Difference between RPC and genset

steamingspud · Jun 24, 2013

Hiyah folks,

I'm doing a project to build a rolling mill using a gearbox that runs on three phase. So, I need something to make three-phase.

I've seen the very many posts on making rotary phase converters, which is probably what I'll end up doing. I understand the theory behind it, and may have already made one in lab (but who remembers that stuff anyways?). My problem isn't so much the design, but my paps...

My dad's an electrician, and he's seen a lot of three-phase cranes with gensets. He's saying to take a one phase motor, belt it to another three-phase motor, in turn making three-phase power as if it were a generator.

He does not understand how a rotary phase converter can work and create the voltage needed to operate the gearbox motor. I hadn't even seen that as an issue, but he assumes creating a genset is superior to a RPC.

Question is, what's the difference? What makes an RPC better than just a mechanical genset? Are there benifits to either? Do I need a transformer if I use an RPC?

Froneck · Jun 24, 2013

First connecting one motor to another isn't going to work. The "generator" motor will have to be excited. You can spin it as fast as you want and it will not "generate" a voltage unless you excite it! It can be done but belting it is not a good idea as V belts get the traction from the slip so if you belt the two with the same diameter pulleys they will not run the same speed, the "generator" will slow down as load increases so the output frequency will lower. Therefore they must be direct coupled but again the drive motor will change speed as load changes. You can find 3 phase Generators with Permanent magnets but they will get expensive. As you get higher in HP single phase motors are not available and expensive! You can not use a 1HP Single phase motor to drive a 2HP "generator motor" and expect to get 2 HP out, you can at best get slightly less than 1 HP out. Simplest way is to make an RPC.
Only time you will need a transformer is if you want to increase (possibly decrease) voltage (run a 480VAC 3 phase motor on a 240VAC single phase) or in some cases make 2 phase!

Frank

peterh5322 · Jun 24, 2013

True three-phase (utility or simulated utility by Phase Perfect) can source and sink power to and from the load. Utility three-phase can source and sink power up to the capacity of the premises service equipment as the power grid simulates an infinite bus (source and sink). Phase Perfect three-phase is capable of significant short-term overloading, although it cannot simulate a true infinite bus.

RPC three-phase can only source power to the load, it cannot sink power from the load. This has implications for so-called regenerative drive systems, which expect, and require the source to dissipate significant power when operating in the two braking quadrants of a four-quadrant drive system. If the drive system dissipates braking power in braking resistors, RPC three-phase will work satisfactorily.

Philabuster · Jun 24, 2013

steamingspud said:
I'm doing a project to build a rolling mill using a gearbox that runs on three phase. So, I need something to make three-phase.

How big of motor you going to use? 1HP? 5HP?

Rolling mill would benefit from a speed control, so kill two birds at once and feed the motor with a VFD. Done.

I am guessing your home brew rolling mill is not going to be installed in a steel mill, but add detail as needed.

steamingspud · Jun 24, 2013

Frank, that was a good explanation. My problem with the horsepower so far has been having the info on hand. I'm not sure the hp rating on the gearbox motor, and the motor I got for the RPC has it listed as SPL, so I'm in the dark as to if it's going to work or not.

Peter, hopefully I'm following you correctly haha. You're saying I need resistors on the other end of the third leg creating the third phase? I assume the braking quadrants are the sections not powering the RPC but contracting flux... I'll admit I'm a little lost in your wording.

Philabuster, nothing so big as the steel mill ones. It's for a blacksmith shop to make even welds on long pieces of stock and tapers for different projects. I don't need the speed adjustment so much, I push the hot stuff in, and it spits it back out at a slow pace.

peterh5322 · Jun 25, 2013

"Peter, hopefully I'm following you correctly haha. You're saying I need resistors on the other end of the third leg creating the third phase? I assume the braking quadrants are the sections not powering the RPC but contracting flux... I'll admit I'm a little lost in your wording."

RPC three-phase and static three-phase is simulated three-phase, not real three-phase.

With specific respect to four-quadrant drives, if (and only if) the drive "regenerates" power into the source, then you will need a source of real three-phase. An RPC cannot "sink" power on the B phase. A static has no connection to the B phase after starting, anyway.

However, if a four-quadrant drive has its own braking resistors, then RPC three-phase is fine as it will never be asked by the drive to "sink" power on the B phase.

Perhaps some definitions may be helpful ...

A two-quadrant drive has these operational modes: forward motoring, reverse motoring, and coasting to a stop (from either forward motoring or reverse motoring), with no, or little control over the motor when coasting.

A four-quadrant drive has these operational modes: forward motoring, forward braking (often to a stop), reverse motoring, and reverse braking (often to a stop), with usually complete control over the motor in all four modes.

JST · Jun 25, 2013

peterh5322 said:
RPC three-phase can only source power to the load, it cannot sink power from the load. This has implications for so-called regenerative drive systems, which expect, and require the source to dissipate significant power when operating in the two braking quadrants of a four-quadrant drive system. If the drive system dissipates braking power in braking resistors, RPC three-phase will work satisfactorily.

