3 phase motors in series

[tcmtech]

If a low viscosity oil like a SAE 10 is used it's not really a problem.

Try it some time. Put an old fan motor or some open frame capacitor start/run type of motor in a coffee can of light oil and let it rip.

Unless it's a pathetically low powered shaded pole type it won't have any problems at all. To be honest try it with a low duty cycle motor like one from a garage door opener in a larger bucket of oil. The thing will run continuous duty without a problem as long as it can get the oil to circulate around the windings and core.

 

[strantor]

The super expensive 42kw oil filled (iso22 hydraulic fluid) subsea motor we use currently, is only 83% efficient (rated). A motor of this size would typically be 90-95+% efficient. The lower efficiency probably is mainly to do with the oil filled nature. I am using 60kW total as a guess right now for the buffer room I'll need to account for the oil. But it will need to be tested for sure.

 

[tcmtech]

So what are you cutting underwater that needs a 42 KW saw ?

And can we have pictures of the saw at least?

 

[strantor]

pm sent

 

[shortbus=]

After thinking on this a while... Wouldn't this be a candidate for some thing you already have some experiance with, that you talked about years ago at AAC. 400Hz motors. The higher frequency would allow higher amperage through the existing cables and motors and generators are off the shelf.

 

[strantor]

After thinking on this a while... Wouldn't this be a candidate for some thing you already have some experiance with, that you talked about years ago at AAC. 400Hz motors. The higher frequency would allow higher amperage through the existing cables and motors and generators are off the shelf.

I've beaten the 400Hz thing into the ground multiple times. I've spoken at length with Engineers at **broken link removed**, Submersible Motor Engineering, and **broken link removed** about having them design us a small form factor, high power density motor based on 400Hz technology. They all told me that you cannot save space and maintain mechanical the same output speed/torque by increasing frequency and poles. The increase in poles negates the decrease in iron. You can get a same-kW motor in a fraction of size/weight by using 400Hz, but it is going to output 10's of thousands of RPM. To get a usable RPM (I need 1500-2000), you need a gearbox, which also negates the size savings of the motor. There is no advantage in size for going with 400Hz in this application. Using 400Hz will only increase the price.

For an EV automobile however, it still makes sense, as the gearbox is already required and already taking up space.

 

[tcmtech]

Now after seeing what you are working with I have to ask why cant the transformer be place down with the rig?

I see by the specs the saw machine weighs in at nearly 2 tons whereas a oil filled 50 KVA 3 phase transformer weighs in at maybe 500 pounds.
Adding in the additional how many hundreds of feet of umbilical cord weight I can't see where hanging the transformer down near the unit a few tens of feet up the cord would cause a problem. Or for that matter putting three smaller units mounted right to the unit or further up the line wouldn't work either.

Given the submerged operation I suspect that given the runtimes and massive amount of direct cooling the water gives that slightly undersized transformer units would work fine in that environment.

Just a thought.

 

[strantor]

Now after seeing what you are working with I have to ask why cant the transformer be place down with the rig?

I see by the specs the saw machine weighs in at nearly 2 tons whereas a oil filled 50 KVA 3 phase transformer weighs in at maybe 500 pounds.
Adding in the additional how many hundreds of feet of umbilical cord weight I can't see where hanging the transformer down near the unit a few tens of feet up the cord would cause a problem. Or for that matter putting three smaller units mounted right to the unit or further up the line wouldn't work either.

Given the submerged operation I suspect that given the runtimes and massive amount of direct cooling the water gives that slightly undersized transformer units would work fine in that environment.

Just a thought.

The tool I sent you in PM isn't the tool we're discussing; it's the overachieving brother of the tool in question.
The one I sent you is a deep sea tool (10,000ft). This one is to be a much cheaper, shallow (<500ft) tool.
The tool I sent you uses a several hundred thousand dollar heave compensated umbilical winch with several multi-KVA motors powering it. This one needs to get by with a much smaller, less sophisticated winch.
These tools and umbilical cables weigh much less in water than in air... until you go adding huge chunks of iron which can't be appreciably lightened with buoyancy material (buoyancy material also costs a lot).
The 50kVA transformer it would need, wouldn't be a 60Hz transformer, it would be a 30Hz (max) transformer, which is twice as big.
The transformer(s) would require an oil filled pressure compensated enclosure which adds weight and cost.

It isn't impossible to do it with transformers, and it's an idea that I still have on the back burner, but it has a lot of CONs associated with it.
It's the only way in which 400Hz technology could shine in this scenario. I could use much smaller toroidal 200Hz transformers with 400Hz motors (w/gearboxes), and that should do the trick. But it would cost a lot.

 

[shortbus=]

OK, another idea to throw into the mix. How about hydraulic motors? With the hydraulic pump in the saw body. Many of the earth moving and other off road equipment and deep mine equipment today use hydraulic motors as drive motors. Like hydrostatic mower drives. And like a steam engine they have a ton of torque at low speeds.

 

[strantor]

OK, another idea to throw into the mix. How about hydraulic motors? With the hydraulic pump in the saw body. Many of the earth moving and other off road equipment and deep mine equipment today use hydraulic motors as drive motors. Like hydrostatic mower drives. And like a steam engine they have a ton of torque at low speeds.

This is one of the typical approaches. This is how an ROV works; most of the power is used for an onboard HPU. This concept has been evaluated for the tool in question and found to be not cost effective. The HPU itself isn't that expensive, but there are a lot of proportional valves required, and electronics, which have to be sealed inside a pressure vessel (friggin expensive vessel), and now you have a communications bus in the umbilical which almost exclusively needs to be fiber optic, which drives up the cost of the umbilical, and so on.

