Temp vs current

[Hipwell]

Hi all,

A question regarding temperature vs current.

For example if I've got a 15kw 3ph motor running in 40 degrees internal ambient temperature, how much less current / resistance will it use if I move the motor outside to say an external ambient 15 degrees.

We are talking about moving all our motors outside (where possible) to reduce running costs as it is very warm in the manufacturing area, roughly about 30+ degrees all year round.

Regards,
Dean

 

[Darkwood]

Welcome to the forum Hipwell

 

[Spoon]

Have you checked with the motor manufacturer, to see what they say?

 

[Barry White]

Can see the theory copper conducts better when it is cooler and therfore the motor in theory will be more eifficient but moving the motors outside ? Will this not provide losses in the drive drain to get the motion where you need it.
This is an interesting discussion please give more details.

 

[Marvo]

With DC motors and synchronous motors operating temperature has a considerable effect on the motor efficiency. With AC induction motors the effect of the ambient temperature on their energy consumption is minimal as long as they're being run within their design specifications. You may see a very small energy saving by relocating them but it's impossible to predict the exact amount. There's also a possibility that relocating them could increase their energy consumption due to voltdrop across longer cable lengths etc.

What has motivated you to explore relocation as being your energy saving strategy? Lower ambient temperatures would certainly increase the MTBF (life expectancy) of your motors but I wouldn't see it as resulting in effective energy saving.

Have you looked at installing drives on the motors? VFD's run motors far more efficiently than connecting DOL or Y-D. Have you considered other options such as power factor correction? Both these options would almost certainly decrease energy consumption far more than lower ambient temperatures.

 

[Hipwell]

Hi,

They are 15kw motors running a vacuum pump.

We are considering moving them outside (under s shelter) because they generate a lot of heat and this creates high internal temperature for the operators, I also want to put across to the factory manager the added running benefits with costs.

We will be taking a low voltage cable to switch a contactor to turn them on and off from the current placement and running a more local 3 phase supply.

Say if they are currently running at 20A in a constant 40 degree temp, what is the expected drop in current if they are in 15 degree temp?

If there is a correction factor calculation for this then I could also use this in further projects.

Thanks in advance

 

[Spoon]

I take it you haven't contacted the motor manufacturer to see what they say...

 

[Marvo]

..........Say if they are currently running at 20A in a constant 40 degree temp, what is the expected drop in current if they are in 15 degree temp?........

It will be negligible. The only way to establish the difference in run current would be to actually do a real life test.

I think you've got to change your mindset from thinking about run current to rather thinking about overall efficiency. Efficiency is about getting the maximum end process for the minimum electrical energy input. Just reducing run current doesn't necessarily achieve this in fact it's possible with some changes that result in a reduced run current could actually result in a drop in overall system efficiency and could increase overall energy consumption.

I'd suggest firstly you need to list your objectives in order of priority. If reducing the temperature in the workplace is the driving priority you may be better looking at a forced ventilation and air movement solution. If energy saving is the top priority then you need to do a full energy audit of the entire process including establishing the efficiency each motor is working at plus power factor etc then look at VFD's and PFC.

 

[Spoon]

It sound like the OP is determined to move the motors and not take any advice.... or maybe it's just me....

 

[Hipwell]

No I haven't contacted the motor company as of yet but its something I will do thank you.

Marvo I understand what you are saying about efficiency's, I totally agree with you.
The move is happening for a number of reason, internal temperature, space, better access, I'm trying to include a cost saying for the ESOS scheme.
Agreed it may be negligible but still potentially a saving?
The motor and cable will have higher resistance running in higher temperature so this is where the small saving will come into place?

What I'm asking for (it may not exist!) is a correction factor for different temperatures, e.g. if a motor specs a 20A design current what is the ambient temperature for this? e.g. 40degrees so if you move the motor to the chilled room of 10 degrees then times the current by 0.9981 to get the new value?

Maybe I'm wrong just thought id ask the question.

Thanks again.

 

[Lucien Nunes]

It is not current that you want to save, it is power, which is what you pay for. An induction motor has variable power factor with load and voltage, so reducing the power might not reduce the current but will save on your bill.

