Electronics > Projects, Designs, and Technical Stuff

Stored energy Induction motor

<< < (4/4)

Terry Bites:
MOVS are used to soak up the occasional transient not the every day ones.
As suggested suitably rated RC snubber will last a lot longer. You can put a MOV across that.
Buy a bigger contactor!

T3sl4co1l:
"AC arc suppression requires the use of a metal-oxide varistor (MOV) [or...]"

Good reference, it supports my point :)

Tim

floobydust:

--- Quote from: Jester on March 21, 2023, 11:30:21 am ---[...] Board space is limited so if I add a snubber it has to be SMT. SMT MOVs tend to be small Joule wise, hence my desire to determine Joules.

--- End quote ---

SMT capacitors 10-100nF rated 250-400V are expensive, huge and very few offerings. Vishay's biggest SMT X2 X7R cap is 12nF, 2220 case. So 5.6x5mm
Power electronics are necessarily big just for the HV spacings and heat. I think you're boxed in with making this Steve Jobs small.

Why is the relay so big? 40A resistive is typically good for 1-2HP motors (UL 873/508). Coil power will be around 1W. If you already have this big through-hole part on the board? so what's wrong with a snubber cap or MOV being through-hole?
If you use a smaller relay with a decent snubber it will last longer than a larger relay with wimpy arc protection.

Furnace and heater (i.e. hot tub, water tank) control boards in the IEC/UL safety standard have an endurance test. It's 100,000 cycles and usually specialty relays are used, you can see some datasheets in the same class are 30K or 100K cycle-rated, different contact alloys.
Other appliance relays, I have replaced zillions of them, there is no snubber at all. I have never seen a MOV used across contacts, out in the wild.

Pic of recently replaced cube relays on a 1/2HP motor, does this look like 5nF was enough? Manufacturer upgraded to 47nF.

Jester:

--- Quote from: floobydust on March 21, 2023, 06:06:33 pm ---
--- Quote from: Jester on March 21, 2023, 11:30:21 am ---[...] Board space is limited so if I add a snubber it has to be SMT. SMT MOVs tend to be small Joule wise, hence my desire to determine Joules.

--- End quote ---

SMT capacitors 10-100nF rated 250-400V are expensive, huge and very few offerings.

Why is the relay so big?


--- End quote ---

Agreed on the SMT capacitors. Small and affordable SMT MOVs are available so if I can find one that is "big enough" it makes for a simplified solution.

The relay is "big" because the primary purpose of this circuit is to trip the motor out (quickly) when it jams and when it jams the LRA is 30A.

The prototype works well from a functional perspective trips in about 8ms) and fits in a common 1110 utility box with the required creepage and clearance.  I might make more of these so I'm exploring cost and size reduction so if a 50c part will work adequately why use a $2 part? The relay is on the bottom of the board and a SMT snubber fits nicely on the top near the contact pins. A through hole snubber will require a larger board, and hence a larger enclosure I would prefer not to do that.

T3sl4co1l:
Not bad :)

I'm not even very impressed by SMT chip MOVs, simply because 7mm discs are much cheaper, and about the same layout area as equivalent chips.  The single-layer footprint does help sometimes though.

You can also get chips in smaller sizes, assuming that's adequate for voltage and energy ratings; an 0603 chip affords extremely robust ESD protection for example.  (Not that a 0603 TVS doesn't either, heh!)

Tim

Navigation

[0] Message Index

[*] Previous page