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Generators & Electric Motors General Discussion

Three phase genset build.. Questions


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  #1  
Old 02-17-2019, 11:29:30 AM
Kurtis Kurtis is offline
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Default Three phase genset build.. Questions

Hi.. I am building a three phase genset so I can bench and use three phase equipment in a neighborhood that only has single phase. I am an old mechanic so I have a fairly good understanding of DC but only the basics for AC and three phase, so I have a few questions.

For a start, when I first put this together and started it, it was putting out 240v on all three lines. When I went back to try it again it wasn't generating current at all except some residule current. However, when I knock the belt off the engine the rpms go way up like the engine was working quite hard to turn the motor. I am spinning the motor the opposite direction it was set for but I didn't think that would matter until it was connected to another machine. The only thing that I changed between trials is that I grounded the capacitor bodies.

Also, I am beginning to think that I don't have a big enough motor to power the piece of equipment that I am trying to bench right now which is an Holz Her panel saw. The pdf for it was to big to post on Smokstak so it can be viewed at http://altpubad.com/Temp/EL2511029.pdf

Any help would be greatly appreciated.. Thanks
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  #2  
Old 02-17-2019, 01:51:55 PM
Kurtis Kurtis is offline
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Default Re: Three phase genset build.. Questions

Here is an image of the electrical panel: http://altpubad.com/Temp/20190215_123648.jpg Looks to me like it runs on single phase, two phase and three phase.
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Old 02-17-2019, 08:11:34 PM
Wayne 440 Wayne 440 is offline
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Default Re: Three phase genset build.. Questions

I claim no expertise, but have some comments

(1) I agree that your "generator" ( a 3.73 kW motor) is probably too small to power the 3kW saw motor.

(2) The drive ratio between your engine and the "generator" is also a problem, unless that engine makes a lot more power at low speeds than I think it does.
The desired speed for your "generator" is (I think) around 1870 RPM, my guess is your engine is turning only about 1000 RPM.

(3) You wrote "...The only thing that I changed between trials is that I grounded the capacitor bodies..." I suggest that you carefully reverse that change, since the voltage collapse happened immediately after.

Have you considered making a rotary phase converter instead?

Last edited by Wayne 440; 02-17-2019 at 08:41:46 PM. Reason: typo
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Old 02-17-2019, 09:26:59 PM
DMeed DMeed is offline
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Default Re: Three phase genset build.. Questions

I'm trying to figure out how a 575V motor is creating 240V.

Is there a quick primer on using induction motors as generators?

Also how is a motor/generator that is rated 5.25 amps is going to supply power to a motor that takes 10 or 12 amps at 240V

I would second the rotary phase converter idea.
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Old 02-17-2019, 09:33:57 PM
dalmatiangirl61 dalmatiangirl61 is offline
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Default Re: Three phase genset build.. Questions

He said 240v per leg, which might be close to 575 between legs? I agree, find a different motor and build a rotary phase converter.
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Old 02-17-2019, 09:42:43 PM
Wayne 440 Wayne 440 is offline
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Default Re: Three phase genset build.. Questions

Quote:
Originally Posted by DMeed View Post
...Is there a quick primer on using induction motors as generators?...
Here is a source I have referred to before.

http://www.redrok.com/cimtext.pdf
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Old 02-17-2019, 09:57:36 PM
Radiomike Radiomike is offline
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Default Re: Three phase genset build.. Questions

This is an induction generator, it depends on having some residual magnetism to start generating with the capacitors proving the leading VARs for excitation. It has to be run at super-synchronous speeds usually around 50 to 100 rpm above nominal synchronous. There is a good wiki article on estimating the required capacitance. https://en.wikipedia.org/wiki/Induct...tor#Excitation it also details the use of DC to start the self excitation process. The value of the capacitors determine the voltage. I note your blue motor has a rated voltage of

If you suspect the capacitors then check then with a "Megger" for ground faults and a series resistance and use three voltmeters to calculate the value of Xc. Note the capacitors have an internal 1Mohm internal resistor. , They should however test clear of ground.

Have you thought of a rotating phase convertor. A bank of capacitors and a pilot motor would be a lot simpler, unless you need an independent source. Another option is a variable speed (VSD) unit these have a 240volt single phase input and a 240V three phase output. https://www.ebay.ca/i/182935786653?c...d=2&dispItem=1

https://www.amazon.com/Variable-Freq.../dp/B07CJXQWVT

I see your blue motor is rated at 575V and 5 amps but the red motor is 6.1 Amps at the highest voltage

Mike
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Old 02-17-2019, 10:00:22 PM
pegasuspinto pegasuspinto is offline
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Default Re: Three phase genset build.. Questions

Youtube is full of people who have sucessfully used VFD's to convert single phase to 3 phase. Many VFD's are even built for the purpose. And amazingly cheap.

I am a little confused what all the circuitry in your hole saw is trying to accomplish, but the VFD may well replace it, as the VFD can do soft start, speed control, and reversing functions internally.

If I recall correctly, trying to use an induction motor as a generator is a bit sketchy, as the field is unstable and may well collapse on a surge load. I think the compensation is to use a considerably larger motor then the load requires.
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Old 02-18-2019, 12:08:35 PM
Radiomike Radiomike is offline
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Default Re: Three phase genset build.. Questions

I assume your Panel saw is like this https://www.youtube.com/watch?v=4MAsVT693E4.

The schematic for the control looks complex but is mostly a series of interlocks to prevent mal and mis-operation and to maintain safety for the operator. There must be an English version available somewhere. Any induction generator will struggle to provide not only the motive power but the correct voltage for the auxilliary circuits, and what appears to be a single phase fan. Using an induction generator will give large voltage swings when the main motor is switched on and off. The control circuit and its transformer will not be happy being subjected to large over-voltages.

The duct tape covered wiring on the main motor is a source of some concern. Has someone been modifying the wiring and bypassing the safety interlocks?

One suggestion is a VFD for the main drive and a single phase supply to the control circuit and exhaust fan. Or a rotary phase convertor which can maintain a closer voltage.

From the schematic it looks possible to separate the single phase parts from the three phase parts.

Mike
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Old 02-18-2019, 09:42:37 PM
Kurtis Kurtis is offline
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Default Re: Three phase genset build.. Questions

Hi Wayne, thank you. I was thinking that I would have to adjust the pulleys somewhat. Trying to keep them as large as possible because of the distance between them. I put the platform together so I could easily switch motors up for testing. I put different caps on it today with no change, just 3-4 volts on each line. Could I, have blown the flashing of it by turning it the wrong direction?
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