Oversizing motor conductors

[autopilot]

We have a 15 HP 208 volt 46.2 amp air compressor motor, 120 ft of #6 copper for the supply. I did the VD calc and it is less than 3%. The building owner does not like the slight dimming of the lights when the motor starts. Does anyone know if going up in wire size to #4 would help or is a soft start the only choice? Any opions would be very appreciated.

 

[don_resqcapt19]

Increasing the size of the motor conductors will likely make the lights dim even more. The increase in size of the motor conductors will let even more current flow to the motor and increase the voltage drop on the system. Smaller conductors actually create a limited form of reduced voltage starting.

 

[cadpoint]

Here's. A nice overview of softstart http://www.eng-tips.com/faqs.cfm?fid=1064.

I frankly would think that the panel is near or nearly over loaded, with a 15 HP starting! Point being if every
light circuit if even running at 16 amps it could well see the dimming.

Most commerical and industrial always seperate the lights and motors, this elimates dimming. This also usually
includes a distrubution panel in front of the various panels.

Just because one can add some size breaker doesn't relieve one from doing load calc's.

You could see if the loads are balanced on the panel, the best they can be. You might even consider a new
cicuirt and move some lights to it but I don't this will get it either.

It's use of the motor that will determine if you can apply the various soft starts.

 

[autopilot]

Thank you for the input very thankfull that I found this site.

Thank you for the input very thankfull that I found this site.

The existing instalation is in a 200 amp sub panel with an avarge ballanced load of 40 to 60 amps A B and C about 5 foot from the 800 amp main I-Line panel. If I move the 15 hp breaker to the main I LINE panel and take out an existing spare 200 amp breaker that is for a future addition and replace it with a 90 amp, that would put the load on the main service. Would this be a posibly sullution for the voltage drop dimming the lights? (if the addition is ever constucted it would be at the same end of the building that the compressor is at now and could be reconnected to the future panel) Opinions would be great.

 

[Dennis Alwon]

The dimming can be caused by many factors and a common cause can be the distance the transformer is from the building. It is very common for large motors to cause dimming or a small draw down on other circuits.

 

[Smart $]

The existing instalation is in a 200 amp sub panel with an avarge ballanced load of 40 to 60 amps A B and C about 5 foot from the 800 amp main I-Line panel. If I move the 15 hp breaker to the main I LINE panel and take out an existing spare 200 amp breaker that is for a future addition and replace it with a 90 amp, that would put the load on the main service. Would this be a posibly sullution for the voltage drop dimming the lights? (if the addition is ever constucted it would be at the same end of the building that the compressor is at now and could be reconnected to the future panel) Opinions would be great.

You would reduce voltage drop to the compressor motor proportional to the amount of resistance contributed by the 5' between main and sub (i.e., very little if any noticeable improvement)... and you may add the voltage drop right back into the equation if you have to extend the compressor motor conductors to reach the main. Simply put, this will likely not reduce any dimming or flickering effect. As mentioned by Dennis, but in other words, the dimming-flickering is a result of transformer voltage sag (i.e. not an overly "stiff" supply voltage) and combined voltage drop of the service and feeders. Measure the voltage at the service before, during, and after starting the compressor. If there is a noticeable voltage drop, the cause of dimming-flickering is ahead of the service equipment.

 

[mnbiker]

would it be possible to get a compressor guy in to do i little plumbing, so the compressor motor runs all the time. but is only loaded when being called on. All Our compressor ( quite a bit larger) all have a load/no load option. this greatly reduces the start up current as the motor is already spinning.

 

[kwired]

would it be possible to get a compressor guy in to do i little plumbing, so the compressor motor runs all the time. but is only loaded when being called on. All Our compressor ( quite a bit larger) all have a load/no load option. this greatly reduces the start up current as the motor is already spinning.

I would think a soft start or VFD would be better choice when it comes to energy consumption. An unloaded motor still consumes a certain amount of power just to keep it going, where a stopped motor is consuming nothing, and a slowed down motor is still working but at a lower rate.