Now, WHY would you say that? It's clearly not true.

The machine is basically linear, and symmetrical as far as input power on any of the windings.... There is no "rectification" that would prevent power flow in one direction.

1) any power on the A-C phase is connected directly to the local single phase bus, and sinks power just fine.

2) Any power on, say, the A-B phase will behave as if the C phase is the "generated leg", and will send power into it. Likewise on the C-B leg, the "A" phase would be the "generated leg". The resultant is going to be the superposition of the various inputs, since the system is basically linear.... as linear as any motor.

The RPC will accept regen power better than most any VFD without a brake resistor, and as well as some that do have them.

Now, is it going to be as efficient at accepting regen as a direct 3 phase power line connection? No, it cannot be, due to extra series impedances, but it may be good enough. In any case, if you have an RPC you don't have input 3 phase power and the question is moot.

Froneck said:
First connecting one motor to another isn't going to work. The "generator" motor will have to be excited. You can spin it as fast as you want and it will not "generate" a voltage unless you excite it!

In practical use, it can and does work, but I would not suggest it for this sort of usage. Excitation for stand-alone generation is normally provided by capacitor loading.

it is not well suited to large suddenly applied loads, which tend to cause generation to drop out unless the motor has a separate exciting winding with the loading on it.

9100 · Jun 25, 2013

How hard would it be to change the gear motor to a single phase one?

An AC motor used as a generator will self excite if connected to a capacitive load. There are also self excited generators available, not permanent magnet. Some have a winding connected to a capacitor. They maintain surprisingly good voltage regulation. Larger ones have separate exciter generators.

Question- has anyone belted a three phase motor to a driving motor to overspeed it and put line voltage on one phase? I have never heard of it being done, but it seems at least possible that it would give a reasonable output on the third leg.

Bill

Froneck · Jun 25, 2013

The SPL rating means special and as far as I know it's use to indicate the motor is less than continuous duty (24/7) Unless it has a very short time rating it should be OK to use as an RPC To test simply pull start the RPC motor are see if it will do as you intend to use it.
Surely there are many type of 3 Phase Generators, some will self excite and others have field windings! If there was an application I'm sure someone made it! However its not a simple task by someone not familiar with electrical power generation to convert a motor into a generator. Just trying to keep it simple! I'm thinking the OP is trying to make due with what he has.
As to belting up a motor to get a third phase how do you plan to synchronize it?

Frank

peterh5322 · Jun 25, 2013

It is indeed possible to generate true three-phase using a single-phase to three-phase motor-generator set.

This combinations could be a single-shaft machine, or, more likely, a three-shaft machine.

The generator portion must, necessarily, be a synchronous machine, one with a wound rotor. A "squirrel cage" rotor just won't due.

In the three-shaft machine, the third shaft is actually the separate exciter, usually a small machine of about 1/3 to 1/2 HP.

The prime-mover motor, and the synchronous generator, actually an alternator, may be in the 5 or even 20 HP range.

With a separate exciter, as described above, the excitation of the synchronous generator may be critically excited (power factor neither leading nor lagging) or may be under- or over-excited (thereby adding or subtracting volt-amps, reactive, to the load).

There is nothing which says the prime mover has to be an electric machine. Heck, even a water wheel will do, or even a Detroit 2-71, as the exciter is almost always a self-excited machine, and it is only the main alternator which requires separate excitation.

9100 · Jun 25, 2013

Froneck said:
The SPL rating means special and as far as I know it's use to indicate the motor is less than continuous duty (24/7) Unless it has a very short time rating it should be OK to use as an RPC To test simply pull start the RPC motor are see if it will do as you intend to use it.
Surely there are many type of 3 Phase Generators, some will self excite and others have field windings! If there was an application I'm sure someone made it! However its not a simple task by someone not familiar with electrical power generation to convert a motor into a generator. Just trying to keep it simple! I'm thinking the OP is trying to make due with what he has.
As to belting up a motor to get a third phase how do you plan to synchronize it?

Frank

It was just an errant thought. A squirrel cage motor generates synchronized back EMF while slipping. An over speeded (slipping the other way) motor should also generate back EMF synchronized with the excitation on one winding. A lot of wind generators work that way. I have never heard of this for phase conversion and it may not work, just wondering.

Bill

peterh5322 · Jun 25, 2013

"Now, WHY would you say that? It's clearly not true."

It IS true, for "Monarch Sidney" 10EE armature-regeneration drives, which I state from experience.

If such a drive, or a similar drive, has been modified to inhibit regeneration on the B phase, then such a modified drive will brake regeneratively, but with reduced performance, about 66.67 percent of its former braking capability, in the limit.

jim rozen · Jun 25, 2013

"...The RPC will accept regen power better than most any VFD without a brake resistor..."

I was gonna say, experiment shows that you can plug reverse a motor being driven by a
rotary converter and it works just fine.

Is this truly regerative, is my meter running backwards? Highly doubtful.

Is the energy in the spindle and load motor going somewhere? Absolutely.
Nothing blows up or catches on fire, and the spindle slows to stop.

