9000BTU too weak?

[SoFarAndNear]

Hi,

 

I have a small room 10sqm with a 9000BTU Samsung Inverter unit in my house. Its on the top floor. Without A/C the room is at 31celsius. A/C runs with full power for hours and its just 28celsius.

 

Sun isn't shining so the walls are cold. I have SCG Stay Cool 75mm ceiling insulation on the top of the room. Should be much colder, right? Or is 9000BTU not strong enough?

Edited June 30, 2017 by SoFarAndNear

[Peterw42]

Should be fine for that size room, I think you defiantly have a problem. Have you tried powering it off at cct breakers and back on, sometimes electronics needs a reset ? Otherwise it may need a regas.

[Crossy]

Yup, you have an issue. Time to call the man.

 

The standard Thai guesstimate for A/C is 6-700 BTU per m² and that does tend to over-size somewhat as it's a catch-all for poorly insulated rooms.

 

The unit is actually running on "cool" (snowflake symbol) you haven't got the A/C on "dry" (water drop symbol) or something like that have you?

 

[KhunBENQ]

Have a similar situation.

If I would set it to full power I would freeze to death

 

[lopburi3]

I have 26sm using 9k and easily keep it at 28c (my comfort temp) and could get a lot cooler on anything but the hottest days.  You do have it set for a lower temp and mode set to cool?

[bankruatsteve]

I have a Daikin inverter and lately with the power outs, I have to go through loops to get the stupid compressor to work.  First, I make sure it is "off" at the remote.  Then, I shut off the breaker (yes, there is a slight power draw even when it is off).  Then, I wait 10 minutes and turn the breaker on, then the remote.  Sometimes the compressor still doesn't kick in but if I go through the "modes", it will then start up - not always which I then repeat the procedure until the damn thing starts up.

 

All this to say, if your inverter "thinks" like mine, make sure the compressor is running because the inside unit will act normal even though nothing is cooling (just fan).

[Jdietz]

Sure you don't need a new 'starter' cap?

[Crossy]

Just now, Jdietz said:

Sure you don't need a new 'starter' cap?

 

it's an inverter unit, so likely has a brushless DC motor (really a multiphase induction motor and a bunch of electronics), no starter cap.

[Jdietz]

You're right. I looked it up, looks more like a stepper motor controller :) So yeah probably an issue with the 'tronics..

 

[bankruatsteve]

1 hour ago, Crossy said:

 

it's an inverter unit, so likely has a brushless DC motor (really a multiphase induction motor and a bunch of electronics), no starter cap.

A DC motor?  What is being "inverted" then?  'Splain it to me Lucy.

[Crossy]

5 minutes ago, bankruatsteve said:

A DC motor?  What is being "inverted" then?  'Splain it to me Lucy.

 

Read the content of the brackets

 

or

 

https://en.wikipedia.org/wiki/Brushless_DC_electric_motor

 

[bankruatsteve]

3 minutes ago, Crossy said:

 

Read the content of the brackets

Brackets?  Where's this going?

[Crossy]

Just now, bankruatsteve said:

Brackets?  Where's this going?

 

(really a multiphase induction motor and a bunch of electronics)

[bankruatsteve]

4 minutes ago, Crossy said:

 

(really a multiphase induction motor and a bunch of electronics)

I hope I'm not stressing your patience.  I've 'googled' inverter consumer products and when it comes to AC I remain befuddled as to what is actually going on.  Is a "multiphase induction" motor AC?  Is a "bunch of electronics" DC?  If so, then I'm back on base.  It threw me when you said "DC motor".  Or, do I still need 'splaining?

[Jdietz]

It looks to me like the electronics do a 2-phase AC -> DC -> 3-phase AC conversion. The 3 Phase AC then can be run at most any frequency (speed) controlled by the microcontroller

 

simplified schematic:

 

The part labeled Power Factor Correction actually looks more like a buck converter to me (Step-down converter)

So AC 240 -> DC [Motor voltage] -> 3 phase. In this configuration both the motor voltage and frequency are freely controllable.

Edited June 30, 2017 by Jdietz
Added some more info / comments

[Jdietz]

Edited above post to add some more details.

 

[bankruatsteve]

Thanks Jdeitz.  Schematics - it's been a while, but that looks like an AC motor to me.  (?)

[Jdietz]

Yeah I think the term DC is a bit inappropriate, and used for the controller part only. The motor is a brushless motor but still needs AC to drive it.. Crossy may be able to add some more details, I usually deal with way lower voltages and currents :)

 

The Inverter technology (DC) is the latest evolution of technology concerning the electro motors of the compressors. An Inverter is used to control the speed of the compressor motor, so as to continuously regulate the temperature. The DC Inverter units have a variable-frequency drive that comprises an adjustable electrical inverter to control the speed of the electromotor, which means the compressor and the cooling / heating output. The drive converts the incoming AC current to DC and then through a modulation in an electrical inverter produces current of desired frequency. A microcontroller can sample each ambient air temperature and adjust accordingly the speed of the compressor. 

[tutsiwarrior]

I've got a 6 month old 24kbtu Mitsubishi inverter to cool a 40m2 ground floor shophouse space, all glass front and no direct sun...I run it at 24C March to May and at 28C other months for the same level of comfort...gotta think about the ambient temps, rate of conductive heat transfer due to the delta T, etc...

 

 

Edited June 30, 2017 by tutsiwarrior

[Crossy]

17 hours ago, Jdietz said:

It looks to me like the electronics do a 2-phase AC -> DC -> 3-phase AC conversion. The 3 Phase AC then can be run at most any frequency (speed) controlled by the microcontroller

 

Pretty much, but the motor sees rapidly pulsed DC rather than true AC. The driver devices are either on or off to minimise power dissipation, they're also invariably IGBTs rather than regular bipolar transistors.

 

The motor itself usually has a permanent magnet rotor, more like a stepper motor than a regular AC induction motor.

[carlyai]

It looks to me like the electronics do a 2-phase AC -> DC -> 3-phase AC conversion. The 3 Phase AC then can be run at most any frequency (speed) controlled by the microcontroller
 
simplified schematic:

 
The part labeled Power Factor Correction actually looks more like a buck converter to me (Step-down converter)
So AC 240 -> DC [Motor voltage] -> 3 phase. In this configuration both the motor voltage and frequency are freely controllable.

To me, it seems not a simplified schematic, but a wrong schematic. It seems like an A.C. full wave rectified to DC with a filter capacitor. Fairly constant DC. I can't see what all those transistors do, except for current supply, but I guess there's something left out. Sorry if I'm wrong.

Sent from my SM-J700F using Tapatalk

[Jdietz]

What's left out is the controller that activates the transistors, causing them to do what it says on the package. Bases of the transistors will be connected to the microcontroller through some drivers.

Also left out is all the feedback. This is just the motor driver board.

 

[kannot]

Ive  got 10000 btu in a 50m2 house but it does have  double  block walls built on the ground and 10cm roof  insulation 12  mm gyproc  ceilings, no problem 25c  all day even when 42 c  outside, cost in hot season 1450 month this  year cool season 8-900 left permanently on day/night