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Title: Air Conditioner amperage draw at low voltages
Post by: Mike Sokol on September 22, 2018, 07:21:37 pm
So I just had another "expert" tell me that the amperage draw of an air conditioner compressor in an RV goes UP if the voltage is too low. Now it will certainly draw a lot more current at start up via the starter capacitor since it won't open the relay quickly. And I certainly understand about back EMF and how a low voltage on a AC-DC motor can lead to excessive current draw. But this is for a sealed air conditioner compressor with an induction motor that has a starter winding and capacitor. I think the current draw will go do as a direct function of reduced voltage, and the air conditioner will need to have a higher duty cycle to keep up with the demand. But what physics could explain current draw on an induction motor going UP when the voltage goes DOWN? 
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Tom Bourke on September 22, 2018, 07:48:25 pm
If I recall, the lower voltage creates less torque.  This leads to lower RPMs.  Lower RPM's means the slip, difference between real RPM and ideal RPM at line frequency, is high.  High slip creates less counter EMF.  Less counter EMF leads to higher current.

The continuation of this is higher current leads to more heat and that burns out the motor.

Title: Re: Air Conditioner amperage draw at low voltages
Post by: Mike Sokol on September 22, 2018, 08:17:41 pm
If I recall, the lower voltage creates less torque.  This leads to lower RPMs.  Lower RPM's means the slip, difference between real RPM and ideal RPM at line frequency, is high.  High slip creates less counter EMF.  Less counter EMF leads to higher current.

The continuation of this is higher current leads to more heat and that burns out the motor.

Sounds plausible, but does low voltage create an actual increase in current draw, or perhaps the current doesn't drop linearly as it would do for a resistive load. I need to see a voltage/amperage curve for an air conditioner compressor. The premise is that too low of a voltage will increase current to where it burns up extension cords. That is, a long extension cord will have enough voltage drop that the compressor will draw more current which will burn up the receptacle.   
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Stephen Swaffer on September 22, 2018, 09:49:29 pm
Tom is on the mark.  Think about it-the reason for the high start op surge is that a stopped motor presents a near short circuit as far as a load is concerned because the only thing impeding current flow is the DC resistance of the windings.  At rated speed, the counter EMF liimits the current to the rated ampo draw.  The counter EMF is the main impedance to current flow-you really aren't dealing with Ohm's law in this situation.

Loading a motor slows it down-leading to a greater amp draw.  Lower voltage tends to make a motor stall or slow down even more.

The AC motor will have a relatively constant load-to get the same power at a lower voltage requires more amps-which leads to a greater voltage drop in the cord-so a snow ball effect.

People have a tendency to look at the ampacity of a wire to determine the size of the cord they need-but as is often discussed on here in audio applications, voltage drop is far more relevant at any distance-and the distance of concern is usually the distance from the transformer.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Mike Sokol on September 23, 2018, 10:07:52 am
Loading a motor slows it down-leading to a greater amp draw.  Lower voltage tends to make a motor stall or slow down even more.

The AC motor will have a relatively constant load-to get the same power at a lower voltage requires more amps-which leads to a greater voltage drop in the cord-so a snow ball effect.


OK, so I've got that in my head right now. So I understand how an induction more draws more current as it's loaded more. Basically it's the flux slippage angle. As the rotor falls behind the rotating flux field it draws more current. If the motor was spun up externally to the same rotational speed and phase of the flux it should draw almost no current. Sound right?

So lets take the same motor with the same load (air conditioner compressor) and put a big Variac on it. As I change the motor voltage from 120 to 110 to 100 to 90 volts, what happens? Is there a linear curve of the motor drawing more and more current as the voltage drops? That is, will it draw maybe 10 amps at 120 volts, and 12 amps at 110 volts and 14 amps at 100 volts and 16 amps at 90 volts (or something to that effect?). I don't expect it to act like a resitive load, but is there some inverse function that will predict extra current draw at lower voltages if the motor is attempting to spin the same compressor load? This is independent of any starter capacitor effects....

