RV Air Conditioner tripping Inverter

[Hedges]

I watched the link and know you have no reason to believe me. But if the run capacitor is increased 5X during start then the LRA peak instantaneous Amps will also increase by 5X. In your case your inverter must be able to handle the higher peak Amps for the shorter time period. A soft start lowers the LRA but iit does take longer to start.

You might keep in mind that your 100 msec meter is 7 cycles of 60 Hz so you are definitely missing the peak.

I don't think 5x cap will increase LRA 5x.

A simple capacitor across the AC line, yes current will be proportional to capacitance value.
But this is 5x increase in a capacitor that is in series with an inductor. Increase capacitance to infinity, and you just have the inductor directly across the line. 5x capacitor move impedance close to that of the inductor.
Similar to having two resistors in series. If you decrease one resistor value to 1/5 it's original value, the series resistance doesn't decrease as much.

There is probably some capacitor value which is optimum for maximum starting torque, providing something like 90 or 120 degree phase shift at 60 Hz with that particular winding inductance. Except, can't get more than 90 degree with one capacitor, and that would deliver zero power. The starting winding likely has a location some angle from the main winding, so one capacitor gives maximum starting torque and a different one maximum running torque.

 

[Jasgeer]

There is essentially no or low inductance when the rotor is not turning. The size of the capacitor determines the reactance at 60 Hz and thus the initial current draw. My understanding is that if you left the larger hard start capacitor in parallel with the run capacitor permanently connected the run winding would overheat from too much current.

 

[eXodus]

There is essentially no or low inductance when the rotor is not turning. The size of the capacitor determines the reactance at 60 Hz and thus the initial current draw. My understanding is that if you left the larger hard start capacitor in parallel with the run capacitor permanently connected the run winding would overheat from too much current.

Source: HVAC School https://hvacrschool.com/start-capacitor-inrush-facts-myths-part-4/
Amps over time with and without Hardstart.

It looks to me that the duration high amperage is needed - is reduced and even the peak amps are going down from 200 to like 180.
The starting duration reduction from 700ms to start to 100ms - is massive - I meant that's 1/6 the "power" needed.

You have to think where the power in a inverter is coming from - the battery and internal capacitors. - It's easier supply a high current for 100ms instead of 600ms.

There is probably some capacitor value which is optimum for maximum starting torque, providing something like 90 or 120 degree phase shift at 60 Hz with that particular winding inductance

that is the case with Hardstart kits - if you oversize it too much - it's not doing any good.
I see a hardstart kit like a gearbox on a car - when everything is right - you can start and get going in 3rd gear. -, but when you up a against a hill (inverter)- no movement. While the hardstart shifts you down in 1st gear - you got massive more torque to get going - but the engine still stays the same - no additional hp. Just more torque at the wheel.

I've put hardstarts on everything as small as a 6000 BTU window, up to a 5ton (64000 BTU) A/C unit which would dim the light in the whole hose while starting. Never got a complain and none of those A/Cs died on me yet. (I think 6 years ago I started working with those)

So when you go through the HVAV boards - there is some agreement that manufacturer omitted Factory Hardstart kits (large starting capacitors) - for cost cutting reasons - and went with the smallest they could get away with. - I mean you safe $10 over hundreds of units? Lots of profit.

When you look - some higher end units with in the 10-12 year warranty range - have those capacitors again these days.

 

[Hedges]

Source: HVAC School https://hvacrschool.com/start-capacitor-inrush-facts-myths-part-4/

"
Conclusion #3

Hard start kits don't decrease starting amps at the moment of start; they can't.

What they can do is reduce the time it takes to get the motor started. So, in a time-averaged sense, a hard start kit very well may reduce amperage and wattage.

That is why a hard start is often specified for long-line applications and non-bleed TXVs. It's also why techs have found that adding a hard start can reduce light-dimming complaints by speeding up the amount of time the compressor remains at LRA.

Conclusion #4

Hard start kits do increase the current on the start winding. In fact, that's essentially all they do differently than a run capacitor. They both provide a phase-shifted current to the start winding; a start capacitor just does MORE of it.
"

The example waveforms showed the start time reduced from several hundred milliseconds to 100 milliseconds. That still exceeds the time capacitors in an inverter could supply current, so voltage drop in cables and from batteries is significant. The inverter needs to be able to provide surge current sufficient for LRA in order to start the motor.

One of the graphs showed about 175A peak current and 30A running current. Another showed 175A peak and 20A running.
Thermal-magnetic breakers typically fast-trip at about 5x their rating, which occurs within 30 milliseconds. The breaker rating would have to be greater than 20% of LRA to prevent fast magnetic trip, and would then use thermal trip to protect in case of a stalled motor.

