In the datasheet, they wrote:
Rated frequency of operation with / without load 6 / 1200 min-1
What's the signification of this line ?
When they talk about Mechanical endurance cycle, is it open to close or open to close to open
First establish what "rated load" and "minimum load" mean
(1) What does "rated load" mean ?
There are 12A and 16A versions.
Considering 12A version.
Max rated switching current is 12 A
Switchable voltage = 250 VAC or 440 VAC .
Max switched VA = 3000
SO at 250 VAC max switched i = 3000/250 = 12A as expected.
But at 440 VAC max amps = 3000/440 = 6.8A
ie it can be used at 12A at 250 VAc but must be derated to 6,8A if 440 VAC is used.
So raed load = 12A at 250 VAc or 6.8A ay=t 440 VAC.
(2) What does "minimum load" mean?
Some relays have a minimum rated switched current for eg contact wetting purposes. I could not see any minimum spec.
Assume that "minimum load" = no load.
" - Max. switching rate at rated- / minimum load - 6 min-1 / 1200 min -1" means
When loaded at 250 VAC, 12A OR at 440 VAc, 6.8A it may be switched at a maximum rate of "6 min-1" = 6 times per minute = not more than once every 10 seconds.
When loaded at zero amps - say perhaps 250 VAC present or 440 VAC present but no load then max switching rate = "1200 min-1"= 1200 times per minute = 20 times per second !!!
Like Russell says powered 6/min, unpowered 1200/min or 20/sec. Mechanical endurance is given as on/off cycles, so a relay with a 10K lifetime can be switched on and off 10000 times; it's not 5000 times on + 5000 times off.
20 cycles per second may seem a lot, but remember that common relays switch on or off in a few ms. So you would expect higher cycling frequencies than the 20 cycles per second. The reason that it's only 20 cycles is because re-energizing the coil too soon may cause the relay not to switch off between cycles, because the magnetic field may not have been deconstructed completely. The 20 per second is just a safe value.
For powered switching the determining factor is the heat generated in the contacts during switching on or off. Cycling at 6 per minute means there's 5 seconds between opening and closing, enough to let the contact cool down. (Keep in mind that while switched on there's also some heat generation in the contact, though less than during switching.)
If those contacts are power contacts and operated at "minimum load" = no load e.g. 2 mA logic.. A few % will fail to operate from corrosion. You need to apply a capacitor discharge circuit to maintain at least 10% rated current even if only for microseconds of cap discharge.
Having learnt the hard way on this in 1978 with a large box with twin contact power relays ( eg. 30A power and 1A aux contact) for a remote SCADA project in aerospace biz, using P&B and Aromat quality relays this failure mode is normal is due to inadequate wetting current.
No load contacts require Au flash plating and are called signal relays or telephony relays. Do not make the same mistake. As I recall I used 5V pullup 1MΩ and 10uF tantalum across the contacts to sense if the contacts were open or closed for these 1A aux contacts. I designed a software loop to sense AUX closure and go to next of 96 relays after verifying closure. It sounded like a gattling gun and then failed within a minute. I'd bump the cabinet and it would continue for a while then stop and fail at another random relay. I diagnosed the cause and then fixed the design. It was rated for 1e5 to 1e6 contact cycles. I verified 1e4 error free on the design change before release. Good enuf.
As far as the rate of switching that is another aspect altogether. At rated current, flashover can burn out the contacts, so rate must be slowed down to guarantee toggling on off.