Does the DRO display even tenths increments 0.0001 0.0002 or 5 digits like 0.0001X
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Inquiring about rotary phase converters and VFDs.
- Thread starter Tom Fitzpatrick
- Start date
Former Member
Guest
RPC or VFD complex question, 25 years ago I would have said RPC hands down no questions asked because VFD's where expensive and limited. Now VFD's are very cost effective and several advantages that RPC's can not provide.
- Reversing
- Speed adjustable
- Low cost
- And most importantly (and why industry is switching to it) it reduces start up currents. Loosely speaking almost the same as running currents. This means lower power requirements, more machines on the same service. Remember as far as the motor is concerned it is doing it's rated speed whether its zero rpm or twice its rated speed. This is why the amperage remains constant on the line side more or less.
My suggestion is VFD hands down.
- Reversing
- Speed adjustable
- Low cost
- And most importantly (and why industry is switching to it) it reduces start up currents. Loosely speaking almost the same as running currents. This means lower power requirements, more machines on the same service. Remember as far as the motor is concerned it is doing it's rated speed whether its zero rpm or twice its rated speed. This is why the amperage remains constant on the line side more or less.
My suggestion is VFD hands down.
Chipper5783
Super User
Tom, obviously there are a few options as far as running 3 phase motors from a single phase supply - they each have benefits and detriments. Perhaps you could provide more details as to what machines you have (hope to have soon), and what you want to do with your shop?
Tom Kitta
Ultra Member
Chinese DROs display tenths but its interpolated from native metric system. The standard scales steps are 5 micron so I would not exactly trust this reading to a tenth. Better scales are 1 micron , so maybe you can somewhat rely on a tenth (there are few more things related to how accurate measurement is, not just one thing).
Former Member
Guest
On selecting a DRO, or anything else (cnc controls, etc) the biggest thing you need to understand is what is your accuracy on your machine.
- Do I have end play and backlash of 0.001 (or more lots of machines do) or is it 0.0001. We can reduce its effects by direction of approach.
- What is the repeatability, again same issues and solution.
- How stiff is your machine (dirty minds out of the gutter please, this is PG rated), without stiffness (and dampening) vibrations effect accuracy.
Understanding this thing determines the best choice for Measurement Tolerance.
Selecting servo's for my machine I had the option of an 800 count per rev or 6400 count per rev ( not much extra cost in the scope of things), it translates as 0.1" (1 rev of handle movement) / 800 = 0.000125" per count or 0.1/6400 = 0.00001563 per count.
Its nice to display to this precision but my machine just after tightening and using every trick in the book in the first 10 minutes at idea temps will achieve 0.0005" get beyond that time frame and I can expect 0.0015 at best. No I didn't upgrade, wasted dollars for no gain.
BTW we didn't even include variations in our system of guidance.
Changes the outlook doesn't. Don't get fooled by smoke and mirrors precision, most are. Worry about accuracy and repeatability, with that you can compensate to get the tolerances you want, just takes longer but gives better results.
So do you checks first, understand what you should expect is possible and choose from there.
- Do I have end play and backlash of 0.001 (or more lots of machines do) or is it 0.0001. We can reduce its effects by direction of approach.
- What is the repeatability, again same issues and solution.
- How stiff is your machine (dirty minds out of the gutter please, this is PG rated), without stiffness (and dampening) vibrations effect accuracy.
Understanding this thing determines the best choice for Measurement Tolerance.
Selecting servo's for my machine I had the option of an 800 count per rev or 6400 count per rev ( not much extra cost in the scope of things), it translates as 0.1" (1 rev of handle movement) / 800 = 0.000125" per count or 0.1/6400 = 0.00001563 per count.
Its nice to display to this precision but my machine just after tightening and using every trick in the book in the first 10 minutes at idea temps will achieve 0.0005" get beyond that time frame and I can expect 0.0015 at best. No I didn't upgrade, wasted dollars for no gain.
BTW we didn't even include variations in our system of guidance.
Changes the outlook doesn't. Don't get fooled by smoke and mirrors precision, most are. Worry about accuracy and repeatability, with that you can compensate to get the tolerances you want, just takes longer but gives better results.
So do you checks first, understand what you should expect is possible and choose from there.
Agree with your post except for the first and last bullets. Reversing is easily achieve with a reversing switch right on the machine and if placed correctly is could be more convenient. As far as industry going to them, the soft start is a great feature for reducing inrush and keeping your demand down. There is a money saving component in this for industry as they are usually billed by VA demand, this is not a factor in a home shop and there is no savings. What a VFD will do is reduce light flicker if you are starting large machines. The money saving component would be in when the RPC/VFD is at idle. This can be reduced by planning your work and keeping the RPC idle to a minimum.
Tom Kitta
Ultra Member
The light flicker is also a function of load - if there is no load on the motor when starting you really need to go quite big to get noticeable light flicking. And depending on connection it will also flicker in few other houses.
