I have a big 240v shop compressor in a compressor room. I'd like to add a circuit that would watch air pressure, room temperarture, and control the pressure that way (in addition to the two mechanical fail safes that are already there, of course).
But I have to guess there's a huge kick load when that AC motor starts. Could I really just drive an Atmel that runs a solid state relay that controls a big 1hp motor?
Or do I need a contactor?
Or do I need a "motor starter"?
It sounds like I ideally want a "motor starter" that I can control electronically. Does anyone know of such a thing?
A single phase 1hp motor draws around 8A @230V. The current drawn at startup will be significantly higher, as you said. Does the compressor have its own circuit off the distribution panel? What current is the fuse/breaker rated for?
If you can find a relay/solid-state relay/contactor with a similar or higher rating, I guess that would be safe.
For example this sold state relay has a 40A rating and could be controlled by the digital output of an Arduino.
I'd probably go-ahead and try a solid state relay rated for maybe 20A (or more). Maybe one like [u]this one[/u].
Electro-mechanical relays (contactors) tend to more "electrically rugged". If you stress them they'll generally have a shorter life but they usually don't "die instantly" like a solid-state relay can. Although they won't "last forever" like a solid state relay (when not abused) they can last a long-ling time... Probably longer than the compressor.
The biggest downside to a relay is that you need a driver circuit. If you get the right solid-state relay you can drive it directly from the Arduino. That's why I'd try the SSR first... It's easier... And if you fry it, it's not a huge expense considering the cost of the equipment you've got already.
Or do I need a "motor starter"?
It sounds like I ideally want a "motor starter" that I can control electronically. Does anyone know of such a thing?
If it doesn't have one now you shouldn't have to add one.
As an aside, I still don't get the difference between a relay and a contactor. They seem the same to me. If the contact rating is over X amps, its called a contactor...
PaulRB:
As an aside, I still don't get the difference between a relay and a contactor. They seem the same to me. If the contact rating is over X amps, is called a contractor...
The "contactors" on equipment I have had used an additional low power set of contacts in series with the coil. It is made for push-on and push-off buttons. The momentary on button closes the contactor and it stays on until the off button opens the holding circuit.
Low power switches such as the light switches in your home are not much more than two pieces of metal rubbing together.
High power "contactors" will have features such as an "arc chute" to guide and control the large spark ("arc") that forms when trying to turn off the equipment. Really big ones which are manually switched require the operator to wind up a crank handle before switching.
I've never seen a good description of what makes the name important.
1/ how big is your motor?
2/ Could you not drive the existing contactor to acheive control?
The main difference between a contactor and an ordinary relay is that it has 2 contacts in series per phase . This makes it much better at breaking large inductive loads.
Be careful how you "tap into" the control circuit as many larger compressors have unloader valves.
Bypass or cause any override of the existing system could result in burnt out motor, or as in larger systems, snapped crankshaft.
Mods would definitely void any warranty, also might be a good move to check any insurance or workplace health and safety violations.
a relay is the more broad term. like the difference between a solid state switch and a transistor or an FET
a contactor often has a coil that is replaceable. and comes in different voltages. many contactors switching a 480V load will have a 120 volt coil, but you can remove that and replace it with a 12V coil, or a 480V one.
contactors are almost always fail open and are NO, Form A. it is rare to find a Form C contractor.
even a dual motor pump controller will have matching contactors with a see/saw lever to prevent both from being used at the same time.
When used with a motor, they generally have 'heaters' a form of current limiting device that heats under load. too much heat and the contactor breaks (opens) when they get too hot.
I would suggest that you use a motor contactor, rated for the motor you want to use.
select a coil of no more than 120 volts, 12VAC or 12VDC may be hard to find.
and use a simple relay to control the voltage to the coil. I jokingly want to say in a Darlington arrangement.
an Arduino can control a small relay that controls the coil of the contactor.
anyone else have their spell checker screaming at them ?
bluejets:
Be careful how you "tap into" the control circuit as many larger compressors have unloader valves.
Bypass or cause any override of the existing system could result in burnt out motor, or as in larger systems, snapped crankshaft.
Mods would definitely void any warranty, also might be a good move to check any insurance or workplace health and safety violations.
in my days of new construction, a typical specification would limit starts and run times. no more than 3 starts an hour and run times of about 20 minutes. that was back in the day when building automation was pneumatic and thermostats and duct damper operators were pneumatic. Variable pitch vanes of 6 foot diameter fans.... before the days of variable speed, the motors ran constant speed, but the fan blades were variable pitch, or inlet vanes of a blower would modulate to cut off the inlet air.
but now she's associated with the lowest point in the series' history.
davepl:
I have a big 240v shop compressor in a compressor room.
.....that runs a solid state relay that controls a big 1hp motor?
1 HP, 230 V is only about 5 amps, with the voltage spike when you start should not be that great.
read the rating of a relay. they use 3 types of loads, resistive, inductive and motors, usually in HP.
The Songle SRD relay we all use has a restive load rating of 10 amps at 120VAC, 7 amps at 28VDC
and has an inductive load rating of 3 amp.
the larger Songle SLA relay, the SLA-05VDC-SL-A (Form A)
has a 30 Amp resistive load rating and a 1hp motor rating at 120V. the Form-C is slightly less on the resistive rating.
the rating for a 250VAC motor is 2 hp for both Form A and Form C.
IMHO, you might eek out a bit more life if you use a Zero crossing circuit when using the relay. But it is not needed for general use.
