The electromagnetic powder brakes and clutches combines the resilience of a fluid clutch with the locked-in stability of a friction clutch. Torque is transmitted by a specially alloyed dry ferromagnetic powder.
The clutchs can sustain continuous slip (within their empirically established heat ratings)at a precise and stable torque value, which is determined by excitation level.
Slip between input and output level is not necessary in order to transmit torque, and provided load torque does not exceed the torque for which a unit excited to transmit locked-in synchronous operation will take place. Conversely, if load torque exceeds the energised torque level, slip will occur in an absolutely smooth manner at the predetermined torque value.
For all practical purpose, static and dynamic coefficients of friction are sensibly equel also output torque is indepentent of speed, or slip speed.
Powder performance is unaffected by temperature rise at the working surface, and clutchs will, at all times, have the characteristics that transmitted torque is directly proportional to current. It should be noted that use of dry powder rather than fluid- suspended powder medium promotes consistency and accuracy of torque control.
Briefly, the clutch comprises two concentric members, the body or input member containing the field coil, and within it, and separated by a small annular gap, an inner rotor, or output member. The annular gap contains a ferromagnetic powder which is activated when the coil is energised.
The resultant flux which is generated, passes through the powder, causing it to align with the flux path, and thereby creates a driving bond between input and output members, the strength of which is solely determined by the value of d.c. current applied to the field coil.
The torque transmitted by powder clutchs is proportional to field current, and is infinitely variable from maximum design rating down to practically zero in all models.
Torque/current characteristic curves can vary by 5% dependent on whether current is increasing or decreasing. This is due to magnetic hysteresis.
For all practical purpose torque is independent of speed, either with or without the presence of slip, and can be maintained within an accuracy of 5% for speeds within the recommended operating range 50 to 3000 rpm. Deenergised drag torque due to residual iron circuit magnetism, bearing and seal friction, is less than 1% of design torque rating for any clutch.
Response time to torque is determined by field coil inductance to resistance ratio plus a magnetic delay due to eddy current loss.
To ensure satisfactory operation clutchs and brakes must be mounted in horizontal attitude.
Power control board for magnetic particle brakes