 

[Jraef]

As mentioned, the first thing to check is that the compressor has an unload valve and that it is functioning properly. Sometimes people dont understand the purpose and believe unloaders are wasting energy, so they disable them.

In some designs, the unload valve is on the receiver and is normally closed (so that the tank hold pressure when the compressor is off) and the solenoid pulls in to open the valve for a few seconds when the motor is first started, then drops out again to let the pressure build. For those, there will be a timer in the circuit. Others have a normally open valve on the compressor head, and when unpowered, vent the compressor cylinder to atmosphere, then the solenoid is energized at the same time the motor is. The valve has a way to make it close slowly to give the compressor time to get to speed first. If you have this type, a distinctive characteristic is that the compressor makes a loud hiss whenever the motor shuts off as the valve vents the residual pressure in the head. With neither of these, the compressor will be difficult to start (assuming reciprocal or screw compressor here). If you don't have an unloader, talk to a compressor dealer about adding one first, because it probably should have one either way and that alone may solve the problem.

If you do have a functioning unload system and it's just a matter of too much burden on the power feed, a soft starter will likely do the job on this because from the sounds of it, the system CAN start it, it just causes too much VD from the starting current. But I agree with the others, increasing the cables or changing the breaker will not likely help and may make it worse. Those solutions would solve a VD at the MOTOR terminals if that were the problem, but not the VD on the entire system, unless the motor VD is making it take so long to start that the up stream transformer is saturating. If that were the case, you would likely also be getting nuisance overload tripping of the compressor (unless that did happen before and someone "fixed" it by upping the overload heater!).

VFDs are not always good on compressors, it depends on the type of compressor, the storage system and the type of lube system it has. Don't go that way unless you can engage someone with a lot of experience in successfully applying VFDs to air compressors.

 

[kwired]

VFDs are not always good on compressors, it depends on the type of compressor, the storage system and the type of lube system it has. Don't go that way unless you can engage someone with a lot of experience in successfully applying VFDs to air compressors.

Yes, I mentioned VFD's, but failed to mention that you can't just put one on any unit, as it can introduce other problems - the unit needs to be designed to be run at variable speeds in order to use one. If it is a reciprocating compressor, it likely is not going to last if run at speeds below its rated speed, short term acceleration excluded, which is cheaper to do with the soft starter.

 

[kingpb]

A simple way to possibly reduce the negative affect is to simply use the transformer taps.

The taps are usually on the HV side of transformer because less rated current, so you use the -2.5% tap, or even -5% tap. This will raise the voltage on the LV side. Cheap fix if it accomplishes what you need.

 

[weressl]

As mentioned, the first thing to check is that the compressor has an unload valve and that it is functioning properly. Sometimes people dont understand the purpose and believe unloaders are wasting energy, so they disable them.

In some designs, the unload valve is on the receiver and is normally closed (so that the tank hold pressure when the compressor is off) and the solenoid pulls in to open the valve for a few seconds when the motor is first started, then drops out again to let the pressure build. For those, there will be a timer in the circuit. Others have a normally open valve on the compressor head, and when unpowered, vent the compressor cylinder to atmosphere, then the solenoid is energized at the same time the motor is. The valve has a way to make it close slowly to give the compressor time to get to speed first. If you have this type, a distinctive characteristic is that the compressor makes a loud hiss whenever the motor shuts off as the valve vents the residual pressure in the head. With neither of these, the compressor will be difficult to start (assuming reciprocal or screw compressor here). If you don't have an unloader, talk to a compressor dealer about adding one first, because it probably should have one either way and that alone may solve the problem.

If you do have a functioning unload system and it's just a matter of too much burden on the power feed, a soft starter will likely do the job on this because from the sounds of it, the system CAN start it, it just causes too much VD from the starting current. But I agree with the others, increasing the cables or changing the breaker will not likely help and may make it worse. Those solutions would solve a VD at the MOTOR terminals if that were the problem, but not the VD on the entire system, unless the motor VD is making it take so long to start that the up stream transformer is saturating. If that were the case, you would likely also be getting nuisance overload tripping of the compressor (unless that did happen before and someone "fixed" it by upping the overload heater!).