Some inverters have surge ratings 200% of continuous for a few seconds, some 300%. That needs to exceed LRA.

It would be interesting to see waveforms for a soft-start. Could be it allows full starting winding current, even uses a capacitor to increase phase shift. But if it switched on voltage to run winding after waveform dropped from peak, that would reduce current. Narrow pulses of voltage might also work, either relying on winding inductance to limit peak current or just reducing during of peak current equal to LRA during each phase.

First link shows diagram of soft-start waveform, second shows scope shots for power control but not the ramp during soft-start.

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Phase Angle Control Module for Solid State Relays with Soft Start and Voltage Limit - SSRMAN-1P

SSR Phase Angle Control Module for Solid State Relays with Soft Start and Voltage Limit

nuwaveproducts.com

 

[eXodus]

What they can do is reduce the time it takes to get the motor started. So, in a time-averaged sense, a hard start kit very well may reduce amperage and wattage.

That is why a hard start is often specified for long-line applications and non-bleed TXVs. It's also why techs have found that adding a hard start can reduce light-dimming complaints by speeding up the amount of time the compressor remains at LRA.

that makes sense.

The house with 5 ton A/C and light dimming issues is the last on the road and powerline.

The example waveforms showed the start time reduced from several hundred milliseconds to 100 milliseconds. That still exceeds the time capacitors in an inverter could supply current, so voltage drop in cables and from batteries is significant. The inverter needs to be able to provide surge current sufficient for LRA in order to start the motor.

that's what I found in my practical tests with various inverters. The surge capacity of the Inverter still needs to be about 3X of the running compressor.
But not 10X anymore compared to none hard start.
Probably has something to do with the time domain - most inverters are defined with like 2-5 seconds surge capacity. The hard start makes the compressor get up to speed fast - before the window of higher power the inverter can provide is exhausted.

 

[Hedges]

Having designed circuits and tried to implement over-temperature protection, I realize that (unless you have a temperature sensor on the die), junction temperature is only inferred by heatsink temperature. What I did was to thermally connect I2C temperature sensors to the copper transistors were soldered to, so software could set an interrupt at some temperature. I also made a circuit that would latch off the transistors if they couldn't achieve regulation of electrical setpoint within some milliseconds. This was for a closed system with known load and operating conditions. In contrast, inverters are exposed to anything the customer plugs in.

For an inverter to be robust, I think the microcontroller should measure heatsink temperature, transistor current, and calculate junction temperature according to theta J-C. That will handle the 3 minute to 30 minute specs for higher power, and derating at elevated temperature. For starting surge, it should calculate power and heating over time considering thermal mass of the die (or response of the system according to previously run thermal simulations.) In this way it can allow maximum surge current while preventing transistors from overheating.

I don't expect low-end inverters to be as carefully designed. Their typical customer is going to power a house with a refrigerator as the largest motor load, or perhaps a small air conditioner. Testing is probably limited to representative loads. Some people have used them successfully for a while powering tools like a welder, then had them blow up. Depending on filtering, an inverter welder could be particularly stressful due current draw being pulses at higher switching frequency.

 

[eXodus]

For an inverter to be robust, I think the microcontroller should measure heatsink temperature, transistor current, and calculate junction temperature according to theta J-C. That will handle the 3 minute to 30 minute specs for higher power, and derating at elevated temperature. For starting surge, it should calculate power and heating over time considering thermal mass of the die (or response of the system according to previously run thermal simulations.) In this way it can allow maximum surge current while preventing transistors from overheating.

I hope that this will be an academic discussion in the future of solar and people are just going to buy everything DC.

It's rather waste of time of improving a technology to support an outdated and dying technology (one stage compressors) instead of going for the root cause and just building a good DC-compressor and omitting inverter altogether.

Hardstart, Softstart, Easystart - get a DC Air conditioner - none of those parts is necessary. For the money people are spending on getting to run antique terrible in efficiency one stage A/C unit? Mindboggling. Go DC

 

[Hedges]

Sounds like a great idea!
We'll just design a motor-drive inverter and place it between the 48V battery and 3-phase motor, call it "Brushless DC"!

While there are some DC traction motors in locomotives, 3-phase motors are used everywhere in industry.
Power-factor corrected variable speed inverter drive compressors in A/C could be good.
For smaller motors I'm happy with induction, shaded pole, universal. Many refrigerators these days are inverter drive, but induction motors work fine given a suitable size inverter to power the house.

 

[HaldorEE]

This one looks interesting for DIY applications. Peak 25A is a lot better than most < $100 soft start products I have seen.

https://www.newark.com/united-automation/ssc-25/control-module-softstart/dp/98K1086?ost=ssc-25

 

[eXodus]

Sounds like a great idea!
We'll just design a motor-drive inverter and place it between the 48V battery and 3-phase motor, call it "Brushless DC"!

