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Old 01-02-2012, 02:53 AM   #41 (permalink)
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More DC to AC inverter tests

Quote:
Originally Posted by thingstodo View Post
...On the agenda for tomorrow - power the inverters from separate 12V batteries, verify that the inverters can be connected in series to get about 280 VDC, and connect this voltage up to a test VFD to ensure that it powers up as it should.
Progress, but not all the progress I was hoping for.

I got the two DC to AC inverters powered from separate batteries. They are now isolated, so I put them in series and I got 281.2 VDC from them. The inverters are taking in from 0.9A to 1.1A at 12.5 VDC. I have not figured out where the variations in current come from. The cooling fans spin up when power is applied, but they stop almost immediately.

I ran into a bit of a snag.

The VFD that I will be powering has a pre-charge resistor on the AC input, but not on the DC bus. I need to rig up a power switch, a pre-charge resistor, a fuse and a bypass contactor to connect the high voltage DC to the VFD.

The pre-charge resistor limits the current drawn by the VFD to charge up it's capacitors when DC is applied. The bypass contactor 'short out' or bypasses the pre-charge resistor after the capacitors have charged up. This allows full DC current to the VFD.


Last edited by thingstodo; 01-02-2012 at 01:10 PM.. Reason: fix typo - big to bit, re-arrange to make explanation clearer
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Old 01-02-2012, 04:36 PM   #42 (permalink)
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More DC to AC inverter tests

Quote:
Originally Posted by thingstodo View Post
On the agenda for tomorrow - power the inverters from separate 12V batteries, verify that the inverters can be connected in series to get about 280 VDC, and connect this voltage up to a test VFD to ensure that it powers up as it should.
More progress. I recycled some parts and a portion of a circuit. There is a self-resetting 5A fuse, a switch, a pre-charge resistor with a minibreaker that can short out the resistor, and a terminal block mounted on a piece of lumber.

The 2 DC to AC inverters are connected in series, that gives me 281.4 VDC. I connect this through the small alligator clips (which will likely melt before I get to 5A) to my lumber-mounted switch and on to my VFD.

The VFD powers up normally. I bypass the pre-charge resistor, then start the VFD outputting 0.9 Hz. All is fine.

I shut it all down and wire the VFD to a small three-phase 240VAC motor, modify the motor wiring to allow my clamp-on meter to show the current for one phase, then power it back up.

The VFD is powered up, the speed setpoint is set to 10 Hz, and the motor is started from the keypad. The clamp-on meter shows 2.5 amps. The DC from the batteries into the inverters shows about 7 amps - not excessive.

The VFD has issues - it's current sensor reads full scale all of the time. This means that it has very little protection, since the current is already at maximum, and that it will only run a motor for 30 seconds before shutting it down and displaying an overload fault.

So I would say this test is a success.

I'm trying out a small video camera - the Kodak touch. I'll try to post some pictures and a schematic. The video is quite disappointing.
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Old 01-04-2012, 11:19 PM   #43 (permalink)
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More DC to AC inverter tests

I scaled back the testing, using one DC to AC inverter.

The efficiency was poor at the low end - under 80% when output is below 0.75 amps. The inverter uses about 11W just sitting there.

As the output power went up, so did the efficiency. My setup topped out at just under 5.5 amps output at 118VDC. Input is 714W and output is 642W for an efficiency of just under 90%.

I'll post a schematic and the data tomorrow.
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Old 01-06-2012, 10:39 AM   #44 (permalink)
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DC to AC inverter efficiency details

Here is a sketch of how the equipment was connected.

