Running a 200-208 V Air Compressor on a 240 V High Leg Delta?

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msjayhawk

New member
I have a "200 VAC" Ingersoll-Rand Air Compressor that I put in a few months ago. It is a good size unit, lubricated scroll, and about 11 horsepower. It is working fine and has about 70 hours run time on it since I put it in. It came with 670 hours (used deal), and I have put the other 70 on it hooked up to a high-leg delta connection (240 VAC, I guess).

My problem is, my compressor people keep telling me I am going to burn it up, due to the high voltage and don't seem to understand the connection (most of them are young).

My ELECTRICIANS, who are old :) tell me it is not a problem at all, and that it is actually a good thing because this motor is 'kind of hard to kick over', and it is not really 240 like you would think of as a 208 or something, I can be missing part of that.

Anyway, I have had 200 VAC motor compressors hooked up to this system for 17 years, and never had a problem with a motor. I replaced the 5 hp with a 7.5 hp for the air I needed, and I replaced the 7.5 after 10 years just due to the need for more air.

I am an ME, so this is all kind of confusing to me once you go past DC current :) The compressor guys are telling me I am going to kill the motor and void my warranty, and the electricians are saying they are 'full of stuff'. I tend to believe the electricians, as I have never had a 200 volt motor burn out on this exact same circuit.

Any comments, Please??!!
 

broadgage

Senior Member
Location
London, England
A 240 volt delta service is intended for single phase 120 volt lighting and small appliances, for single phase 240 volt loads such as water heaters, and for 240 volt 3 phase loads such as motors driving A/C units or perhaps compressors.

Although 208 volts nominal is present between the high leg and the grounded neutral, such services are not really intended for 208 volt loads.
In practice it works fine provided that the 208 volt load is small in relation to the service.
I would not regard your compressor as small in relation to the service, unless it is a very large service.

When current is drawn between the high leg and the neutral, it passes through 2 transformer windings with voltage drop and losses in each.
This can result in excesive voltage drop, and may overload the transformer bank, despite a total load that appears to be OK.

If it works, then it is probably acceptable, though not ideal. I would urge that any future replacement motor or complete compressor be designed for 3 phase 240 volts.
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
110.3(B) requires us to follow manufacturers instructions. So if they're telling you it should be on a 208V supply....

What about warranty?
 
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kwired

Electron manager
Location
NE Nebraska
A 240 volt delta service is intended for single phase 120 volt lighting and small appliances, for single phase 240 volt loads such as water heaters, and for 240 volt 3 phase loads such as motors driving A/C units or perhaps compressors.

Although 208 volts nominal is present between the high leg and the grounded neutral, such services are not really intended for 208 volt loads.
In practice it works fine provided that the 208 volt load is small in relation to the service.
I would not regard your compressor as small in relation to the service, unless it is a very large service.

When current is drawn between the high leg and the neutral, it passes through 2 transformer windings with voltage drop and losses in each.
This can result in excesive voltage drop, and may overload the transformer bank, despite a total load that appears to be OK.

If it works, then it is probably acceptable, though not ideal. I would urge that any future replacement motor or complete compressor be designed for 3 phase 240 volts.

I assumed he meant he is operating a three phase motor with a 208 volt rating on a 240 volt supply. Voltage to ground on one phase means nothing to the load - in fact it will operate the same if the system had a grounded phase.

You may have shorter life on this motor than if operating it on a 208 system. Chances are the motor will last longer than any warranty however. If the compressor fails but not the motor during warranty period, good luck they will try to find any reason to void the warranty.

If you want to operate the motor at its rated voltage you will need a transformer to change the voltage, or your service transformer(s) changed to supply 120/208. Note that changing the supply to the building may change performance of other equipment you already have.

Another possibility is VFD controller for the compressor motor. Set the V/F settings so that you have around 208 volts at 60 Hz. If the compressor is designed for operation at lower speeds you can also save some energy when the unit is "idling" by running it at lower speed.
 

broadgage

Senior Member
Location
London, England
I assumed that the O/P was working a single phase 200/208 volt motor between the high leg and the grounded neutral of a 120/240 volt high leg delta system.
If this is the case, then I stand by my remarks.

It appears however that I am probably mistaken, and that the O/P is in fact working a 3 phase 200/208 volt motor from a 3 phase 240 volt delta service with a high leg.
If this is the case, then the presence of the high leg is of no consequence, it is a simple case of a motor intended for 208 volts being used on 240 volts.
Might be acceptable, might not be. A lot would depend on the actual voltage, if the 240 volts nominal is on the low side at perhaps 230 volts actual at the service and down to 220 at the motor, then it should be fine.
If however the supply is actually nearer 250 volts, with little drop between service and motor, then I would advise against it.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
A pair of buck/boost transfomers are relatively inexpensive, but being an old guy I am inclined to agree with 'ignore the nonexistant problem'. Take some actual voltage checks. Not sure how the BBs with installation would cost in comparison the mentioned VFD. The drive would be a heck of a lot simpler.
 

