Transformers (step up/down/balancing) autotransformers

mike95490

Not Decepticons vs Autobots

120 vs 240 In phase/out of phase To balance or not to balance

So trying to run a house off grid, with only 1 or 2 large loads, all at 120Vac, I'm wondering about using a balancing transformer.

They do have standby losses, and I'm wondering if I should try to rig up a timer and contactor (relay) or SSR to only switch it on for the hours I'm likely to be using it, or if the"power up" surge of the transformer is likely to take a toll off the lifetime of the inverter ?

Thoughts ? Suggestions ? Stack another 6048 in the system ? I doubt I'll ever be using the FULL capacity of the inverter, but concerned about un-even loading of the 2 legs, or will careful choices of the splitting loads (washer:A, dryer:B) to phases, be enough? Anyones experiences with an inverter kicking offline from un-equal loading ?

XW6048 with a couple of 240VAC loads (well pump, welder)

120VAC Dishwasher, Front load washer & dryer, garbage disposal, hair drier, electric kettle (will be loaded with hot tap water)

RCinFLA

Re: Transformers (step up/down/balancing) autotransformers

The XW6048 can take a 75% imbalance or 41 amps @120 vac.

First approach would be to balance the 120vac load connections as much as possible.

If you need full 6kw 120vac (50 amps) you can re-jumper the unit for 120 vac only operation. There is an app note on the Xantrex site showing how to do it.

mike95490

Re: Transformers (step up/down/balancing) autotransformers

I've got to keep the 240 for the pump.

Not knowing deep details about large transformers, would a zero crossing SSR, be a way to not have a huge surge as the windings are energized, or maybe best to just leave it ON all the time and have good balance and eat the 50w 24/7 loss ??.

Just scheming, trolling for ideas. Wife is partially trainable to not turn everything on at the same time. Overload induced blackouts will cure that quickly I hope.

BB.

Re: Transformers (step up/down/balancing) autotransformers

You could switch in a PTC (positive temperature coefficient) resistor... When you want to turn the transformer off, put a PTC in series with the transformer. Current flows and PTC warms up--reducing current flow to near zero. Which would then leave the transformer with no polarized steel core (demagnetized). After a few seconds, turn off the AC power and short out the PTC.

Another way (frequently used on computer power systems)... Put a NTC (Negative Temp Coefficient) Resistor in series with the transformer input. You turn on the power, NTC is cold, little current flows, then as heats up, pretty much full current flow for your loads (Soft Start).

When power is turned off, just let it cool for a few handful of seconds before turning back on.

Using NTC's are very common and probably your best option for a cheap and easy to implement method of surge control. Just not for fast on/off/on cycling (needs time to cool down).

Here is one PDF spec. sheet for NTC's.

And here is another PDF NTC surge protection presentation.

-Bill

RCinFLA

Re: Transformers (step up/down/balancing) autotransformers

I am still trying to understand the problem you are trying to fix.

A 240 vac to 120 vac step down transformer is only going to give you another 20% current capability over XW single sided loading. Unbalance loading on XW does not cause any issue on XW. At full 75% imbalance (41 amp 120vac load on XW) you will have a couple of percent lower efficiency due to a bit extra transformer secondary wire loss but an external transformer will likely lose more.

An external 240v to 120v transformer is only going to give you 50 amp 120v load capability (9 amps more the single sided load max on XW).

The XW inverter transformer secondary is a 240vac center tapped winding. The primary side of the transformer and tranformer magnetic core only sees a load from secondary. It doesn't know if it is from single sided 120vac load on secondary or 240 vac load on secondary. Just the V*A matters.

mike95490

Re: Transformers (step up/down/balancing) autotransformers

I'm sorry - I was not really clear, I am mussing over all the different ways I can go, but I must maintain the 240VAC configuration for the water pump.

I'm thinking ahead, if I need to solve a problem with un-balanced loading on the inverter. I won't know till house is finished, exactly how things will work out. Maybe I won't have any issues, and predicting which loads will be on individually, or in pairs, will work out.