Title: Re: Air Conditioner amperage draw at low voltages
Post by: Ron Hebbard on September 23, 2018, 01:57:09 pm
So I just had another "expert" tell me that the amperage draw of an air conditioner compressor in an RV goes UP if the voltage is too low. Now it will certainly draw a lot more current at start up via the starter capacitor since it won't open the relay quickly. And I certainly understand about back EMF and how a low voltage on a AC-DC motor can lead to excessive current draw. But this is for a sealed air conditioner compressor with an induction motor that has a starter winding and capacitor. I think the current draw will go do as a direct function of reduced voltage, and the air conditioner will need to have a higher duty cycle to keep up with the demand. But what physics could explain current draw on an induction motor going UP when the voltage goes DOWN?
  If your "expert" were correct then an applied voltage of 1 volt would draw an unbelievable Amperage and the current consumption at zero volts would be truly unbelievable.  I suspect there's a flaw in his reasoning. 
Toodleoo! 
Ron Hebbard
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Tom Bourke on September 23, 2018, 02:37:59 pm
f
  If your "expert" were correct then an applied voltage of 1 volt would draw an unbelievable Amperage and the current consumption at zero volts would be truly unbelievable.  I suspect there's a flaw in his reasoning. 
Toodleoo! 
Ron Hebbard
At some point the motor no longer has the torque to maintain rotation and will slow to a stop.  Depending on the motor dc resistance the current draw will drop as voltage drops. At some point in the voltage X current curve the heating effect will be safe for the motor and wire.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Stephen Swaffer on September 23, 2018, 03:28:33 pm
Indeed.  Some motors are actually designed as "impedance protected"-if the bearings lock up even full voltage won't overheat it-it just basically turns into a heater.  Usually seen on small blower motors or cooling fans.

The exact relationship depends on the motor design-more efficient motors are more efficient because they have less slip, etc.  VFD's actualy vary foltage aand frequency to keep current within design limits.  Honestly, I have only read enough on motor theory to make me dangerouos beyond this point-enough to understand the practical aspects so I can set up VFD's for relatively simple applications-but I am thinking the volts/amps/speed relationship is more complex than a linear relationship and really requires specific motor design criteria. 

We usually look at voltage/phase/HP/amps/frame on a motor-but a nameplate has a lot more information for a reason-AC compressors will also spec LRA-"locked rotor amps"-the reason all that info is on a nameplate is that itells the reader about the motor-if the reader is smart enough to understand the information.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Mike Sokol on September 23, 2018, 06:35:24 pm
I am thinking the volts/amps/speed relationship is more complex than a linear relationship and really requires specific motor design criteria.

The reason for this exercise is the knee-jerk response I get from many RV technicians who say that low voltage will increase current draw and cause an RV's TT-30 plug to burn up. Think you have problems with low voltage at a show? I'm getting field reports showing static voltage down around 93 volts for extended periods in the afternoon when hundreds of RV owners turn on their air conditioners.

I just don't believe the current draw will go UP when the voltage goes DOWN. My WAG is that is that the current may not go down linearly, but I don't think it will go UP (unless the air conditioner starter winding kicks in). In any event, if the current draw does go UP, there's still an upstream 30-amp circuit breaker to protect the 30-amp receptacle and cord-set wiring. That's why I believe there must be some other reason for the overheated connectors.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Marc Sibilia on September 23, 2018, 09:00:28 pm
I just don't believe the current draw will go UP when the voltage goes DOWN.

The speed of the motor (before accounting for slip) is determined by the frequency, not the voltage of the AC.  The mechanical power to the load doesn't change much because the speed doesn't change much.


More here:
Autotransformers, anyone? (http://forums.prosoundweb.com/index.php/topic,165180.msg1522536/topicseen.html#msg1522536)

Marc
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Jonathan Johnson on September 24, 2018, 01:11:09 am
...That's why I believe there must be some other reason for the overheated connectors.

Receptacles with weak grip, due to repeated use and repeated overheating. Weak grip can also present an increased electrical resistance, which leads to localized heating. Replace those weak receptacles.

Burnt receptacle or plug contacts due to connecting and disconnecting while under load. Burnt contacts = higher resistance = heating. This is why you should flip the breaker feeding the receptacle OFF before connecting or disconnecting.

Frayed wires inside cordsets where they meet the plug broken strands due to the extra flexing (work hardening/metal fatigue) that happens when cords are connected and disconnected. Fewer intact strands = higher resistance = heating. Grip the plug, not the cord, when connecting and disconnecting.

These are the most common causes of a cordset overheating when anything near the rated load is applied. It can all be avoided with proper handling (grip the plug not the cord!), use (turn the breaker off!), and maintenance (replace those weak receptacles!).

The plug connection fails because of damage, not because of the load. It would be like trying to tow a 10,000 pound trailer with a 15,000 pound rated hitch that's been cut halfway through, then blaming the trailer when the hitch breaks.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: John Roberts {JR} on September 24, 2018, 10:06:48 am
That's why I believe there must be some other reason for the overheated connectors.
I had to replace an old 2 pin outlet with new in my house because the plug for a small auxiliary heater was getting noticeably hot in it...  Contact resistance matters, even wire type can matter, with manufactured home fires caused by aluminum wires oxidizing and becoming elevated resistance (P=I^2 xR).

