Inverters don't do sine

I recently built a simple PLC card tester to test cards for IDEC FC-3A series. I have a customer who uses several of this style PLC, and we want to keep them going for years to come even though they are officially obsolete.

The tester consists of backplanes, a power supply, and a CPU with 3 simple routines programmed in to make an output card do tricks. The output card is wired to an input card externally, and the signals from the output card are received by the input card and evaluated by the PLC program and the operator.

For example: one of the subroutines sends 500 100ms pulses out on each output, then compares counters on the INPUTS to make sure all pulses made the round trip. Like I said: simple.

Anyway, to switch between subroutines I just have a rotary selector switch and a 120vac INPUT card in a dedicated slot to handle the tester inputs: Start, selector switch setting, reset....

(By the way, this is all built into a tool-case/briefcase and looks EXTREMELY suspicious. I hope I never have to talk my way through an airport......)

The other day, I put down my tailgate, plugged the suitcase tester into my inverter, and started trying to test some cards.

I had all indications of power (power supply and CPU indicator lights looked normal) but the dedicated 120vac input card wasn't getting my control signals. After dragging out the meter, much testing, card swapping and head scratching, I took it inside and plugged it into the WALL, and it worked fine.

All this description boils down to: MY IDEC FC-3A series 120vac input cards work FINE on REAL 120vac, but not on my truck inverter. (I went into so much detail because most responses to new threads are for more information.)

I was wondering if anyoune ELSE has had this problem.... And I wanted to share it with everyone ELSE in case you run into it so you'll have a little heads up.

And has anyone built similar testers?

Stationmaster

(By the way, this is all built into a tool-case/briefcase and looks EXTREMELY suspicious. I hope I never have to talk my way through an airport......)
I'd suggest if you do go through an airport to have at least 2 lawyers travelling with you.
I get pulled over for carrying a couple of serial cables and a few adapters in my bag.

Steve,

Yes, there is smooth sine-wave current, and there is rounded-off modified square-wave current. You need a real 120 volt alternating-current pure sine-wave inverter to make it work from a vehicle 12-volt DC circuit. Perhaps something like this:

http://partsonsale.com/samlexinvercharge.html

Here is a research paper that explains the different methods, and a new one to produce pulse-width modulation.
http://www.google.com/search?hl=en&q=120+volt+sine-wave+generator&btnG=Google+Search&aq=f&oq=

Thanks Lancie, I appreciate the input, but the inverter I'm using now was about $80.00...... For the difference I can buy a LOT of extension cords. lol (I'm saving every penny I can.....)

Years ago I made a receptacle with alligator-clip leads so I can tap into a panel if need be, (observing load limits of course).

So far I haven't run into a situation where I've HAD to work on PLC components where there was no juice, though I know it could happen.

Because of the appearance of this contraption and the potential chaos it could cause, we've taken to jokingly referring to it as "The Suitcase B*mb". I'm REALLY going to have to nip that in the bud.......

: )

Stationmaster

Stationmaster said:

s "The Suitcase B*mb"
: )

Stationmaster

Click to expand...

In an airport you can't even whisper that word.

Mickey said:

In an airport you can't even whisper that word.

Click to expand...

I know, RIGHT? We'll have to come up with a CODE name.....

If you noticed, I didn't even dare SPELL it all the way out.

Station..... er, um....

How about bombastic??

An alternative when you RARELY have to do testing and need clean sine wave in the boonies. GE and others made 12VDC 120VAC single phase MG sets for boats. 750 to 1000 watt (??). Since we still have a few boats here in Seattle I may be able to scrounge one up. Guess on cost 2 to 300 dollars (dont need clams have plenty on beach).

Dollar for dollar if you really need this then a "pure" sine inverter is way to go.
Dan Bentler

Sort of related to this topic but how would one calculate the amount of dc needed to supply 120 vac at 1000 or 2000 watts continuous as some of the units from lancies link are rated for?

How would I know if my car, truck or RV system has that kind of dc output?

How can you convert from dc power usage to ac power usage and back?

Another example is solar panels I have been looking for info on how to determine how much solar equipment Dc output I would need to run certain ac loads in my house

I have not been successful in finding a real straight forward way to calculate this.

Power = Voltage * Amperage

or more commonly

P=VI

1 Watt = 1 VDC * 1 Amp
1 Watt = 1 VAC RMS * 1 Amp

You'll have some losses in the conversion from DC to AC that will need to be accounted for, but basically:

250 Watts = 120 VAC RMS * 2.083 Amp
250 Watts = 12 VDC * 20.83 Amp

The Plc Kid said:

Sort of related to this topic but how would one calculate the amount of dc needed to supply 120 vac at 1000 or 2000 watts continuous as some of the units from lancies link are rated for?

How would I know if my car, truck or RV system has that kind of dc output?

How can you convert from dc power usage to ac power usage and back?

Another example is solar panels I have been looking for info on how to determine how much solar equipment Dc output I would need to run certain ac loads in my house

I have not been successful in finding a real straight forward way to calculate this.

Click to expand...

One thing Monkeyhead forgot to tell you is that on inverters or MG sets for that matter power out = power in (less losses).
OR IxE = IxE
Otherwise he gave you all needed calcs as above.

On a vehicle you generally do not have a large enough alternator (or generator) and some load if not most will be supplied by battery.
More homework
GIVEN 1,000 watt inverter 12 VDC supply with a 120 volt AC output.
Assume no voltage droop on battery and 12V 100 amp hour battery (100 amp for one hour - no more)
AC Load does not vary and is 950 watt resistive only. Inverter has a 95% efficiency - loss is in form of heat only.
Truck supplying power has a 35 amp alternator. Assume full output of alternator can go to battery (ie no other loads)

FIND
AC line amps
DC line power to inverter
DC line amps
Allowable time to run inverter

DUE Sunday night??
Dan Bentler

ANSWERS TO ABOVE

DC VOLT 12
AMP HRS 100
ALTERNATOR 35
AC VOLTS 120
AC LOAD 950
INV EFFICIENCY 0.95

ANSWERS
INVERTER LOSS 47.50
DC LINE WATTS 997.50
DC LINE AMPS 83.13
BATTERY AMPS 48.13
TIME ON BATT (hrs) 2.08

Figure this out and then we will do solar. Almost identical with a couple more wrenches in gears.
Dan Bentler

Lancie1 said:

Steve,

Yes, there is smooth sine-wave current, and there is rounded-off modified square-wave current. You need a real 120 volt alternating-current pure sine-wave inverter to make it work from a vehicle 12-volt DC circuit.

Click to expand...

By the way..... It doesn't surprise me at ALL that my cheap inverter doesn't make a pure, smooth sine wave. I'm surprised that the input card can tell the difference.

Stationmaster

Stationmaster.
I suspect that the problem is not how clean your sine wave is but that the output from your inverter is not actually AC. The output from the inverter will not fall below 0V (ground) so the current will not alternate. The output voltage will be modulated as a sine wave but will always be positive with respect to 0V/Ground. The solution would be to use a small isolating transformer with the secondary grounded on one side. I would connect the transformer directly to the inverter output without a switch/breaker so that prevent the transformer inrush from damaging the inverter output devices.