[Pictures coming soon]
Several AC units are down in Building 1.
Have a list from one of the Mechanics with which units are faulty and what he thinks may be wrong. Spare parts in his locked cabinet. Motors and pressure switches he says. It's not that I don't believe him; I just don't want him taking all my fun out of diagnostics.
I laugh a little when I hear one of the bosses was baking all week in his office; I feel sorry for him but I keep thinking of that Seinfeld episode where Kramer was using butter as suntan oil, and the time I put bacon grease on a tanning bed as a teenager to make it smell like someone was cooking. I try to concentrate, but I'm in a great mood from finally getting some overtime.
The units should be running all the time, he says. I assume the temperature is controlled by the Andover system, which it is. I pop off a cover in the office for one of the thermostats, there's no illumination at the LED. Well, its not wired to that half.
I get a 6-foot ladder and bring my tool cart. A Rubbermaid cart with my tools, some fittings, wirenuts, colored tape, some hardware, my screwgun. Everything neatly sorted and waiting to tackle what they were manufactured for.
My unit isnt running. Its an Envirocon II 480v AC unit. My Journeyman says to replace the motor. While I might agree that since the other motors are failing from age, this one could probably be replaced anyways. I know these motors are $270 so I'll test it first. The screw gun quickly removes the 1/4" head sheet metal screws and the access panel comes off. This is the side with the compressor, suction/accumulator/dryer/heat exchanger; I'm not exactly sure, as I've never sen this unit before and there are NO ELECTRICAL DRAWINGS. Well, no problem. I check for voltage before I even open the disconnect/fuse box. 480v line to line, 277 to ground on each line at the contactor (no neutral). That's good.
If they should be running all the time, the contatcor should be pulled in. I know how auto AC systems work and know some of the variants quite well, and how different types of Freon® operate - in many systems 1 PSI = 1 degree, hence the necessary actions of the compressor.
I push my screwdriver in on the contatctor, and the compressor starts. Okay, not getting power to the contactor coil...which is the same orange colored wire on EVERYTHING. It's too dirty to see the printed circuit numbers, but I dont need them anyways. I trace to a relay I've seen before on the VanD-Land project, and that goes to an "ice cube" relay, and that goes to what is probably a low-pressure switch to prevent the compressor from turning on if it's low on Freon®...it just opens the circuit to the first relay, turning off everything downstream so nothing runs (maybe; thats a simple way it CAN work, but I'm here to fix it, not reverse-engineer it right now).
I don't think this is the problem, so I check the 480-to24-volt Transformer for power. Nothing out of the secondary.I cut the power at the disconnect and open the disconnect/fusebox door to visually see if the contacts are open and the 480v is shut off. Dying wouldn't be good today, just like any other day. I use my meter to check the fuses for the heck of it, and for power on the panel side. Nothing.I check power on the feed side to make sure the meter is working. 480v from L1 to L2, L2 to L3, L1 to L3. 277 to ground from each.
I cut the yellow and blue leads on the Transformer, strip the ends, wirenut my test leads to each end, I should get 24V AC when I turn the power back on. Nope. Turn off again. I know the Transformer is getting power (at least to the point where the windings start and I cant see any more) as its tapped off the feed side of the contactor...thats also how I know it's 480 volts.
The motor is a "Capacitive run" motor, meaning it has a capacitor wired in by the motor so that it can change the phase angle of a motor to make it more efficient, or to operate at a different torque at a given speed, many reasons and methods, see Wikipedia for more info. This one is marked 5 uF. My meter can check these, so I disconnect one lead and check. 5.05 uF. Good, at least while not under a load. Rare that they change without failing, I've never seen one.
To satisfy my Journeyman, I cut the 2 power leads on the motor (it has several for low, medium, high speed, select by color and the legend on the motor.)
The meter reads 46 volts. I know right away its just a transient voltage; if you hooked up a load to it, say a 48-volt light bulb, it wont even light. I have a "Wiggie" to test it (an electronic one; the mechanical ones are suicidal to use) but it's at home. I assure him there's no "juice" behind that voltage, maybe just a lost eddie current. Go home, Eddie. Nothing to see here.
I wire the motor back up and put the panel back on. I look online for a witing diagram, I can only find a parts list, but its a DHW series, and 030 is the "size" of the unit. I see a transformer on there and print out the sheet. No diagrams anywhere. Thats a first. I feel like Google has failed me again. It has.
I check Sunday for a 480 to 24 volt transformer. What size to get? Well, I can estimate by measuring the resistance of the coils and use Ohm's Law and my knowledge of transformers to solve this, even with a damaged transformer. But how, you ask? Just pretend you've asked. Simple! Measure the primary, and...Oh, a billion ohms. Thats bad. Also a number we cant use. Measure the secondary wires (the Cub Scout yellow and blue ones on the 24-volt side). 0.8 ohms.
Ohm's law (disregarding transformer inefficiencies and manufacturing defects, and a not-perfect Faraday effect but darned near close, assuming the secondary isn't damaged) says V=IR. Volts equals amps times ohms.. That equates to volts divided by ohms equals amps. Roughly 3 amps.
Transformers are measured in "volt-amps", for this demonstration here just think of them as "watts". How do we get volt-amps? Same way you get horse-apples. Multiply horses by apples, you get horse-apples.
24V times 3A = about 75VA.
This is a "step down" transformer, meaning the voltage is reduced. Now, we cant destroy energy too easily, so as a benefit, we get a big increase in amperage...which would be the 3 amps. That means a fraction of that is being used on the 480 volt side. 75 volt-amps divided by 480 volts = 0.15625 amps. can you calculate what the resistance SHOULD be? Yes, about 3000 or so. There might be extra turns on each side to compensate for losses or for manufacturing inefficiencies (causing losses) so if you're close, you've probably calculated right.
What we do with the voltage, the opposite happens with the current (amperage). This final amount is determined by the number of "turns" of wire on each side. The size of the wire is different (except in isolation transformers which don't want to change values much, depending on application) on each side; larger wire handles more current, so it becomes easy for you to recognize which side is which if you know what the transformer is supposed to do (if it works, unlike mine).
Now, the Grainger catalog lists an 80 voltamp 480:24 transformer for $68. But wait, can't we just use a 277 volt transformer and hook one hot lead to the transformer lead, and ground the other lead, making 277 volts, and save $40 and use one of the dozens of 277:24 transformers in the stock room?
Sure, if you want to kill someone.
Hooking a neutral to ground at a component is in effect making the ENTIRE metal assembly all the way back to the ground bus in the main panel its fed from INTO A CONDUCTOR. Not only that; it might work but now you've compromised the ability for any GFI on the circuit as well as the breaker to be unable to operate, or may trip instantly (depending on how it's wired. I think it's fun to draw that out & discovered many things that might happen if a hot is cut on a shared neutral in a multi-volt power distribution system
Neutral is NOT GROUND. Ground is NOT NEUTRAL. Should there be any break in the conduit or a high-resistance along that path, a loose fitting, etc...you could touch it and give it a much-wanted path to ground the power wants so badly. It will do it at the expense of your life.
I can even say that NEUTRAL is not NEUTRAL in a 1-phase circuit. It's a return but also a supply path half of the time.
Don't ever be lazy and hook up a ground to be a neutral because you don't feel like running another wire, or if the job is too big or too expensive. What's your life worth?