JR
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Peter Morris on September 24, 2018, 11:03:57 am
So I just had another "expert" tell me that the amperage draw of an air conditioner compressor in an RV goes UP if the voltage is too low. Now it will certainly draw a lot more current at start up via the starter capacitor since it won't open the relay quickly. And I certainly understand about back EMF and how a low voltage on a AC-DC motor can lead to excessive current draw. But this is for a sealed air conditioner compressor with an induction motor that has a starter winding and capacitor. I think the current draw will go do as a direct function of reduced voltage, and the air conditioner will need to have a higher duty cycle to keep up with the demand. But what physics could explain current draw on an induction motor going UP when the voltage goes DOWN?

The motor is more or less required to deliver the same power to drive the compressor. To do that if the voltage is reduced the current must go up.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Tom Bourke on September 24, 2018, 08:14:39 pm
I tried to find a graph of current vs voltage on an induction motor.  Unfortunately my google foo has failed me on this.  However I did turn up many diagrams that show current draw vs speed.

The exact characteristics depend on the motor but over all there were torque peeks around 80 to 90% speed.  Locked Rotor Amps is around 600% load amps and the curve was mostly high till a knee around 80 to 90% speed.  Also efficiency drops as voltage goes above or below design spec.

Overall it looked like a current spike would be at that 80 to 90% voltage of around 150 to 200% design current.  Compare that to a breaker trip curve and you have the potential for a semi consistent 150% overload on the distribution wiring and connections that will almost never trip a breaker.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Mike Sokol on September 24, 2018, 08:59:36 pm
Overall it looked like a current spike would be at that 80 to 90% voltage of around 150 to 200% design current.  Compare that to a breaker trip curve and you have the potential for a semi consistent 150% overload on the distribution wiring and connections that will almost never trip a breaker.

Per your hints I found a paper which describes overheating and torque curves of an induction motor as a result of under and over voltage conditions. I don't see a direct correlation graph between lower voltage and increased current on an air conditioner compressor, but this is getting much closer to revealing the inner workings.

https://aceee.org/files/proceedings/2001/data/papers/SS01_Panel2_Paper27.pdf (https://aceee.org/files/proceedings/2001/data/papers/SS01_Panel2_Paper27.pdf)
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Stephen Swaffer on September 25, 2018, 03:31:33 pm
So, 2 things on the chart for a typical motor.

10% increase in current at rated load- no doubt an AC compressor is designed to run at rated load.

23% increase in temp rise- the AC system has to get rid of that heat as well making the system work harder.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: David Buckley on September 25, 2018, 10:00:23 pm
If it is an inverter A/C then as the voltage drops the inverter will draw more current to maintain the wattage.

Having said that, almost all the A/C I've seen in the USA is not inverter tech, its just direct-on-line start compressors with the associated light dim as it starts.  Outside the USA where Japanese manufacturers dominate, its all inverter A/C.

Title: Re: Air Conditioner amperage draw at low voltages
Post by: Peter Morris on September 26, 2018, 11:09:33 am
The motor is more or less required to deliver the same power to drive the compressor. To do that if the voltage is reduced the current must go up.

Here is a bit more of an explantation ... http://www.icrepq.com/icrepq07/346-romero.pdf
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Mike Sokol on September 28, 2018, 12:50:27 am
Just got this from Dometic engineering today. They make rooftop air conditioners for RVs, so they must know something about this. Note there is indeed a 2 amp or so increase in the compressor motor current when the voltage is reduced from 115 to 105 volts. But the shaded-pole fan motor has a decrease in current when the voltage is reduced from 115 to 105. So the effect is real and substantial.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Mike Sokol on September 29, 2018, 12:04:54 pm
I just talked to an application engineer at Dometic, and they don't have any data on what happens to one of their air conditioners when they're run on very low voltage, which I'm getting reports down to 94 volts. Their marketing department might send me one of their air conditioners to play with, so I could get a 3kva variac and do some experiments to gather real world data. 
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Art Welter on September 29, 2018, 03:08:41 pm
Note there is indeed a 2 amp or so increase in the compressor motor current when the voltage is reduced from 115 to 105 volts. But the shaded-pole fan motor has a decrease in current when the voltage is reduced from 115 to 105. So the effect is real and substantial.
The roughly 10% increase in amperage with a 10% reduction in voltage Dometic noted seems to correspond  well with what " Desmostylus" wrote 4/26/2005:
"Reduce voltage by 1%, slip increases by 2%, reducing impedance by 2%, current increases by 1%."

