I have a question on the effect of inverter technology on relay lifetime

[mamech]

question on effect of inverter technology on relay life time

hello

I knew that the life of the relay becomes less when there are sparks. so my question is: if I eliminate the source of these sparks, I could get much longer life of the relay , that exceed the described values in datasheet?

I ask about this because we are designing a board that may be able to control Air conditioning units, so I knew that most of air conditioners implements inverter technology , so this means that the load will be controlled using the soft starters (if I understand correctly) , which will not cause the sudden high current in the beginning.

moreover , we are intending to make on and off of the relay during zero crossing.

I conclude that the nature of controlling the load (using inverter), and the time of switching (during zero crossing) will enhance the life time much??

thanks

 

[BradtheRad]

Re: question on effect of inverter technology on relay life time

we are intending to make on and off of the relay during zero crossing.

This might be possible with a solid state relay but for a mechanical relay you may find it difficult. Once the coil is energized, you will need for the time to be predictable to the milli-second, as to the moment when contacts touch.

 

[kathmandu]

Re: question on effect of inverter technology on relay life time

For an inductive load (air conditioner compressor), zero crossing point of the voltage means (theoretically) maximum load current.

Practically, the load current might not be maximum but almost there thus you better switch the relay off during the voltage peak (when the load current will be almost zero).

But, like @BradtheRad did mention, the usual relay switch time it's in the miliseconds range. A 50 Hz AC voltage (current) has a 20 ms period thus it may take more than a period from the "on" command till the relay contacts will actually close/open.


PS: An inverter type A/C has a DC motor thus it might have some sort of PFC input stage thus the load current and the voltage might be "in-phase".

Anyway, a solid state relay would be best (as previously recommended).

 

[mamech]

Re: question on effect of inverter technology on relay life time

This might be possible with a solid state relay but for a mechanical relay you may find it difficult. Once the coil is energized, you will need for the time to be predictable to the milli-second, as to the moment when contacts touch.

may be this can be resolved by an early actuation signal for the relay so it compensates the delayed response of it?

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For an inductive load (air conditioner compressor), zero crossing point of the voltage means (theoretically) maximum load current.

regarding the existence of inverter as an intermediate stage between the power relay and the load, this makes the starting current of motor to be more limited (soft starting) and so this may cause longer life for the relay?
I think that the ratings that exist in relay datasheet does not take into account the existence of soft starters as an intermediate stage before the load.