Three phase load preference
- Thread starter markt63
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: Three phase load preference
Depends on a lot of factors including just what you mean by "large". Usually it is most cost effective to use 480V motors rather than 208V motors if you have 480V available. OTOH, the utility often charges more to install a 480V service than it does a 208 or 230V service. Thats why you often see 208V or 230V services with transformers inside the plant to increase the voltage up to 480V to service 480V loads. Seems silly but fairly common.
The motor does not care if it is wye or delta. Industrial plants that do not want to be shut down due to a ground fault use delta. Most commercial establishments would be wye.
Depends on a lot of factors including just what you mean by "large". Usually it is most cost effective to use 480V motors rather than 208V motors if you have 480V available. OTOH, the utility often charges more to install a 480V service than it does a 208 or 230V service. Thats why you often see 208V or 230V services with transformers inside the plant to increase the voltage up to 480V to service 480V loads. Seems silly but fairly common.
The motor does not care if it is wye or delta. Industrial plants that do not want to be shut down due to a ground fault use delta. Most commercial establishments would be wye.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Three phase load preference
It also depends on what you mean by ?best.? Lowest initial cost? Lowest operating cost? Lowest maintenance cost? Easiest to install? Easiest to remove, should the need arise? Most readily available from local suppliers? Easiest access to replacement parts? Most nearly compatible with other equipment in the inventory (i.e., compatibility with regard to replacement parts)? These are some of the questions that I would ask, in the process of selecting the ?best? design alternative.
It also depends on what you mean by ?best.? Lowest initial cost? Lowest operating cost? Lowest maintenance cost? Easiest to install? Easiest to remove, should the need arise? Most readily available from local suppliers? Easiest access to replacement parts? Most nearly compatible with other equipment in the inventory (i.e., compatibility with regard to replacement parts)? These are some of the questions that I would ask, in the process of selecting the ?best? design alternative.
charlie tuna
Senior Member
- Location
- Florida
Re: Three phase load preference
i would suggest the 480 volt service---remember the customer pays for the energy he consumes. if you start out with a 120/208 supply and transform it up, especially for a motor load because the transformer will be oversized, you pay for the heat and energy used by the transformer. the motor will run cooler on 480 volts. the operating cost will be less at 480 volts!
i would suggest the 480 volt service---remember the customer pays for the energy he consumes. if you start out with a 120/208 supply and transform it up, especially for a motor load because the transformer will be oversized, you pay for the heat and energy used by the transformer. the motor will run cooler on 480 volts. the operating cost will be less at 480 volts!
charlie
Senior Member
- Location
- Indianapolis
Re: Three phase load preference
Sorry Charlie, I don't understand either one of those statements.
wirenut1980
Senior Member
- Location
- Plainfield, IN
Re: Three phase load preference
charlie tuna, I believe the motor will still be using the same amount of power, but will draw less current at 480 V than at 208 V. So operating cost should be the same in either scenario (charged by KW-hr). Actually if the customer has 208 V, the operating cost would increase if they installed a step-up transformer to 480 V because of added losses in the transformer. As for operating at a cooler temperature, that may be true for an existing feeder that once had to supply amps at 208 V, and then supplies less amps at 480 V.
charlie tuna, I believe the motor will still be using the same amount of power, but will draw less current at 480 V than at 208 V. So operating cost should be the same in either scenario (charged by KW-hr). Actually if the customer has 208 V, the operating cost would increase if they installed a step-up transformer to 480 V because of added losses in the transformer. As for operating at a cooler temperature, that may be true for an existing feeder that once had to supply amps at 208 V, and then supplies less amps at 480 V.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Three phase load preference
As to running cooler, the things that make a motor run hot are primarily friction (we can assume that is the same for both motors) and ?I squared R? losses within the motor windings. Since the 480 volt motor runs on lower current, it will have less ?I squared R? losses, and will therefore run cooler.
As to operating costs, if all other factors are the same (i.e., HP, power factor, and efficiency), then the KVA drawn by both motors will be the same. Power is power, and what we pay for is power. There might also be a utility charge for power factor, but for this discussion I have assumed they are the same. The only difference is that the 480 volt motor will have less ?I squared R? losses within the feeder and branch circuit conductors. So it is cheaper to operate, but I think the difference is small.
However, if the service is 208 volts, and if you install a step-up transformer to supply a 480 volt motor, then Charlie Tuna and Wirenut 1980 are both right in saying that there will be an added operational cost in the energy losses within that step-up transformer.
I think that Wirenut 1980 agreed with the first statement, but not with the second. I agree with both (sort-of).
As to running cooler, the things that make a motor run hot are primarily friction (we can assume that is the same for both motors) and ?I squared R? losses within the motor windings. Since the 480 volt motor runs on lower current, it will have less ?I squared R? losses, and will therefore run cooler.
As to operating costs, if all other factors are the same (i.e., HP, power factor, and efficiency), then the KVA drawn by both motors will be the same. Power is power, and what we pay for is power. There might also be a utility charge for power factor, but for this discussion I have assumed they are the same. The only difference is that the 480 volt motor will have less ?I squared R? losses within the feeder and branch circuit conductors. So it is cheaper to operate, but I think the difference is small.
