New Information
The results of the previous experiment were troubling, as they didn’t support conventional wisdom regarding fast contact opening. To gain more knowledge we will try the experiment again. This time using the large 3-phase contactor as shown in this picture.
Note that it was necessary to remove the bidirectional diode assembly from the contactor’s body. This diode can be seen on the box next to the contactor. Also, it was necessary to add an auxiliary contact module to the top of the contactor. This provided access to the N.O. and N.C contacts.
The results are stunning as shown in the next figure. With the bidirectional diode removed and a 1N4001 connected across the contactor’s coil, it took about 105 ms for the relay to open. The “flight time” defined as the time it takes for the N.O. contact to open and the N.C. contact to close was about 10 ms.
Improved Response
Rather than follow the L/nR method, I used the active clamp as introduced in a follow up article involving an Active Clamp MOSFET Driver. The results are shown here.
Observe that the horizontal time frame is not the same between these two images.
When compared to the flyback diode, the active clamp opens the contactor in 18 ms. That is nearly a six-time speed increase. The flight time is reduced by a factor of 2 to about 4.5 ms.
Conclusion
Myth confirmed!
The opening time for a contactor is very much dependent on quick demagnetizing of the coil. The velocity appears to be faster which should result in faster arc extinguishing for contacts that are under load. Finally, as shown in these arguably limited experiments, the physics of the mechanical assembly matters. It would appear the effect is greater for the large contactor.
Best Wishes,
APDahlen