1 phase 460v cond. motor?

[jarganda83]

Today I was zapped bt the chassis of a condensing unit. I changed a cc heater on this unit because it shorted out in a puddle of water. I energized this unit after installing the new cc heater and the cond fan grill zapped me. I removed the 24v control wires, so the unit was not running. I put one meter lead on the roof flashing and the other lead on the cond motor grille, the meter showed 260v. I shut down power and removed the cond motor leads. I rechecked for chassis voltage and found none. I ohmed this motor out: c-s 74, c-r 76, s-r 82. I picked up a new motor and installed it with no chassis voltage. I ohmed the new motor before I installed it and the resistances were very similar.

How could a single phase motor ohm out all the same? The motor only uses 2 legs of power. L1 to common, L2 to run cap to run, other side of run cap to start.

???

[SBKold]

You gotta ohm it to ground.

[timebuilder]

Today I was zapped bt the chassis of a condensing unit. I changed a cc heater on this unit because it shorted out in a puddle of water. I energized this unit after installing the new cc heater and the cond fan grill zapped me. I removed the 24v control wires, so the unit was not running. I put one meter lead on the roof flashing and the other lead on the cond motor grille, the meter showed 260v. I shut down power and removed the cond motor leads. I rechecked for chassis voltage and found none. I ohmed this motor out: c-s 74, c-r 76, s-r 82. I picked up a new motor and installed it with no chassis voltage. I ohmed the new motor before I installed it and the resistances were very similar.

How could a single phase motor ohm out all the same? The motor only uses 2 legs of power. L1 to common, L2 to run cap to run, other side of run cap to start.

???


I have a more important question.

Why was the chassis not bonded to the panel? That overcurrent device should have opened on the fault.

[jarganda83]

This unit was grounded. It's supplied with 3 leg @ 460 and 1 ground wire. The cond motor is stud mounted. I did check the motor from each lead to the motor frame and found no resistance.

[GT Jets]

This unit was grounded. It's supplied with 3 leg @ 460 and 1 ground wire. The cond motor is stud mounted. I did check the motor from each lead to the motor frame and found no resistance.

Well there you go then....:whistle:

[jarganda83]

So.... the old motor was not grounded. Right? But, it was, I replaced it and now the is no more voltage on the cabinet.

Maybe my meter is not powerful enough the detect that small of a resistance between the motor lead and the frame? But still be enough to pass voltage. Is that possible? I should also mention that the comp. has one leg with continuous power. So the comp. could be allowing some voltage to pass to the cond. fan motor when neither motor is running.

What about the motor winding resistances? I've never seen a single phase motor ohm out like that. I would think that common-start and common-run have to add up to start-run. How can the not? The new motor runs under it's rated amps.

[jarganda83]

This unit is a Carrier
M/n: 38YCC036630
S/n: 1002E30967

[GT Jets]

So.... the old motor was not grounded. Right? But, it was, I replaced it and now the is no more voltage on the cabinet.

Maybe my meter is not powerful enough the detect that small of a resistance between the motor lead and the frame? But still be enough to pass voltage. Is that possible? I should also mention that the comp. has one leg with continuous power. So the comp. could be allowing some voltage to pass to the cond. fan motor when neither motor is running.

What about the motor winding resistances? I've never seen a single phase motor ohm out like that. I would think that common-start and common-run have to add up to start-run. How can the not? The new motor runs under it's rated amps.

You misinterpreted your meter reading in your post... "Found no resistance" IS a direct short", you should have stated that the resistance was greater than what your meter was capable of reading, unfortunately for many meters, this is a mere 200ohms.

Just because your meter did not read any "CONTINUITY" does not mean the motor was good.

I am also betting that the grounding on the line side of the machine has been compromised in one way or another. Betting it is not bonded at all. (If it was, you would not have gotten "zapped". Also there needs to be a separate ground wire from the chassis of the condenser fan motor to a KNOWN good ground.

Several years ago there was a facilities guy knocked unconscious by getting shocked and falling and hitting his head on a gas line. I often use my Volt Alert during PM's to test the sheet metal before I pull them apart, especially if they have a history of tripping breakers.

GT

[jarganda83]

Im sorry, I ment to say my meter read "OL" when I checked from the motor leads to the motor frame.

This motor came from the factory with just 3 leads (common, start and run) and no ground wire.

If the unit was grounded properly then all of the leaking voltage should have gone straight to ground and none to me? I know there is a ground wire coming in with the line voltage, it's mounted in a lug near the contactor. I didn't check where it's coming from, maybe I should...

