3 Phase Converter Schematic. (Miller system)

[tcmtech]

Sounds like the long way around to doing something simple.

Two speed motors work just fine on VFD's provide you never switch motor speeds while the VFD is active and the motor is turning.

After that if it was me I would probably just pull the 415-volt two-speed motor off and put a more common 7.5 HP single speed with a VFD unit on it and skip the whole step up and multi motor concept altogether.

Now as for getting a normally three phase fed 7.5 HP VFD unit to run on single phase that's not all that hard. With most all you need to do is put in a larger rectifier set and double or triple the main power filtering capacitor bank size.

I've done a few that way and never had any problems with them afterward.

 

[Overlord]

Hi thank you for the information two questions dose rpm matter some use 1750 but I was told to use a minimum of 3490. And will a 10hp power a weilder or is a 20hp better. I have a photo of specs

 

[tcmtech]

RPM is not all that important. the higher speed units have a bit better surge capacity due to the extra stored energy in the rotor but for a welder it not critical by any means.

Yes 10 HP should run that welder just fine. However you may have to play around with the values of the two main run capacitors that ballance out the phase to phase voltages to be within a reasonable range of each other at load.

 

[RODALCO]

Just found this thread re the RPC's and interesting discussions going on here.
For start Capacitors I have successfully used Electrolytic Capacitors salvaged from old CRT TV's.
If they are wired back to back ( pos-neg-neg-pos ) they work fine on AC 50 Hz.
These Caps are usually 400 or 450 Volts rated and I put two in series.
Values are 220 or 330 uF which yields in half the value when back to back.
I am the dude from NZ who got a system running, pretty rough but it works.
For stable voltages Capacitors have to be fitted across the three phase output.
A bit of trial and error is required here to keep the voltages stable.

 

[tcmtech]

I've done the same before as well. My 15 HP air compressor has a pair of 2600 uF 450-volt electrolytic's set up back to back for the start circuit. Been that way for 5 - 6 years now as best as I can recall.

 

[Overlord]

So I have been using a 7 HP 3000 RPM 3 phase motor for my phase converter and just starting it with a pulley and another motor no start capacitors. I spin it to 1700 RPM and turn on power, works great. But I just bought a GE 15 HP 3460 RPM 3 phase motor to upgrade and I can't get it to start, I am even spinning it to 3000 rpm with no luck. Before I send it in to get looked at is it able to start this way or dose HP not mater as long as it's up to RPM

 

[tcmtech]

HP is not relevant but power source capacity is.

I have built units up to 25 or 30 HP now that have been in service for a number of years now without problems but they are close to 200 amp service boxes that have 4/0 feedlines coming in.

Most often the problem with larger motors not being able to get up to speed is a result of insufficient supply power. There is just too much voltage drop on the power supply side to be able to get the motor up to proper speed and lock into its correct rotational speed.

A 15 HP 230 VAC motor like the one on my main shop air compressor on hard startup can easily draw 150+ amps and needs a power source and related wiring capacity to be able to keep the voltage up around 200 VAC or better at that current draw to start reliably.

Any idea what your voltage is pulling down to at the motor when you are trying to start it and do you have any phase balancing capacitors on it as well?
If you don't have sufficient power source capacity and properly sized phase balancing capacitors on the motor it may refuse to run on single phase at all despite not having anything wrong with it.

 

[Overlord]

 

[Overlord]

I have no capacitors trying to make sure it worked before buying a bunch and 200 amps at pole ran through 60 amp breakers

 

[tcmtech]

Without knowing what the actual voltage is at the motor and what amps it's drawing, preferably with the properly sized running capacitors on too, I can't give you much for suggestions.

 

[ClydeCrashKop]

Without run capacitors, what do you have connected to the third leg?

If you are trying to make a 3 phase motor run on 1/3 of its windings, you may have to spin it up to its 3525 RPM.

If you just want to know if it works, take it somewhere that has real 3 phase & plug it in. Even a service station may have 3 phase for their air compressor.

 

[Overlord]

Thanks for all your help guys just found out motor is no good so back to the hunt.

 

[tcmtech]

Been there done that more than once.

 

[customMotion]

working from the schematic at the beginning of this thread i have built the 20 hp converter per the diagram above. i have started it three times now, thankfully with the cover on the box. i close S1, close the 100 amp breaker, the motor goes to up speed in about half a second and then 3 to 5 seconds later C1 explodes. i am as sure as i can be that there is not a short anywhere. i am sure i am doing something stupid (beyond having replaced C1 and trying again before identifying a problem). i have a 100 amp single phase service with good wiring, no problem getting the motor up to speed. the motor is a baldor M2515T. any other ideas?

 

[tcmtech]

C1 explodes because its motor start not motor run type capacitor. The back bakelite case and remains of the guts are a dead giveaway.

Also being C3 looks to be a single polarized electrolytic (DC use only) which if it is it will blow up on the first heavy startup you do with it.

To work correctly you would need two more of the 100 uF metal can type motor run type capacitors in place of each of the 200 uF motor start units you blew up assuming you need 300 uF of balancing capacitors to compensate for both the motor and whatever loads you are powering with it. If not any where form 150 - 200 uF should get your phase to phase voltages to be within reasonable working range.

As for the single electrolytic you can use two of them back to back to make a 1200 uF AC motor start capacitor and two sets of that to get your 2400 uF total you need assuming you have a high mechanical load on the motor and it not working just as a phase converter unit. If it just doing phase converter work you might get by with just two of the 2400 uf capacitors ruining back to back.

My 15 Hp shop compressor has been set up with two 2400 - 2800 uF electrolytics placed back to back for the start circuit for years now and has no trouble starting even in -10F or a bit low winter temperatures.

 

[customMotion]

thank you sir, i think i know where i can get more of the metal can type.

 

[customMotion]

i am familiar with running two electrolytics back to back in ac aplications but i had no idea there were both run and start caps. can you point me toward a source of info that might advance my knowledge?

 

[tcmtech]

Run capacitors are typically found on things like HID lighting ballasts and HVAC compressor and fan motors and rarely come in anything but a metal can design.
The majority under 100uF but can be anywhere between 200 - 1000+ VAC rated. Most are rated around 200 - 370 VAC nowadays that have to work on common 120 - 277 VAC systems.

 

[customMotion]

while we are still speaking, can i ask your opinion on the attached? i have these three large electrolytics on the shelf. do you think this might last for a start capacitor? And further to the difference between run and start copacitors, since a run cap with the same mF and voltage rating is ~2x the size of an equal start cap, can i assume the difference size difference is required for heat dissipation?

 

[tcmtech]

The three capacitor setup will work for a while but at some point I would expect the mismatch to have caused enough reverse conduction in the pair of 1000 uF capacitors to cause problems at some point. No idea when.

As for the run Vs start capacitor designs the simple answer is that run capacitors have much more surface area on their respective plates and a thicker/different separator material and dielectric (light oil Vs paiste) so as to reduce their ESR (Equivalent Series Resistance) than a start capacitor has. That's why they don't overheat.

If yo want to learn more there is a lot of online technical information on the various aspects of capacitor design and application. You just need to do some digging to find the information most relevant to what you are working with otherwise it could get a bit overwhelming to do a cold start on the subject.