I started with a 6A battery charger on the deep cycle battery. I didn't get very far into the test before the inverter shut down on low voltage. (40% on the variac, under 200W output)

So I replaced the 6A 12V charger with a 40A 12V power supply, cranked the +/- 10% adjustment on the 12V as high as it would go - 13.33V, and connected it to the battery terminals. After the battery was charged up, I got through more trials but was not able to run the 900W heater all the way up. I got a trial at 94% output on the Variac, 714W output on the WattsUpPro to work. 95% on the variac dropped the voltage at the terminals of the DC to AC inverter below 10.8V, where the inverter shut down

As noted, this data includes the losses from the 140VDC to modified sine wave output stage. I have not soldered the additional wiring or fusing inside the DC to AC inverters to allow me to connect loads to the high voltage DC output. The WattsUpPro was available and gave me voltage, current and power measurements - it was convenient. It gives me confirmation that the previous efficiency estimate of 85% was a worst-case scenario. The losses are pretty consistent at about 10% from 366W output to 714W output. I have no reason to expect that relationship to change drastically as the output goes higher. As the transistors heat up, there will be some additional losses.

The output voltage at V2 dropped from 140 VDC to 118.7 VDC at the end of the test. Since the test stopped well under continuous rated power and far under peak power, I will assume that droop was caused by the battery voltage drop into the inverter. I'll try to get the test re-run with an additional parallel battery. The high voltage DC began to droop when the battery voltage dropped below 12.0 VDC.
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Last edited by thingstodo; 01-06-2012 at 10:41 AM.. Reason: typo
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Old 01-14-2012, 07:20 PM   #45 (permalink)
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An overdue Overview

I have likely confused many people about what I'm trying to do. I know that I use a lot of acronyms and that makes it harder to follow. So I'm going to step through the parts and pieces that I have collected and try to explain what my goal is. Then I'll back up a bit and explain how the tests that I'm doing fit into the system.

The 40 Horsepower (HP) General Electric (GE is just a brand name) electric motor is an AC motor like you'd find in many mills and other industrial installations all over the world. This one happens to have a rated voltage of 575 Volts of Alternating current (VAC) three phase. Connected to a motor starter that uses the normal 60 Hertz (Hz) electricity from the grid, the motor will spin at 1770 revolutions per minute (rpm), driving a load of about 120 foot-lbs. Foot-lbs is a measurement of the force that the motor will 'push' at a distance from the center of the motor shaft. At this load, the motor will draw about 38 Amps (A) current. If the load is lower, the motor will spin slightly faster, but will not exceed the 1800 rpm that the electricity is 'driving' the motor at.

The Variable Frequency Drive, or VFD, is not exactly a controller like a Curtis or a Soliton. As designed, the input is 60 Hz three phase, or three wire, electricity. The VFD uses diodes and electric filters composed of capacitors, inductors and resistors to make that input electricity into Direct current (DC), similar to the battery current. It then uses power electronics (transistors or IGBTs) to 'make' a rough version of AC that is fed to the motor. The Frequency (f) of the AC is what speeds up and slows down the electric motor. There are many ways to 'tell' the VFD what speed you'd like the motor to turn. Most VFDs have a 'keypad' mode where you type in a number from 0 to 120 or 200 and the VFD speeds up the motor, then controls it at that speed. It is also common to use a 0 - 5V signal to change the speed. So the VFD takes care of the 'throttle' or gas pedal part, and it has a lot of parameters, or numbers that you can change, to allow you to specify how fast you'd like to motor to speed up, slow down, how much current it can put into the motor, when the motor will overheat and should be shut down ... there are many modern VFDs that have 600 - 1000 parameters. I don't use the AC part. The batteries connect up to the 'middle' of the VFD, where the AC has been converted to DC. The DC batteries are connected in series, or the + of battery 1 to the - of battery 2, the + of battery 2 to the - of battery 3, and so on, to add up the required voltage and the VFD works as it would in a mill. It is NOT intended to be used in the way that I am using it ... but it works.

