How to take advantage of the magnetic slip mechanism

Jon Lawson

Magnetic gears have the additional functionality of a slip mechanism. In many applications this slip function can provide protection to other parts of the system. For example, magnetic gears are used in the Zilift Torque Drive system to couple a progressive cavity pump with a high speed permanent magnet motor. As well as providing a gear ratio between the motor and the pump the magnetic gear protects the whole system from damage if the pump becomes blocked with debris. When this happens the magnetic gear simple slips without damaging itself or any other parts of the system. Once the slip is noticed by the operators they can intervene with motor reversals to clear the blockage. 

The slip torque level is fully customisable during the electromagnetic design stage. Typically, the slip torque is higher than the largest torque value seen by the application being targeted. A specific slip-torque value can also be accommodated during the design phase. 

The magnetic gear has 2 rotors, the input high speed rotor (HSR – low torque side) and output pole piece rotor (PPR – high torque side) – this is the output rotor of the magnetic gear which connects to the load. For torque to be transmitted from the input shaft (HSR side) to the output shaft (PPR side), a relative angular displacement between the HSR and PPR needs to exist. Figure 3 shows the torque characteristic for a magnetic gear designed for an actuator. The transmitted torque of the gear is increased with the relative angular displacement between the 2 rotors. When the angular displacement between the two rotors reaches 90 degrees, the maximum torque transmission capability of the gear will be reached and the pole-slip behaviour will occur. This is an ‘abrupt phenomenon’ and is passively triggered. The magnetic gear will then re-engage normal operation when the excess torque has been removed from the output shaft.

The other figures show the prototype of a gear going through a pole-slip event, where the torque on the output shaft (PPR) is higher than the maximum torque transmission capability of the magnetic gear. During overload, sinusoidal torque pulsations will be present on both the PPR and HSR, with the amplitude being equal to the pull-out torque of the device. Basically, the magnetic gear is constantly trying to re-engage normal operation and transmit power between the input and output shafts, but cannot do that until the overload condition has be removed. Thus, during overload it is constantly going through the stable and un-stable cycles, until the overload is removed.  

Figures 4 & 5 show what happens to the speed of the two rotors of the magnetic gear. During overload, the PPR is stopped (i.e. 0 rpm triggered by a high torque demand on the output shaft), while the HSR continues to spin at a constant angular velocity. When the overload is removed, the gear re-engages normal operation and the speed of the load increases back to the demanded value. 

This functionality is also being used in actuator systems that incorporate a magnetic gear into a Pseudo Direct Drive or PDD. Instead of the there being a clutch or torque fuse again the actuator simply slips if there are foreign objects or if control is lost and the actuator hits the end stop.

A PDD is the combination of a magnetic gear and a permanent magnet electric machine. The magnet gear is mounted inside a normal wound stator. This stator has the same number of poles as the inner (high speed) rotor of the magnetic gear. This results in a very compact and efficient low speed high torque machine. This electric motor or generator also, of course, includes the slip characteristic of a magnetic gear. In order to protect the out array of magnets in the gear the stator has a relatively low current density. This brings further benefits in terms of long stator life and removes the need for secondary cooling circuits.

As well as having applications in actuators these PDDs make ideal generators for large wind turbines. They have excellent efficiency even at part load which is where the bulk of operations take place. As a result, they provide significant benefits in terms of the cost of the electricity generated. This performance benefit has recently been confirmed during testing at the ORE Catapult in Blythe.