Between the increased amperage from the air conditioner and refrigerator compressor motors, sunlight directly heating connectors, and increased electrical resistance on worn connections, not surprising to see connectors burn/melt at low voltage in the campground.

Art
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Mike Sokol on September 29, 2018, 09:01:48 pm
The roughly 10% increase in amperage with a 10% reduction in voltage Dometic noted seems to correspond  well with what " Desmostylus" wrote 4/26/2005:
"Reduce voltage by 1%, slip increases by 2%, reducing impedance by 2%, current increases by 1%."

It will be interesting to see what happens to the amperage as the voltage drops down to 93 volts, which is the lowest reported campground voltage so far. It obviously can't be linear, with a 50% reduction in voltage causing a 50% increase in amperage. But it's possible that dropping the voltage down into the mid 90's could increase the amperage by 20% or so, and that will probably lead to a lot of internal heat buildup in the compressor.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Mike Sokol on September 29, 2018, 09:05:28 pm
Between the increased amperage from the air conditioner and refrigerator compressor motors, sunlight directly heating connectors, and increased electrical resistance on worn connections, not surprising to see connectors burn/melt at low voltage in the campground.

While there's an increase in the amperage draw, but since the wiring is protected by the upstream circuit breaker, I'm not sure it would cause the connector to overheat. But that's EXACTLY when my readership suggests...
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Tom Bourke on September 30, 2018, 06:44:32 pm
While there's an increase in the amperage draw, but since the wiring is protected by the upstream circuit breaker, I'm not sure it would cause the connector to overheat. But that's EXACTLY when my readership suggests...
That breaker is going to have a trip curve.  The cycling of the compressor is probably just enough to keep the breaker from tripping.  However the connectors and wire are still being over heated each cycle.  I think your seeing cumulative damage.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Tim McCulloch on September 30, 2018, 09:55:25 pm
While there's an increase in the amperage draw, but since the wiring is protected by the upstream circuit breaker, I'm not sure it would cause the connector to overheat. But that's EXACTLY when my readership suggests...

They're talking about the shore cord 14-50 at the pedestal or related CS series connector at the coach?  Or something else?

I've read a number of electrical threads at the "Miss Manners" RV forums and some of the crap people think is real simply astounds me... Then there are a couple of retired electricians who set some of them right...
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Mike Sokol on October 01, 2018, 07:56:27 pm
They're talking about the shore cord 14-50 at the pedestal or related CS series connector at the coach?  Or something else?en there are a couple of retired electricians who set some of them right...

Yes, but it's usually a 30-amp TT-30 (Travel Trailer) receptacle and cordset that overheats.
Title: Re: Air Conditioner amperage draw increases at low voltages
Post by: Art Welter on October 01, 2018, 10:10:41 pm
Yes, but it's usually a 30-amp TT-30 (Travel Trailer) receptacle and cordset that overheats.
Because most every receptacle in the campground has been serially abused by people plugging in with their AC and refrigerator on, arcing and pitting them out, covering the connections with carbon deposits..
Title: Re: Air Conditioner amperage draw increases at low voltages
Post by: Tim McCulloch on October 01, 2018, 10:56:41 pm
Because most every receptacle in the campground has been serially abused by people plugging in with their AC and refrigerator on, arcing and pitting them out, covering the connections with carbon deposits..

Yepper.

De-energize the outlet, connect, re-energize.  Turn on AC or water heater, charge house battery(ies).

NO hot connections - why is it a difficult concept.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Tim McCulloch on October 01, 2018, 10:58:17 pm
Yes, but it's usually a 30-amp TT-30 (Travel Trailer) receptacle and cordset that overheats.

I'm with Art, I was just trying to figure out *which* connections were doing the Joan of Arcing...

Also my mind is in Class A 50 amp land (my next RV, sometime, maybe) so a hard-wired TT30 cord set didn't immediately present itself as the connector under suspicion.  :-[
Title: Re: Air Conditioner amperage draw increases at low voltages
Post by: Jonathan Johnson on October 02, 2018, 12:20:51 pm
Yepper.

De-energize the outlet, connect, re-energize.  Turn on AC or water heater, charge house battery(ies).

NO hot connections - why is it a difficult concept.

Why hasn't anyone designed an interlocking switch/receptacle panel where the RV plug can't be connected or disconnected with the switch on?

