Lock rotor amps experiment

[Geo-TH,In]

In the scientific community a person has an idea, he conducts an experiment, then publishes the results for other people to confirm or disprove. I challenge anyone to conduct my experiment, publish your results before you say it won't work.

I've read generators don't like motors because of the lock rotor amps (LRA) may be 3x the full load amps (FLA). I discovered the LRA for my 4 hp electric chain saw is at least 6x the FLA .

I conducted my experiment on a portable self-cleaning workbench (truck's tailgate), in my heated attached garage. My chain saw is a Worx 18 in 4 hp which I purchased at Sears. My genny is a Honda clone, champion 3500 watt RV 120v 30 amp out.

I carry 275 of 12 g extension cords in truck. I used the 120v outlet in garage which is about 100 ft (12g wire) from load center in house, because I don't have my genny with me today. I had to use 25 ft of cord to connect chainsaw to 120v. I measured the volts at that point and added more extension cords between the 25 ft cord and the 120v of the garage.

Here are my results,
25 ft cord, no load volts (NLV), 122.8v full load volts (FLV) with saw on 115.3v ,
lock rotor amps (LRA) 59.75a
full load amps (FLA) 9.5a

I increase extension cord lengths in 50 ft increments. The results were as I had expected.

275 ft of 12 g extension cord added to the estimated 100 ft 12a house wiring produced the following results:
NLV = 122.4v FLV = 105.6v LRA = 41.4a FLA = 9.5a

I think you can see the LRA for about 125 ft ( 100 ft of house wire + 25 ft of cord) of 12g wire is almost 60 amps, twice the amps the genny is rated at. No wonder the instruction manuals for generators say you shouldn't use them on motors.

In conclusion, 375 ft of 12g wire ( 100 ft of house wire + 275 ft of cord) reduces the LRA almost by 1/3 and the voltage dropped about 16 volts to 105.6v

So there are times when power girds are under stress, the line voltages may drop to 105v, producing a brown out. I will use extension cords to reduce the LRA of my saw from 60a to at least 40 amps if not more.

I also don't think running a universal motor on 105v will harm the motor. My concern is not to harm the genny with 60 amps.

I have been using a 50 ft, 14 g cord on saw. Only problem is I don't have any 14g cords or my genny with me today, it's at my other home in country. I will be doing actual experiments with genny, 14g cords and chainsaw when the temps warm up a little. Publish those results later.

I challenge others to conduct my experiment and publish your results.
George

 

[tomturkey]

So please tell me, when I get my generator going because of power failure, and hook it up so my furnace will run:
1.) I should use as short a cord as possible between the genny and the hook up point.
2.) I should use 12ga or heavier
3.) My fan/blower motor won't like it but will run anyway
4.) Any other conclusions to draw Mr. Edison lol

 

[JMOR]

(quoted from post at 14:12:31 01/24/14) So please tell me, when I get my generator going because of power failure, and hook it up so my furnace will run:
1.) I should use as short a cord as possible between the genny and the hook up point.
2.) I should use 12ga or heavier
3.) My fan/blower motor won't like it but will run anyway
4.) Any other conclusions to draw Mr. Edison lol

dison was the DC guy.........Tesla & Westinghouse were the AC guys, and Geo is messing in their sand box.
As an aside, resistance motor starters are common in the large motor world.

 

[Geo-TH,In]

tomturkey,
I would suggest you get an ammprobe and measure your fan's LRA. I got a cheapie HF digital, which works better than my old analog probe for measuring lock rotor amps. Keep in mind, some furnaces don't like the noise produced by a gennys. It messes with their electronic brain boxes.

If your motor uses less than the amp rating of your genny use 12g cord. In my case the LRA is twice the rating of my genny, so I'm installing dc resistance, many extension cords, which seems to limit the LRA. The chain saw doesn't know the difference, it starts like normal.

Another experiment will be to measue the LRA of well pump. My goal is to do no harm to genny and still have the pump work.
George

 

[Geo-TH,In]

Jessie,
Some say Edison's greatest discovery was Tesla. Edison thought there was no need to re-invent the universal motor. Tesla may be thought of as the father to induction motors.

I've seen some very large 3 phase motors that had what we called a slip start. It was a 2 stage starting procedure. Don't remember just how they did it.

You gave me an idea. Like the current relay on a refrigerator, during start the LRA is great enough to pull in the start windings. When compressor comes up to full speed, current decreases and the relay drops out the start windings.

