Synchronous or hysteresis coupling
Both types transmit torque contactlessly via a magnetic field, but they function fundamentally differently. The permanent-magnet coupling (synchronous coupling) has permanent magnets on both sides that couple pole to pole: It runs synchronously and slip-free until its pull-out torque is reached—beyond that point, the coupling disengages abruptly. The hysteresis coupling, on the other hand, transmits its torque via a hysteresis material that is constantly remagnetized in the rotating field. The torque generated remains nearly constant, regardless of speed and slip—continuous slip is designed into the system and is safe as long as the heat remains manageable.
Containment shell: hermetic seal
Between the inner and outer rotors sits a non-magnetic wall, the containment shell—it hermetically seals the pumped medium from the drive side and is therefore the safety-critical component. Stainless steel is the pressure-resistant standard for non-critical media, but it generates eddy-current losses in a rotating field (heat that increases with speed, field strength, and wall thickness). Hastelloy offers the highest corrosion resistance; ceramics are loss-free (non-conductive) but susceptible to brittle fracture; and PEEK is lightweight and corrosion-resistant at limited pressures.
Calculate slip power
When a hysteresis coupling operates with slippage, all of the power transferred during slippage is converted into heat—in the case of continuous slippage, this heat must be reliably dissipated:
P_S = M_Last · Δn / 9.55
P_S = Slip power (W) · M_Last = transmitted torque (Nm) · Δn = slip speed, difference between drive and output (min⁻¹)
Even 5 Nm at a slip speed of 500 min⁻¹ results in approximately 262 W of continuous heat—a magnitude at which the design no longer fails due to torque but rather due to heat dissipation. With a permanent-magnet coupling, this calculation does not apply in synchronous operation: Δn = 0, meaning there is no slip heat.
Mnemonic
Comparison with the mechanical seal
A mechanical seal seals the shaft against a moving sealing surface—by design, this results in low, but never completely zero, leakage, and as a wear part, it requires regular replacement. The magnetic coupling, on the other hand, has no shaft passage: As long as the containment shell remains intact, it is leak-free. For non-critical media, the mechanical seal often remains more cost-effective over the life cycle; for applications involving toxic, aggressive, or hard-to-access media, the total cost of ownership (TCO) calculation frequently favors the magnetic coupling.
Worked example: Slip power
A hysteresis coupling transmits M_Last = 8 Nm at a slip speed of Δn = 300 min⁻¹.
- P_S = M_Last · Δn / 9.55 = 8 · 300 / 9.55 ≈ 251 W
For verification: Magnetic coupling calculator (opens in a new tab)
Knowledge check
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