amendez

Member
I have a similar situation however, nothing is currently connected to the high leg (208 volt single phase). My question is along the same lines. The service to the customer is 120/240 V, three phase delta. Most of the existing HVAC equipment is dual rated 230/208V, 1-phase. They are all currently connected to 2-pole breakers (@240 Volts). In the end, placing a meter on all three phases, the current draws are as follows: A-phase (200A), B-phase (238A) and C-phase (44A). As you can see, there is a huge imbalance. What I was wondering was, is it even legal per the NEC to take some of the dual rated condensing units off the 240 volt 2-pole breakers and connect them onto the 208V (L-N) leg to redistribute the load?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I have a similar situation however, nothing is currently connected to the high leg (208 volt single phase). My question is along the same lines. The service to the customer is 120/240 V, three phase delta. Most of the existing HVAC equipment is dual rated 230/208V, 1-phase. They are all currently connected to 2-pole breakers (@240 Volts). In the end, placing a meter on all three phases, the current draws are as follows: A-phase (200A), B-phase (238A) and C-phase (44A). As you can see, there is a huge imbalance. What I was wondering was, is it even legal per the NEC to take some of the dual rated condensing units off the 240 volt 2-pole breakers and connect them onto the 208V (L-N) leg to redistribute the load?

Your first problem might be finding a suitable single pole CB that can operate at 208 volts that fits in this existing panel.
 

erickench

Senior Member
Location
Brooklyn, NY
There is something about this problem that I don't understand. If the machine is operating as three phase then how can you draw power from a single phase 120/240 volt system? If it's a single phase machine and the 120/240 volt power supply is taken from a three phase 240 volt delta with a neutral center tap then you must find some way on how to make use of the high leg. This would give you 208 volts which would put you closer to the voltage rating of the machine. But you would have to hire an electrician to run the high leg wire into your area. Usually a 120/240 supply does not give you access to the high leg.
 

stew

Senior Member
I would really like to know what the motor current is under load as it relates to the nameplate. For the most part if you are drawing at or less than nameplate current there should not be a problem.The smoke will stay in if you dont draw more than nameplate current.
 

sgunsel

Senior Member
What is the exact nameplate info (voltage, phases, amperage, HP)?
Most motors are not marked "about 11 HP".
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
You tagged onto an old thread and it can confuse people when you do that. I suggest that next time start a new thread with a new issue and reference the older thread if you think it necessary.

I have a similar situation however, nothing is currently connected to the high leg (208 volt single phase). My question is along the same lines. The service to the customer is 120/240 V, three phase delta. Most of the existing HVAC equipment is dual rated 230/208V, 1-phase. They are all currently connected to 2-pole breakers (@240 Volts). In the end, placing a meter on all three phases, the current draws are as follows: A-phase (200A), B-phase (238A) and C-phase (44A). As you can see, there is a huge imbalance. What I was wondering was, is it even legal per the NEC to take some of the dual rated condensing units off the 240 volt 2-pole breakers and connect them onto the 208V (L-N) leg to redistribute the load?

A 120/240V 3 phase 4 wire "high leg" delta system is intended to be used where 120V loads are a "minor" part of the load, because they are inherently difficult to balance. But if most of the loads are TRULY* 240V and require 2 pole breakers, then they should be arranged so as to be balanced between the 3 phases as much as possible. Looks to me as if someone got scared of the "high leg" issue and loaded everything onto 2 of the 3 phases. Not good.

As long as the load is using 240V ONLY then it makes no difference which 2 phases are feeding it, you will get 240V to it. You should investigate this further and try to move breakers around to balance it. To not do so it to put thermal stress on the transformer and if they have any 3 phase loads, the voltage balance will suffer, making the motors run hot and die early.

* The tricky part is when you feed 240V to a machine or appliance, and INTERNALLY, they are expecting to be able to tap off 120V by going line to neutral. A common example is something with a 120V control system like a time clock, etc. If you tap the line from the high leg, you will have 208V line to neutral instead of 120V, which will likely kill the controls. If ALL of your 240V single phase loads are like that, they should not have used a 120/240V 3Phase 4 Wire service.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
What is the exact nameplate info (voltage, phases, amperage, HP)?
Most motors are not marked "about 11 HP".
You are commenting on the original post from back in March, this thread was opened up again with essentially a new post.

Hence (amendez) the confusion you can cause by tagging onto (hijacking) old threads.
 

kwired

Electron manager
Location
NE Nebraska
I have a similar situation however, nothing is currently connected to the high leg (208 volt single phase). My question is along the same lines. The service to the customer is 120/240 V, three phase delta. Most of the existing HVAC equipment is dual rated 230/208V, 1-phase. They are all currently connected to 2-pole breakers (@240 Volts). In the end, placing a meter on all three phases, the current draws are as follows: A-phase (200A), B-phase (238A) and C-phase (44A). As you can see, there is a huge imbalance. What I was wondering was, is it even legal per the NEC to take some of the dual rated condensing units off the 240 volt 2-pole breakers and connect them onto the 208V (L-N) leg to redistribute the load?

Why not just put one side of as many 240 volt loads as you can or as necessary to try to balance the phases? Do not do this with 120/240 loads but just straight 240 loads. Remember you have to use a 240 rated breaker if connecting to high leg and not the typical 120/240 rated breaker.
 

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
Many delta systems are not designed to be balanced. In my area most delta systems are fed by only 2 transformers (open delta). The 120/240 volt transformer is often much larger that the 240 volt transformer feeding the 3rd leg. I have seen a 100kva 120/240 volt transformer with a 15kva 240 volt transformer. The load on the 3rd leg is expected to be much lower that the other legs. Most of the load is expected to be single phase connected to the main transformer.
 
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