on "strong excitation at startup." The capacitor energy storage discharges rapidly into the opening coil, causing the coil current to rise quickly. Under the action of a relatively large electromagnetic force, the moving iron core overcomes the permanent magnet holding force and the contact spring reaction force, forming a relatively high initial acceleration. At this time, the opening coil should be under full voltage or short-time strong excitation, but it must not be maintained for a long time; otherwise, the speed in the later stage will continue to rise.

The second stage relies on "weak flux or current cut-off." After the moving iron core has gained kinetic energy, PWM chopping, current limiting, or rapid cut-off of the opening coil current is used to reduce the electromagnetic force. In the three-coil structure, the auxiliary/holding coil can participate in magnetic flux adjustment according to the magnetic circuit design to correct the resultant electromagnetic force. The specific polarity and timing must be determined by the magnetic circuit and prototype tests and cannot be simply applied by analogy.

The final stage relies on mechanical buffering. A hydraulic or spring buffer absorbs the kinetic energy of the moving iron core, while the contact spring and permanent magnet force participate in deceleration, causing the speed curve to decrease in the latter half of the stroke. Ultimately, the opening speed curve should be measured with a displacement-speed sensor, and the capacitor voltage, discharge time, PWM duty cycle, and buffer parameters should be set so that both the just-after-separation speed and the end speed meet the requirements.

Products Description

The factual basis is that the dynamic characteristics of a permanent magnet actuator are determined by the combined electromagnetic-mechanical action, and both the opening coil current and buffer parameters are controllable and measurable.

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