Testing DC inverter compressors

[frankv1@maltane]

Hi

Is it possible to test compressors used in DC inverter units using a standard Inverter.
From what i have learned so far the unit electronics are converting the input 240v AC to DC and outputing a variable voltage 3 phase to the compressor.
The problem i have is with some Hitachi units which just stop as soon as the compressor starts to ramp up. Installation , charging , supply voltages are all ok.
In fact i have another unit on the same electrical supply and is working fine

Any suggestions

Fra

[Thermatech]

Yes you can use industrial type inverter to run air conditioning unit compressors. The inverter must be rated for the KW / HP of the compressor you want to test.

I use a Mitsubishi Electric industrial inverter to test compressors.
You can programe it for any start sequence you like for forward or backward rotation & up to any speed you want.
It has a display for the amps & hz so you can monitor what amps the compressor is pulling as it ramps up.

1/ Disconnect the 3 ph power to the ac unit & connect to the industrial inverter instead.
2/Then make the output cable connections to the compressor.
3/ Power up. Select the start & run peramitors & hit the start button.
4/ Monitor the amps & hz on the inverter display as the compressor starts & ramps up.

You need to have some data for what amps a good compressor will draw during the start up sequence with the industrial inverter.
So the more compressors you test the better you will get at making a pass / fail judgement for any compressor.

The only problem with this is the cost of the industrial inverter & most service managers just will not spend that sort of money for a sevice tool which will only be used once in a while.

With Hitachi systems you first need to test the unit inverter output
Safety first power off & let capacitors discharge.
Make setting SW1-1 on the inverter board to disregard low amps & disconnect the output cables to the compressor.

Then operate with inverter circuit as stand alown measure the 3 ph output. It should be ballanced voltage.
Measure earth or N to each output ph U,V&W. Then as double check you can measure between each output ph U-V , U-W & V-W.
If the output voltage is ballanced then you are 99% sure the inverter is ok.

I also have very small 1/4 hp 3ph motor which I connect instead of the compressor for this test as it is good to see the motor shaft turning in the correct direction & see the speed ramp up.
9/10 times if the inverter will run my little motor then its ok.

If the inverter is ok but the compressor trips on high amps then possible mechanical problem with compressor & industrial inverter can only confirm the high amps & you will need to replace the compressor anyway.

[frankv1@maltane]

Thanks for your reply.
My concern is that there are now in use brushless 3 phase DC motors with
permanent magnet rotors.Will these work with a normal inverter.
I have experience with industrial inverters and have installed quite a few on various machines. I am just not sure that these will work on these
motors

Fra

[Thermatech]

Not had any problem up to now

I mainly look at Mitsubishi ElectricCity Multi VRF , Hitachi Set Free VRF & Daikin VRV.
I have been using the trouble shooting methods described including industrial inverter for more than 10 years & it still seems to work on the latest R410a equipment.

[frankv1@maltane]

Have set up the compressor in my workshop. Tried to start it up with an inverter. The compressor ramps slowly to abou 10 Hz it vibrates a lotand the inverter trips on over current. I have set motor parameters in inverter to match the compressor.The compressor is a rotary type.

[Thermatech]

Most inverter compressors will ramp up to minimum speed of 20 to 30 hz at start up.
So if your compressor has overcurrent before getting up to minimum speed there must be a problem with it.

So you will need to replace the compressor.
Perhaps as you have it in the workshop you can cut it open & try to see what is causiing the problem.

[frankv1@maltane]

Thanks for the help.
Will chop the beast and see what's inside.

Fra

[totalacedude]

hi,

i know this is an old post, but this hasnt really clarified much to me! lol

im not a refrigeration engineer and all i do all day is repair inverters and dc drives.

to me the word 'dc inverter' is really a bit of a clash, as an inverter chops up the dc back into ac lol

i think what they are doing is something called 'vector mode' which alot of standard inverters have!

after looking at the motor cut out everything points to it being a servo motor, but even me after my years of working with inverters etc i am baffled at how exactally they are driven!

any know how???

i am going to pull the u v and w off my mitsi and put bulbs where the compressor goes and watch what happens, then i can prob figure it all out!

am quite interested in knowing result because this will mean that a normal ac inverter in vector mode should run the thing!

the main reason im so interested is that i had 'compressor lock' reciently on my mums unit, swapping phases on the compressor a few times actually unlocked it, (i was lucky) but the output from the ipm wasnt to great at low speed!

what im thinking is if an inverter would work with these then you could set the inverters up to give more of a kick than any IPM would normally do and run it in wrong direction (to try and pull away from the direction it locked)

if this saves having to go through all the ball$ache of having to replace the compressor and only works in every 1 in 10 sureley its worth it???

i will play over hols and report back what i find.

merry xmas all

ian

[Yuri B.]

Hello.

this is an old post

but it is the theme I was thinkikg about precisely at logging in today.
Testing an induction motor (compressor) of an A/C by a portable invertor seems not to be a problem, but, I am keanly interested in whether an BLDC motor (compressor) can be tested by the "conventional" induction motor drive - not by a BLDC drive? I am afraid, I think it cannot - too different methods of control (wave forms different, etc).
That is why
frankv1@maltane had

The compressor ramps slowly to abou 10 Hz it vibrates a lotand the inverter trips on over current.

[VRVIII]

Hi

Is it possible to test compressors used in DC inverter units using a standard Inverter.
From what i have learned so far the unit electronics are converting the input 240v AC to DC and outputing a variable voltage 3 phase to the compressor.
The problem i have is with some Hitachi units which just stop as soon as the compressor starts to ramp up. Installation , charging , supply voltages are all ok.
In fact i have another unit on the same electrical supply and is working fine

Any suggestions

Fra

Hi Frank,

I can’t say for definite, but very much doubt that it’s possible to test digitally controlled inverter compressor with a standard inverter.

I’m not expert on motors but know the basic principles;

The motor windings are located in the stator and the magnets in the rotor, the current flowing through coils will be reversed/switched depending on the rotor position.



IE.

• Current flows in on U1 and out on U2 - A fraction of a second later, during the negative alternance, the current is reversed and will flow from U2 to U1.
• Again a fraction of asecond later, “U” is still positive and “W” is still negative but “V” became POSITIVE. The current flow in the stator has turned. And so does the magnetic rotor.

Obviously the inverter electronics must detect the rotor position to have this level of control and to be able to apply the correct voltage to the proper coil.
The motors are wired in star and have an additional N terminal which is connected to the star point, the inverter electronics monitor this connection to determine the rotor position. However the latest digitally controlled motors only have 3 terminals, these are capable of determining the rotor position by monitoring the UVW phases only.
These motors are far more efficient than a traditional AC motor, especially when operating at low to med speeds and they also have a high starting torque.


A standard inverter will only vary the frequency and will not reverse the coil polarity as required by these motors. I know that if a 415v 50Hz supply is connected directly the motor will try to drive in 2 directions, drawing excessively high current instantly tripping the mains cicuit breaker .

[skags]

has anybody found a way to run the compressors at a low hz just to check current draw,
what type of industrial inverters have you been using , i was thinking of three phase vsd some how, we have a problem with seized compressors taking out PCB's and not having a way to test for a seized compressor, it would be nice to have something that could jist run the compressors interested to kno what people are using