voltage drop

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sparky91

Member
hi folks , pretty new here
my dad and i are electricians .....me 23 years , my dad 40 years ,
i havent seen what im going to explain, its only work of mouth from my dad.
his friend has a fairly new ac compressor at his house on a 2-pole 40 amp breaker. He told me when utility company lowers voltage during extreme heat, 2-pole breaker trips. I assume that the drop in voltage causes an increase in amperage . One of my friends told me that he put a shunt trip on HIS own compressor ,as to not to damage the compressor or capacitor on the unit .
One idea i had that was shot down was to check wire size and see if a bigger breaker (5amps More) would stop breaker from tripping, but i was told that that would damage the capacitor being that it would allow a lower voltage into the unit and ruin it. My dad told me years ago he installed something MANY YEARS AGO, so that the reduced voltage would not affect the compressor.
He said it MIGHT BE CALLED A BUCK N BOOSTER OR A STABILIZER
Evidentally the homeowner wants the ac to still cool the house even with the brown-out. Is there a product that he can install that cant keep the compressor running at normal voltage . Its single phase 220 volt system , maybe i could increase the voltage to 240 , so that if they lower the voltage , id still be in the 220 volt ballpark
As i said , i havent physically seen it yet , but the breaker definitely trips when they lower the voltage.
i know its alot of typing , but wondering if anyone can help.....THANKS
 
kwired

kwired

Electron manager
Location
NE Nebraska
sparky91 said:
hi folks , pretty new here
my dad and i are electricians .....me 23 years , my dad 40 years ,
i havent seen what im going to explain, its only work of mouth from my dad.
his friend has a fairly new ac compressor at his house on a 2-pole 40 amp breaker. He told me when utility company lowers voltage during extreme heat, 2-pole breaker trips. I assume that the drop in voltage causes an increase in amperage . One of my friends told me that he put a shunt trip on HIS own compressor ,as to not to damage the compressor or capacitor on the unit .
One idea i had that was shot down was to check wire size and see if a bigger breaker (5amps More) would stop breaker from tripping, but i was told that that would damage the capacitor being that it would allow a lower voltage into the unit and ruin it. My dad told me years ago he installed something MANY YEARS AGO, so that the reduced voltage would not affect the compressor.
He said it MIGHT BE CALLED A BUCK N BOOSTER OR A STABILIZER
Evidentally the homeowner wants the ac to still cool the house even with the brown-out. Is there a product that he can install that cant keep the compressor running at normal voltage . Its single phase 220 volt system , maybe i could increase the voltage to 240 , so that if they lower the voltage , id still be in the 220 volt ballpark
As i said , i havent physically seen it yet , but the breaker definitely trips when they lower the voltage.
i know its alot of typing , but wondering if anyone can help.....THANKS
Click to expand...

How low is voltage dropping? I bet your AC is rated 208-240 volts and will handle a fair amount of voltage drop without any trouble.

Low voltage will result in higher current but I would expect the overload on the compressor to open circuit before the breaker will trip if breaker is sized according to nameplate data. The nameplate will typically call for a breaker that is at least twice rated full load of compressor plus the fan or other auxilary equipment.

You could have breaker tripping problems at startup if it is taking too long to start, but I highly doubt you have running problems.

If that is the case you could install hard start capacitor kit. They are easiest to find at HVAC suppliers. Most compressors are PSC motors. The hard start kit just adds a higher value capacitor that boosts the starting torque then is removed from circuit after it starts. It will overheat windings if left in circuit for too long.

You may also want to make sure the compressor is not 'short cycling' for some reason and trying to start with head pressure in the refrigerant. Simple anti short cycle timers can help with this problem also available from HVAC supplier maybe even only $10 or less, but you also need to make sure there is not a problem in control circuit causing the short cycling.
 
augie47

augie47

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Location
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Occupation
State Electrical Inspector (Retired)
First, I would take note of the first sentences in kwired's post:

How low is voltage dropping? I bet your AC is rated 208-240 volts and will handle a fair amount of voltage drop without any trouble.

Low voltage will result in higher current but I would expect the overload on the compressor to open circuit before the breaker will trip if breaker is sized according to nameplate data.

Check your nameplate data and see if the breaker is properly sized. If it is undersized the lower voltage may change the load enough to cause it to trip.
Once you have done that follow the remaining advice.
 
V

Volta

Senior Member
Location
Columbus, Ohio
sparky91 said:
... He told me when utility company lowers voltage during extreme heat, 2-pole breaker trips. ....
Evidentally the homeowner wants the ac to still cool the house even with the brown-out....
If it is a brown-out from too many A/C units running on the grid, he is part of the problem. :cool:


Is there a product that he can install that cant keep the compressor running at normal voltage . ...
A generator would do it.

As i said , i havent physically seen it yet , but the breaker definitely trips when they lower the voltage.
i know its alot of typing , but wondering if anyone can help.....THANKS
Click to expand...

:)
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Notwithsatnding kwired's excellent suggestions, you may not want to take this too far.

