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Three-phase alternating current generated by one phase alternating current?

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Physikfan

Thu Apr 20 2017, 07:43AM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

For the motors of my vacuum pumps I need 400V three-phase current (1-2 kW), but unfortunately I have not such a current source in my home.

I see three possibilities:

1. I could use a capacitor to establish a phase shift for a "third phase".
2. I manufacture a circuit according to an already proven circuit diagram with IGBTs.
3. I purchase a commercial ready circuit or device with housing via Ebay.

Please, who can give me good advice?

Sulaiman

Thu Apr 20 2017, 08:05AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

something like this

EDIT: inserted after reading the post below
the cost, weight, size of passive components, or a motor-generator set, required for 1 - 2 kW make a VFD attractive
the starting torque of a vfd is adjustable to greater than nominal
a VFD has many benefits, and uses

Dr. Slack

Thu Apr 20 2017, 08:11AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Ideally you would use a capacitor and an inductor, and may need a transformer as well for amplitude changes. This sort of phase shifter only works well for a small range of loads, but is quite respectable over a 2:1 range.

Once a 3 phase motor is started, it will often run surprisingly well on a single phase. Depending on what static load the pump puts on your motor, you may or may not be able to start it with a simple phase shifter, and then disconnect it for running.

One type of single to 3 phase converter that is commonly used is a large 3 phase motor, run off load. It's started with a simple phase shifter, and then run from a single phase. You take the three phase supply from its terminals.

Obviously a VFD (variable frequency drive) will do the job properly, giving your motor full starting and running torque.

It really depends on how over-specified your vacuum pump motor is, how well it will tolerate short-cuts in powering it.

radiotech

Thu Apr 20 2017, 11:11AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

If you have several motors, an artificial three phase stem may be set up.

Search Phase converter.

System works well. I have set up a few over the years.

You must know your motors well.

1492686705 2463 FT179585 Brook Motors Ltd Wee Book From Long Ago

klugesmith

Thu Apr 20 2017, 06:41PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

+1 on what Dr Slack and Radiotech both mentioned. Often called "Rotary phase converter". It's been traditional in amateur shops for many decades. My friend Tom has one to run his milling machine and lathe. No semiconductor parts required.

Here are a couple more DIY-oriented links:

"how-to-build-a-rotary-phase-converter"

"projects/Phase-Converter":

To begin, you would scrounge a three-phase induction motor. "... called an idler, and needs to have a 20-30% higher horsepower rating than the largest equipment motor that you will be using".
p.s. long ago my employer needed to test a 25 kW product on 50 Hz power, before shipping it to a customer in Europe. We had no shortage of three-phase power at 60 Hz. Rented a rotary frequency converter, aka a motor-generator set. Speed ratio set by a v-belt connection.

Others have mentioned solid-state Variable Frequency Drive units, which commonly have three-phase outputs. VFD's that come with three-phase AC inputs can usually be used on single-phase power. Vacuum pumps with 50/60 Hz motors will suck faster at 60 Hz.

Physikfan

Fri May 05 2017, 07:52AM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Next Monday I hope to get "real" three-phase current finally.
I will start the frequency converter next Thursday with a small pump with the help of an electrical engineering specialist, then I will know more.

Physikfan

Sun Jun 25 2017, 08:01PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi Sulaiman, Dr. Slack, radiotech and klugesmith

Thanks for your posts.

Meanwhile, I have a working combination of a single-phase fed frequency converter and a three-phase motor.
However, I will not use the pump motor, since I would not necessarily expect the pump to have a wrong direction of rotation and I also have unintended oil fountains already enough.
Exciting will be the experiments with variable frequency, I will also use an oscilloscope to study the corresponding curve forms and phase differences.

Regards

Physikfan

johnf

Mon Jun 26 2017, 09:21AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

These are very common
like

But nobody said cheap unless you have an inefficient large three phase motor lying around to use as an idler

Physikfan

Fri Jun 30 2017, 06:48PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi Sulaiman, Dr. Slack, radiotech, klugesmith and johnf

When a motor is powered by a frequency converter, the pulse-width-modulation (PWM) voltage applied to the motor induces additional magnetic noise at the clock frequency and also a harmonic distortion of the clock frequency.

Therefore it should be possible to detect acoustically wether a three-phase AC motor is driven by a frequency converter or operated by "normal" three phase current.

This would justify measurements with a good microphone and a FFT analyzer.

I will therefore carry out acoustic measurements with such an analyzer parallel to electrical current-voltage measurements.

Sulaiman

Fri Jun 30 2017, 08:29PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

older drives used audio frequencies for the pwm clock
newer drives with faster semiconductors often have ultrasonic pwm clocks
so an audio microphone may not work.

Smaller drives often rely soley on the inductance of the motor windings to produce a sinusoidal current from the pwm signal
a 'scope can 'see' the switching.
Many drives have output inductors internally so it is difficult to see the switching with a motor connected.

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