While there are some DC traction motors in locomotives, 3-phase motors are used everywhere in industry.
Power-factor corrected variable speed inverter drive compressors in A/C could be good.
For smaller motors I'm happy with induction, shaded pole, universal. Many refrigerators these days are inverter drive, but induction motors work fine given a suitable size inverter to power the house.

I just would like to reduce boxes.

Battery (box) - Inverter (box) - A/C Unit (box)

instead of going - Battery -> A/C how every you do the internal design - I don't care, call it whatever you want.

Almost everything in my RV runs on 12V. There are three appliances which need AC at the moment - Microwave, A/C and Coffeemaker. Just got a propane coffeemaker and for the Microwave and I can buy a DC https://www.roadtrucker.com/12-volt-microwave-wavebox/12-volt-dc-microwave.htm

So the only item to solve left - A/C

 

[Supervstech]

I hope that this will be an academic discussion in the future of solar and people are just going to buy everything DC.

It's rather waste of time of improving a technology to support an outdated and dying technology (one stage compressors) instead of going for the root cause and just building a good DC-compressor and omitting inverter altogether.

Hardstart, Softstart, Easystart - get a DC Air conditioner - none of those parts is necessary. For the money people are spending on getting to run antique terrible in efficiency one stage A/C unit? Mindboggling. Go DC

The problem with this is with large air conditioners, the wattage needed to compress the refrigerant is massive… to use a 48V DC motor would require #2 AWG conductors… the inverter dc motors on variable speed systems use 300VDC inside the cabinets…

 

[Zwy]

I hope that this will be an academic discussion in the future of solar and people are just going to buy everything DC.

It's rather waste of time of improving a technology to support an outdated and dying technology (one stage compressors) instead of going for the root cause and just building a good DC-compressor and omitting inverter altogether.

Hardstart, Softstart, Easystart - get a DC Air conditioner - none of those parts is necessary. For the money people are spending on getting to run antique terrible in efficiency one stage A/C unit? Mindboggling. Go DC

After researching this subject heavily, and reading every thread on this board, I've come to the conclusion that DC powered air conditioning is not that efficient at this time compared to mini split AC powered systems.

You just don't see the capacity with the DC powered systems. One might ask why? The reason is the amount of watts required to power a unit. DC will require much larger cable and many more amps to achieve the same cooling btu/watt ratio.

It's just not going to happen.

 

[Zwy]

The problem with this is with large air conditioners, the wattage needed to compress the refrigerant is massive… to use a 48V DC motor would require #2 AWG conductors… the inverter dc motors on variable speed systems use 300VDC inside the cabinets…

You were posting while I was typing. That's exactly what I have determined. Just takes too many amps on DC.

 

[eXodus]

The problem with this is with large air conditioners, the wattage needed to compress the refrigerant is massive… to use a 48V DC motor would require #2 AWG conductors… the inverter dc motors on variable speed systems use 300VDC inside the cabinets…

I agree, but we are in the vehicle mounted section of this board - and none of those have particular large A/C units 13-15K BTU. With around 1kw running.

You just don't see the capacity with the DC powered systems. One might ask why? The reason is the amount of watts required to power a unit. DC will require much larger cable and many more amps to achieve the same cooling btu/watt ratio.

After researching this subject heavily, and reading every thread on this board, I've come to the conclusion that DC powered air conditioning is not that efficient at this time compared to mini split AC powered systems.

There is no development dollars in DC technology yet. Most RV A/C units with AC power are producing about 10BTU per Watt.

An AC minisplit produces 14-15 or so. But a DC MiniSplit has the potential to get above 20BTU per watt.
So in a vehicle setting - you are reducing that powerdraw - instead of looking at 1000w for one roof unit - which would require - rather large cable - we are at 500w at 48V that's a a meager 10A. You could use the existing 12V wiring to the roof A/C.

We don't have those units yet, but the market is heating up all around the world - more and more countries instating anti idle laws - electric busses and construction machines are getting the norm. So there will be more development money going that route. Just a matter of time.

 

[eXodus]

This one looks interesting for DIY applications. Peak 25A is a lot better than most < $100 soft start products I have seen.

https://www.newark.com/united-automation/ssc-25/control-module-softstart/dp/98K1086?ost=ssc-25

A $10-15 Hardstart from Supco which you can pickup almost everywhere. Solved usually all my inverter start issues.
Easy to source, easy to install. Just make sure that you match the correct size.

