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3 phase power question

3 phase power question

3 phase power question

First the history. I have an older 7.5 hp 3 phase 220V motor that I need to power in my shop at home. I have looked at the VFD, but at this hp level they are require a 3 phase input, not single phase. This has me lead to build a RFC. The idler motor that I had planned on using was toasted, so for testing this weekend I used a 3 hp motor as the idler motor with the 7.5 hp motor turning a variable speed transmission and two separate gear boxes without any load applied.

These are the measurements that I recorded during testing which make no sense to me. Measurements were made with a Fluke 30 clamp on amperage meter and Fluke 73III voltage meter.

Single phase from breaker panel
L1 current is 8.1 amps
L2 current is 8.3 amps
L1-L2 voltage is 245

3 phase to the 7.5 hp load
L1 current is 10.5 amps
L2 current is 10.4 amps
L3 current is 10.9 amps
L1-L2 voltage is 245
L1-L3 voltage is 245
L2-L3 voltage is 244
L1-L3 capacitor bank is 120 mfd
L2-L3 capacitor bank is 70 mfd
L1 current at 3 hp idler motor is 2.5 amp
L2 current at 3 hp idler motor is 2.6 amp
L3 current at 3 hp idler motor is 2.8 amp

Checking the in coming power meter indicates that running the central a/c at 19.5 amps is pulling nearly four times the amount of power than the RPC and the 7.5 hp motor.
It appears to me that since my idler motor is so small, that I have basically made half a static convertor and half a rotary convertor. But this does not explain the measured in coming current being less than the measured load current with the voltage equal.

RE: 3 phase power question


Please check Thread237-102768.

You cannot add currents arithmetically; your KWHR meter is telling you the truth, your A/C is consuming more power than the motors. There is an important reactive component in your input and output currents in your setup that do not contribute to the real power consumption, with an ammeter and a voltmeter you cannot resolve how much of your readings is real or reactive currents. This can explain your apparently conflicting readings.
Why you are using an idler motor?

Check this site too: http://www.metalwebnews.com/howto/ph-conv/ph-conv....


RE: 3 phase power question

Just so you know, you CAN use a VFD, even though you only have 1 phase input. Most 230V VFDs do not have Phase Loss detection (or it can be easily defeated), so it does not "know" that the AC power is coming from 2 wires or 3, as long as the voltage level is suffficient. The only trick is to double the size of the VFD for the motor, i.e. for your 7.5HP motor, buy a 15HP VFD. That is because a) the "front-end" compponents which convert AC to DC, must handle all of the 3 phase current through only 2/3 of the devices, so those devices see 1.73 x the motor current: and b) you need additional capacitance in the DC link to smooth out the extra ripple caused by converting from a 1 phase source. So by just doubling the size you accomplish both requirements.

"Venditori de oleum-vipera non vigere excordis populi"

RE: 3 phase power question

I'd say it looks good. The currents are close enough. You still can't get a full 100% of rated HP from it though.

RE: 3 phase power question

These motors are running practically with no load. The input single phase incoming ( 1.984 kVA) is providing basically the losses of the motors (real power). The capacitors are providing the reactive power consumed by the motors.

Remember that energy is not created nor destroyed, only transformed.

RE: 3 phase power question

Mirus International also makes a version of their LINEATOR (R) that will operate a 3 phase drive off of single phase power and give you the harmonic current distortion of an 18 pulse rectifier.

If a 3 phase motor is operated as a permanent split capacitor motor it will only produce 2/3 of nameplate horsepower. Th idler motor just simply adds another voltage source and enough inductance to use with a capacitor to get full output from the larger motor.

A so called static converter for a 3 phase motor such as an Add A Phase ( I read their patent) uses a current or watt relay to switch in a helper inductance and capacitor when the motor power level goes above 2/3 of nameplate rating.

In actuality almost ALL phase converters use both rotary and static conversion principles. A rotary phase converter is just simply a big 3 phase motor that is acting as a single phase to single phase motor generator in 1 box using its ability to provide a 3 phase voltage source when energized from single phase power and also rotating at full speed. The current in each lead of this helper motor should be limited to 2/3 of nameplate rating because the rotor has a very powerful reverse sequence electromagnetic field running against it.

A static converter, also known as a Steinmetz Balancer Set, is that you connect a single phase inductance in your  example L2-L3 and an equal KVAR capacitance L1-L3. Depending on the phase rotation this will balance off a single phase load or source connected L1-L2.

Your L2-L3 capacitor will counteract any static conversion from your L1-L3 capacitor. Your 70 microfarad capacitor should be connected L1-L2 if what you want to do with it is power factor correction.

You also need a bigger helper motor OR a switched inductance and capacitance to help both motors when needed. Otherwise, the 7.5 HP motor will not develop full torque and the 3 HP helper motor will tend to overheat.

Mike Cole, mc5w@earthlink.net

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