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Sign inMagnetising shot duty cycle
A magnetic particle bench delivers thousands of amperes, and almost all of that energy ends up as heat in the transformer, the rectifier stack, the contact heads and the cables. The equipment is rated by a duty cycle: the fraction of a repeating on-off period during which current may flow without the windings exceeding their thermal class. A 10% duty cycle at 5 000 A does not mean 10% of 5 000 A, it means current for one tenth of the time.
The rest interval follows directly. If the shot lasts t_on and the rating is D percent, the full period must be t_on x 100/D, so the machine must sit idle for t_on x (100/D - 1) before the next shot. Ignoring that is the usual cause of a bench tripping thermally halfway through a batch, and of contact heads and cable lugs annealing and losing their grip.
Shot length is bounded from below by the technique rather than the machine. In the continuous method the bath flows over the part and the current is applied while the particles are still mobile, with the flow stopped just before the current ends so the indications are not washed away. The current must be on long enough for the particles to migrate to the leakage field, which is why the standards call for a minimum shot duration of about 0.2 s and at least two shots.
Short shots are also why magnetising current is measured with a peak-reading or averaging meter built into the machine rather than a clamp meter. A 0.5 s shot does not give a handheld instrument time to settle.
Duty cycle D = on-time / (on-time + rest) x 100 Minimum rest = t_on x (100/D - 1) Cycle period = t_on + minimum rest Shots per minute = 60 / cycle period Continuous method: at least 2 shots, each at least 0.2 s
- Duty cycle is a thermal limit on the equipment, not a limit on the technique. Exceeding it trips the bench and anneals contact hardware.
- In the continuous method the bath flow is stopped just before the current ends, so indications are not washed away.
- Residual technique - magnetising first and applying particles afterwards - is only permitted on materials with sufficient retentivity and where the governing code allows it.
- Read the magnetising current from the machine's own peak-reading meter. A handheld clamp meter cannot settle in a half-second shot.
- Long parts and multiple directions multiply the shot count. Plan the rest intervals into the throughput figure.
Reference: Duty cycle is the equipment manufacturer's thermal rating; use the value on the rating plate. The continuous-method requirement for a minimum of two shots of at least about 0.2 s is given in ASTM E1444/E1444M-22 and ASTM E709-21; ASME BPVC Section V (2023 Ed.), Article 7 requires the continuous method with the current applied while the particles are still mobile.