I need a current relay so when it sees 60 amps it install a small resistance during start mode, Then drop out when current levels get below 30 amps. Or it may be easier to use a timing relay that will install a small resistance for just a fraction of a second during the start mode. It was pre 1985 my father was in the hospital with a heart attack and I had to build a phase converter to run two 25 hp 3 phase motors.
It was better to use a timing relay to drop out the start capacitors. I think we used about 1 second for the start capacitors.

What do you think? Do I need more sand?
George

 

[JMOR]

You can Google 'resistance motor starters' & find more than you want to look at.

 

[tomturkey]

You guys are so far over my head, I see the George Marsh constellation and now the JMOR constellation. LRA is probably beyond my mental capabilities. And I wonder, Do I need to know so I would not ruin a generator? Or is this all in that elec eng/theory stuff. gobble

 

[Mike M]

Would you not have to in effect lock the chain saw chain from turning to get this reading ?

And if so how are you going to LOCK the well pump to test it ? unless you have an extra one not in the well.

 

[JMOR]

(quoted from post at 15:30:26 01/24/14) You guys are so far over my head, I see the George Marsh constellation and now the JMOR constellation. LRA is probably beyond my mental capabilities. And I wonder, Do I need to know so I would not ruin a generator? Or is this all in that elec eng/theory stuff. gobble

take the simple approach here............if the gen will start the motor, then all is well. If it won't, then get a bigger gen, don't try to use that motor/gen combination, unload the motor for starting, do as Geo it trying, attempt to reduce starting current.

 

[Geo-TH,In]

MikeM,
Your defination of LRA is different from what I'm familiar with in the hvac business. On an AC compressor it lists the FLA and LRA. The LRA is the current the motor uses when you first apply power and the rotor is stationary. There is no way to actually lock up a compressor or well.

Definition for "Locked rotor amp"
Definition for locked rotor amp
Locked rotor amp - (Noun)
LRA, or locked rotor amps, is a specification given to refrigeration compressors which indicates the current (amperage) being drawn by the electric motor within a refrigeration compressor when its electric motor is "locked up", such that the motor’s rotor is not turning. The LRA value is used to determine what gauge wire and circuit breaker should be used when wiring the compressor to the circuit breaker panel. Typically, the running amps (current, RA) is the current being drawn when the refrigeration compressor is running normally. The running current for small compressors is around 25% less of the locked rotor current.

To measure the LRA, there is a button on an ammprobe you push on. The probe then measures the largest current. The LRA current starts out large and within less than a second usually is close to the FLA.
George

 

[Geo-TH,In]

Jessie,
I like building things that I can't buy. Like the energy management system I came up for the genny. Got to thinking how I can make a circuit to reduce the LRA of my chainsaw and well. I'll post the results when I get a working model.

Went to my other place in the country where I keep the genny. I put the genny outside, ran an extension cord inside the attached warm garage and conducted some measurements. Results for genny, 25 ft of 12g wire and 50 ft of 14g wire. The term I use as FLA and FLV, full load amps are the measurements I make with the saw running and it's really not under load, just running. It would be impossible to put a uniform load of a saw.
NLV 126.4v FLV 120v LRA 66.6a FLA 9.7a

100 ft of 14g and 125 ft of 12g wire produced the following

NLV 126.3v FLV 113.8v LRA 43.7 FLA 9.6a

The genny shakes very little with 43.7a, unlike the stress the genny is under with FLA of 66.6a.

Final experiment was using 100 ft of 14g and 175 ft of 12g wire. Results
NLV 126.3v FLV 112.3v LRA 37.1a and FLA 9.4a

The genny really likes the added wire, no stressing, no shaking.

So this gives me a place to start from. I'll add 100ft to 200 ft of 12g ballast wire and then in less than a second short out the ballast wire. I think 200 ft will work perfectly for my chain saw. May have to use less ballast wire for the well. I could add a switch, one direction for well and the other for chainsaw.

I measured one well, it's powerfactored to 0.99, a 3/4 hp 120v LRA 53.7a FLA 9.6a.

I think I can, I think I can, I think I can. Let you know later when I get-er-done.
George

 

[Steve@Advance]

Been thinking about this all afternoon... My conclusion is TANSTAFL theory: (There aint no such thing as free lunch)

Here we go...

By adding the resistors (extension cords) you are relieving the load on the generator. But at the same time, you are reducing the end result power the motor delivers. It's like a soft start that never kicks into run.

For the saw, that may be OK, you're there controlling it, bog down, back off; run fast, push it harder.

My concern is a motor you have no control over, that is designed to need FLA at rated HP for extended peroid of time to do it's job, like a well pump, compressor, etc. The snowball effect: Load goes up, amps go up, volts go down, performance goes down, motor heats up, extension cord heats up, but the generator is happy. Problem is the work isn't getting done.