A Programmable Logic Controller or PLC is an industrial computer that is used to control many groups of motors in modern industry. I'm using an older model to take all of the inputs - accelerator, brake, motor speed, battery pack voltage, motor and battery current, and a few dozen more ... and drive the VFD, drive gauges on the dash of my truck, log data, and keep me safe. Many of the dashboard gauges accept a voltage output that I can approximate by turning the PLC outputs on and off quickly, called Pulse Width Modulation or PWM. The programming language of the PLC allows me to do many things that used to require relays and timers. My main reason to use the PLC is to over-ride the speed signal to the VFD under certain conditions. And the PLC allows me pretty much full access to anything that I want to log, or record. I plan to log as many things as I can so that when something goes wrong, I have as much information as possible to figure out what went wrong and how to fix it. So the PLC is sort of my controller, but the combination of the PLC and the VFD is what a Curtis or a Soliton does. I have looked at what optional inputs there are in the Curtis and the Soliton. I'll check out any other controllers that I can find information on to assist me in determining with what options I want to use. Again, we'll see.

Getting back to my testing. I have parts of a VFD that is large enough to drive my 40 HP motor, and that is the end game - check what the 40 HP motor will do. But it is difficult to test. I don't have a bank of batteries large enough to drive that motor for any realistic testing period. I have 240 VAC to my house like everyone else, so producing 575 VAC or 750 VDC is difficult. So I'm doing some testing with a smaller 5 HP motor, and I'm using equipment that can be driven with 240 VAC and batteries that I have. These motors are called T frame motors and they have specifications that are published by a standards organization - NEMA. Any particular motor from a name brand can exceed those specifications, but they must meet the minimum. And a lot of the specifications deal with the shaft size, the distance between the mounting bolts, the size of the mounting bolts ... the mechanical details that let you buy a replacement motor from another vendor when the first motor fails.

If the NEMA specifications gave me the information that I want, I would not need to test. But the specifications were developed for use in industry, years ago, so they don't include quite enough information. Because T frame motors have certain specifications, I'm testing a smaller one so that I can apply that information to the larger one. For example, I'd like to know what the maximum torque is that my motor will produce. That is the central factor that determines how fast it accelerates, and is the difference between driving a big golf cart that takes 4 minutes to get to 60 mph and driving the original Internal Combustion Engine (ICE), where it took about 25 seconds to get to 60 MPH. Will it perform better that the ICE and get to 60 mph in 15 seconds? likely not without a transmission. If the acceleration is too low I have a problem to solve - I may have to get a bigger motor, or use a transmission, or something else. If NEMA listed that torque, with the current that is required to produce that torque, I would not need to test. The graphs that are published show about 220% torque as a maximum or 'breakdown' torque. There is no mention, that I can locate, of what current is required to produce that torque, nor what slip rpm it is produced at.

Another factor is how much current the motor takes when it is accelerating at it's maximum. I need the battery bank to be able to supply that current, and the VFD to be able to put out that current, and the cooling system to be able to get rid of that heat so things stay running and are reliable.

So that's how the 5 HP torque testing fits into the truck design.

The Direct current to Direct Current (DC/DC) converter or Direct Current to Alternating current (DC/AC) inverter that I've been playing around with is an attempt to boost the voltage from my battery pack up to the voltages that the VFD can use. The 575 VAC VFD will run with a minimum of 750 VDC, but that's 60 batteries at 12.8V each. The weight is just not practical. If I can take a larger battery, like a fork truck battery, and boost the voltage from 12VDC to 140 or 150 VDC, then I can wire those 140V outputs together in series to get the voltage as high as I need it to be. The efficiencies I've seen so far make me think that this loses about 10% of the battery power as heat. That's a pretty steep cost, needing an extra 10% battery power. I doubt that this is a practical solution, but I will check it out since I want to learn and most of the parts have been surplus, ebay, or free. I want to explore some options before I begin to sink large amounts of money into this project.

So that's how the DC/AC inverter (The higher voltage DC portion is between the 12V DC input and the 120VAC inverter output) helps make my truck move.
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Old 01-15-2012, 04:39 PM   #46 (permalink)
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Plan for the weekend

I'd like to go back to testing the maximum torque that the 5 HP motor will put out, with my newly acquired and larger output current VFD. But I need to power up my DC to AC inverters, and use them to power up the VFD.

