Does undervolting damage an electric motor?

[fleabag]

I was reading reports of people who complained that their electric leaf blowers were crap because after X amount of uses or something like after 2 years, one day they found it sparking, causing the housing of their leaf blower to melt. I then read a post that the reason why this happens is because people use an insufficient gauge of wire to power their electric leaf blowers.

So now I'm wondering, is that really true? I mean I can believe that a 12a load on 50 or 100ft of 14 or even 16ga wire would cause a voltage drop but would too low of a voltage cause sparking and a fire? I'd have figured that too low of a voltage would just result in a leaf blower that didn't blow very well at all, not complete and total failure.

 

[eldorado99]

I could be wrong but undervolting an electric motor shouldn't matter a bit. But depending on where the wire is that is too small a gauge it could burn out and cause problems...

 

[Colt45]

change the brushes when they're worn maybe...

 

[boomerang]

Things that are made in China are not necessarily cheap solely because of slave labor. Many manufactured goods regardless of where they're made are garbage. They're designed and manufactured using cheap materials and are destined to have a short life. The throw away society thing.

 

[StageLeft]

Maybe, but I also don't see how. I imagine it's just akin to running a motor slowly and with little amps, which means less heat buildup (which is good for motors).

BTW, if you don't already have the leaf blower please make Baby Jesus happy and buy a gas one. I firmly believe and will argue until the day I die that corded yard tools are Satan's mistress. I still lament having bought a corded blower. The day I replaced a perfectly good corded trimmer with a gas one still stands as one of the better points in my life.

Can you believe that some people actually used to use corded lawn mowers? LOL

 

[Analog]

Undervoltage is defined as a condition where the applied voltage drops to 90% of rated voltage, or less, for at least 1 minute. Low-voltage conditions occur when a facility asks for more power than the line can deliver. But take note that it's not always an issue of the electric utility not having adequate capacity ? transformers can act like a choke, restricting how much total power gets through to your system. Perhaps the facility (or a neighbor on the same electric utility transmission line) added new equipment and increased the load beyond what the line could deliver. In isolated cases, maybe the facility added a motor with a long cable run, and the voltage drop in that circuit resulted in low voltage at that motor. Brief low-voltage conditions can also happen when someone starts a large load without notifying the power company, or when the power is shorted to ground or to another line. Even loose cable connections can cause low voltage.

Fig. 1. The torque of an electric motor changes as the square of the voltage is applied to its terminals. As the nominal applied voltage drops, so does the torque level.

To illustrate why low-voltage levels are unhealthy for electric motors, let's review the relationship between motor torque and applied voltage (Fig. 1).

Low-voltage lowdown. For an electric motor, torque changes as the square of the voltage applied. A 10% increase in voltage, for example, will boost torque 21% (1.1×1.1=1.21). Conversely, at 90% of rated voltage, the motor suffers a 19% reduction in torque (0.9×0.9=0.81). A more severe undervoltage condition, 20% below rated value, would reduce the motors torque to only 64% of rating (0.8×0.8 =.64). The effects are a 156% overload and catastrophic failure.

If torque decreases below the torque required by the load, the motor would stall. At that point, the only product of the motor is heat. However, a 100-hp motor, with locked rotor, becomes a 500kW resistive heater.

Fig. 2. Insulation life is halved for every 10°C rise in temperature. In this example, insulation life drops from four years to less than six weeks for a 50°C change in temperature.

Excess heat is a problem for motors because insulation life is halved for every 10°C increase in temperature (Fig. 2). At the same time, the temperature of the winding will rise 10°C to 15°C for each 10% drop in voltage. That means the insulation life of a motor that operates on 10% lower voltage will decrease to only 50% to 75% of its expected life.

Undervoltage events that last long enough to increase winding temperature cause irrevocable damage to winding insulation and unexpected failures later. A sustained low-voltage event can damage the insulation system of every electric motor running during the event. The only motor that's safe during a prolonged undervoltage event is one that's turned off.

http://ecmweb.com/mag/electric_save_motors_during/

 

[StageLeft]

Originally posted by: Analog
Undervoltage is defined as a condition where the applied voltage drops to 90% of rated voltage, or less, for at least 1 minute. Low-voltage conditions occur when a facility asks for more power than the line can deliver. But take note that it's not always an issue of the electric utility not having adequate capacity ? transformers can act like a choke, restricting how much total power gets through to your system. Perhaps the facility (or a neighbor on the same electric utility transmission line) added new equipment and increased the load beyond what the line could deliver. In isolated cases, maybe the facility added a motor with a long cable run, and the voltage drop in that circuit resulted in low voltage at that motor. Brief low-voltage conditions can also happen when someone starts a large load without notifying the power company, or when the power is shorted to ground or to another line. Even loose cable connections can cause low voltage.

