[modeleng] Re: 3 phase

Julian

Here's a copy of a reply I sent in response to a similar question back in
2006.  NOTE :  Some of the Cap values and motor ratings will be different
for you but the calculation factors (uF per hp) are correct.

Pierre

You're trying to make a Static Phase Converter.

First point to remember is - there isn't a single value of capacitor(s) to
suit all motors of a particular size.  Tuning for optimum performance is
usually required, because motors from each manufacturer have slightly
different parameters and the application can affect the motor as well.

Secondly - various designers of home built (and commercial) Static
Phase-Converters also disagree on the need, or otherwise, for more than one
running capacitor.  I favour the two running capacitor design.

Your motor sounds, from the current ratings, to be circa 1/2 HP
(0.375kW) output rating.  A kW or HP rating on the data-plate should confirm
this and, if it's bigger or smaller, may affect the capacitor values I'm
suggesting

Personally, I'd buy a suitably rated Single-phase to 3-phase Inverter /
Variable Speed Drive unit from E-bay.  I've seen these go for as low as 30
to 60 Euros in the size you would require - which would probably be less
than the cost of the Capacitors in a Static Converter.  I have one on a
milling M/C and one on a lathe (both in the 1.1 to 1.5kW size) and would not
use anything else. 

Anyway, if you want to carry on down the Static Converter route, a good
start would be as follows:-

Connect your motor in Delta config.
Label the three motor terminals U,V & W
Connect the Single Phase input - via a suitably rated contactor or on/off
switch - between U & V Connect a Running Capacitor between U & W Connect a
Running Capacitor between V & W Connect a Starting Capacitor via a
"Self-releasing Push-on" Switch between V & W

Value of Capacitors.
The U to W Running Capacitor in microfarads = 4 to 5 times the motor rating
in horse-power.  In your case 2uF to 2.5uF The V to W Running Capacitor in
microfarads = 10 to 15 times the motor rating in horse-power.  In your case
5uF to 7.5uF Starting Cap anywhere between 40uF and 100uF.  Circa 80uF
should give you a quick enough start.  NOTE:  If the motor takes more than
about 2 seconds to get up to rated speed, you are in danger of burning out
one of the windings.  The Start Capacitor size should be chosen to achieve
less than 2 seconds and bigger Caps = quicker start.  However, bigger Start
Cap also means a larger start current.
The Run Capacitors should both be oil-filled types rated at 330 to 370 Volts
AC and specified for the application.
The Start Capacitor can be an 250v to 300v AC Rated Electrolytic to save
costs but, if this type is used it is only suitable for short duration
starting and should never be left connected for long running periods (hence
the use of a "Self-releasing Push-on" start switch)

Testing
Push and hold the "Start Switch" and connect the motor to single-phase mains
using the contactor or on/off switch.
If the motor doesn't reach running speed within approx 2 second or less,
disconnect the mains immediately and increase the size of the start
capacitor.
When the motor is running at rated speed, release the "Push-button Start
Switch" to disconnect the Start Capacitor.
Using a suitable meter, check the values of  voltage  U to W and V to W.
U to V should be your normal single-phase voltage ~230V.  The aim is to get
all three phases as near as possible to 230V (ie perfect conversion).  If
you achieve near 220 to 240V on each that's ok - so values like 242v and
218v are ok.  250+ is to be avoided.

Possible Bad Results.
1.  The motor may not have enough torque using a static inverter to start
the M/C "on load" - if this is so and you haven't got the ability to reduce
the load (eg a mechanical clutch) then you will probably need to make a
Rotary Phase Converter by adding a pilot motor.
2.  One of the phase voltages is very much higher than 230v (maybe ~300v).
If so, switch off immediately and change the values of the running
capacitors.  Try first reducing the value of the V to W capacitor.
3.  Even after playing around with Start Capacitors and Running Capacitors
you still can't get the motor to run at rated speed or it slows considerably
on using the M/C.  This is because the motor torque using a Static Converter
is only 70% to 80% of rated value and, consequently, is not enough to power
the M/C.  In this case you need to use, a larger motor OR a Rotary Converter
OR a Phase Inverter/VFD.

Have fun.

Andy

-----Original Message-----
From: modeleng-bounce@xxxxxxxxxxxxx [mailto:modeleng-bounce@xxxxxxxxxxxxx]
On Behalf Of julianharrison7@xxxxxxxxxxxx
Sent: 20 May 2008 09:46
To: models
Subject: [modeleng] 3 phase

Hi Guys,
Have aquired a couple of fairly old 3 phase pillar drills from a local
engineering works. The cosst is I have to fetch them which seems fair. They
are in very good condition and still being used to finish off a large
contract. Don't know the make but most of the rest of the works they came
from last year were "Senior" machines for this smaller stufff. They are mine
when job done. Rather than changing the motor I hope to keep the one on
them. I have heard it is possible to fit a capacitor to act as the third
phase. If this is correct can someone please tell me what capacitor, where
to connect etc

Many thanks

Julian.

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