Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Jonathan Johnson]

...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.

[John Roberts {JR}]

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

[Peter Morris]

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.

[Tom Bourke]

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.

[Mike Sokol]

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

[Stephen Swaffer]

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.

[David Buckley]

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.

[Peter Morris]

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

[Mike Sokol]

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.

[Mike Sokol]

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. 

[ProSoundWeb Community]