Fig. 1. The torque of an electric motor changes as the square of the voltage is applied to its terminals. As the nominal applied voltage drops, so does the torque level.

To illustrate why low-voltage levels are unhealthy for electric motors, let's review the relationship between motor torque and applied voltage (Fig. 1).

Low-voltage lowdown. For an electric motor, torque changes as the square of the voltage applied. A 10% increase in voltage, for example, will boost torque 21% (1.1×1.1=1.21). Conversely, at 90% of rated voltage, the motor suffers a 19% reduction in torque (0.9×0.9=0.81). A more severe undervoltage condition, 20% below rated value, would reduce the motors torque to only 64% of rating (0.8×0.8 =.64). The effects are a 156% overload and catastrophic failure.

If torque decreases below the torque required by the load, the motor would stall. At that point, the only product of the motor is heat. However, a 100-hp motor, with locked rotor, becomes a 500kW resistive heater.

Fig. 2. Insulation life is halved for every 10°C rise in temperature. In this example, insulation life drops from four years to less than six weeks for a 50°C change in temperature.

Excess heat is a problem for motors because insulation life is halved for every 10°C increase in temperature (Fig. 2). At the same time, the temperature of the winding will rise 10°C to 15°C for each 10% drop in voltage. That means the insulation life of a motor that operates on 10% lower voltage will decrease to only 50% to 75% of its expected life.

Undervoltage events that last long enough to increase winding temperature cause irrevocable damage to winding insulation and unexpected failures later. A sustained low-voltage event can damage the insulation system of every electric motor running during the event. The only motor that's safe during a prolonged undervoltage event is one that's turned off.

http://ecmweb.com/mag/electric_save_motors_during/

What you're saying makes sense in a certain circumstance. E.g. if a motor is responsible for moving a certain load at all times, regardless of how many volts are going through it. As they drop the motor has less torque and if they drop far enough the motor is no longer moving and producing heat.

However in this application, the load drops directly in line with the volts going to the motor. It's not like a leaf blower says come heaven or high water i'm going to spin this blade at 3000 rpm and the motor is responsible for doing that no matter what, forcing everything it can from the motor with some constantly changing gear set. In this case since the load is directly attached to the motor, so as the motor slows so does the load.

Again, I may be wrong, but this makes sense to me I have worked a lot on model airplanes and in my experience if you have a propeller attached to a motor (which is all a leaf blower is) and you reduce the watts sent to that motor from the speed controller the motor will always be under less stress and less load (and thus it cools down); as you increase the juice to the motor it spins the prop faster, the load is thus higher, and the motor heats up.

 

[sao123]

Originally posted by: Skoorb

Originally posted by: Analog
Undervoltage is defined as a condition where the applied voltage drops to 90% of rated voltage, or less, for at least 1 minute. Low-voltage conditions occur when a facility asks for more power than the line can deliver. But take note that it's not always an issue of the electric utility not having adequate capacity ? transformers can act like a choke, restricting how much total power gets through to your system. Perhaps the facility (or a neighbor on the same electric utility transmission line) added new equipment and increased the load beyond what the line could deliver. In isolated cases, maybe the facility added a motor with a long cable run, and the voltage drop in that circuit resulted in low voltage at that motor. Brief low-voltage conditions can also happen when someone starts a large load without notifying the power company, or when the power is shorted to ground or to another line. Even loose cable connections can cause low voltage.

Fig. 1. The torque of an electric motor changes as the square of the voltage is applied to its terminals. As the nominal applied voltage drops, so does the torque level.

To illustrate why low-voltage levels are unhealthy for electric motors, let's review the relationship between motor torque and applied voltage (Fig. 1).

Low-voltage lowdown. For an electric motor, torque changes as the square of the voltage applied. A 10% increase in voltage, for example, will boost torque 21% (1.1×1.1=1.21). Conversely, at 90% of rated voltage, the motor suffers a 19% reduction in torque (0.9×0.9=0.81). A more severe undervoltage condition, 20% below rated value, would reduce the motors torque to only 64% of rating (0.8×0.8 =.64). The effects are a 156% overload and catastrophic failure.

