Relay contact life

[rawelk]

When reading through manufacturer's specifications for electromagnetic relays there is almost always a 'mechanical life' spec, often an electrical life spec for non-inductive loads, and sometimes one for inductive loads.

In this case, I'm looking at a Phoenix Contacts #2961105 relay used in their new "PLC" line of extremely small DIN rail mounted relay terminal blocks. It also has what appears to be a Tyco/Schrack part# (REL-MR- 24DC/21), uses AgSnO contacts rated for 6A/250VAC, and is given a mech life estimate of 20 million operations.

They also have an 'opto' (transistor) output DC module #2965595 rated for 3 amps.

In my application (driving a small pneumatic solenoid valve; don't know the specs, but it is 24 VDC, and looked to be on the order of 10 to 15 watts) it'll be cycled on about 4200 times per hour with a duty cycle on the order of 20 to 40%.

The machine was originally equipped with relay modules, but I'm planning on replacing them with the opto modules - relays just aren't going to hold up

I'm wondering about several things ...

1). What does a manufacture consider 'life'? Is this similar to how bearing life is specified (i.e. - "B10" life, where 10% of the units are estimated to have failed by the specified time)?

2). Is there any rule-of-thumb for derating mechanical control relays subject to high cycle rates?

3). Solid state modules don't have a mechanical cycle life spec, and very few are given even an electrical one. For purposes of EM vs. SS estimated life comparisons I've typically used 300 million operations for a solid-state module. Does this sound like it is in the right ballpark?

 

[Mike2468]

4200 cycles per hour is more than once every second. The electrical life of a mechanical relay will be limited. The solenoid is inductive, the electrical life will be further limited.

Use solid state. Google on: odc modules -- for several vendors. Install a high speed diode across the solenoid for reduction in the surge during turn OFF. The cycle life of a solid state odc module is massive.

 

[rawelk]

Thanks, Mike.
Yes, indeed, 4200 operations per hour is 70 cycles per minute.

I will be using their #2965595 opto-isolated module for replacement of the existing EM relays, and the engineer I spoke with indicated they've built a suppression diode into the module.