The tempco of copper is 0.39%/K, so a 15 degree difference will affect the winding resistance by 5.8%. But you won't save 5.8% of the power, nor even 5.8% of the loss (which might be 5.8% of 10% i.e. 0.58%.) It would only affect the copper loss, perhaps half the total, the remainder being iron, cooling fan, bearings etc. On that basis you might save 0.3% due to lower copper temp. Unless you go massively overboard with piping and cabling, you will probably add much more than this in extra losses outside of the motor.

By contrast, a lot of processes run with motors of completely the wrong size working inefficiently, and resizing the motor or pump or adding VFDs or some other performance improvement, can often save tens of percent.

 

[Marvo]

.....I'm trying to include a cost saying for the ESOS scheme.
Agreed it may be negligible but still potentially a saving?

Yeah, I'm with Lucien on this. I think you're on a hiding to nowhere to try claim energy savings were made because the motors are at a lower ambient temperature, you might get it past a maintenance manager but it's going to be a hard sell to anyone who knows motor theory. The only way I'd be buying into it is if comprehensive energy consumption figures were available for before and after the move to back the claim up.

The motor and cable will have higher resistance running in higher temperature so this is where the small saving will come into place?

What I'm asking for (it may not exist!) is a correction factor for different temperatures, e.g. if a motor specs a 20A design current what is the ambient temperature for this? e.g. 40degrees so if you move the motor to the chilled room of 10 degrees then times the current by 0.9981 to get the new value?

You're still trying to sell a reduced run current as an energy saving, I²R losses are only a small window into a bigger picture with many other variables, some of which are far more likely to have a far more profound effect on energy consumption.

If you have your heart set on relocating the motors I'd strongly suggest you motivate using reliability (>MTBF) and ease of access for maintenance or for replacement which translates as less downtime, also operator comfort (temperature/noise levels) would also be relevant motivation but unless you're adding PFC or VFD's or substantially reducing cable lengths I'd leave possible energy saving out of it if someone is likely to ask for your supporting data.

 

[marconi]

Hi,

They are 15kw motors running a vacuum pump.

We are considering moving them outside (under s shelter) because they generate a lot of heat and this creates high internal temperature for the operators, I also want to put across to the factory manager the added running benefits with costs.

We will be taking a low voltage cable to switch a contactor to turn them on and off from the current placement and running a more local 3 phase supply.

Say if they are currently running at 20A in a constant 40 degree temp, what is the expected drop in current if they are in 15 degree temp?

If there is a correction factor calculation for this then I could also use this in further projects.

Thanks in advance

Reading your posts #1 and #6 I think you are not studying the current energy costs of running a number (unspecified in your posts but more than 1) 15kW vacuum pumps and the options this project provides for replacing them with fewer lower powered but more efficient equally effective machinery. And, there are the maintenance costs too.

15kW, 8 hours a day, 5 days a week 49 weeks a year( 52 - one week each for Christmas, Easter and a maintenance week) is 29400kWh. At circa 9p for Business rate kWh (a figure I obtained from a quick Google search) the electricity cost for one machine is 29400 x 0.09 = £2646

I did a quick Google search 'energy efficiency vacuum pumps' and found some case studies of schemes to replace old technology with new, energy efficient machines. The one I read replaced three running 7.5kW machines with one 4kW Liquid Ring Technology machine. Doing the sums that reduced the annual energy bill from 3 x £1323 = £3969 to £705. Less maintenance, noise, space, heat too?

Just a thought for an avenue of investigation using a through-life cost analysis.

 

[Besoeker]

Hi all,

A question regarding temperature vs current.

For example if I've got a 15kw 3ph motor running in 40 degrees internal ambient temperature, how much less current / resistance will it use if I move the motor outside to say an external ambient 15 degrees.

We are talking about moving all our motors outside (where possible) to reduce running costs as it is very warm in the manufacturing area, roughly about 30+ degrees all year round.

Regards,
Dean

Not enough, by a considerable margin if any, to justify moving the motors