If torque decreases below the torque required by the load, the motor would stall. At that point, the only product of the motor is heat. However, a 100-hp motor, with locked rotor, becomes a 500kW resistive heater.

Fig. 2. Insulation life is halved for every 10°C rise in temperature. In this example, insulation life drops from four years to less than six weeks for a 50°C change in temperature.

Excess heat is a problem for motors because insulation life is halved for every 10°C increase in temperature (Fig. 2). At the same time, the temperature of the winding will rise 10°C to 15°C for each 10% drop in voltage. That means the insulation life of a motor that operates on 10% lower voltage will decrease to only 50% to 75% of its expected life.

Undervoltage events that last long enough to increase winding temperature cause irrevocable damage to winding insulation and unexpected failures later. A sustained low-voltage event can damage the insulation system of every electric motor running during the event. The only motor that's safe during a prolonged undervoltage event is one that's turned off.

http://ecmweb.com/mag/electric_save_motors_during/

What you're saying makes sense in a certain circumstance. E.g. if a motor is responsible for moving a certain load at all times, regardless of how many volts are going through it. As they drop the motor has less torque and if they drop far enough the motor is no longer moving and producing heat.

However in this application, the load drops directly in line with the volts going to the motor. It's not like a leaf blower says come heaven or high water i'm going to spin this blade at 3000 rpm and the motor is responsible for doing that no matter what, forcing everything it can from the motor with some constantly changing gear set. In this case since the load is directly attached to the motor, so as the motor slows so does the load.

Again, I may be wrong, but this makes sense to me I have worked a lot on model airplanes and in my experience if you have a propeller attached to a motor (which is all a leaf blower is) and you reduce the watts sent to that motor from the speed controller the motor will always be under less stress and less load (and thus it cools down); as you increase the juice to the motor it spins the prop faster, the load is thus higher, and the motor heats up.

again... its the stall point which would be the critical factor.

 

[PottedMeat]

Aren't leaf blowers usually brushed AC/DC universal motors so undervolting -> higher current draw -> higher temperature rise -> quicker failure?

 

[iamwiz82]

My company's plant here has been undervolted a few times, anywhere from 25-75% of the proper voltage on each phase. It does some wild things to anything electrical. LCD screens do some wild acrobatics, motors burn up, Fluorescent lamp ballasts are eaten. Fun stuff.

 

[ebaycj]

Originally posted by: boomerang
Things that are made in China are not necessarily cheap solely because of slave labor. Many manufactured goods regardless of where they're made are garbage. They're designed and manufactured using cheap materials and are destined to have a short life. The throw away society thing.

Yep I just had to have my $1000 5 year old washing machine fixed, because it was designed with a $5 wear part in the agitator (part in the middle that sloshes the clothes around). I was told it's designed to wear out in approximately 3-5 years of light-normal use.

If they would have made the EXACT same part out of steel instead of soft nylon, it would very likely never break, ever, under normal use. Probably would have added $5 worth of cost to the washer.

 

[dullard]

16 gauge wire could give you a ~12V drop on a 100 foot line in good conditions (more in bad conditions). That is 10%. Where 12AWG will give less than a 5V drop. So, there is a noticible difference in voltage.

I think most people should consider the alternative that they are always warned against: Just mow up the leaves. Use them for garden compost. Or even mulch them. Sure, mulched leaves might change your soil pH a bit. But my thickest/greenest lawns are always where I mulched leaves. Your soil may very will need that pH change. If it works in your area, you'll save countless hours of labor and hundreds of dollars of a good leaf blower's cost.

 

[Paperdoc]

A leaf blower does not run under constant load, so the detailed notes on 3-phase 100HP motors driving a constant load has limited relevance. While it is true that using too small an extension cord (a 100 foot cord with only 16- or 18-gauge wire is too small for many devices!) can drop the voltage a lot at the end of the cord, the magnitude of the drop depends in large part also on the appliance. A leaf blower draws what current when at full speed? I really doubt it is over 10 amps! That would be a damned big heavy motor to lug around the yard! But even supposing the net result is a 10% voltage supply drop, I would not expect the result to be any very big increase in motor heating.