However, if the service is 208 volts, and if you install a step-up transformer to supply a 480 volt motor, then Charlie Tuna and Wirenut 1980 are both right in saying that there will be an added operational cost in the energy losses within that step-up transformer.
Re: Three phase load preference
Posted by Charlie B:
But I agree with the I^2R losses being higher in the feeder for a 208V motor.
Steve
Posted by Charlie B:
Don't forget the 480v motor will have smaller wire in the windings (higher R). So we can't really say a 480V motor runs cooler. It all depends on what temperature the manufacturer designs it to run at.
But I agree with the I^2R losses being higher in the feeder for a 208V motor.
Steve
charlie tuna
Senior Member
- Location
- Florida
Re: Three phase load preference
we are talking 480 volt -three phase
vs
230 volt - three phase!
typical motor or standard motor nameplate data states say as an example:
480 volts---------10 amps
230 volts---------20 amps
then add the ten per cent adder for 208 volts 208 volts---------22 amps
more current more heat= more operating costs!
we are talking 480 volt -three phase
vs
230 volt - three phase!
typical motor or standard motor nameplate data states say as an example:
480 volts---------10 amps
230 volts---------20 amps
then add the ten per cent adder for 208 volts 208 volts---------22 amps
more current more heat= more operating costs!
dnbob
Senior Member
- Location
- Rochester, MN
Re: Three phase load preference
This brings up an interesting point for me. We recently changed a dairy farm over from a 120/240 v. 3 phase system to a 480Y/277 v. 3 phase system and the customer noted that his electric bill was reduced by almost 50%, based on the same pumps & augers etc. running their normal amounts. All of the power is stepped down to 208Y/120 3 phase via an isolation transformer. I guess my question is if "power is power", why did he notice such a significant drop in utility charges?
The reason for the change over was to eliminate his open delta which was causing so much stray voltage his cows were not producing and was losing 60 or more head per year.
This brings up an interesting point for me. We recently changed a dairy farm over from a 120/240 v. 3 phase system to a 480Y/277 v. 3 phase system and the customer noted that his electric bill was reduced by almost 50%, based on the same pumps & augers etc. running their normal amounts. All of the power is stepped down to 208Y/120 3 phase via an isolation transformer. I guess my question is if "power is power", why did he notice such a significant drop in utility charges?
The reason for the change over was to eliminate his open delta which was causing so much stray voltage his cows were not producing and was losing 60 or more head per year.
Re: Three phase load preference
charlie tuna:
I see you are refering to a dual voltage motor ran below the rated voltage. I was refering to separate 480v vs. 208v motors. Im not sure which Charlie B had in mind.
Apples and oranges
Steve
charlie tuna:
I see you are refering to a dual voltage motor ran below the rated voltage. I was refering to separate 480v vs. 208v motors. Im not sure which Charlie B had in mind.
Apples and oranges
Steve
charlie
Senior Member
- Location
- Indianapolis
Re: Three phase load preference
Dnbob, We would not change from an open delta to a 480Y unless he had a significant amount of load or he wanted to pay for the change. If the load was there and he was changed at no cost, his rate may have been wrong as well. If that was the case, perhaps his rate was changed to a more favorable rate so that his bill went down.
The real question is, "What was the change in energy consumption?"
Dnbob, We would not change from an open delta to a 480Y unless he had a significant amount of load or he wanted to pay for the change. If the load was there and he was changed at no cost, his rate may have been wrong as well. If that was the case, perhaps his rate was changed to a more favorable rate so that his bill went down.
The real question is, "What was the change in energy consumption?"
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Three phase load preference
I am not sure I understand how a properly functioning open delta would cause stray voltage. Perhaps there was something wrong with its installation. In any event, if there is stray voltage, and if it impacts the cows (i.e., by virtue of their feeling the current), then it is clear that energy was being wasted (i.e., by sending current through the dirt). When the system was upgraded, that energy waste was eliminated.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Three phase load preference
I had presumed that the motors had not yet been purchased, and that there was a choice of buying a 480 or buying a 208.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Three phase load preference
charlie, charlie b, and dnbob
I was called to inspect a dairy with excessive stray voltage. By the time I got there, the utility had changed a 120/240 3 phase 4 wire delta to a 120/208 wye and the stray voltage had disappeared.
The delta secondary with a grounded wye primary, drives currents back thru the utility neutral, and will impart some current in many parallel paths around the farm.
Additionally a grounded delta feeder from one service run to more than 1 farm building and installed without a ground wire per 250.32(B)(2)will often put neutral currents on all sorts of metallic piping, etc.
I strongly encourage all utilities who serve dairies with 120/240 delta services to look closely at changing them to 120/208. It seems that deltas are often a problem with stray voltages.
Jim T
charlie, charlie b, and dnbob
I was called to inspect a dairy with excessive stray voltage. By the time I got there, the utility had changed a 120/240 3 phase 4 wire delta to a 120/208 wye and the stray voltage had disappeared.