I do not get anything with 15hp rpc start and also nothing when starting from it a 10hp lathe - even at full speed. But I recon there could be some issues if I went bigger, say starting a 25hp RPC with a fly wheel. or when starting 15hp lathe at maybe less then full speed but with very heavy load of say 1000 lbs.
Agree with saving $ at idle. Through lights in the shop use at least 2x as much power as RPC at idle & large VFD also is not exactly not using any power at idle.
Former Member
Guest
The in rush current and light flicker is caused because the motor is catching upto the rotational phase of 60hz this miss match between phase rotation motor rotation creates resistance that increase current. For example 17 amp motor running is closer to 25-30 amps at start up and can last 1-5 seconds or more depending on loading. This peak draw causes lots of problems and brown outs momentarily. Additionally because of how Hydro measures power usage it also great increases your costs falsely. Additionally, the light flick is caused by by a sudden draw of power and it balancing on the system, reducing voltage.
Remember your Ohm's Laws. Change one of the 3 in the equation one or both of the others must change to maintain status quo.
VFD's prevent this because the motor is never catching up if set up correctly.
Remember your Ohm's Laws. Change one of the 3 in the equation one or both of the others must change to maintain status quo.
VFD's prevent this because the motor is never catching up if set up correctly.
Tom Kitta
Ultra Member
Light will only flicker if the load on the circuit is causing significant voltage drop - this happens when you draw too many amps on a too thin wire. This is why when you turn hair dryer in your bathroom on same circuit as lights in said bathroom and its only 15amp circuit with wires sized accordingly & hair dryer is also 15amps you are likely to get a little brown out.
But most RPCs are set on separate circuit to say lights in the garage, and down line wire, where they meet is for 100amps (or more). Even with large inrush at least in my case there is no light flicker in the whole garage.
Down at the pole they should have also enough spare capacity to prevent few houses to flicker - I guess until one or more of my neighbors buys a large welder and starts taking out serious amps.
My mill light - which is powered via 240-24v transformer from RPC does flicker if I turn the mill on above around 1200 rpm.
When my lathe starts with its 10hp engine driving larger chuck at speeds over say 1000 rpm I would not be shocked to see 15-20V drop. Too much of a drop can be bad for the motor.
Before VFDs there were delta - star switches & more friendly machines simply had clutches.
In a factory I doubt they would pay more for electricity - as they had many motors (relatively small to overall supply) and I don't think they all started at the same time. Maybe you mean just a waste through reactive power - multiple too big motors for application running at say only 20% of load say a set of conveyor belts - that would be bad indeed. this is why old welders went out the door - they had, even with bank of capacitors, lots of reactive power & thus did cost factory arm and leg in electrical costs for no work done. None of this applies to home, we do not pay (I hope) for power we don't use.
Former Member
Guest
You would be surprised, a friend had a business that would monitor the exact power used and compare it to what Hydro charged, he got a percentage of the difference saved for the companies.
Tom Kitta
Ultra Member
Oh I don't doubt that - they have specialists whose job is to pick correct motor size so its 80% loaded not 20% loaded. Imagine a 100hp pump motor loaded 20% vs. 25hp pump motor loaded 80%. Say oil company, many motors. Commercial. Difference in costs would be I imagine more then 2x (!) If it was some remote area and company had to pay for power to get there costs difference would be in millions.
Its not just motors but everything else that has reactive power - there are companies that just do this - lots - for example google shows https://reactivepowerspecialist.com/ and I bet there are 1000+ other companies just doing this.
I have the same issue with light flicker when if start my mill on the higher RPMs. In rush current on startup as a rule of thumb is double the name plate and that is why VFD/ soft starts have become so popular.
You are right at present residential is not billed for reactive power and most commercial is not as well. Most residential and commercial sites are resistive loads. In industrial and commercial kVA is used where the load is predominately reactive power.
Sorry for subject tangent but do static phase converters have any design/circuitry to ramp in power kind of like micro-soft start, or is it basically 'instant on' more or less like a switch? Also, if I understand one of the reservations of SPC is some power loss vs. motor rating. I can see on certain machines where the motor replacement might be more specialized mount or shaft or whatever. But ay I have a plain vanilla 2HP 2P motor & buy a 3HP 2P motor with SPC, wouldn't it work out in the wash?
You can probably tell electrical stuff is not my forte LOL
You can probably tell electrical stuff is not my forte LOL
An SPC in its basic form is just capacitors converting single phase to three phase and it is disconneted once the motor is running. This cofiguration is used in an RPC to start the motor and that is what it as does in a SPC. With an SPC you have a minimum loss of 1/3 of the hp rating of the motor. The SPCs that I have seen did not have soft start or the ability to to regulate motor speed There might be some out there that can do this but they probably would cost more then a VFD.