Motor start current draw 2

[hpuckett]

Question- background
I have a 20 hp 3 phase motor powering an reciprocating air compressor.
When the air dryer goes into regenerate mode we use alot of air which leads to the motor cycling on and off, maybe 30 times an hour, for several hours. This is obviously not a good situation on several levels, but what I am wondering about at this point is power consumption. I am contemplating putting in a relay that keeps the compressor on while the regeneration is taking place, to eliminate the repeated, needless and excessive restarts.

Actual question--
Does anyone have a feel for how to analyze the tradeoff in power consumption, i.e. the compressor running continuously for 2 hrs, loading and unloading as necessary vs restarting as needed, (as it does now) for maybe 60 times in a 2 hour period. We have a relatively low air consumption in general and we are probably oversized. Normal compressor start/runtime cycle is around every 5 minutes, starting unloaded and running loaded for maybe a minute. Additionally, I was wondering about the tradeoff in just having the compressor run all the time, loading and unloading as necessary, in terms of power consumptions given what seems to be excessive cycles, even under normal consumpion conditions. I know there are other mechanical concerns, but at this point and for this forum I am only wondering about power consumption directed at reducting electrical consumption. Anyone got any ideas?

 

[Marke]

Hello hpuckett

Do you have any indication what the off load power consumption is? this would have a major bearing on the results.
From my experience, the off load power consumption can still be significant and would lead to increased consumption if the commpressor is left running, but a comment on this figure would enable us to give a better appraisal.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

Starting a motor 30 times per hour is excessive. Yet staring it every 5 minutes is a high starting rate (your normal condition).

Energy consumption could not be so important since probably the motor starts in 2 to 3 seconds. The starting KVA are , around 6 kVA per Hp, as it has been discussed in this forum the starting condition is independent of the load, real power in kW ( kWH is what you pay) is calculated as KVA*PF ( power factor at starting is low, lets assume PF=0.4)

For a 2 hour period 30 starts per hour the energy consumption due to starts is:
kWh = HP*6*0.4*3(sec)*30 *2/3600 =2.4 KWH

The motor running 2 hours at full load will consume (assuming 85% efficiency):

kWh = 20*.746/.85*2 = 35.1 kWh

The point here is that the lives of the motor and starting device are under accelerated wearing due to the high stress imposed by starts.

In my opinion your storage tank is too small or the allowed drop of pressure is to tight, If the air storage tank is large enough and the allowed pressure drop is reasonably large, the compressor will start less frequently and will stay under load for extended time avoiding the destructive start stop cycling.

 

[jbartos]

Suggestion: Sometimes, this problem is solved by a large compressed air reservoir with high air pressure and various pressure reducers according to compressed air needs.

 

[jraef]

Assuming the pneumatic solution which jbartos pointed out is not feasible for some reason, the approach most often taken is to use a "short cycle timer". It keeps the motor running for a minimum ammount of time each time it is started, avoiding the rapid on-off cycling. You can use standard timers if you know what you are doing, but if not, the refrigeration industry uses them all the time for the same purpose (think of a teenager opening the fridge door every 5 minutes to see if there is anything he missed!). They call them just that, Short (or rapid) Cycle Timers.

You will significantly shorten the motor life with that duty cycle. An AC motor cools off much faster when running than it does when off. You are severely stressing the insulation in your windings.

As to power consumption, there is no better way to save than turning it off, and starting power is insignificant (cost wise) in comparison unless you are charged for instantaneous peak demand, a rarety with most utilities.

A short-cycle timer is a good compromise. It allows it to turn off sometimes to save some energy, but stay running during the high cycle times.



"Venditori de oleum-vipera non vigere excordis populi"

 

[skiier]

How is it that if you are "oversized" your unit runs too much? Jbartos has it right. You need a "cushion" tank of some sort for all that air. Pump up your tank. Don't just pump up the lines feeding your equipment. If you size the tank right your compressor may only run a few times an hour.

The energy required by the plant in terms of air is reflected to the electrical power supply. The compressor supplys this energy and can do so in an efficient manner or a not so efficient manner. Take your pick.
I think you probably don't need as big a cushion tank as you think.