What can happen in a brown out condition is that the lower voltage results in lower torque in the motor (lower by the square of the voltage drop), which, if the compressor and fans are sized close to the bone, results in an overload OF THE FEEDER CIRCUIT, but not necessarily immediately of the individual branches IN the circuit, such as the compressor and fans separately. What happens is, lets say you have a 20% VD from the PoCo, the motors lose 36%of their HP (.8 x .8= .64 torque, which means .64HP), but the LOAD on the motors remains constant, which means unless the motors were over sized by 36%, they will overload. But most likely it's a race to see who trips first, the feeder CB or the individual OL protection devices inside the AC unit, and the CB is losing.

That's not necessarily a bad thing, it's called FIRE PREVENTION. If you scheme too hard to somehow circumvent this, be aware that all you will HOPEFULLY do is shift the problem down stream and cause the individual OL protection devices to trip instead. But unfortunately you might be removing the added margin of safety and cause a fire.

Adding a buck-boost transformer would have to come with a control system to have it NOT be in the circuit when the voltage is normal, otherwise you OVER excite the motors and cause damage that way. So to do that, you have to have a set of interlocked contactors, voltage sensing devices, some sort of logic to make the decisions, circuit protective devices for the components, an enclosure to put it all in and if you are in a state that requires it, an NRTL label on the whole shebang. Figure around $2K plus your installation cost. Then if your cool customer buys off on that, you will also have to increase the circuit size feeding this monster, because the boost transformer is only going to solve the problem for the AC unit end, it is STILL going to be drawing more current from the supply. So let's say you have that 20% VD, that will result in an increase in the current draw by 36% and your 40A circuit must now be a 60A circuit, which means replacing the wire and breaker feeding the boost transformer contraption.

If the things that kwired suggested are not in play here, in my opinion the generator idea is the only SAFE bet. If the PoCo delivers a brown out, have a standby genset with an ATS that kicks in and powers all the essential circuits, such as the AC, flat screen TV, espresso machine, margarita blender and porn-surfing laptop in the den.
 
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sparky91

Member
WELL , my initial theory was, if the breaker trips on voltage drop, the breaker must be sized pretty close to the actual amperage. So when that voltage does drop, it pushes the breaker over the limit. First i thought of increasing breaker size by 5 amps (if wire is sized correctly).
But i was texting an ac contractor friend of mine and he said i need a single phase "PHASE LOSE MONITER" .....havent looked it up yet BUT if it maintains correct voltage for compressor it probably would work
Im going to look it up and get back to you guys
thanks fellas
 
iaov

iaov

Senior Member
Location
Rhinelander WI
If the voltage drops I can understand the motor not working efficiently but I don't understand where you guys are getting an increase in current. The frequency has not changed so therefor the impedence of the motor has not changed either and if we look at what Mr. Ohm says about the current being proportional to the voltage and the resistence we will see that the current goes down not up. Is there something I'm missing here??:eek:hmy:
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
iaov said:
If the voltage drops I can understand the motor not working efficiently but I don't understand where you guys are getting an increase in current. The frequency has not changed so therefor the impedence of the motor has not changed either and if we look at what Mr. Ohm says about the current being proportional to the voltage and the resistence we will see that the current goes down not up. Is there something I'm missing here??:eek:hmy:
Click to expand...


That works for a purely resistive load, but motors are different.

Power or Horse Power remains a constant value for a motor, so if voltage goes down, current goes up.
 
kwired

kwired

Electron manager
Location
NE Nebraska
sparky91 said:
WELL , my initial theory was, if the breaker trips on voltage drop, the breaker must be sized pretty close to the actual amperage. So when that voltage does drop, it pushes the breaker over the limit. First i thought of increasing breaker size by 5 amps (if wire is sized correctly).
But i was texting an ac contractor friend of mine and he said i need a single phase "PHASE LOSE MONITER" .....havent looked it up yet BUT if it maintains correct voltage for compressor it probably would work
Im going to look it up and get back to you guys
thanks fellas
Click to expand...

Circuit breakers supplying a motor or hermatic compressor need to be sized large enough to to not trip during startup. This value is easily 2 to 2.5 times the normal running current without any questioning of the breaker size. For a motor to trip the breaker it will need to be drawing 2 to 2.5 times rated current or more. Thermal overload that is part of the compressor will trip before the breaker is loaded long enough to trip. The breaker is intended to only open circuit for short circuits and ground faults.

That is assuming the breaker size is properly selected .

As far as a single phase "phase loss monitor" I don't see what value there is of even making such a device. The device that was mentioned was more likely a voltage monitoring device, which would still give a fault output if a line was completely lost, but is not really the same thing as a three phase, phase monitor. It will not maintain voltage but is intended to interrupt the control circuit if the voltage level is outside of predetermined settings, which is what three phase monitors will also do. The same type of device is used in automatic transfer switches to initiate transfering sequence if incoming power is lost or even partially lost.
 
T

T.M.Haja Sahib

Guest
jumper said:
That works for a purely resistive load, but motors are different.

Power or Horse Power remains a constant value for a motor, so if voltage goes down, current goes up.
Click to expand...

For a motor driving a fan,if voltage goes down, current goes down.Correct?
 
T

T.M.Haja Sahib

Guest
ActionDave said:
It's not that simple for motors. If voltage goes down there is less current available, but the motor will still want enough current to do the work that it is being asked to do.
Click to expand...
If the work is also less to do,then the motor should be content with less current.
 
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