I would think all generators would or should come with proper overload protection, as in time delay circuit breaker, that will ignore instantaneous overload, and only react to a long term, overheat situation, therefore preventing damage to the generator. Granted, a generator run at or near capacity will experience a shorter life, especially the portable models.

Sometimes it's just not feasible to size a generator for the worst case possible load. But in that case you and I would know to use common sense and take action to prevent a total shutdown or burning something up. But, unfortunately there are people out there that have no clue. For them, the generator manufactures just say "don't run motors" to cover their butts.

I haven't ever sized a generator. I'm wondering if the same 125% FLA for motor loads applies or if it's higher?

My 2 1/2 cents worth! LOL

 

[Bill in IN]

Something tells me the cords are still coiled in your experiment. That being the case you are probably doing more reactance than resistance...

 

[Hanky]

Starting large motors has been done many ways, have seen some that actually use "reduced voltage starting" had a transformer to drop the voltage while it came up to speed, "resistance start" with big resistors in series with the motor windings that are shunted out when it gets up to speed. " wye start delta run" was another common method, and now we have VFD"s, and soft starters.

Craziest one I ever saw was a 16,000 HP motor that had the rotation stated by a smaller thru a huge electric clutch, and when it got up to speed the smaller motor dropped out and ithe bigger motor went across the 13,800 VAC and finished coming up to speed, a lot of unusual equipment out there.

 

[Dusty MI]

(quoted from post at 15:58:42 01/24/14) Been thinking about this all afternoon... My conclusion is TANSTAFL theory: (There aint no such thing as free lunch)

Here we go...

By adding the resistors (extension cords) you are relieving the load on the generator. But at the same time, you are reducing the end result power the motor delivers. It's like a soft start that never kicks into run.

For the saw, that may be OK, you're there controlling it, bog down, back off; run fast, push it harder.

My concern is a motor you have no control over, that is designed to need FLA at rated HP for extended peroid of time to do it's job, like a well pump, compressor, etc. The snowball effect: Load goes up, amps go up, volts go down, performance goes down, motor heats up, extension cord heats up, but the generator is happy. Problem is the work isn't getting done.

I would think all generators would or should come with proper overload protection, as in time delay circuit breaker, that will ignore instantaneous overload, and only react to a long term, overheat situation, therefore preventing damage to the generator. Granted, a generator run at or near capacity will experience a shorter life, especially the portable models.

Sometimes it's just not feasible to size a generator for the worst case possible load. But in that case you and I would know to use common sense and take action to prevent a total shutdown or burning something up. But, unfortunately there are people out there that have no clue. For them, the generator manufactures just say "don't run motors" to cover their butts.

I haven't ever sized a generator. I'm wondering if the same 125% FLA for motor loads applies or if it's higher?

My 2 1/2 cents worth! LOL

Steve, very well said. I was thinking along that line, but could not put it in words.

Dusty

 

[Indiana Ken]

George,

I am assuming you want to reduce the LRA in rush to allow using your generator on the chain saw...?

Can you replace the switch on the saw with a two position trigger switch. To start, you pull to the first position and allow the saw to start with no load. Once it is running at part speed then pull the switch to the second position and begin your cut. I was playing with a similar thing with my shop vacuum to allow my genny to run it. I used a diode in the first switch position and the second position bypassed the diode. The LRA with the diode with be less than 60 amps so it may work for you. My shop vacuum is not 4 Hp.

 

[Geo-TH,In]

Hanky,
You and JMOR seem to be the only ones who understands what I'm trying to accomplish. Before I did this experiment, everyone thought I was a bubble off, it can't be done, I'll damage this or damage that. They tell me why it can't be done, yet they haven't tried to do it.

I'm 100% convinced it will work.

I'm waiting on a delay on make timer to add my version of a soft start circuit add to my energy management circuit. Should complete it in about a week.

George

 

[MarkB_MI]

George, let's back up a bit. As you'll remember from your eighth-grade science class, every experiment starts with a hypothesis. What's your hypothesis?

I thought you wanted to prove you can take a motor with high starting current and by adding resistance, reduce its inrush current to the point where you could start it with a generator that otherwise couldn't start it. If this is your hypothesis, I don't see where you've proved it. Get out your generator and find a motor it can't start. Your air compressor might be big enough. Then start adding extension cords and see if you can add enough resistance to get it to start. I don't think you'll be successful, but I've been wrong many times before.