Softstart works - https://www.rvtravel.com/rv-electricity-hard-or-soft-start-caps/ just hard to justify the cost. Further they are way more complex. So just another part that might fail and is not easy to replace on the road

 

[HaldorEE]

A $10-15 Hardstart from Supco which you can pickup almost everywhere. Solved usually all my inverter start issues.
Easy to source, easy to install. Just make sure that you match the correct size.

Softstart works - https://www.rvtravel.com/rv-electricity-hard-or-soft-start-caps/ just hard to justify the cost. Further they are way more complex. So just another part that might fail and is not easy to replace on the road

A hard start is to deal with a specific condition in compressors. It does make it possible to start motors on an inverter that you would not otherwise be able to, but it is the opposite of a soft start (full current for a shorter period) and this puts more stress on the motor and gear train (if one is in the system). A hard start is exactly that and it wears out the system faster.

Hard and Soft Starters in the HVAC Industry – Which Is Right For You?

Starters are used in the HVAC industry for single-phase compressors. These compressors use permanent split capacitor (PSC) type motors for ratings above 0.5kW. Start assist devices can be the “hard start” or “soft start” type. But

www.briskheat.com

I do understand why a hard start is attractive after seeing $300+ pricing for soft start kits. That is why I was excited to see find an $80 soft start module.

 

[eXodus]

A hard start is to deal with a specific condition in compressors. It does make it possible to start motors on an inverter that you would not otherwise be able to, but it is the opposite of a soft start (full current for a shorter period) and this puts more stress on the motor and gear train (if one is in the system). A hard start is exactly that and it wears out the system faster.

Hard and Soft Starters in the HVAC Industry – Which Is Right For You?

Starters are used in the HVAC industry for single-phase compressors. These compressors use permanent split capacitor (PSC) type motors for ratings above 0.5kW. Start assist devices can be the “hard start” or “soft start” type. But

www.briskheat.com

I do understand why a hard start is attractive after seeing $300+ pricing for soft start kits. That is why I was excited to see find an $80 soft start module.

The question is - what does "wear-out" faster? bearings? wiring?

I got a 5 ton 21 year old unit running with a hardstart, the A/C in my RV is 18 years old.
That window A/C is almost 10. (and that's in Florida, - means it runs all year round, almost every day)
Most of the units I place hardstart are well beyond their "expected" life and are chugging happy along.

My conspiracy theory is that A/C makers don't like people using those and rather buy new systems.

If any of those dies, I'm going to buy inverter units and don't have to mess with single stage anymore

 

[Supervstech]

The question is - what does "wear-out" faster? bearings? wiring?

I got a 5 ton 21 year old unit running with a hardstart, the A/C in my RV is 18 years old.
That window A/C is almost 10. (and that's in Florida, - means it runs all year round, almost every day)
Most of the units I place hardstart are well beyond their "expected" life and are chugging happy along.

My conspiracy theory is that A/C makers don't like people using those and rather buy new systems.

If any of those dies, I'm going to buy inverter units and don't have to mess with single stage anymore

All trane systems come with a hard start capacitor and potential relay system at least as an option.
The higher end units ALL come with it, unless it is a variable speed inverter drive compressor.
Unlikely the kit shortens the life of the system.
What damages windings is sustained torque on the start winding. HSK provide the boost voltage to pull from the cap AND the L2 line, aiding starting torque of the compressor. As a compressor piston or swash plate operates, wear will increase load on the compressor. The start kit helps extend the life of a compressor, and keeps the system operational longer.
I'm not sure it would help one start on a weak inverter though.

 

[eXodus]

I'm not sure it would help one start on a weak inverter though.

It's common knowledge with the Inverter-Gasoline Generator Users that Hardstartkit allow small generators to start A/C units which would normal not run. This doesn't mean it's helping with to small of a system

Small History lesson, early 2000s many people like the Honda Eu2000 Suitecase style generators - which have trouble starting a standard 13500 BTU RV A/C unit. The Honda puts out about 1600w and 2000w surge.

Buy adding $10 hard start kit - you don't need to upgrade to the next larger generator size. which is a EU3500 - something you can't carry alone anymore.
From there the Hardstartkit for Inverter knowledge spread - just go on the Supco Reviews on Amazon - about half of them are about inverters.

My electrical Thesis is that a HSK keeps the starting compressor duration under the maximum time a Inverter can supply surge current. If the surge current is to low - it will still not start. My 1200W Inverter will not start my 600W window A/C when the Fan is off, the surge of starting the Fan and the compressor is to much... and without HSK it didn't start at all. (hm maybe ad another HSK to the Fan motor capacitor...)

 

[Supervstech]

FYI, the suppc hard start kits are inferior.
521 makers of the csu1-3 series hard start kits use a true potential relay that matches to the load of a broad range of compressors.