[GT Jets]

Im sorry, I ment to say my meter read "OL" when I checked from the motor leads to the motor frame.

This motor came from the factory with just 3 leads (common, start and run) and no ground wire.

If the unit was grounded properly then all of the leaking voltage should have gone straight to ground and none to me? I know there is a ground wire coming in with the line voltage, it's mounted in a lug near the contactor. I didn't check where it's coming from, maybe I should...


Now we're getting to the meat and potatoes.

A higher end meter very well may have been able to prove the motor partially grounded. What make/model meter were you using?

I agree that the ground issue needs to be investigated further. My bet is that the breaker panel the power is coming from has lost continuity with the ground stake at some point. Either that or it was never completed (both equally plausible).

For god sakes be careful....

[timebuilder]

This unit was grounded. It's supplied with 3 leg @ 460 and 1 ground wire. The cond motor is stud mounted. I did check the motor from each lead to the motor frame and found no resistance.

The unit needs an electrically acceptable low impedance ground to the overcurrent device that protects the unit. ALL current should have flowed through the equipment grounding conductor, as opposed to you, wearing work boots and touching the fan grille. If you felt discomfort when touching the grille, the impedance of the ground path is too high.

[timebuilder]

Im sorry, I ment to say my meter read "OL" when I checked from the motor leads to the motor frame.

This motor came from the factory with just 3 leads (common, start and run) and no ground wire.

If the unit was grounded properly then all of the leaking voltage should have gone straight to ground and none to me? I know there is a ground wire coming in with the line voltage, it's mounted in a lug near the contactor. I didn't check where it's coming from, maybe I should...

If you got "zapped," then yes, the entire ground path from the service SHOULD be checked. Similarly, the connection in the unit disconnect should not rely on three sheet metal screws to ground the cabinet of the unit. A grounding conductor should run from the disconnect to a grounding lug somewhere in the unit power section.

Note to all:

when using screws for grounding or bonding, only a machine thread screw is acceptable. Typical sheet metal thread screws are not acceptable in the NEC.

A ground lug, attached with one or more machine screws, is best.

[thermojohn]

The unit needs an electrically acceptable low impedance ground to the overcurrent device that protects the unit. ALL current should have flowed through the equipment grounding conductor, as opposed to you, wearing work boots and touching the fan grille. If you felt discomfort when touching the grille, the impedance of the ground path is too high.

Great explanation.

[timebuilder]

Now we're getting to the meat and potatoes.

A higher end meter very well may have been able to prove the motor partially grounded. What make/model meter were you using?

I agree that the ground issue needs to be investigated further. My bet is that the breaker panel the power is coming from has lost continuity with the ground stake at some point. Either that or it was never completed (both equally plausible).

For god sakes be careful....

The problem would not be with the grounding stake or rod. The ground path begins at the panel with the system bonding jumper, which connects the ground bar to the cabinet and the neutral. This is the ONLY point in a system where the neutral and grounding conductors meet, so as to avoid what they call "objectionable current," that is, where current typically is flowing through the ground conductors. That current should ONLY flow during a fault. That's why they changed from three wire to four wire plugs for dryers, because we used to tie the ground (cabinet) to the neutral at the terminals in the back of the dryer. Now, only the cabinet connects to the fourth wire, and never carries current until something in the dryer shorts to the cabinet.

The grounding conductors carry fault current to the neutral at the panel, so as to assist the overcurrent device to open. The ground rod outside is there to take any lightning potential in the building wiring system to a low impedance earth ground, which only lightning uses as its sought potential. We can read voltage potential to ground because the building neutral and the utility transformer neutral (the center tap of the secondary) both tie to ground rods for lightning protection of the grid and structure.

[ararams]

Keep in mind all your electrical connections including ground is only as good as it's weakest or apparent connection. I your unit is grounded you may want to check the five or six screws on the fan guard to top cover since carrier uses screws with a rubber washer.

[timebuilder]

Keep in mind all your electrical connections including ground is only as good as it's weakest or apparent connection. I your unit is grounded you may want to check the five or six screws on the fan guard to top cover since carrier uses screws with a rubber washer.

AND, the motor should have a grounding conductor going to the unit cabinet, under a machine thread screw, such as a self-tapping 10-32 screw. Green coloring is not an NEC requirement, BTW.

[jarganda83]

Timebuilder, does this "bonding jumper" weaken when there is a large flow of current through it? The cc heater in the unit

[jarganda83]

Sorry... the wire nuts that attatch the the cc heater leads were laying in water in the botttom of the unit. So, that did trip the breaker at the roof panel and at the breaker down stairs.