My suspicion is more in line with boomerang's. EVERY element of a cheap product is cost-reduced, not just the labor. The copper in the windings may be a slightly lower grade. It very likely is smaller gauge winding wire - that would consume less copper and make a smaller physical size as well as weight, but increase the heat generation AND reduce the ability of the heat to escape, resulting in higher internal temperatures. The varnish insulation on the winding wire may be thinner, allowing failure due to overheating to occur sooner. Many motor internal parts may be molded plastic rather than molded metal, reducing heat escape. Poor design of fan blades on the motor shaft, coupled with poor case design for air flow, may also affect heat removal. The choices in brush size, material and mounting, and in commutator materials, will affect their lifetime and also their current-carrying capacity and the heat generated at the commutator. By the time you put all these factors together, you can make a cheaper product with a known shorter lifetime.

Why do that? First of all, people learn and change their behavior much more effectively from immediate feedback than from delayed. So it is naturally hard for us to remember that the leafblower only lasted two years, when you're in the store faced once again with prices ranging from $29.95 to $89.95. That one on the end sure does not LOOK three times better, and you have NO way to know whether it will last more than three times longer. Well, you actually do have SOME information on this question - there's a general "rule of thumb": THERE IS NO FREE LUNCH! If you deliberately buy the cheapest item all the time, your odds are high that you will get poor performance or short product lifetime. Beyond that, however, quantifying more precisely is difficult. I'm quite prepared to believe that, at the top end of the pricing range, some products are overpriced for what they deliver. Somewhere in the range there is an optimum balance of price / performance / product lifetime. Trying to find that takes a bunch of time to research other people's reports. And sometimes all you can use as a guide is the reputation of the maker's name on the item. Personally, I'm more inclined to look towards the middle of the price range if there is NO performance review info available. I'll also look closely for design and construction details. If I see what look to me like short-cuts or questionable design in some areas, I am more inclined to think that same philosophy was used in the entire product.

One of my favorite quotes in politics (from Britain, I believe) is: In a true Democracy, the electorate deserves the government they get. The same applies to consumers in a true free market. You get what you pay for, in a rough sense, AND what you find available in that market is heavily influenced by the past history of consumer choices. Remember that as you make your own choices.

 

[hanoverphist]

Originally posted by: iamwiz82
My company's plant here has been undervolted a few times, anywhere from 25-75% of the proper voltage on each phase. It does some wild things to anything electrical. LCD screens do some wild acrobatics, motors burn up, Fluorescent lamp ballasts are eaten. Fun stuff.

why are those systems on a circuit that is backed up? with a UPS you should never see under or over voltage situations occur. they act as a filter as much as a back up.

 

[boomerang]

Originally posted by: ebaycj

Originally posted by: boomerang
Things that are made in China are not necessarily cheap solely because of slave labor. Many manufactured goods regardless of where they're made are garbage. They're designed and manufactured using cheap materials and are destined to have a short life. The throw away society thing.

Yep I just had to have my $1000 5 year old washing machine fixed, because it was designed with a $5 wear part in the agitator (part in the middle that sloshes the clothes around). I was told it's designed to wear out in approximately 3-5 years of light-normal use.

If they would have made the EXACT same part out of steel instead of soft nylon, it would very likely never break, ever, under normal use. Probably would have added $5 worth of cost to the washer.

A friend of ours bought a new refrigerator. $2300 for it and the salesman told her to not expect it to last past 8 years. Meanwhile, I'm thinking it might be time to replace our 22 year old refrigerator. The question is, how much should I spend for something that will last 1/3 as long yet use less energy?

I don't know the answer to this question.

 

[dullard]

Originally posted by: Paperdoc
I'm quite prepared to believe that, at the top end of the pricing range, some products are overpriced for what they deliver. Somewhere in the range there is an optimum balance of price / performance / product lifetime. Trying to find that takes a bunch of time to research other people's reports. And sometimes all you can use as a guide is the reputation of the maker's name on the item. Personally, I'm more inclined to look towards the middle of the price range if there is NO performance review info available.

You do need to be careful with that "pick the middle" strategy. There is a clever pricing strategy that you might fall for with that strategy.

Suppose I had a restaurant and you came in for a burger. Suppose I had a cheap burger for $5 and a "high-end" burger for $8. The high-end burger is just window dressing and isn't actually any better. Both cost me about the same in materials, overhead, and labor. So the profit is far bigger on the $8 item. I want everyone to buy the $8 burger. The ultimate value for customers is the same (both satisfy hunger). Thus, there would be a substantial number of people buying the $5 burger just to save money. The quality-only customers will buy the $8 burger, but the value-conscious buyers will buy the $5 one.