So I will need to have some batteries charged to run the inverters for a reasonable period of time. I have some old NiCd batteries that were retired from an electrical substation. They are nominally 1.2V, so 11 of them in series is 13.2V. This is a good solid input to for a DC to AC inverter. If I can use three strings in parallel, I should be able to drive the amps that the inverters will require.

I need to find a convenient way to charge them, fairly quickly and with little manual input from me. When I have some batteries charged, I can get the motor mounted on a frame, strap it to a large pulley, then use a linkage to measure the output torque. That's likely be next weekend since I still need to figure out how to charge 71 NiCd batteries
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Old 01-15-2012, 04:44 PM   #47 (permalink)
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DC to AC inverter as a battery charger?

Can I use the DC/Ac converter as a charger for the NiCd battery bank? At 10.99V input, the output is 118.7V I can use a useless 6V or 12V lead-acid battery in series to 'drop' the voltage. It's worth a try. Using the 40A 12V power supply as the input to the inverter and adjusting the output to float where the batteries should be when the NiCd are 'full' will do a slow trickle charge. It's not the way it should be done - Saft recommend 0.2C for charging. That's 0.2 * 39A = 8A. Also recommended is 1.45 - 1.7V per cell. That sounds high so I guess I'll check - maybe I can't even charge the whole string with 140 VDC. 140 / 1.7 = 82. 140 / 1.45 = 96. But there is 71 - which would be 103 - 120.7 volts. Add 13.8V for a 12V battery - that gives 135V. Add another 13.8V for another 12V battery - that gives 148V. No way to adjust the current so I hope that it is somewhere around 8A. I can drop the voltage on the 40A power supply down to 11.06V and get an output of about 120.2V ... I guess we'll find out.

I adjusted the power supply down to 11.3VDC, which appears to be as low as it goes with no load. It remains at 11.3V when connected to the DC/AC inverter. The output voltage is 140.4V out of the DC section of the inverter, with no load.

I connected all of the NiCd cells in series. They were very discharged (to the point of battery abuse. It's been a while since I looked that these). They should be nominal 1.2 V per cell, or 85.2V and they were at 69.1V. Actually, they were lower than that. Several cells were at 0.5V which is far below fully discharged. None were 0V. In the time it took me to connect the cells together and check them a couple of times, the voltage rose from the original 55V (adding up the individual cell voltages) to 69.1 so I expect that the temperature rise from -5C outside to 15C in the garage helped a bit as well.

I connected the 140 VDC across the NiCd batteries - and the inverter shut off. Could be over-current and could be under voltage on the input. I guess it was too much load. No fuses blew.

I added a mostly-charged but still unservicable lawn tractor battery in series with the string to limit the current. The lawn tractor battery is not serviceable - not enough current delivered to turn the starter - and I have not traded it in yet so I'll use it as a current regulator. It's worth a shot.

The lawn tractor battery worked too well. There is a drop of 30VDC across it and the charging current does not register well on my clamp-on meter. Somewhere between 0.2 and 0.3 amps. Even after an hour, the current still has not risen. The voltage across the NiCd pack is up to 101V, which is 1.42V per cell or the minimum recommended floating charge voltage. I was looking for at least 4 amps, and would have liked 8 amps. Something about stirring up the electrolyte with bubbles?
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Old 01-15-2012, 04:52 PM   #48 (permalink)
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DC to AC inverter as a charger - sort of

I switched out the lawn tractor battery to an older and less useful small car battery that floats at about 7V. It should take a bit more current.

Well, actually it takes a lot more current. The DC/AC converter trips on low input voltage. The clampon shows somewhere over 12A just before the inverter shuts down on low input voltage. That's a bit higher than I wanted to go.

So one 12V battery gives me 0.3A. Another gives me 14A. I'd like somewhere between, at about 4A.

After searching through my junk pile a bit more, thinking about what I could use to limit the current to charge the batteries, I decided to try an old toaster.

I plugged it into an outlet first and heated it up. Then, after it popped, check the resistance with a meter. It's about 16.4 ohms when I get the meter on it. I'm sure it was hotter before that. When I connect that in series with the NiCd batteries to limit the current, instead of the 12V battery, it appears to work.