[timebuilder]

Timebuilder, does this "bonding jumper" weaken when there is a large flow of current through it? The cc heater in the unit

Generally speaking, no. It is sized by the manufacturer of the panel to be sufficient for its purpose.

[jarganda83]

When the unit "zapped" me it was only a tingle.

So, if the unit was properly grounded I should have felt absolutely nothing?

The breaker only trips when there is high amperage, right? So, if the short to ground in motor was very small then maybe it wasn't over amping?

[ararams]

AND, the motor should have a grounding conductor going to the unit cabinet, under a machine thread screw, such as a self-tapping 10-32 screw. Green coloring is not an NEC requirement, BTW.
[ I energized this unit after installing the new cc heater and the cond fan grill zapped me. This unit was grounded. The cond motor is stud mounted. This motor came from the factory with just 3 leads (common, start and run) and no ground wire. ]
These four statements confirm the ground was lost at the cfm grill to cabinet connection.

[timebuilder]

When the unit "zapped" me it was only a tingle.

So, if the unit was properly grounded I should have felt absolutely nothing?

The breaker only trips when there is high amperage, right? So, if the short to ground in motor was very small then maybe it wasn't over amping?

A short is an unintentional connection from line potential to a grounded body or conductor, bypassing a load. Shorts can carry significant amperage. There may have been a high resistance bleed of current from the windings to the motor hosing, and not a true "short."

That said, there are several ideas to consider. For example, were you touching the cabinet when you touched the fan grille, or were you not touching any part of the unit except for the contact where you felt some current?

In the first example, where you would be touching a grounded cabinet, and then with the other hand, you touched an improperly grounded motor that was leaking current to its housing, you were completing a circuit to ground, and the unit was grounded, yet the motor and fan grille were not.

In the second example, if you were standing on a dry surface in work boots and you STILL could feel current, you narrowly escaped a deadly shock, probably due to the boots. In that situation, it would be likely that the unit is not properly grounded to the power system by an EGC.

Bottom line is this: if the unit and its components were properly grounded, it unlikely that you would feel a current bleed from a defective motor, if that is the only contact, i.e, the hand that felt the current. If the motor had a real short, enough current would flow through the equipment grounding conductor back to the panel to open the overcurrent device.

So, the answer is in exactly how you contacted the unit, and how well the unit and the motor are tied to the equipment grounding conductor.

Remember, it only a takes an extremely small current flowing through your chest to send your heart into a fibrillation that only a cardiac device such as an AED, can correct.

[R123]

You really need to use a megger to check for grounded motors. If the motor has a very mild ground, the 9 volt battery inside your standard ohm meter isn't powerful enough to complete the circuit. A megger, which produces 250 or 500 volts, will be alot more accurate. There has been many times my high quality Fluke 87 won't show a ground but the megger will pick it up.

[jarganda83]

You guys are absolutely right! When I got zapped I was repositioning the lid because I removed it to install the cc heater. The motor leads were still connected and the screws to the lid were not installed and the unit was supplied with power.... You guys are good.

I know your right because I checked voltage at the grille and at the cabinet. I was not showing any voltage at the cabinet because it is properly grounded.

[timebuilder]

The problem would not be with the grounding stake or rod. The ground path begins at the panel with the system bonding jumper, which connects the ground bar to the cabinet and the neutral. This is the ONLY point in a system where the neutral and grounding conductors meet, so as to avoid what they call "objectionable current," that is, where current typically is flowing through the ground conductors. That current should ONLY flow during a fault. That's why they changed from three wire to four wire plugs for dryers, because we used to tie the ground (cabinet) to the neutral at the terminals in the back of the dryer. Now, only the cabinet connects to the fourth wire, and never carries current until something in the dryer shorts to the cabinet.

The grounding conductors carry fault current to the neutral at the panel, so as to assist the overcurrent device to open. The ground rod outside is there to take any lightning potential in the building wiring system to a low impedance earth ground, which only lightning uses as its sought potential. We can read voltage potential to ground because the building neutral and the utility transformer neutral (the center tap of the secondary) both tie to ground rods for lightning protection of the grid and structure.

I caught myself using an incorrect term, so I want to correct it for all of you. My bad.

The jumper for bonding at the panel is the MAIN bonding jumper. The System bonding jumper is used where to have a transformer in a store (for example) where the service is 460v, and the transformer is there to supply power to the outlets, lighting, equipment, etc, that need 120v. In the transformer you have the SYSTEM bonding jumper, as it is a "separately derived system."

[jarganda83]

How could a single phase motor ohm out all the same? The motor only uses 2 legs of power. L1 to common, L2 to run cap to run, other side of run cap to start.