So, I'm considering a 240V CT transformer as a balancing transformer, to equalize the load imbalance between the phases, to keep the inverter from shutting down at 75% imbalance.
I've got a 3KVA transformer, that I would wire up like the Trace T-240 (4.5KVA, 12W idle) :

RCinFLA

Re: Transformers (step up/down/balancing) autotransformers

The hookup shown isn't really doing anything over what the transformer in the XW is doing. It is of little benefit.

mike95490

Re: Transformers (step up/down/balancing) autotransformers

RCinFLA wrote: »

The hookup shown isn't really doing anything over what the transformer in the XW is doing. It is of little benefit.

Well, the benefit would be, that I could not worry about the last 25% of load imbalance shutting the inverter down. As I understand it, the XW's internal transformer CAN'T manage a severe imbalance when in the stock, 240V mode.

The external transformer is a crutch, that consumes power ( 10-40watts apx) even when idle, but allows near total immunity from load imbalance.

So I will be trying the system without the transformer first, and only bring it online, if I had issues. I was just "testing the waters" to see if anyone has had any imbalance issues, and it appears it's pretty rare.

jtdiesel65

Re: Transformers (step up/down/balancing) autotransformers

I currently have an sw4024, but have been contemplating an xw4024 or moving to a different brand with 120/240.

I have the same well pump problem which I solve by using a transformer with an http://www.ameritron.com/Product.php?productid=ICP-120 to solve the in-rush current problem.

Anyway, one thing I was wondering about the xw4024 or any 120/240 inverter is the balance question. If it's 4000, does it mean it's 2000 per leg or is all 4000 available on one leg if there is nothing on the other leg?

From the original question, it sounds like it must be somewhat balanced.

BB.

Re: Transformers (step up/down/balancing) autotransformers

With the XW, if configured for 120/240 VAC, you can pull up to ~75% of the rated power on one 120 VAC circuit.

If you reconfigure the XW to 120 VAC only (minor reconfiguration and possible a different firmware), then it will do 100% at 120 VAC.

-Bill

RCinFLA

Re: Transformers (step up/down/balancing) autotransformers

For XW's there are two 120 vac secondary windings. They are normally wired in series 240/120v configuration or can be re-jumpered to run in parallel for full power 120vac only operation (there also may be a software change required).

The 120 vac, 75% rated VA 120vac load imbalance limitation is based on wire size used for secondary winding of transformer. Wiring the two windings in parallel effectively doubles the wire diameter giving back the full VA rating at 120 vac only operation.

The transformer core (size) performance limit and primary side transformer loading is based on VA rating so it doesn't know whether the VA load is from half side 120 vac or full 240 vac secondary loading. Only the secondary winding wire sees the increase current and associated wire resistance loss when running the 75% rated output from half the secondary winding.

Normally a transformer is designed to balance the losses between core loss and wire resistance loss. The XW transformer core is designed for a bit more leakage inductance then normal for a typical power transformer. This is usually undesirable for a power transformer, but for XW it allows for an effective high frequency PWM filtering scheme by just adding a capacitor across secondary.

There is a rough 'lbs of iron' per VA for transformer core size. The greater the leakage inductance, the softer the saturation curve is on transformer allowing a greater peak VA for a given core size, but higher average core loss over operating VA range. An auto-transformer configuration can be run at twice the VA compared to a total isolation wiring configuration.

The original T-240 autotransformer is a bit skimpped on core size for its 3.9 kVA (auto-transformer config) power rating. A good quality transformer for 60 Hz, 2 kVA (4 kVA auto-transformer config'd) should weigh about 50 lbs. The T-240 transformer weighs about 35 lbs. It is likely designed with more leakage inductance core to soften the core satuation overload performance. This makes its core losses higher. It has a spec of <12 watts of no-load power loss.

You have to be a bit careful about using an isolation transformer configured as an auto-transformer hung on output of XW for 120 vac power balancing. Most isolation transformers have a slightly higher then 1:1 turns ratio to make up voltage drop due to transformer losses. When configured as auto transformer the two sides are slightly different, like 1 to 1.02 ratio. This can cause extra idle current flow between XW transformer and balancing transformer creating extra losses.