It could be as simple as a switch handle that covers the receptacle and plug when on. I think every RV I've ever seen has a right-angle plug, so it shouldn't be too difficult.
Title: Re: Air Conditioner amperage draw increases at low voltages
Post by: Chris Hindle on October 02, 2018, 12:32:55 pm
Why hasn't anyone designed an interlocking switch/receptacle panel where the RV plug can't be connected or disconnected with the switch on?

It could be as simple as a switch handle that covers the receptacle and plug when on. I think every RV I've ever seen has a right-angle plug, so it shouldn't be too difficult.
Sell it as a weather cover. Open cover, power is off. Close cover, .....
Chris.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Jay Barracato on October 02, 2018, 07:42:58 pm
Sell it as a weather cover. Open cover, power is off. Close cover, .....
Chris.
Because the RV crowd and gadget industry will immediately start selling some "convenience" device to bypass it.

Sent from my Moto Z (2) using Tapatalk

Title: Re: Air Conditioner amperage draw at low voltages
Post by: Tim McCulloch on October 02, 2018, 11:20:22 pm
Because the RV crowd and gadget industry will immediately start selling some "convenience" device to bypass it.

Sent from my Moto Z (2) using Tapatalk

I think guys just want gadget bragging rights around the camp fires.  "Hey guys, check this out... lets you plug in to those fancy self-closing boxes!"
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Chris Hindle on October 03, 2018, 08:23:05 am
I think guys just want gadget bragging rights around the camp fires.  "Hey guys, check this out... lets you plug in to those fancy self-closing boxes!"

Ya, who cares about "safety", "Fire" and "not killing people" once beer is involved....
Oh well.
If it were easy, it would already be done.
Chris.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Stephen Swaffer on October 03, 2018, 01:21:51 pm
So the smallest service I can legally put in a home is 100 amps-but ignoring that limitation, if I figure a service based on an oven, hot water heater, and AC unit roughly half the size of a house then do rest of my required calculations I come up with a required service size of somewhere around 60 amps-and that will be connected to the POCO through (hopefully) torqued lugs. And service conductors sized for 3% voltage drop.

Is it any wonder that RV's on 30 or 50 amp receptacles with a long cord plugged into tired receptacles from X number of plugins each summer, abused by weather 24/7 and multiple thermal cycles have issues with plugs melting down?
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Tim McCulloch on October 03, 2018, 02:16:24 pm
So the smallest service I can legally put in a home is 100 amps-but ignoring that limitation, if I figure a service based on an oven, hot water heater, and AC unit roughly half the size of a house then do rest of my required calculations I come up with a required service size of somewhere around 60 amps-and that will be connected to the POCO through (hopefully) torqued lugs. And service conductors sized for 3% voltage drop.

Is it any wonder that RV's on 30 or 50 amp receptacles with a long cord plugged into tired receptacles from X number of plugins each summer, abused by weather 24/7 and multiple thermal cycles have issues with plugs melting down?

I'm surprised there aren't more problems at the RV end.  Folks with shore power on retracting reels have little idea of what can and do go wrong.  Hot & loaded connection of CS-series at the coach.  Vibration-loosened lugs and terminals in auto transfer switches, bad relay contacts in ATS; loose connections at inverter/converter/charger...  There's a lot of manual maintenance to be done on RVs and there are plenty of owners who get surprised by just how much of they need to do themselves.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Mike Sokol on October 05, 2018, 05:28:20 am
There's a lot of manual maintenance to be done on RVs and there are plenty of owners who get surprised by just how much of they need to do themselves.

The big difference between your brick house and your RV is that the RV bounces down the road thousands of miles every year and plugs into perhaps dozens of different power services, while your brick house electrical system is one and done.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Tim McCulloch on October 05, 2018, 11:09:28 am
The big difference between your brick house and your RV is that the RV bounces down the road thousands of miles every year and plugs into perhaps dozens of different power services, while your brick house electrical system is one and done.

Both of my grandfathers were farmers so I became acquainted with vibration and impact issues at a young age.  It's younger RV owners who will be surprised - those owners who can't change the tire on their car or know where the crankcase dipstick is... or why you'd care.
Title: Re: Air Conditioner amperage draw at low voltages
Post by: Mark Cadwallader on October 05, 2018, 04:16:11 pm
Both of my grandfathers were farmers so I became acquainted with vibration and impact issues at a young age.  It's younger RV owners who will be surprised - those owners who can't change the tire on their car or know where the crankcase dipstick is... or why you'd care.

Why fix it if you can just trade it in on a new (bigger/better) one?