Moral of the story is, anytime you (or I) come up with what seems like a great idea to do this efficiently and cheaply, some turd floats to the surface of the punch bowl. I haven't seen any turds in this series motor punchbowl yet, but so far the idea is just a fart in the wind. Once I start applying voltage we will see where the turd surfaces.

 

[tcmtech]

Another thought I had today about your concept and wanting to go as high of voltage as you can with relatively stock motors would be to look for the ones built for 600 volt power systems which are fairly common in large industrial and mining applications. Stack two of those end to end you and you have the capacity for a 1200 volt feed system.

At a 75 KVA load that would get you down to ~21 amps per phase which at 500 feet on 14 ga copper wire would get you a ~28 volt drop per line at the motor or ~ 2.3 % voltage loss per line.
Add that loss both ways and your normally 575 volt rated motors would be working almost dead on in their rated voltage range.

That's how I would consider approaching the final build design.

 

[tcmtech]

This is one of the typical approaches. This is how an ROV works; most of the power is used for an onboard HPU. This concept has been evaluated for the tool in question and found to be not cost effective. The HPU itself isn't that expensive, but there are a lot of proportional valves required, and electronics, which have to be sealed inside a pressure vessel (friggin expensive vessel), and now you have a communications bus in the umbilical which almost exclusively needs to be fiber optic, which drives up the cost of the umbilical, and so on.

Moral of the story is, anytime you (or I) come up with what seems like a great idea to do this efficiently and cheaply, some turd floats to the surface of the punch bowl. I haven't seen any turds in this series motor punchbowl yet, but so far the idea is just a fart in the wind. Once I start applying voltage we will see where the turd surfaces.

What all does the hydraulic system need to power other than the drive motors for the saw and some cylinders?
I'm curious being that to make a single pump run multiple motors at the same speed plus supply auxiliary flow to a valve spools is not all that complicated nor should be expensive and I can't see where a hydraulic system that runs at 2000+ PSI would need much protection from outside water pressures of a few hundred PSI.

If it was me I would be designing the whole hydraulic system and its electrical components to sit inside it's own hydraulic reservoir tank and have that pressure balanced with the outside. All the electrical solenoids are oil insulated and the whole system is pressure balance with the outside making it possible for the tank to be light and cheap to make.

To bad you're half way across the continent away. I would love to do some hands on work and testing with this sort of system design.

 

[strantor]

What all does the hydraulic system need to power other than the drive motors for the saw and some cylinders?
I'm curious being that to make a single pump run multiple motors at the same speed plus supply auxiliary flow to a valve spools is not all that complicated nor should be expensive and I can't see where a hydraulic system that runs at 2000+ PSI would need much protection from outside water pressures of a few hundred PSI.

If it was me I would be designing the whole hydraulic system and its electrical components to sit inside it's own hydraulic reservoir tank and have that pressure balanced with the outside. All the electrical solenoids are oil insulated and the whole system is pressure balance with the outside making it possible for the tank to be light and cheap to make.

To bad you're half way across the continent away. I would love to do some hands on work and testing with this sort of system design.

The application demands variable and tightly regulated speed, so it's not just dumb coils on bang-bang valves. It's analog proportional valves with LVDT spool position feedback loops into hysteresis dithering valve control cards inside of wheel speed monitoring feedback loops controlled by a PLC. The problem is that you can't pressure compensate the electro-hydraulic circuitry. The electronic circuits are fine to be immersed in oil, but the pressure is the problem. Many components (like electrolytic caps) have internal voids and will implode. So all that crap has to go inside a pressure vessel. There are some companies out there that make valve controls designed for subsea oil compensation but they are one-off expensive affairs. On top of that the valves are also super high flow rates (~60GPM on our hydraulic tool) to get the job done. The valve hardware itself is something NASA would balk at.

But let me just give myself an "out" here - when I said "it's been evaluated" - I didn't do the evaluation. I haven't beaten this one into the ground personally, so there is a chance you're right, and a hydraulic system could be cheaper. But I doubt it, and I can't spare the time to research it in depth.

 

[tcmtech]

Okay I follow that part with the electronics components being pressure sensitive but as for the majority of the hydrostatics systems they shouldn't have to much concern with external pressures if everything is in a somewhat balanced design.

Now for ~60 GPM well now you're talking my language! I've worked with large industrial equipment for most of my life so to me hydraulic systems that have nothing less than 1 1/2" lines and 50+GPM flow rates is not new. Granted it's primarily related to industrial earth moving and oilfield fraccing equipment and like equipment but I have had enough hands on dealings with high flow rate high pressure electric over hydraulic systems and their components to not be afraid of them by any means. Kind of miss it actually.

At my last job all of our major equipment that did the mixing for the frac fluids was electric over hydraulic so I am fairly familiar with how it's set up and controlled.
Granted the odds are you are using something of a different operating system but still it's based on a PLC of some sort that's looking at the various pressure and flow rates of each pump or motor and making adjustments from there.

 

[gary350]

What is your plan???

If they are 100% identical, current and voltage it will work but if not it will be like putting two 120 volt 75 watt and 100 watt light bulb is series and trying to run them of 24o volts, the 75 watt bulb will burn out. If 1 of your motors is under more load than the other motor it will be like the 75 and 100 watt light bulbs.