connecting a VSD/VFD to an AC compressor

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I was wondering about using a Variable Speed AC Drive for two air
conditioners.

One is a window unit, with rotary compressor that uses single phase
with a capacitor to phase shift and three wires to the motor.  Can
this be made to run as a three phase motor without the cap?  As far as
I'm aware "inverter" (hvac-speak for VFD drive) aren't available for
window units, in spite of their higher efficiency.

The second is a mini-split with a three phase motor that does have an
inverter style drive and ramps between 2 amps and 10 amps when heating
or cooling.  (from a hardware perspective cooling is the default valve
position)

My mini-split has control issues and they want more for a replacement
board than a new unit costs, and they ain't cheap....  When it chooses
to work it works very well, but generally the controller starts
sending the outside unit conflicting heat-cool/set-point commands with
rapid fire succession, and I haven't been able to find the cause
except to say it seems to be humidity related.

Both units are 230 VAC.

Commercially available VSD's are reasonably priced and will go from
single to three phase, and I kinda like the idea of designing my own
thermostat that will ramp the VSD depending on temperature.

AND the big question (I've installed a lot of VSD's in industry in
pharmaceutical manufacturing) how do I know, preferably before
wrecking the hardware, that the motor is turning in the right
direction.  SOP in industry is to look at the motor, but with a sealed
compressor that isn't an option.  (and there doesn't appear to be a
convention anywhere that I'm aware of for which phase is A-B-C.. Do
motor wire colors offer a clue?).

Re: connecting a VSD/VFD to an AC compressor
On 4/29/2017 6:53 AM, default wrote:
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  Having little knowledge other than when you slow down a motor with a  
VFD, more heat is produced. My experience is these sealed  
motor/compressor units already run hot, I wouldn't want to add any more  
heat.
   My one data point, is with the electric motor rewind industry, years  
ago it came out with a higher temperature insulation for motors used  
with VFD.
                                          Mikek



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Re: connecting a VSD/VFD to an AC compressor
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With the older VFD the motors would run hot.  They often depend on the  
speed of the motor to use the internal fan blades to cool the motor.  As  
the motor slows down the motor runs hotter.  Some of the newer VFDs use  
a different power scheme to help this out.  Some even have a setting for  
an internal fan, or external cooling of the motor.

Where I worked they burnt out 2 of the 30 HP motors whena VFD was put on  
them.  I told them to add an external fan to cool the motor and whenthat  
was done the motor never failed again.

I don't know so much about the compressor part of the Air Conditioner,  
but of then the air handler fan is ramped up and down.  This seems  
mainly just so they can get a better efficency  number to put on the  
unit.

Re: connecting a VSD/VFD to an AC compressor
wrote:

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The single phase motor cannot be run as a three phase motor because it
is a two phase motor. There are only two windings, not three as in a
three phase motor so changing connections cannot make it a three phase
machine. Single phase motors that do not use a run capacitor have a
really lousy power factor when lightly loaded, it gets better when
fully loaded.. These motors can be made more efficient by lowering the
voltage when lightly loaded and there used to be for sale a device
that does this. There still may be I just can't remember the name.
Single phase motors with a run cap have a better power factor but it
still can be improved. The main advantage for making the motor more
efficient is just to reduce heat in the windings. You won't save any
money. Is the cap in your unit switched out of the circuit when the
motor is up to speed? If it is then I would suspect that the motor is
running pretty much fully loaded and there is not much you can do that
is economically feasible.
Eric

Re: connecting a VSD/VFD to an AC compressor
On Sat, 29 Apr 2017 10:24:00 -0700, snipped-for-privacy@whidbey.com wrote:

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"These motors can be made more efficient by lowering the voltage when
lightly loaded and there used to be for sale a device that does this.
There still may be I just can't remember the name."

Aren't you referring to the NASA attributed "power factor corrector?"
https://spinoff.nasa.gov/spinoff1997/er5.html

I have one on a window fan and one for my well-pump.

I did something like that with a large 48" belt driven commercial
cooling fan.  The original motor used 5.7 amps, the replacement (split
phase capacitor type) I got for it used 3.2 amps and I could shave a
little more by reducing the voltage (had an ammeter on it and would
turn down the voltage until the current started climbing again).

The voltage control was from a schematic I found in one of the
"Encyclopedia of Electronic Circuits" -Marcus.  It was originally
intended as an AC voltage regulator for commercial theater projectors
and used a pair of SCR's to regulate the voltage.  (just slightly more
complicated than a lamp dimmer)

Re: connecting a VSD/VFD to an AC compressor
On Sat, 29 Apr 2017 10:24:00 -0700, snipped-for-privacy@whidbey.com wrote:  

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Potential relays energize windings and capacitors using back electromotive forces (which is efficient).

Re: connecting a VSD/VFD to an AC compressor
On 29/4/17 7:53 pm, default wrote:
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For a piston type compressor rotation direction is not important but for  
a scroll type it is. If it's a scroll and is not performing then change  
the direction.

Re: connecting a VSD/VFD to an AC compressor
On Sun, 30 Apr 2017 07:52:48 +0800, Rheilly Phoull

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Can it run in the wrong direction long enough (say a few minutes)
without damage, to determine if it is cooling?

Re: connecting a VSD/VFD to an AC compressor
On 30/4/17 6:03 pm, default wrote:
Quoted text here. Click to load it

I wouldn't like to say since there are probably models that would but  
anecdotally it would seem there are some that may displace a seal.
I would google for specific info id in that situation.

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