JST · Jun 25, 2013

The key to regen is RATE...... translating to current

There is an extra impedance in series on the B phase, so you cannot put energy BACK INTO the RPC any faster than you can get it OUT OF the RPC.... In each case there is a voltage change, in one case up, in the other case down.

It's perfectly possible to dump more energy and have the effective B phase voltage go up and trip something in the source, although I'd be a little surprised, since teh A-C phase wires are still directly connected, providing a direct path for considerable current...

Bottom line is that heavy regen is about the same thing as a hard-starting load, and BOTH need an oversized RPC.

Before I'll agree that an RPC "cannot" accept and transfer regen power, someone must show me the "one-way valve"........ I think that will be a rather difficult task.

A generator is not always a wonderful sink either.... the reason the mains are a generally good sink is that there is a lot of hungry load out there. Dump 100MW in at the wrong place suddenly, and see what happens on the mains.

A local generator will likely urp when 75% of its load is coming back as regen.... or maybe 150% for a short while as stored kinetic energy is dumped from a load.

steamingspud · Jun 25, 2013

Hahahaha, guys, sorry but this is kinda funny. I don't mean to cause too much debate, but it's healthy I'm sure.

I certainly won't be using a water wheel, and I don't need a diesel generator, though the phasor torpedo would be pretty cool.

I still want to use three-phase, and my paps is still convinced that rotary phase converters aren't as good as a generator with one-phase as the motive source. I've heard both answers here, that the genset idea will and will not work. I know a few of you think I should just make it simple, but I'm not in it because I'm a blacksmith, but more as a student.

So what I might end up doing is building one of the RPCs on this forum, which are excellent sources by the way, and just... tryin it out.

JST · Jun 25, 2013

Generator can work. RPC can work.

With an RPC, it's an induction generator for one phase wire only. The other two are direct from power line, no losses.

With a generator, ALL the power has to come through the genset, so there are generating losses on all power, not on basically 1.3 of it. Smaller generators have significant impedance, which means voltage drop during a surge.

But an RPC isn't a wonderful generator, so it's losses are not negligible, and it ALSO has drop on a surge, but only on the B phase..

My guess is that if you use an RPC that is decently oversized for the load, you'll do fine. I have no frame of reference for your rolling mill..... when I think of one stand I think of something pretty large, which is probably nothing like what you intend.

But I also think of fairly heavy loading, and maybe a 3:1 oversize on the RPC, not the usual 1.5:1 is appropriate?

jim rozen · Jun 25, 2013

"...my paps is still convinced that rotary phase converters aren't as good as a generator with one-phase as the motive source."

If he's the one doing the work then do it his way. I've never done this and it seems unneccesarilly complicated. I do have
a rotary converter that I made 25 years ago that powers my three phase equipment in my shop so in that sense my
experiment disagrees with your dad's theory. Have your dad show you a picture of why a rotary converter won't work, and
then show him a picture of one that does work, and has been doing so for the past 25 years. Total cost for the converter
was about 50 bucks for wiring devices.

The smaller motor is to get the 5 hp converter motor up to speed before switching the line across two phases.

Your dad might ask to see what kind of voltage waveform the converter produces, is it distorted, is it truly three phase
power? This is a true and accurate measurement of the voltages provided by the converter shown above"

He might ask, what good is it? What kind of machines does it power? Show him the lathes and milling
machines run by the converter - those two lathes and the hardinge miller are powered only by that
converter shown above.

Then ask him to explain what he's going to build for you and how much it will cost.

steamingspud · Jun 26, 2013

Jim, excellent explanation and graphics! And a nice lookin shop too.

I'll be building this, which is exciting. He's grabbing the motor mounted on a gearbox from an old job of his, but the project is my own.

I'm gonna keep sifting through circuit layouts for RPCs, probably capacitor start. I have a single-phase motor kickin around I might use as a starter, but why waste a perfectly good motor?

peterh5322 · Jun 26, 2013

"It's perfectly possible to dump more energy and have the effective B phase voltage go up and trip something in the source"

Which is what happens in a "Monarch Sidney" armature regeneration type of drive.

There are six SCR "power blocks", three for motoring and braking FORWARD, three for motoring and braking REVERSE.

No issues when in the two quadrants where motoring are employed.

However, when in the two quadrants where braking is employed, the A and C phases are OK as the single-phase L1 and L2 can "sink" any applied current due to such braking. But, the B phase is NOT OK and it will see a phase over-voltage and trip the drive out.

It is a complication that the drive's general purpose supply, for the TTL and ECL packages, is also three-phase, derived from a rather small three-phase transformer and a "six phase" rectifier.

Again, a modification to the logic at TTL levels which inhibits SCR "power block" triggering whenever operating in the two braking quadrants would solve this particular failure mode.

However, the simplest solution for those who have spent $100,000 on their 10EE is to simply throw another stack of "Jacksons" at the problem, and buy a Phase Perfect.

jim rozen · Jun 26, 2013

Nobody's mentioned this yet, but if he's interested in variable speed for his
machine, and it's going to be a one-off, then a simple VFD would probably
be best for the application.

Difference between RPC and genset

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