The current is 26A into the inverter, and about 2.1A out of the DC of the inverter. The output voltage is only 138.1V. The battery string measures 101V. So the difference, 37V at 2.1A, calculates to around 17.5 ohms in the toaster heating element.

I'd like more charging current, so I boost the 12V into the DC to AC inverter. The output voltage climbs to 144.5V and the current rises to 2.4A. I forgot to check the battery voltage, but they are a large load so it's pretty safe to assume they are still 101V. 43.5V at 2.4A gives 18.1 ohms. That seems odd.

Going back and thinking about this, the toaster - with 120VAC across the resistor, allows about 12A. That works out to 10 ohms when the toaster gets really hot. So as current rises, the resistance goes down, but only to a point. The resistance should not have risen to 18.1 ohms fro 17.5 when the current went up. I'll have to think about that one.
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Old 01-15-2012, 04:55 PM   #49 (permalink)
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DC to AC inverter as a manual battery charger - OK

2.4A is far below the 4A I expected to see and even further below the 8A I'd like to see. But I don't have an easy way to regulate the current. I could build a circuit, but that's a bit of a diversion. It looks like I won't be getting the batteries charged up and tested today. I need to get them out of the garage and put my car in there. We got 6 inches of snow last night and I'm not cleaning off the car all week before going to work.

39 A-h at 2.4A will take the better part of 12 hours, with the batteries gassing quite a bit toward the end. These batteries are about 15 years old, so they won't have the capacity they started with - but I don't know how that affects the length of time to charge, either. The voltage on the batteries does not rise as they near 'full', they just produce a lot more gas as the water boils off ... at least, that's what it looks like to me.

I'll leave it run for a few hours and make sure that the batteries are not swelling, that the current stays about where it is. I plan to use the batteries in banks of 11 (13.2V) to power the inverters, which will power my VFD. So I will have 6 strings of 11, 3s3p for each of my 2 DC to AC inverters. The next time I'm charging the bank, I won't have as many batteries to charge so the current will likely be higher. I can play around with that and perhaps get the input current for the inverter up to 40A. That should get the output to 140V and about 3.5 amps. I guess I won't make it up to 4A output unless the output voltage drops a bit.
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Old 01-15-2012, 04:59 PM   #50 (permalink)
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DC to AC inverter as a manual charger - OK so far

I checked after about an hour. The output current is down a bit at 2.3A. The NiCd battery string voltage has risen to about 101.3.

Just for the heck of it I started to increase the output voltage of the 12V power supply and I got it up to 13.8V Strange that it would not go over 13.33V earlier. Perhaps the adjustment on the voltage is not as wide when there is no load, or when the power supply is cold? Increasing the voltage to 13.8V raises the output current of the power supply to 36.5A. I tried a bit higher and the power supply shut down. It's close to high current, I guess, so 13.8V and 36.5A it is.

The output voltage is interesting. It is no longer 140VDC but has gone up to 154.4 VDC. Again, it has never been that high during my previous testing. But I was doing my adjustments when everything was unloaded and the electronics had not been running for long so nothing was warm. It is certainly warm now. The tests I did with the 120 VAC loads earlier had much higher current output - perhaps the two are related. The DC to AC inverter does not appear to regulate the DC intermediate voltage well, but the output voltage is very steady at 120 VAC RMS.

The DC output current is now up to 2.9A, and the voltage across the batteries has risen again, to 101.7V. With 71 batteries in series, that is 1.43V each. Still not the 1.45V minimum for doing a charge, and nowhere near the 1.7V I was shooting for, but I guess I'd need more input current to the DC to AC inverter for that as well.

Right now, I am wasting 154.4 - 101.7 = 52.7V at 2.9A or 153W ... over 1/3 of my available charging power, on a toaster to limit the current and allow the batteries to charge at all. I will have to address that, but for now this works. Efficiency is good, but that's an optimization. Things have to work first.

I'm trying to keep focused on the task - I want to test the 5 HP motor again, and that requires charged batteries.

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