???

Has anyone ever ohmed out a single phase motor and got 3 similar resistances?

[SBKold]

I caught myself using an incorrect term, so I want to correct it for all of you. My bad.

The jumper for bonding at the panel is the MAIN bonding jumper. The System bonding jumper is used where to have a transformer in a store (for example) where the service is 460v, and the transformer is there to supply power to the outlets, lighting, equipment, etc, that need 120v. In the transformer you have the SYSTEM bonding jumper, as it is a "separately derived system."

Timebuilder I have wondered this for some time now - about the seperate neutral 4 wire range and dryer cords.

I understand what you mean about not using ground as current path.

At the dryer electrical connection there is a jumper from ground to neutral so what did this really accomplish?

[timebuilder]

At the dryer electrical connection there is a jumper from ground to neutral so what did this really accomplish?

The intention of that equipment jumper was to place the chassis of the dryer at the same potential as the "grounded conductor," which we routinely call the "neutral" conductor.

(Note: the NEC refers to the "hot" wires as the "ungrounded" conductors, the neutral as the "groundED conductor, and the ground wire as the "groundING conductor. They are working on changing the verbiage for most grounding conductors to BONDING conductors, since the purpose of an Equipment Grounding Conductor is to "bond" the metal parts to the panel via wires and conduit so that any short results in the overcurrent device being opened, which cuts off power to the offending appliance.)

Today, we use a separate conductor that is intended to carry NO current unless there is a fault, and that is the fourth wire. It eliminates the possibility that the cabinet can become energized due to neutral currents in the circuit. This raises the level of safety, which is what the code is all about.

Does that help?

[timebuilder]

Has anyone ever ohmed out a single phase motor and got 3 similar resistances?

Yes. It is usually caused by a winding-to-winding short circuit.

[jarganda83]

Yes. It is usually caused by a winding-to-winding short circuit.

But the resistances were that way on the old and the new motor. The new motor runs within it's rating.

[timebuilder]

But the resistances were that way on the old and the new motor. The new motor runs within it's rating.

I would have to see the method used. Perhaps that would explain it.

A three phase motor would have the same resistance for all windings. Are you perhaps talking about a three phase motor??

[R123]

I would have to see the method used. Perhaps that would explain it.

A three phase motor would have the same resistance for all windings. Are you perhaps talking about a three phase motor??
I agree, something is not right here. Either you have a 3 phase motor, your meter is not working properly, or your not ohming it out right. Was the motor completely disconnected and isolated when you took the readings?

[SBKold]

The intention of that equipment jumper was to place the chassis of the dryer at the same potential as the "grounded conductor," which we routinely call the "neutral" conductor.

(Note: the NEC refers to the "hot" wires as the "ungrounded" conductors, the neutral as the "groundED conductor, and the ground wire as the "groundING conductor. They are working on changing the verbiage for most grounding conductors to BONDING conductors, since the purpose of an Equipment Grounding Conductor is to "bond" the metal parts to the panel via wires and conduit so that any short results in the overcurrent device being opened, which cuts off power to the offending appliance.)

Today, we use a separate conductor that is intended to carry NO current unless there is a fault, and that is the fourth wire. It eliminates the possibility that the cabinet can become energized due to neutral currents in the circuit. This raises the level of safety, which is what the code is all about.

Does that help?

Thanks for the reply and explanation. My conclusion is then it is a backup conductor to the single grounding conductor we used in the past. Because depending on which conductor has the least resistance for the neutral currents it may be using either one.

[timebuilder]

Thanks for the reply and explanation. My conclusion is then it is a backup conductor to the single grounding conductor we used in the past. Because depending on which conductor has the least resistance for the neutral currents it may be using either one.

With today's four wire cord, you do NOT hook up the strap. They want NO connection between the neutral and the equipment grounding conductor. The strap was only used when there were only three wires.

[jarganda83]

I ohmed the new motor right out of the box. The motor only has 3 leads, black, yellow and brown. I just checked from one lead to another. I checked the old motot the same way, with the motor leads off.

I guess there is a chance my meter was wrong, but I ohmed a different motor the same day because I wanted to compare numbers and it was about right. The only difference was the voltage. The old and new motor were 460 single phase. The compared motor was 208/240 single phase.

I knew somthing was up...

[timebuilder]

With today's four wire cord, you do NOT hook up the strap. They want NO connection between the neutral and the equipment grounding conductor*. The strap was only used when there were only three wires.

*In the dryer.

There IS a connection between the neutral and ground, but that ONLY happens at the service entrance panel.

