Eddy Current Magnetic Couplings – Eddy Current (Class 2)
This coupling is an asynchronous version that relies on a speed mismatch between the driver and follower to produce a force. An array of alternating pole permanent magnets (N-S-N-S) is placed on either the driver or follower, and an electrically conductive material (typically aluminum or copper) is placed on the mating component. When the driver is translated with respect to the follower, an electrical current is induced in the conductive material, which results in a magnetic field that opposes the permanent magnets and “couples” the two components. Amperes Law governs the relationship between the induced electric and resultant magnetic fields.
The magnitude of the resultant force is directly linked to:
1. Speed differential between the two components
2. Magnetic material characteristics
3. Resistivity of the conductive media
4. Separation distance between the driver and follower.
Unlike the Synchronous Coupling (Class 1), this asynchronous version is a “lossy” device and prone to Ohmic loss heating resultant from the induced electric fields. Eddy Current Magnetic Couplings
Eddy current couplings and brakes
Eddy Current Magnetic Couplings – In contrast to the drive and brake elements described previously, the torque in eddy current clutches and brakes is only produced by a relative speed between drive and driven sides.
Thus the transferable moment increases with the relative rpm. Fig. 10 shows the torque gradient for two different air gaps. In practice rings or segments in permanent magnet material e.g. HF 24/20 are magnetised on one side with several poles and on the other hand we find a copper disc 2-5 mm thick, which for magnetic reasons has a soft iron backing of 2-6 mm thickness. Table 4 compares the torques, which can be achieved by eddy current clutches and brakes for 3 different relative rpm. and various air gaps.
The given values relate to room temperature which was set by measurements of corresponding cooling of the copper discs. In eddy current clutches and brakes the temperature coefficient of the copper is considered along with the temperature coefficient of the magnet. Eddy current clutches and brakes heat up considerably due to the development of eddy currents with increasing rpm.; with temperature increase the value of the torque attainable decreases considerably. If cooling is not provided, temperatures up to 200°C at relative rpm of 1000/min can occur on the copper disc whereby the torque decreases by 50%. The losses thereby incurred are partly irreversible. They can only be recovered by remagnetisation. If the temperature is kept below 50°C, the decrease in torque is only about 10%. magnetic hysteresis couplings
In certain rpm ranges eddy current clutches show a roughly proportional or constant torque – rpm. – characteristic. This means, eddy current clutches are suitable for use in coil machines where constant band tension and constant band speed are required. For details please refer to special literature.
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