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3 Phase Converter Question?

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gary350

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**broken link removed** I have a milling machine with a 1 hp 3 phase motor that I want to run on 240 VAC single phase. I have L1 and L3 connected to 240 volts in my garage. If I temporarily connect a 90 MFD capacitor from L2 to one of the 240 VAC wires with a N.O. push button switch, then turn on the power switch and the motor starts. Motor runs good on 2 windings once I get it started. If the 90 MFD capacitor stays in the circuit the motor will buzz very loud. A smaller 30 MFD capacitor causes less buzzing. The motor runs at 2/3 it power rating running on 2 windings. Everything I read online says, keep a 30 MFD capacitor between the unused winding and one of the 240 VAC power lines but that does not seem to improve the performance of the motor it just buzzes and uses more electricity.

My question is about the capacitor in the first circuit. Do I really need a capacitor connected to the extra winding motor run good without it???
 
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If the motor is running OK, and you are not loading it to the maximum load, it should be fine running without the capacitor.

https://en.wikipedia.org/wiki/Phase_converter#Conversion_systems

There are lots of single-phase motors running as capacitor-start motors, where the capacitor is disconnected when the motor gets to speed.

The phase angle of the current is not perfect on a 3 phase motor with 1 phase disconnected, but I doubt that it would be perfect with any capacitor. There is a phase difference of 120° between two windings on a 3 phase motor. However, as the current is flowing into L1 at the same time that it is flowing out of L3, that makes one of the winding reversed, so that changes the phase angle by 180°, so the phase difference is only 60°. That means that one winding is running 30° leading from the optimum, and the other lagging 30°. That is not enough to cause a problem unless the motor is heavily loaded.

It will use more electricity than a if it were run from a 3-phase supply, but unless you are using it a lot, it would not be worth buying an inverter.
 
I have been using this one for decades.
Phasecon.jpg
 
**broken link removed** I guess I did not explain this well enough. This is what I had for many years. Then I notice the Phase-a-matic 3 phase converter uses only a Start capacitor, it has NO run capacitor. 90 MFD to start 1 HP and ZERO to run. So I changed my 60 MFD start cap to 90 MFD and removed the 30 MFD run capacitor. Now all I have is 90 MFD to start and ZERO to run. I have been using it for 2 years and I can not tell any power difference. Both give me all the power I need. It seems to me the machine is still running at 2/3 power either way.
 
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Yes, you lose 1/3 of your power with this. That is what I was told when I bought the first one from an electrician.
So now your L3 is open circuit when running?
 
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the motor
Yes, you lose 1/3 of your power with this. That is what I was told when I bought the first one from an electrician.
So now your L3 is open circuit when running?

What I want to know is, in theory is the run capacitor suppose to give the motor more power? I assume no one knows because no one has answered. Technically if you put power to 2 windings a 3 hp motor will produce 2 hp. Several places I read, a motor running on a static converter over heats pulling higher amps. LOL. Duh over loading the motor will do that.

A 3 phase motor running on 3 phase pulls on average about 1 amp per hp on each winding at 480 vac and 2 amp per hp on each winding at 240 vac. If your running only 2 windings on a static converter and your pulling more than 2 amps your over loading the motor. A 3 hp motor is now 2 hp your can not over load it and get 3 hp. The motor wire is sized for 2 amps at 240 vac if your pulling more than that your over loading the wire. Over heating the motor has nothing to do with it running on 1 less winding. Your 3 hp motor is now a 2 hp motor so don't expect it to produce more than 2 hp.

I worked in industry for 45 years we always sized the 3 phase motor starter heaters for the correct amp load. You can over load the motor for a few minutes then it trips the over load and shuts down the motor. If you do not size your over load correctly the motor will over heat and burn up running on 3 phase. If your motor is running on 1 less winding all the same RULES apply, don't over load the motor.

My amp clamp shows current flow in L3 with a static 3 phase convert and a 30 MFG run capacitor. Current flow is low the amp clamp will almost not read so I have to guess it is about 1/10 amp with the motor running at idle speed and about 1/4 amp with an undetermined load. If I change the capacitor to 20, 15, 10 and 5 MFD the amps on L3 changes too. Amps on L3 with a capacitor is NEVER = to the amps on L1 and L2 according to the amp clamp.

Is the amp clamp giving me a true amp reading or is this some type of fake reading. If it is true amps then is it adding HP to the motor considering it is not true 3 phase?
 
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Gary,
In your irst post you show 3 diagrams. Take the one with the switch S1.
This arrangement without the switch and with the capacitor permanently wired, will give you perfectly satisfactory operation. You have to be careful to select the value of the capacitor to balance the 3 separate phase voltages to be the same. For the star connected motor, your phase voltages will be 138 volt. Thus the design voltage of the motor would be 138 volt for each phase winding.
The size of the capacitor would need to be large enough to carry the required current. A typical figure for capacitance is around 70 microfarad per horse power.
I have a siemens 6 HP 440 volt DC variable speed motor with an external cooling fan. The fan motor is 3 phase of about 30 watt. The series capacitor I use is about 2.9 microfarad and gives almost identical phase voltages. The capacitor I have is a paper block capacitor and handles the fan current easily. I was lucky to have a few of these 3 uF caps and selected one with the right capacitance to balance the phase voltages.
Functionally, the capacitor gives a leading current into the particular phase. The power factor of the motor windings gives roughly a 60 lagging phase and the leading phase is set to give a 60 degree leading phase. The sum of these phase currents gives a line current of zero phase. The perfection of the system depends on how close to 120 degree the phase angle can be set between the 2 input phases. For my small fan motor, the lagging phase is almost exactly 60 degree.
Hope this helps.
 
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