minimum voltage to run A/C

wwest wrote:
LOTS of bad, incorrect, information here, throughout the thread...

Malarkey, not desirable, but the more critical issue is whether or not the compressor motor starts reliably each and every time without the internal self-resetting CB "popping".

The misinformation is mainly from you...

The motor's load may be considered fixed, so the same work (or
power/energy delivered) is performed when operated.

No, the A/C compressor motor's load changes, increases, as head pressure rises. Turn off the A/C while the compressor is cycling, turn it back on quickly, the motor likely will not restart. The motor's internal HEAT sensitive CB will begin cycling on and off until the head pressure declines enough to allow the motor to restart. Or the house CB pops..

This statement was made in an effort to keep the analysis simple since the motor was sized to operate, or trip off, under these various conditions.




When the motor's input voltage is low, the input current draw must increase in an effort to perform the same work.

YES!

Whew, we agree...




This increased current draw causes increased internal heating and reduced motor efficiency, which overheats the windings and core and can cause premature failure.

NO! P=IE, POWER (WATTS) = AMPS times VOLTAGE, reduce the voltage and increase the current, POWER doesn't change,....

But PF, Power Factor does.. The increase in current flow results partially from low power factor, the load begins to look more inductive than resistive. So, yes, current flow increases, but since that current flow is now "more" out of phase with the applied voltage, apparent power (derived strictly from voltage and current flow measurements) is greater than ACTUAL power consumption.

Example: Connect a high value bi-polar capacitor directly across the 115 volt AC line. A measure of power, current times voltage, will indicate power consumption when there is virtually NONE!

Actually, when the losses increase due to increased current through the windings and core, the efficiency goes down which means more input power is required to perform the same output work.

We are not talking VAR's here but real power (work)...

Power factor in a motor is a function of the motor's inductance, resistance, losses, and work performed.

Can you give me one example of how the motors inductance changes just by increasing current?

A lower PF would mean that inductance has increased, or less losses and less work is being performed which doesn't happen under normal operations.

None of this malarkey occurs.

PF increases with increased load and decreases when unloaded...


I think you need to revisit some of your hypothesis statements.

Wrong!
The distribution system is Wye. Three phase power is different then single/split phase.
Three phase 120/208 is standard for some light services. More common is three phase 277/480 for heavy industries.
Single/split phase is 120/240 for most services of this type. There is some installations that have 240/480.
Transmission lines are Delta.

Wish I was in the AC repair business :(

You ought to see the almost instantaneous temperature rise in motor windings with undervoltage starting exercises. The old rules, They Designed It, They Engineered It, They Manufactured It, They Warranty It, still is pretty potent wampum in my book. To each his own...

deleted

LOTS of bad, incorrect, information here, throughout the thread...

ken white wrote:
Should a motor ever be operated at lower than rated voltage?

The motor's load may be considered fixed, so the same work (or
power/energy delivered) is performed when operated.

No, the A/C compressor motor's load changes, increases, as head pressure rises. Turn off the A/C while the compressor is cycling, turn it back on quickly, the motor likely will not restart. The motor's internal HEAT sensitive CB will begin cycling on and off until the head pressure declines enough to allow the motor to restart. Or the house CB pops..

When the motor's input voltage is low, the input current draw must increase in an effort to perform the same work.

YES!

This increased current draw causes increased internal heating and reduced motor efficiency, which overheats the windings and core and can cause premature failure.

NO! P=IE, POWER (WATTS) = AMPS times VOLTAGE, reduce the voltage and increase the current, POWER doesn't change,....

But PF, Power Factor does.. The increase in current flow results partially from low power factor, the load begins to look more inductive than resistive. So, yes, current flow increases, but since that current flow is now "more" out of phase with the applied voltage, apparent power (derived strictly from voltage and current flow measurements) is greater than ACTUAL power consumption.

Example: Connect a high value bi-polar capacitor directly across the 115 volt AC line. A measure of power, current times voltage, will indicate power consumption when there is virtually NONE!

So, never operate any motor below its minimum voltage specifications or it will fail early.

Malarkey, not desirable, but the more critical issue is whether or not the compressor motor starts reliably each and every time without the internal self-resetting CB "popping".

How the electric companies set up their grids, for purposes of this thread and all other AC power consumption threads, is irrelevant. Compressor motors used in this application are single phase motors, run accordingly and require a minumum voltage for a power source. There is a reason the AC manufacturers designate a minimum voltage that the units can run on and not sustain damage. When they design their equipment they are not concerned about the utility company's latest and greatest configuration. As far as Dometic and coleman products, that number is 103VAC. Our units will not even start on 90V. I've been a dealer for Dometic's marine products for almost 20 years and that minimum number has always been 103V. Our digital controls shut off the system if less voltage is sensed for 3 minutes.
Everyone needs to keep in mind that this is an RV forum, not an engineering one.

From Coleman(RVP). Doug

Measure Line Voltage To The A/C
Line voltage should be checked at the
unit where the coach wiring connects to the
air conditioner while the air conditioner is
running. The correct operating range is
between 103.5 and 126.5 VAC.

lawrosa wrote:
In the US its 120v and 240V.Always has been and always will be.

Anyone that says 110v or 220v is wrong.

You should not have 112 volts from the home... Call the poco...

By the way have you checked the Hz?? being 60 hz is more important then the voltage.

I would not run anything in the camper. As well as some things in the home...Motors and such will fail sooner being subject to low voltage.

Not correct. Power companys have changed from the old "Y" 3 phase power distribution system to a "Delta" system with an "unbalanced" leg/phase for "legacy" power requirements.

If the A/C motor runs on 102 Volts, of even as low as 90 volts, it will be fine. The critical matter is compressor motor start up power and commercial power distribution systems have an infinite supply of that, relative to say, a 3000W genset.

Should a motor ever be operated at lower than rated voltage?

The motor's load may be considered fixed, so the same work (or power/energy delivered) is performed when operated.

When the motor's input voltage is low, the input current draw must increase in an effort to perform the same work.

This increased current draw causes increased internal heating and reduced motor efficiency, which overheats the windings and core and can cause premature failure.

So, never operate any motor below its minimum voltage specifications or it will fail early.

restlesswind wrote:
Just be real carefull about running anything else along with the AC.

The label on the motor that I just replaced on our RV AC reads "min.volts 103"
We have run the AC on less,but never again.That motor cost me $212.00 plus shipping and labor. The AC does not like to share a 30 amp circuit.

(I know,someone is going to tell me that I paid too much for the motor)

I don't know how you can make that generalization. There are countless numbers of 30A rigs out there that are sharing 30A service with other appliances. The criteria is that you have less than a 30A total load on the electrical system. It requires some energy management, but we do it all the time.