VFDs are not always good on compressors, it depends on the type of compressor, the storage system and the type of lube system it has. Don't go that way unless you can engage someone with a lot of experience in successfully applying VFDs to air compressors.

Too bad that individual posts can not be rated. The OP is an interesting topic, but not necessarily merit a high rating. However this response is an excellent and comprehensive response to the fundamental issue.

 

[autopilot]

Will Do voltage check.

Will Do voltage check.

Thank you. I will check voltage on the next trip there. Info on VD to the main by the 5 foot of feeder was my question, thanks for the answer.

You would reduce voltage drop to the compressor motor proportional to the amount of resistance contributed by the 5' between main and sub (i.e., very little if any noticeable improvement)... and you may add the voltage drop right back into the equation if you have to extend the compressor motor conductors to reach the main. Simply put, this will likely not reduce any dimming or flickering effect. As mentioned by Dennis, but in other words, the dimming-flickering is a result of transformer voltage sag (i.e. not an overly "stiff" supply voltage) and combined voltage drop of the service and feeders. Measure the voltage at the service before, during, and after starting the compressor. If there is a noticeable voltage drop, the cause of dimming-flickering is ahead of the service equipment.

 

[autopilot]

Unload Valve

Unload Valve

The unload valve that I call the head preasure releaf valve (hope we are talking about the same?) was working when I was last there and is a mecanical type. Good thought, maybe there is something wrong there because it has been weeks that the commpressor has been in service and am just hearing about it now? Let me know your opinion? Note: Very cool that I can tap the know how of everyone on this site.

As mentioned, the first thing to check is that the compressor has an unload valve and that it is functioning properly. Sometimes people dont understand the purpose and believe unloaders are wasting energy, so they disable them.

In some designs, the unload valve is on the receiver and is normally closed (so that the tank hold pressure when the compressor is off) and the solenoid pulls in to open the valve for a few seconds when the motor is first started, then drops out again to let the pressure build. For those, there will be a timer in the circuit. Others have a normally open valve on the compressor head, and when unpowered, vent the compressor cylinder to atmosphere, then the solenoid is energized at the same time the motor is. The valve has a way to make it close slowly to give the compressor time to get to speed first. If you have this type, a distinctive characteristic is that the compressor makes a loud hiss whenever the motor shuts off as the valve vents the residual pressure in the head. With neither of these, the compressor will be difficult to start (assuming reciprocal or screw compressor here). If you don't have an unloader, talk to a compressor dealer about adding one first, because it probably should have one either way and that alone may solve the problem.

If you do have a functioning unload system and it's just a matter of too much burden on the power feed, a soft starter will likely do the job on this because from the sounds of it, the system CAN start it, it just causes too much VD from the starting current. But I agree with the others, increasing the cables or changing the breaker will not likely help and may make it worse. Those solutions would solve a VD at the MOTOR terminals if that were the problem, but not the VD on the entire system, unless the motor VD is making it take so long to start that the up stream transformer is saturating. If that were the case, you would likely also be getting nuisance overload tripping of the compressor (unless that did happen before and someone "fixed" it by upping the overload heater!).

VFDs are not always good on compressors, it depends on the type of compressor, the storage system and the type of lube system it has. Don't go that way unless you can engage someone with a lot of experience in successfully applying VFDs to air compressors.

 

[Jraef]

Too bad that individual posts can not be rated. The OP is an interesting topic, but not necessarily merit a high rating. However this response is an excellent and comprehensive response to the fundamental issue.

Thanks. I used to do commisioning and troubleshooting of soft starters, my company had contracts with Gardner Denver, Sullair, Champion and a few others, so I did a LOT of compressors. Unfortunately, I did a lot of them where people retrofitted a soft starter and the problems didn't go away, so I would get called in to "fix the soft starter". 95% of the time it actually was an unloader valve issue and the soft starter was actually unnecessary (for that purpose). I was usually there working on behalf of the compressor company, but their own techs never seemed to bother checking the unloader, they would get it in their mind that it was an electrical problem and got stuck in that rut. I learned more about unloaders in compressors than I ever thought I would have to.

