How bad is low voltage for your electric motors?

[Wayne]

Before I owned a travel trailer with air conditioners, I never really considered how bad low voltage is for electric motors.

I have a friend who is professional electrical engineer. One time we stayed in a RV park that had voltage measured at our 120v outlet in the trailer of 108 volts without the air running and 105 volts with the AC running. Hearing that low voltage shortens the life of motors, I asked him how 'bad' this for the longevity of the AC motors. For the next hour I heard lots about Ohm's law and some math that I don't remember from college. What I walked away with is when you calculate additional heat cause by the increase of amps when voltage is decreased, you overheat the internals of the electric motors and shorten their lifespan. The low voltage also reduces the available torque, so your AC may not even be able to start the compressor, leaving you overheated.

This all got me thinking about hooking my trailer up to a 100' drop cord when we are loading up in front of the house for a trip. We usually only run resistive loads like the water heater and the fridge, but on occasion we will start an air conditioner to cool things off while we are working in the trailer. I found an calculator that shows how much voltage drop I get from using a 100' 14 ga cord to power the trailer.

Voltage Drop Calculator

This shows that just the 100' of drop cord causes a 6.3 volt drop. The voltage in the trailer with AC running is about 110, so a 10v drop from the 120v at my house. This causes about 9% more amps, which causes the motor internals to heat more than with 120v. I stopped running my AC while plugged into my house!

 

[dnewton3]

I cannot speak to electrical appliances in the RV that would be unique for 12v ops, but to be UL recognized most any 115/230 equipment has to be certified to run on minor voltage drops and still function. However, that does not speak to longevity. Once the volts drop by 20%, it's likely to be an issue. The longevity concern is a function of two things; how much voltage drop there exists, and how long it's exposed to that low-volt condition. A drop of 15% for a few minutes is unlikely to be a big deal. A drop of 20% over many hours can cause concerns for degradation.

Let's not forget that the "average" voltage in a home is 115/230. That's because it's commonly accepted that single phase will run between 220V to 240V in most distributions systems depending on many variables. That approximate 230v +/- 10% range is considered "normal". Most 220 systems such as AC units must be able to start and run on "low voltage" per UL; as low as 208 volts. Again, that may affect longevity, but it's not going to make a unit die in 20 minutes; it's not that severe.

Of course, the other variable is the quality of the electronics of the equipment and not just the supply. Decades ago when electrical stuff was made here in the US, the standards were fairly robust and things could take a beating. Now, the cost-savings mentality of throw away devices and appliances makes for cheaper internal components and designs; hence they are more susceptible to low-volt operation.

Also, don't forget that most any equipment that is reasonably expensive is likely to have an internal motor re-set breaker which, if it gets overloaded will drop out the circuit, and then re-energize once it's cooled off. That protects the unit from extreme short term damage such as severe volt drop. Running your AC in the RV from that household cord is not as big a deal as you think. It should be able to run with only 100v. Admittedly not desirable, but doable. At 110v, you're perfectly fine.

 

[Wayne]

My EE buddy pointed out that of voltage drops 10% then starting torque drops about 18% and running current goes up 10%. Heat = current² so 1.1² = 1.21 which is 21% more heat. The rule of thumb is for every 20°F increase in motor insulation temperature the life is cut by 50%. The question is how many hours runtime at 10% low voltage would a RV A/C motor survive? With the cheaply built motors these days, I'm not sure.