Re: Static phase converter
- From: pentagrid@xxxxxxxxx
- Date: Fri, 06 Feb 2009 12:22:50 +0000
On Thu, 05 Feb 2009 16:00:10 -0600, Ignoramus3187
<ignoramus3187@xxxxxxxxxxxxxxxxxxx> wrote:
On 2009-02-05, pentagrid@xxxxxxxxx <pentagrid@xxxxxxxxx> wrote:
You can switch and run a 3 phase motor in the way you describe
but it's a bit unkind to the switching contacts.
Well, let's set this question aside for a minute. Do you think that I
could get full power out of the motor?
If you separately switch in start and run capacitors the
maximum peak switching current is only the normal current drawn
by the motor windings.
If you switch the start capacitor in parallel with the already
connected run capacitor you are instantaneously paralleling two
capacitors each of which may be charged to a different voltage.
The peak current that then flows is only limited by the internal
series resistance of the capacitors and is typically MANY times
steady state winding current.
Sure. But that is a question that can be addressed. The question that
I wanted to find out was, could you get the motor to run at full
nameplate power.
If the run capacitor is selected for current balance
(more accurate than voltage balance) at the nameplate motor load
the short answer is yes. Under these conditions the supply to the
motor is as well balanced or better than the same motor operating
from the more elaborate rotary converter/idler setup.
The following repeat of an earlier post gives more info.
It's true that static converters (start and run capacitor systems
with no idler) can deliver the full rated power of the motor for
surprisingly long periods but that is not the whole story.
A converter of this type is basically a capacitor/inductor
phase shift system which produces an open vee 3 phase system.
This phase shifter is a series resonant circuit and when it is
set up to give the 60 deg phase shift it is working a long way
below its natural resonant frequency. 60 deg is of course the
correct phase angle between the two legs of an open vee system.
The motor(s) is the inductor in the system and
unfortunately the apparent inductance of the motor changes with
rotor speed. For any particular rotor speed greater than about
90% of synchronous speed (the lower limit varies a bit with motor
type) it is possible to choose a capacitor combination which
produces a pretty close approximation to balanced 3 phase at the
motor terminals.
For near the full load rated speed of the motor, large run
capacitance is needed with most or all of it as a single
capacitor feeding the phantom phase from supply live. At light
load the impedance of the rotor rises and if the capacitor value
is chosen to achieve the right phase angle the phantom phase
voltage will be excessive. This could be corrected by feeding the
capacitor from a lower voltage single phase source but this would
mean feeding it from an auto transformer across the supply.
It is much simpler (and of course everybody does this) to
use two capacitors arranged as a voltage divider to
simultaneously achieve the correct phase angle and phase voltage.
The effective capacitanceof the two capacitors connected in
series across the supply is the sum of the capacitances because
the source impedance of the supply is zero and this effectively
parallels the two capacitors.
Because the they also act as a voltage divider, this sum
capacitance is effectively fed from a voltage of supply voltage
times C1/(C1+C2) where C1 is the top capacitor and C2 is
connected phantom phase to neutral.
Because it looks nicely symmetrical there seems to be a
tendency to believe that C1 and C2 should be equal and any
inequality in their optimum value must result from some strange
second order effect. This is NOT true. There is nothing magic
about equal C1 and C2. It simply results in a capacitor of value
C1+C2 fed from half the supply voltage. At this low effective
supply voltage it is only possible to get close to balanced
operation at no load or light loads which enable the rotor to
operate close to synchronous speed. As the load increases with
consequent slowing of the rotor speed the total capacitance needs
to increase with both more in C1 and less in C2. By the time full
load is reached the optimum value for C2 is usually zero.
These effects are very noticeable if you're using a
single motor on a variable load up to near rated full load power
and some compromise necessary. The saving grace is that
industrial motors are surprisingly tolerant of reasonable
overvoltage when operating at light loads. The trick is to size
the capacitors for at or near full load and to accept some
overvoltaqe at light loads. This increases the motor losses at
light load but the total motor losses still remain below the
losses at rated full load so temperature rise is acceptable.
Summing up - if you need to cope with heavy loads on a
static converter throw away the bottom capacitor and be sure to
choose C1 for operation near full load.
None of this helps with starting torque - this is
inherently poor with the static converter arrangement however
large the starting capacitor. This is because correct low speed
phasing requires the capacitor to be fed fed from a voltage many
times the supply voltage.
Jim
.
- References:
- Static phase converter
- From: Ignoramus3187
- Re: Static phase converter
- From: pentagrid
- Re: Static phase converter
- From: Ignoramus3187
- Static phase converter
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