hermetic relays

[lowcool]

Heres the scenario,came across a brazillian made ae9440es 3/8 hp compressor with a burnt out relay.25 ohm on the start and 4 ohm on the run,with windings totalling ok.at the time i didnt even know its capacity.So measured relay coil resistance and found it to be .5 ohm from memory.So found another old second hand unit with similar size winding on coil and found that it had same resistance as the old one.Fitted it and found current draw to be the same with compressor restarting well underload.I have done similar before.

What is the correct way to size a relay to match compressor motor windings?

[nike123]

If it is AC1 rated, than size it 6 times MRA.
If it is AC3 rated, than size it according to maximum absorbed power.

http://www.sea.siemens.com/step/pdfs/cc_3.pdf

[lowcool]

cheers nike,but i was refering to the type of relay fitted similar to your domestic type hermetic compressor.
thanks for the reply

[nike123]

cheers nike,but i was refering to the type of relay fitted similar to your domestic type hermetic compressor.
thanks for the reply

I know, these are usualy AC1 rated and their rated current should be 6 times maximum operating current.

[Frikkie]

We have generic replacement start relays available here for domestic and small commercial compressors. They work for a set HP range so you just buy the right one according to your compressor size.

[lowcool]

i always had the impression that relay coil resistance was relative to motor winding resistances as in as to when the start winding resistance equalled that of relay coil resistance that the relay would drop the start winding out of circuit.so i am wondering how are the resistances relative to each other.
cheers frikkie at the time of fitting the relay i had no idea of compressor horses,once the local wholesaler tracked down the ratings of the compressor and was quoted $85 exc compared to a $40 something part.i have used solid state relays etc with half reasonable success,but prefer original rating parts as compressor replacement down the track is very little say one out of ten compared to five out of ten

[nike123]

Ok, I was totally misunderstood you. I taught that you speak about relay for switching on/off compressor and not about starting relay. Sorry!

[Frikkie]

i always had the impression that relay coil resistance was relative to motor winding resistances as in as to when the start winding resistance equalled that of relay coil resistance that the relay would drop the start winding out of circuit.so i am wondering how are the resistances relative to each other.

The start relay is a current operated relay. The full load current of the compressor always flows through the relay coil so the wire in the coil of the relay will vary in thickness depending on the normal running current of the compresser. In theory the larger the compresser the thicker the relay coil wire will be and therefore lower the resistance the relay winding will be. I don't think you can draw too much from this realationship though, there are other variables involved especially if the relay is gravity operated back to its normal open condition.

It's about the relay coil being sufficiently magnetic at start current to operate the relay by raising the internal armature and being insufficiently magnetic to hold the armature agains gravity when normal running current is approaching.

With a relay coil it is about reactance and ultimately impedance more than pure resistance due to it being an inductive device. Also the resistance or reactance it presents is only relevant if measured at it's operating voltage and with the operating current flowing. The coils ability to be magnetic is not necessarily proportional to it's resistance in any way.

Most multi testers use a very low test voltage at the probes when measuring resistance and almost no current will be flowing during the test so their measurements are not a reflection of what happens under normal operating conditions. You can only test these devices whilst they are operating normally and the equipment required to do these tests is fairly speciallised.

I hope my English make sense, you can say if it makes you confused and I can try to tell it differently for you.;)

[lowcool]

thanks frikkie,no confusion at all.you have cleared up something that has always been at the back of the old brain box,as you say the only way to test is to run and as you say by specialised equip,my thoughts exactly.dropped in and had a look yesterday no overheating starts well,looks like i jagged it again.

cheers have a good weekend fella