Quantifying Power Consumption

shaunn

Member
Join Date
Sep 2005
Location
Orapa
Posts
39
Morning All,

Seeing as we are all becoming more energy conscious of late, (especially in Africa) we had a suggestion to automatically switch off certain motors (e.g: conveyors)and other electrical devices in our plant if they happen to run without any material for more than a certain amount of time. (Amongst other criteria as well.)
I was wondering if there was a feasible way of quantifying the power saved by doing this, to determine if it was a worthwhile exercise or not?

All our motors are linked to PLC's via simocodes, so % of full scale current readings are readily available and motor ratings are available from the name plates. Is it as simple as calculating run time x kw rating x cost/kWh and using this as an indicator of money saved?

Any suggestions will be most helpful.

Regards,
Shaun.
 
The savings can be calculated as you indicated. For long term energy savings around the plant purchase a device to measure the usage. Something like Amprobe's DM-II Pro will measure all the variables you need to calculate the cost (except your cost per KWH, get it from your electric bill)

Modify one machine after having measured the electric usage on a typical day, than measure the usage after modification.

Hope this helps.
 
Shaun,

There are several offestting factors that will work against this scheme. Whether these factors are big or small depends on the the type of load, the size of the motors, and how often each starts.

1. Extra power used for extra start/stops.
2. Additional wear on the electrical contactors.
3. Increase in the plant power factor.

You may need to calculate or measure the extra power used for starting each motor. During start-up from 0 to full speed, a motor can use 1.5 to 2 times more power than for normal running. If the start/stops are going to be frequent, say every 5 minutes or less, then the extra start-up power will be a factor that offsets some of the savings from shutting off the motors. The extra wear on the contactors is also another cost that will ofset the gains from this scheme.

This method of power saving could cause your power factor to increase. If your plant is paying a power-factor charge, the increased power factor and demand factor will be another offset to the savings, unless additional correction capacitors are installed. In this case, the best power-factor control is probably to install a capacitor on each motor contactor, so that when the motor runs, the capacitor is energized.

Many electric utilites bill for "KW demand" based on the highest KW in a time period, say 15 minutes. If your plant is billed like this, you may want to set the allowed run time to be greater than the demand-factor time period, so that the KW demand factor is not increased by the additional start-ups. One method is to stagger the start-up of largest motors, so that only 1 of them is allowed to start in the demand-factor time period.
 
Last edited:
Shaun

I agree with Lancie shutting down machinery and restarting frequently may not be the best thing to do. Shutdown for 1 minute and then restart may run your maintenance costs up and eat into power savings. Shutdown and restart an hour later should be just a little more maintenance but with much less "eat into" power savings.

Go about this in an organized manner.
Start with recording your utility bills ie total Kw demand factor power factor and cost for each billing item and TOTAL COST. In my mind it is easier to convince management using money out of their pocket than Kw.

Start with biggest loads first. See how long they run and how long they are really needed. Then next biggest "class of load sizes"

LIGHTING can be a very large consumer. Get most efficient lighting you can. Frequent off on will decrease lamp life on fluorescant but in most industrial applications this is not a problem since the lights are needed for hours at a time. Shut em down on off shifts. Costs nearly nothing (just labor and maybe shorter contact life) to turn things off.

After all this is done then start looking into how to salvage energy wasted ie hot water down the drain. Use water cooled aftercoolers on air compressors to remove moisture from air and to preheat hot water for building heat or hot water consumption.

Dan Bentler
 
One thing to remember about demand charges is that (in the US typically) they are based on the peak 15 minute kW demand for a month at a facility. So, unless you can go an entire month without starting a given motor, the demand charge isn't that big a consideration. If you start a motor once a month during on peak hours or if you start it ten times a month your demand charge will probably not change.

Consumption is cumulative, so the savings from shutting down motors will probably be significant. If this is tied into an automated system it should be possible to program the system to only start one motor at a time and to space the starts 15 minutes apart as indicated by Lancie. That could actually decrease demand charges compared to manual operation. Similarly, you could use a time delay to inhibit the no load shutdown from momentary or short term load changes. This will minimize the wear and tear factor as well.
 
So, unless you can go an entire month without starting a given motor, the demand charge isn't that big a consideration.
It is where the motors are large and many. Say it is a fertilizer plant and there are five 2000 horsepower ammonia compressors (not uncommon at all), plus about 200 smaller motors from 200 hp. and down. Usually these large compressors are started once and run until a planned or forced shutdown. During a start-up event, if they all go online at the same time, the KW demand factor for the month is much higher than if they start 1 every 15 minutes. Starting all at the same time would also be a terrible strain to put on the plant's main transformer and switchgear.

In these cases, I put an Auto Start-up routine in the PLC that starts the largest, waits, starts the next largest, and so on for all the motors that the process constraints will allow to be put into this pattern.

Either way you start-up, the power bill at one of these plants runs in the $100K range each month.
 
Last edited:
i think maybe inverters can help with this. either you can set the energy saving function to lower amps or entirely shut them off for longer periods.. however, the problem comes with "waking" up from energy saving mode... many are encountering stall or sometimes unable to cope with intantaneous demand.
 

Similar Topics

Hi All, I am trying to figure out a way to quantify the power required for our machines VFD braking resistors. We don't currently have a way to...
Replies
1
Views
1,304
We normally ground the -24VDC side of a control systems power supply. What are you folks thoughts about grounding the -24VDC side of a class 2...
Replies
1
Views
84
Hi all. Would anyone happen to have the Modbus register datasheet for the Honeywell/EnergyICT EIMFlex Modular Meter v1.11 We've 40 of these that...
Replies
0
Views
50
I have an issue with Power Flex 525 during running processing, the VFD stopped suddenly while the PLC and VFD connection ok, VFD does not have any...
Replies
1
Views
92
Good day Forum Members I got a older Lincoln welder and hoping to make it work at our shop. Welder in question is the Lincoln Power Wave 455M...
Replies
4
Views
145
Back
Top Bottom