 

[Jraef]

The unload valve that I call the head preasure releaf valve (hope we are talking about the same?) was working when I was last there and is a mecanical type. Good thought, maybe there is something wrong there because it has been weeks that the commpressor has been in service and am just hearing about it now? Let me know your opinion? Note: Very cool that I can tap the know how of everyone on this site.

Yes, sounds like the same thing. On older machines, that valve simply worked on the head pressure itself rather than an electrically operated solenoid. But the old "M" type mechanical unloaders were notorious for getting stck and not venting, especially if a compressor sat unused for a period of time. Debris / rust / loss of lubrication all contribute to their demise, people seldom think of including them in PM work. The electrically operated versions were a response to that because you could tell when they were not working (in theory), but a lot of companies stick with not having to wire anything else and use the mechnaical unloaders. They typically last out the warranty, but not by much.

 

[kwired]

Thanks. I used to do commisioning and troubleshooting of soft starters, my company had contracts with Gardner Denver, Sullair, Champion and a few others, so I did a LOT of compressors. Unfortunately, I did a lot of them where people retrofitted a soft starter and the problems didn't go away, so I would get called in to "fix the soft starter". 95% of the time it actually was an unloader valve issue and the soft starter was actually unnecessary (for that purpose). I was usually there working on behalf of the compressor company, but their own techs never seemed to bother checking the unloader, they would get it in their mind that it was an electrical problem and got stuck in that rut. I learned more about unloaders in compressors than I ever thought I would have to.

It doesn't matter what kind of machine it is, if you have motor overloads tripping there is something wrong with the motor:happyyes:.

Just had one recently - brand new (well one year old) grain elevator, we have an ammeter on the leg motor so operator can tell how much they can load it. Of course owner was there when I was there and gave them a max load allowed according to the meter, owner says - we need to be able to unload faster than that. Well, I can set it so it will trip at a higher level but you will be replacing a burned out motor eventually, and during busy harvest season when trucks are lined up to unload, a few more minutes per truck wait time is better than waiting for a motor to be replaced, especially one that is 100 feet up in the air.

Faster unloading is not my issue, it should have been (and probably was) addressed with equipment people when you purchased this equipment. Faster unloading likely would mean bigger leg, bigger motor, bigger everything = more money when it was first installed. People just don't realize machines have limitations.

 

[weressl]

It doesn't matter what kind of machine it is, if you have motor overloads tripping there is something wrong with the motor:happyyes:.

Just had one recently - brand new (well one year old) grain elevator, we have an ammeter on the leg motor so operator can tell how much they can load it. Of course owner was there when I was there and gave them a max load allowed according to the meter, owner says - we need to be able to unload faster than that. Well, I can set it so it will trip at a higher level but you will be replacing a burned out motor eventually, and during busy harvest season when trucks are lined up to unload, a few more minutes per truck wait time is better than waiting for a motor to be replaced, especially one that is 100 feet up in the air.

Faster unloading is not my issue, it should have been (and probably was) addressed with equipment people when you purchased this equipment. Faster unloading likely would mean bigger leg, bigger motor, bigger everything = more money when it was first installed. People just don't realize machines have limitations.

Not to mention that a failing motor can catch fire and ignite all around it. There are few cases I would allow overload to be set higher than required, but potentially explosive or fire areas are not one of them.

 

[kwired]

Not to mention that a failing motor can catch fire and ignite all around it. There are few cases I would allow overload to be set higher than required, but potentially explosive or fire areas are not one of them.

That risk was not that high on an leg located outdoors. Grain handling equipment in confined areas and indoors - is an explosion hazard. You just are not finding those kinds of installations in simple storage and transfer operations, reduced explosion hazards of having things outdoors not only makes installation cost less, but makes insurance cost less also.

 

[Haji]

Increasing the size of the motor conductors will likely make the lights dim even more.

But if only the size of its feeder conductors is increased, this would solve the lights dimming problem.

Smaller conductors actually create a limited form of reduced voltage starting.

But this may impact on the induction motor stability.