scottf200
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Wiper inefficiencies - intermittent or low speed better

Wed Nov 13, 2013 4:25 pm

So in my laymen terms: Because of start up load of a motor is it more efficient to run the wipers at low speed instead of intermittent as the motor (I presume) is starting and stopping every X seconds?

Video showing continuous motor running when wipers are on: http://www.youtube.com/watch?v=PMxBKlfQ1IY


Background:
http://hvac-talk.com/vbb/showthread.php ... st13654251
LRA - Locked Rotor Amps: The current you can expect under starting conditions when you apply full voltage. It occurs instantly during start up.
RLA - Rated Load Amps: The maximum current a compressor should draw under any operating conditions. Often mistakenly called running load amps which leads people to believe, incorrectly, that the compressor should always pull these amps.
FLA - Full Load Amps: Changed in 1976 to "RLA - Rated Load Amps".

How many watts will a 2.2kw motor draw on starting?
http://answers.yahoo.com/question/index ... 851AAjid2j
Your compressor will draw about six times there normal running current when it starts. So lets get that first.

Assumptions I have to make since you did not provide adequate information are these:
1)Starting current is 6 times running current
2)Operating power factor for this motor under normal load is 0.80
3) The single phase rate voltage to operate this motor is 240 volts

You will have to adjust your numbers based on what the actual figures are.

- Running kVA = kw/power factor = 2200 / 0.8 = 2,750 VA
- Current = 2750vA / 240 volts = 11.46 amps.
- Starting current = 6 * 11.46 amps = 68.8 amps.
- Your inverter must supply this staring current and not drop the supplied voltage any more than 10% or go no lower than 216 volts during start up..
- The inverter rating will have to be about 16.5 kVA for starting this motor load. Especially is the compressor does not have an unload-er valve for the compression pistons.
Last edited by scottf200 on Wed Nov 13, 2013 5:02 pm, edited 3 times in total.
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mwalsh
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Re: Wiper inefficiencies - intermittent or low speed better

Wed Nov 13, 2013 4:40 pm

Seems to me the load is either pretty much the same with each sweep or of insignificant difference to the starting one. In any event, I only run the wipers when not doing so is dangerous. And I only allow them a single sweep until the next time I feel the windshield needs it. Unless it's completely pissing down.
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scottf200
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Re: Wiper inefficiencies - intermittent or low speed better

Wed Nov 13, 2013 5:03 pm

mwalsh wrote:Seems to me the load is either pretty much the same with each sweep or of insignificant difference to the starting one.
But from the video there is not "each sweep" as the motor is running continuous at low speed.
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Re: Wiper inefficiencies - intermittent or low speed better

Wed Nov 13, 2013 6:55 pm

There is a spike in the energy meter each time the wipers sweep, including and equal to the first one. Now even knowing that the energy meter is a pre-programmed approximation, to me that means it's using a similar amount of energy for all sweeps.
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smkettner
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Re: Wiper inefficiencies - intermittent or low speed better

Thu Nov 14, 2013 9:21 am

Run as appropriate. Efficiency delta will not get you 100' extra range in 100 miles driven.
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Re: Wiper inefficiencies - intermittent or low speed better

Thu Nov 14, 2013 9:29 am

what is the best wiper replacements?
do rubber inserts fit or do you need the whole support mechanism?
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Re: Wiper inefficiencies - intermittent or low speed better

Thu Nov 14, 2013 12:01 pm

smkettner wrote:Run as appropriate. Efficiency delta will not get you 100' extra range in 100 miles driven.


Yeah, just from the size of the fuse you can pretty much not worry about it from a practical matter. I figured the OP was just for academic curiosity.
I noticed you're still working with polymers.

scottf200
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Re: Wiper inefficiencies - intermittent or low speed better

Thu Nov 14, 2013 2:07 pm

Nubo wrote:
smkettner wrote:Run as appropriate. Efficiency delta will not get you 100' extra range in 100 miles driven.
Yeah, just from the size of the fuse you can pretty much not worry about it from a practical matter. I figured the OP was just for academic curiosity.
Correct. I was not worried about *EV range.

When I've thought about this it generally is between the quickest intermittent setting and the lowest continuous wiper blade setting.
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Re: Wiper inefficiencies - intermittent or low speed better

Thu Nov 14, 2013 2:31 pm

I have doubts that a small 12 vdc motor start amps can be compared to a half HP 120 vac induction motor.
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Re: Wiper inefficiencies - intermittent or low speed better

Fri Nov 15, 2013 12:36 pm

Brushed DC motors don't have appreciable inrush currents as compared to AC induction motors. The peak startup current is due mainly to the inertia of the mechanical system connected to the motor.

The accessory part of the energy screen is faked. There is no actual current measured.

I also agree, regardless of wiper speed, no discernible change in range will be detectable.

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