Now suppose I add in a $15 burger to the menu. I seriously don't think anybody will ever pay that much for a simple burger. And in fact, very few people do. Even the people who previously bought the $8 burger probably won't be willing to pay nearly double the amount. If anyone does buy the $15 burger, I'm happy for the large profit margin, but that isn't why the burger is on the menu. Suddenly the bulk of the value-conscious buyers buy the $8 burger. To them, It is only a bit more money over the $5 burger and is a great deal compared to the $15 burger. I just moved a big fraction of buyers from the $5 item to the $8 item just by putting in a red herring in their decision algorithm.

I got people just like you to buy the expensive item (the $8 burger that I wanted to sell). With your logic, you got a good value on the middle priced item. But in reality, you just bought the high priced item of the two real contenders when the $5 burger may have been just as good.

Sorry fleabag for going so off topic, but that is the name of this forum.

 

[Modelworks]

Another consideration is the cooling of the motor. Most AC motors use a fan either attached to the rotor or made into the fan body to cool the windings. Those fans are designed with the full speed of the motor in mind and do not work the same way if you undervolt the motor which could lead to overheating. Leaf blower motors are in the 7 - 10Amp range Something else to watch for is the size of the extension cord. Lots of cords are on the market that look really big for outdoor use, but the actual wire inside is tiny compared to what is required at the 50-100 ft length. For 100ft I wouldn't use anything less than 12AWG that keeps the loss at under 2%. If people are using cheap cords that will cause early failure because the startup of the motor is pulling current that the wiring cannot provide so that startup is prolonged adding heat and wear.

 

[Dirigible]

Originally posted by: Skoorb

[Skoorb blathers on]

Can you believe that some people actually used to use corded lawn mowers? LOL

I :heart: my corded lawn mower. It's the best. Much better than a gas or cordless electric mower for my situation. If it dies I'll buy another.

 

[StageLeft]

Originally posted by: boomerang
A friend of ours bought a new refrigerator. $2300 for it and the salesman told her to not expect it to last past 8 years. Meanwhile, I'm thinking it might be time to replace our 22 year old refrigerator. The question is, how much should I spend for something that will last 1/3 as long yet use less energy?

I don't know the answer to this question.

8 years for a fridge, that is crazy they are supposed to laugh forever.

I my corded lawn mower. It's the best. Much better than a gas or cordless electric mower for my situation. If it dies I'll buy another.

I was going to retort to you in awful and horrific ways but I laughed when you replaced my ramblings with "Skoorb blathers on", so I won't subject us to more of it.

 

[StageLeft]

OK I can't help it. How can you use a corded lawn mower? Aren't you mocked by your neighbors? Answer: Yes, does that bother you? It doesn't bother me when mine laugh at me. So next question: isn't it inconvenient? Answer: Yes. Don't you want to throw the thing at the house or into a trash heap dealing with that cord?

 

[eldorado99]

Originally posted by: Skoorb

Originally posted by: boomerang
A friend of ours bought a new refrigerator. $2300 for it and the salesman told her to not expect it to last past 8 years. Meanwhile, I'm thinking it might be time to replace our 22 year old refrigerator. The question is, how much should I spend for something that will last 1/3 as long yet use less energy?

I don't know the answer to this question.

8 years for a fridge, that is crazy they are supposed to laugh forever.

I my corded lawn mower. It's the best. Much better than a gas or cordless electric mower for my situation. If it dies I'll buy another.

I was going to retort to you in awful and horrific ways but I laughed when you replaced my ramblings with "Skoorb blathers on", so I won't subject us to more of it.

I know what you mean about the fridges. My parents got one as a wedding present and it is still our main fridge. (They have been married for 38 years).

 

[Dirigible]

Originally posted by: Skoorb
OK I can't help it. How can you use a corded lawn mower? Aren't you mocked by your neighbors? Answer: Yes, does that bother you? It doesn't bother me when mine laugh at me. So next question: isn't it inconvenient? Answer: Yes. Don't you want to throw the thing at the house or into a trash heap dealing with that cord?

Can Skoorb's minuscule brain comprehend what I'm about to tell him? Answer: No. Skoorb's minuscule brain is so incredibly teeny-tiny that some scientists speculate it doesn't actually exist.