Just to be 100% clear.

[timebuilder]

I ohmed the new motor right out of the box. The motor only has 3 leads, black, yellow and brown. I just checked from one lead to another. I checked the old motot the same way, with the motor leads off.

I guess there is a chance my meter was wrong, but I ohmed a different motor the same day because I wanted to compare numbers and it was about right. The only difference was the voltage. The old and new motor were 460 single phase. The compared motor was 208/240 single phase.

I knew somthing was up...

A single phase motor that uses a run cap has three wires, and two windings. One is a run winding, and the other is usually referred to as an "aux" winding, which has a function similar to a start winding. The brown wire goes to the aux winding.

What readings did you measure?

[Hrdworkingacguy]

I ohmed the new motor right out of the box. The motor only has 3 leads, black, yellow and brown. I just checked from one lead to another. I checked the old motot the same way, with the motor leads off.

Essentially Black is common, Yellow is run, and Brown is start...Brown to yellow "Should" have the most resistance. Black to Brown the second highest, and Black to yellow the least...A volt meter can pick up on a shorted motor, a grounded motor is a different animal...A short is a broken wire...A ground is a crack in the lacquer in the windings causing power to "leak" to ground...You need meg-ohm meter for that...The OP got blasted by a grounded motor, a shorted motor would blow the fuses or the breaker...:eek:

BTW most of this is my experience, there is no hard fast rule on wire colors, the schematic or hook up diagram doesn't lie...

[Slatts]

You really need to use a megger to check for grounded motors. If the motor has a very mild ground, the 9 volt battery inside your standard ohm meter isn't powerful enough to complete the circuit. A megger, which produces 250 or 500 volts, will be alot more accurate. There has been many times my high quality Fluke 87 won't show a ground but the megger will pick it up.

Couldn't agree more R123.
Mechs who rely on a multimeter to test resistance to ground (earth) run the risk of killing themselves or worse, their paying customer.

Here are a couple of links of links to the type of meters we're talking about:

http://www.hioki.com/product/field.html

http://www.myflukestore.com/c538/megohmmeters.php?currency=USD

They may be expensive but how much is your life worth to you and your family?

[Slatts]

Timebuilder I have wondered this for some time now - about the seperate neutral 4 wire range and dryer cords.

I understand what you mean about not using ground as current path.

At the dryer electrical connection there is a jumper from ground to neutral so what did this really accomplish?

From what I've read here I assume you don't use residual currant devices much over there?

The more I learn about the power reticulation system used in the States, the more it horrifies me. :eek:

We joke about using the earth wire as a neutral return path over here but nobody would actually do it.

It sounds positively 19th century...

[timebuilder]

From what I've read here I assume you don't use residual currant devices much over there?

The more I learn about the power reticulation system used in the States, the more it horrifies me. :eek:

We joke about using the earth wire as a neutral return path over here but nobody would actually do it.

It sounds positively 19th century...

In the US, the neutral is a grounded conductor, and the service neutral is bonded to an earth electrode for surges and lightning.

Earth can never be used in place of a neutral to the power transformer. Here, that neutral is a transformer secondary center tap, so that the full secondary voltage is 220v, and one end of the secondary referenced to the center tap is 120v, which is our basic residential voltage for lighting and small appliances. We send the two ends of that transformer secondary and the center tap neutral from the pole to the home. A two pole breaker provides 220v, and a single pole breaker referenced to the neutral gives 120 volts.

Does that help?

[david chamberla]

3 phase moter will have resistance the same on all 3 legs .If not moter bad ,which i worked on a older trane which was a 208 230 volt 3 phase which hinding in the corner was a step up transfprmer 460 v 1 phase .running condenser fan '

[COOLJIM]

archaic is a good word , do not see rcd, s, still see lots of fuses of all styles on main supplys , do not even see disconnects ( isolators ) next to equipment , a recent service call at a hospital to replace a thermostat on a freezer room , two seperate systems but a single stat mechanical type double throw, so to isolate requirers access to the roof 3 floors up and a key to access the roof and a ride in the elavator and a lot of walking from one end of roof to the other , and you need to turn off both sets of equipment as you dont know which is the supply to the stat , by the way in evey call i make if you want to start a manual defrost , you also need to find the condensing unit which is inevetiably on a roof 20 ft high is common , as the clock and i mean the old style paragon type is still in daily use here isalways locted at the condensing unit , they have not heard of a control panel defrost control etc and an isolator adjacent to the freezer room , , some of there stuff takes alot of getting used to , and then there are wild legs but that is a different story ,