The hysteresis coupling and the permanent magnet coupling are both non-contact magnetic couplings — yet they differ fundamentally in torque behaviour: the PMC runs synchronously and slip-free up to its pull-out torque; the hysteresis coupling delivers a constant slip torque independent of speed and slip, and may slip continuously. Both transmit torque contactlessly via a magnetic field — without any mechanical connection between drive and output sides.
Choosing the wrong design risks either uncontrolled overload or inaccurate speed synchronization—depending on the specific requirements of the application. This article explains both operating principles, compares them side by side, and provides clear selection criteria for practical use.
Key takeaway: Permanent magnet coupling (PMC) = synchronous, efficient, precise speed transmission, abrupt overload response. Hysteresis coupling = constant slip torque regardless of slip and speed, smooth start-up, ideal for capping machines, winders, and torque limiters.
Permanent Magnet Coupling: Function & Features
In a permanent magnet coupling, permanent magnets arranged alternately on the drive and driven rotors are positioned so that their magnetic poles face each other and attract one another. When the drive rotor rotates, it pulls the driven rotor along in sync—pole to pole—without any mechanical contact.
The transmitted torque is load-dependent: It increases with the load until it reaches its maximum—the slip torque. This slip torque depends heavily on the air gap between the rotors; a larger gap reduces the transmissible torque disproportionately.
If the slip torque is exceeded, the magnetic coupling breaks abruptly: The coupling slips completely, disconnecting the drive and the driven components. This behavior acts as a built-in overload protection mechanism, but it also means that there is no smooth transition in the event of an overload.
- Advantages: Precise speed synchronization, high operational efficiency, reliable overload protection, and good scalability up to high torques.
- Disadvantages: Abrupt overload behavior, torque highly dependent on the air gap, continuous slip operation not possible.
Hysteresis Coupling: Function & Features
The hysteresis coupling operates on a fundamentally different principle: A magnetic rotor (drive side) rotates relative to a ring made of hysteresis material (output side). The changing magnetic field continuously magnetizes and demagnetizes the hysteresis ring—this constant reversal of magnetization generates the transmitted torque.
The key difference: The torque generated in this way is almost constant and independent of slip and speed. Whether the input and output sides run in sync or experience continuous slippage, the torque remains stable at the set level. Slippage is a design feature and poses no risk as long as the resulting heat remains manageable.
- Advantages: Smooth, vibration-free startup; defined and stable slip torque (ideal for sealing machines, winders, and web tension control); continuous slip possible; torque-limited drives without abrupt shutdowns.
- Disadvantages: Continuous slippage generates heat and reduces efficiency; typically, these couplings can transmit lower torques than permanent magnet couplings, and they do not provide exact speed synchronization.
Comparison at a Glance
The following table directly compares the key features of both types. Both are based on contactless torque transmission, but they differ fundamentally in terms of torque curve, slip behavior, and primary application.
| Feature | Hysteresis coupling | Permanent magnet coupling |
|---|---|---|
| Torque curve | Constant, independent of speed | Load-dependent, synchronous |
| Slip | Continuous slip possible | Only until pull-out |
| Acceleration characteristics | Soft / gradual | Direct (synchronous) |
| Overload behavior | Smooth (continuous slip) | Abrupt (magnetic pole break) |
| Speed synchronization | No (slip) | Yes (exactly) |
| Efficiency | Low (slip heat losses) | High |
| Typical torque range | Low torques | Small to large |
| Typical application | Capping machines, winding machines, torque limiters | Pumps, agitators, precision drives |
Choosing: Which type for which situation?
The choice between the two designs depends primarily on whether the application requires exact speed synchronization or a defined, stable slip torque.
Choose the permanent magnet coupling if …
- synchronous speed transmission without slip is required,
- high efficiency and low losses during continuous operation,
- precise positioning or a constant output speed are required,
- the application requires hermetic separation via a containment shell (e.g., chemical pumps, agitators).
Choose the hysteresis coupling if …
- a defined, constant slip torque is required, regardless of speed and slip,
- a gentle, vibration-free start is important (e.g., delicate goods, plastic film),
- a constant web tension must be maintained during the winding or processing of materials,
- overload situations should be absorbed smoothly and continuously, without the coupling disengaging abruptly.
Typical applications
Permanent magnet couplings dominate wherever hermetic sealing and synchronous transmission are required simultaneously. Classic applications are hermetic pumps and agitators in chemical, pharmaceutical, and food processing, where the air gap is realised via a containment shell. The containment shell design and its impact on material selection and efficiency are discussed in detail in the article Magnetic coupling containment shell.
The hysteresis coupling is the preferred choice for capping machines (consistent tightening torque regardless of capping speed), winders and unwinding systems (uniform web tension), as well as torque limiters that must reliably maintain a constant torque even under continuous slippage. TEA's permanent magnet couplings (PMKC) cover a wide torque range and are also engineered for custom applications on request.
Practical tip from TEA:
As a general rule: if an overload must be absorbed with a smooth, continuous slip while torque stays constant, a hysteresis coupling is the right choice. If instead the overload event should trigger a clean cut-off and speed synchronisation must be maintained right up to that limit, a permanent magnet coupling is the better choice. If in doubt, it is worth consulting our application engineers — the design depends on torque, slip frequency, and thermal load in each specific case.
How the permanent magnet coupling compares to mechanical sealing solutions is covered in the guide Magnetic Coupling vs. Mechanical Seal.
Find the right magnetic coupling?
Our engineers can assist you in choosing between a hysteresis coupling and a permanent magnet coupling — from your initial inquiry through to a fully engineered solution.
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