3 Phase Converter Schematic. (Miller system)

i appreciate the info, i will abandon the idea of mismatched caps. i have already cleaned up enough confetti. i was looking for a shortcut as usual but instead i will delve into the internet as i need more info.

i don't know whether or not it is acceptable to share sources here, if not please chastise and delete.
anyway, here is an excellent source for run capacitors and many other items for phase converters.
i have been buying welding cable and other hardware from them for years. i wish i had thought of them at the start of this project.
**broken link removed**

I've got a three-phase motor that I've got to run using the circuit and capacitors as recommended by Tcmtech. It's a 380 / 220 V motor, 3 kW.

So far, I've switched the starting capacitor manually.

Any suggestions where I could get a suitable voltage sensitive relay? Would a time delay relay be easier to obtain, as this is a hydraulic lift that gets to speeds in fractions of a second?

Diver300 said:

... Would a time delay relay be easier to obtain ...

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I would say yes.

Perhaps something like this. 220VAC supply, 220VAC, 10A capacity control relay and adjustable delay OFF. I'm not sure, however, how you select the timing range, i.e., the "0 to ≈10 S" option, given your "fractions of a second" timing requirements..

There are others, of course.

Yes, in a fixed load application time delay relay would work but most of those cost as much as a voltage sensing relay.

tcmtech said:

Yes, in a fixed load application time delay relay would work but most of those cost as much as a voltage sensing relay.

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Have you got any specific suggestions for a voltage sensing relay?

Not really. I typically shot for ones with drop-out voltages a bit below the line voltage. The reality is the vast majority of VSR are easily disassembled and adjusted to raise or lower their drop-out voltage point if the range they have is showing issues with either dropping out too soon or not at all.

The alternate name for them are HVAC Potential relays and they cost around $25 - $30 new or about half that on online auctions now if you know where to look. The older style 3 terminal (spade or screw) are the ones I prefer.

I built this based on your system. I am having a problem with it coming up to speed. It starts but turns slowly. Any ideas?
Thanks

tink111 said:

I built this based on your system. I am having a problem with it coming up to speed. It starts but turns slowly. Any ideas?
Thanks

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Are your K2 and or K3 relays switching the start capacitor out too soon? The start capacitor needs to stay in the circuit until the motor is nearly up to speed and to do that the start relay/contactor has to be a special type that does not pull in until a specific voltage is reached.

BTW, your way you have it wired as your circuit shows is very questionable in a number of ways.

Tomorrow I am going to wire the motor just like your diagram without the potential relay. If the motor runs that will verify that there is no problem with the motor or capacitance issues. I also have a 7.5 HP motor that is a Wye configuration.

I bypassed the potential relay by tying the coil of k2 to neutral. So the start caps are in the circuit until I press the stop button. The motor speed is about 40 rpm.
What do you suggest?

What are the voltages and phase relationships of L1 and L2?

Tcmtech's circuit has only two input wires. Using the notation on your diagram, he has the start capacitor between L1 and PH2 when it is connected and the potential relay measures the voltage between the same two points. On yours circuit, the start capacitor is between L1 and PH2 when it is connected, but the potential relay measures the voltage between L1 and N.

Also it seems odd to have the run capacitor, C1, disconnected for startup. It would be better to have it directly between PH2 and PH1

tink111 said:

What do you suggest?

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Don't deviate from the design by adding things not shown or using the wrong type of parts in the wrong places.

I'm assuming, given the capacitor values, its a standard 10 HP 230 VAC 50 - 60 HZ induction motor and that the capacitors are the correct type (motor start and run or power factor correction, not DC electrolytics) and working as well?

Hi just joined the forum so hello.Found this thread really interesting and I am going to try and build a rotary converter.I have this motor which I am going to use for my trial run but when I opened it up I saw this wiring.The motor is from a tread mill and was operated with a VFD.What is the wireing set up can I assume it is a delta set up and how do I prove this.What are the other small wires for?

I guess that the small wires are thermistor wires.

It is odd to see just three wires on a motor rated for both star and delta. I would have expected 6 wires to the terminal block, to allow it to be wired either star or delta.

Have you got the VFD? What voltage was the VFD supplied at? If it was supplied at 230 V single phase, it is likely that the motor is wired for 230 V. If that is the case, it doesn't matter if that is star or delta.

If you run a motor designed for 400 V at 230 V, it will work but its maximum torque will be much less.

Hi,thanks for reply.After all the reading the 3 wires sort of stumped me.I do have the VFD but it is not working. A pic of the plate.Diver300 do you know of anywhere in the uk who would fix VFD?They seem to be fairly cheap now so may not be worth it.I have read that the transformer from a welder which has 215/415 can be used as a step up transformer to give me 415v has anyone done this or could point me to a detailed description?

The VFD clearly outputs the same voltage as its input voltage, and it's maximum input voltage is 240 V, so it should be OK on UK mains voltage.

If the motor was running from that VFD, it will be designed to run on 230 V between phases. It doesn't matter whether it is star or delta, as it would be just about impossible to change the configuration, and therefore the voltage, unless both ends of each winding were available.

Finding someone to repair the VFD is likely to be difficult. Posting a few more details on this forum would be the best bet.

A transformer can be used to increase the voltage from 230 to 400 V, but you wouldn't be able to use that motor at 400 V. If you used that motor at 230 V with a miller system to generate 3-phase at 230 V, you could get 400 V from that with two 230 - 230 V transformers. To do that, you would connect the primaries of the two transformers as delta-configured loads on the two phases that aren't the incoming mains, and connect the outputs star-configured. I put that in a drawing. The motor and capacitors are from the original posting in this thread.



This thread https://www.electro-tech-online.com/threads/three-phase-loads-and-sources.152013/#post-1306245 says why you don't actually want three 440 V single phase sources which would have to be delta-connected, and you would be better with three 230 V sources.

What are you trying to run on 3-phase? Can whatever it is be run at 230 V or do you need 400V?

I have a boxford 280 lathe and a milling machine which are 415 three phase the milling machine also has a two speed motor and both coolant pumps are 415.I know I will have to get a bigger idling motor to do the job but was going to try the motor I have first just to see if I could get it going and get a balanced 240 three phases

Have you looked in the terminal boxes of the various motors to see if they can be changed from star to delta?

The boxford can easily be changed to delta the mill can not.Thank you for posting the drawing of the transformer I have to admit I don't fully understand it due to lack of knowledge of circuit diagrams.Is there any way you could point me to a simpler explanation?Did you ever use a welder transformer?Wondering if maybe I would be better buying a single transformer that would do the job?The miller drawing seem straight forward have you built one with this design in uk and found any problems?

On the diagram, the incoming mains is on the left. The star-connected windings are the motor, and the capacitors are as suggested bu tcmtech. The two devices on the right are two separate but identical 230 - 230 V transformers. They will need to handle the entire load current for each phase. The dots by the transformers show which windings are in-phase. The data sheet of the transformer should show the dots, as this one https://www.farnell.com/datasheets/...65.447265222.1508763134-1700957338.1508488853 does.

The idler motor will only generate one phase, it can't increase the voltage. If you need to transform the voltage you can do it before the idler motor or after, but if you have a 230 V idler motor that can't be changed to being 400 V, then you have to transform afterwards. If you are transforming afterwards, and using single phase transformers, you need separate transformers for each phase. The scheme that I suggested has the incoming phase as one of the three output phases, so there is no need for a transformer for that phase.

This is going to end up costing a lot. The transformers will be £500 each. As tcmtech said earlier in this thread, there gets to be a size and complexity where it's not worth doing, depending on your situation.