The delta secondary with a grounded wye primary, drives currents back thru the utility neutral, and will impart some current in many parallel paths around the farm.
Additionally a grounded delta feeder from one service run to more than 1 farm building and installed without a ground wire per 250.32(B)(2)will often put neutral currents on all sorts of metallic piping, etc.
I strongly encourage all utilities who serve dairies with 120/240 delta services to look closely at changing them to 120/208. It seems that deltas are often a problem with stray voltages.
Jim T
charlie
Senior Member
- Location
- Indianapolis
Re: Three phase load preference
As I heard someone say, "Tain't eazy McGee." The problem is the availability of the third primary phase wire and the fact that the farmer is required to rewire his service. In other words, without a complaint and documentation that there is a problem, we are not doing it.
As I heard someone say, "Tain't eazy McGee." The problem is the availability of the third primary phase wire and the fact that the farmer is required to rewire his service. In other words, without a complaint and documentation that there is a problem, we are not doing it.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Three phase load preference
Charlie
I hear you, and understand where you are coming from. I worked for electric utilities first as a lineman and finally as an engineer for over 20 years. Without a complaint from the farmer, you might have an arugment for not finding and fixing that source of stray voltage, but based on my experience, a simple complaint from the farmer would render a utility liable, even if the farmer didn't want to pay for the extra phase.
Further, if you know about delta's causing stray voltage and you don't hunt them down, many attorneys will have their way with you. I suspect that attorneys on your side will strongly suggest that you do just that. A law suit, even if the utility wins, is usually much more costly than the $20,000 or so to pull in an additional phase.
Jim T
Charlie
I hear you, and understand where you are coming from. I worked for electric utilities first as a lineman and finally as an engineer for over 20 years. Without a complaint from the farmer, you might have an arugment for not finding and fixing that source of stray voltage, but based on my experience, a simple complaint from the farmer would render a utility liable, even if the farmer didn't want to pay for the extra phase.
Further, if you know about delta's causing stray voltage and you don't hunt them down, many attorneys will have their way with you. I suspect that attorneys on your side will strongly suggest that you do just that. A law suit, even if the utility wins, is usually much more costly than the $20,000 or so to pull in an additional phase.
Jim T
charlie
Senior Member
- Location
- Indianapolis
Re: Three phase load preference
But first he has to complain, then we gather facts, and last he has to be willing to rewire to accept the new service configuration.
But first he has to complain, then we gather facts, and last he has to be willing to rewire to accept the new service configuration.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: Three phase load preference
Jtester
A 120/240 3 phase 4 wire delta will still have a grounded tap between "A" & "C" phases and out buildings that use the neutral for ground fault path will still produce stray voltages on everything that is bonded to the neutral wether it is a 120/240 3 phase 4 wire delta or a 120/208 Y. This did not make any sense?
Even a grounded "B" phase delta service would have this problem if the "B" phase delta was used to bond the grounding at a out building.
Edited to add:
I have to add that the most common cause of stray voltage is the connection between the primary MGN to the secondary neutral at the transformer. This is why they are allowed to use a arc gap aresstor in this connection for farm's
[ December 23, 2004, 09:03 PM: Message edited by: hurk27 ]
Jtester
A 120/240 3 phase 4 wire delta will still have a grounded tap between "A" & "C" phases and out buildings that use the neutral for ground fault path will still produce stray voltages on everything that is bonded to the neutral wether it is a 120/240 3 phase 4 wire delta or a 120/208 Y. This did not make any sense?
Even a grounded "B" phase delta service would have this problem if the "B" phase delta was used to bond the grounding at a out building.
Edited to add:
I have to add that the most common cause of stray voltage is the connection between the primary MGN to the secondary neutral at the transformer. This is why they are allowed to use a arc gap aresstor in this connection for farm's
[ December 23, 2004, 09:03 PM: Message edited by: hurk27 ]
dnbob
Senior Member
- Location
- Rochester, MN
Re: Three phase load preference
There have been a multitude of complaints from one of the local utilities which had open delta's feeding dairy farms. This particular farm had I believe 6 amps flowing back on the neutral. From earth to a grounded waterpipe I believe there was 85 milliamps approx. Once they added the 3rd phase, the milliamps were in the low 20's, below the sensitivity of the cattle. That is why we installed an isolation transformer, so there was no tie to the utility neutral. I am not an expert on stray voltage, but the utility has corrected numerous farms recently over a wide spread area, of course at the farmers expense as well as loss of productivity.
There have been a multitude of complaints from one of the local utilities which had open delta's feeding dairy farms. This particular farm had I believe 6 amps flowing back on the neutral. From earth to a grounded waterpipe I believe there was 85 milliamps approx. Once they added the 3rd phase, the milliamps were in the low 20's, below the sensitivity of the cattle. That is why we installed an isolation transformer, so there was no tie to the utility neutral. I am not an expert on stray voltage, but the utility has corrected numerous farms recently over a wide spread area, of course at the farmers expense as well as loss of productivity.
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