A couple of questions about your test:

How did you measure the locked-rotor amps of your saw? Did you clamp it in a vice so the chain couldn't move? I'm surprised you were able to get almost 60 amps out of a 120 volt wall socket without tripping your breaker.

How did you measure full-load amps? There needs to be a heavy load on the motor; sawing through a 10 inch diameter oak log ought to do the trick.

You really can't extrapolate your results from wall power to generator power; they are quite different. Generators have internal resistance which limits their current; that resistance may actually be greater than the resistance of your power cords. The wall power in your shop should have very little internal resistance. Generators are powered by engines that slow down under load while the power grid supplying your shop is effectively limitless.

I suspect you'll get different results using an ac capacitor-start motor rather than a brush-type universal motor.

One last comment: In the end, the purpose of an electric motor is to do work. Work requires power, which is of course voltage times current. Reducing the voltage to your motor means it needs more current to do the same amount of work, the more current it draws the greater the voltage drop until it stalls. Voltage drop is almost always a bad thing.

 

[Geo-TH,In]

MarkB_MI,
Read the posts at bottom to Hanky and JMOR. My goal is to make an energy management system for my genny and do what some of you thing is impossible. I want to make my genny start motors that have large LRA and not damage either genny or motor.
George

 

[MarkB_MI]

George, George, George,

I think you're starting with a flawed assumption: that you're necessarily going to damage your generator and/or motor if the generator isn't up to the task. Even if this is true (and I don't think it is), that doesn't mean the situation will improve if you starve the motor of the current it needs.

When you hook up a generator to a loaded motor, one of two things will happen: either the motor will start and run, or it won't. If it doesn't start, that doesn't necessarily mean you're damaging the motor or generator. And if it DOES run, that still doesn't ensure you won't overload the motor or generator because the generator can't provide enough current.

A few years ago (before I owned my own generator), I borrowed my brother-in-law's 750 watt generator and attempted to use it to run our refrigerator. It couldn't do the job; the refrigerator compressor would labor for a minute or so, then its thermal protection would trip. You're suggesting the problem was that I didn't have a long enough extension cord. Well, I had about fifty feet of 12 gauge cord running to the fridge, so how much did I need? 100 feet? 200? You know as well as I do that there is no way I was going to get the refrigerator running with that undersized generator.

 

[Geo-TH,In]

MarkB_MI, MarkB_MI, MarkB_MI,
I'll post my energy management device when I get a timer in about a week. Please just wait. I'll show you how it works. It's very much like a soft start or a slip start that has been used on very large compressors for decades.
George

 

[WantACaseLASomeday]

(quoted from post at 13:41:47 01/24/14) Jessie,
Some say Edison's greatest discovery was Tesla. Edison thought there was no need to re-invent the universal motor. Tesla may be thought of as the father to induction motors.


Just MHO but Tesla was an absolute genius, and Edison was an over-rated puffed up fool who had nothing better to do, so he stumbled upon a number of relatively obvious truths.

 

[Geo-TH,In]

Totally agree, Edison got credit for all the people who worked for him. Tesla was the brains, but Edison got the credit.

 

[JMOR]

(quoted from post at 15:47:33 01/25/14) Totally agree, Edison got credit for all the people who worked for him. Tesla was the brains, but Edison got the credit.

00 years later there isn't really any difference. Go to work for any corporation, as I did, and part of the employment agreement that you & I did/will sign, turns over any inventions, copyrights, patents, etc. that you make or file for, to the company. Same then & now.

 

[missouri massey man]

(quoted from post at 13:12:59 01/24/14) Would you not have to in effect lock the chain saw chain from turning to get this reading ?

And if so how are you going to LOCK the well pump to test it ? unless you have an extra one not in the well.

Mike, typically "lock rotor amps" is the term used to define the amperage consumed for just the short period in time it takes to initiate rotation form a dead stopped state. At that point it is at its "peak" and rapidly subsides to "rla" or "running load amps", or more specifically the amperage it takes to pull the load attached to it. That term used to be "fla" or, "full load amps", but recently has been generally changing to minimize confusion of what fla really is....

I worked with a guy that would charge refrigerators, freezers (commercial and residential) and central a/c's (commercial and residential) to the full load amperage of the compressors.....at ANY ambient temperature. 99% of the time he was OVERcharging.

George, I think the slip-start you are referring to may be "part-winding" start on larger three phase equipment....???? The contactors would pull in almost imperceptibally different times. Some had a larger delay and the difference in motor sound during start up was VERY noticeable...

You sure do some cool stuff sir.....I wish you lived closer.