My front yard has a patch of lawn about 15'x20'. My back yard lawn is smaller. That shit is ridiculously tiny but it still has to be cut every few months. Gas mower? What a stinky pain in the ass to deal with for a tiny lawn. Cordless? I'll never remember to plug the damn thing in to charge it. Corded? Yessir! No hassle with gas/oil or charging. With a patch of grass almost as small as Skoorb's brain, the cord doesn't need to be very long, nor is it difficult to keep the cord out of the way of the mower.

If I had a big yard, the cord would be a pain and it might be worth using a different type of mower. If I had my shit together and remembered to charge it in advance, a cordless electric might be slightly nicer. If I weren't one of the laziest people on the planet and mowed my lawn more frequently than bimonthly I could use an unpowered push-mower. But for my yard and my lazy disorganized self, corded is the way to go.

(P.S. My neighbors look on me with envy and bow to my superior mowing prowess.)

 

[fleabag]

Originally posted by: dullard
16 gauge wire could give you a ~12V drop on a 100 foot line in good conditions (more in bad conditions). That is 10%. Where 12AWG will give less than a 5V drop. So, there is a noticible difference in voltage.

I think most people should consider the alternative that they are always warned against: Just mow up the leaves. Use them for garden compost. Or even mulch them. Sure, mulched leaves might change your soil pH a bit. But my thickest/greenest lawns are always where I mulched leaves. Your soil may very will need that pH change. If it works in your area, you'll save countless hours of labor and hundreds of dollars of a good leaf blower's cost.

But isn't the voltage drop greater under a high load vs. no load? i.e the voltage drop when you attach a multimeter's probes to the end plug that the blower plugs into after 100feet of wire vs. having a 1500 watt load from the blower and then checking the voltage at the plug that is at the end of that 100 feet of wire? It is this thought that is why my electrical company when there is a voltage issue checks the voltages before a load is applied and when a load is applied (such as a portable heater).

 

[Analog]

Originally posted by: fleabag

Originally posted by: dullard
16 gauge wire could give you a ~12V drop on a 100 foot line in good conditions (more in bad conditions). That is 10%. Where 12AWG will give less than a 5V drop. So, there is a noticible difference in voltage.

I think most people should consider the alternative that they are always warned against: Just mow up the leaves. Use them for garden compost. Or even mulch them. Sure, mulched leaves might change your soil pH a bit. But my thickest/greenest lawns are always where I mulched leaves. Your soil may very will need that pH change. If it works in your area, you'll save countless hours of labor and hundreds of dollars of a good leaf blower's cost.

But isn't the voltage drop greater under a high load vs. no load? I.e the voltage drop when you attach a multimeter probes to the outlet after 100feet of wire vs. having a 1500 watt load and then checking the voltage at the outlet? It is this thought that is why my electrical company when there is a voltage issue checks the voltages before a load is applied and when a load is applied (such as a portable heater).

There should be no voltage drop under no load, since the voltmeter has an input impedance of greater than 1M ohm.

 

[fleabag]

Originally posted by: Analog

Originally posted by: fleabag

Originally posted by: dullard
16 gauge wire could give you a ~12V drop on a 100 foot line in good conditions (more in bad conditions). That is 10%. Where 12AWG will give less than a 5V drop. So, there is a noticible difference in voltage.

I think most people should consider the alternative that they are always warned against: Just mow up the leaves. Use them for garden compost. Or even mulch them. Sure, mulched leaves might change your soil pH a bit. But my thickest/greenest lawns are always where I mulched leaves. Your soil may very will need that pH change. If it works in your area, you'll save countless hours of labor and hundreds of dollars of a good leaf blower's cost.

But isn't the voltage drop greater under a high load vs. no load? I.e the voltage drop when you attach a multimeter probes to the outlet after 100feet of wire vs. having a 1500 watt load and then checking the voltage at the outlet? It is this thought that is why my electrical company when there is a voltage issue checks the voltages before a load is applied and when a load is applied (such as a portable heater).

There should be no voltage drop under no load, since the voltmeter has an input impedance of greater than 1M ohm.

But there is always a voltage drop when you run a device, even if the outlet is on a completely different circuit in the house, but the voltage drop in your house on two outlets that don't share the same circuit in the subpanel should be far less than a voltage drop near the source of the load. The reason for the voltage drop is because of the increase in resistance of the wire due to the higher load. In the case of the house, that increase in resistance is in the wire from the street to the main panel opposed to just the wire going into the outlet and then the load.