Why is there an inverter on an air conditioner?

[caribconsult]

Almost all the current room air conditioners are now min-splits, which is great because it puts the main noice outside. But why do they have and inverter? An inverter converts DC to AC, and since's already AC (usually 220) already there, what's the purpose of this inverter? Is it really a rectifier and just using the wrong name? Converting the line AC to DC and using that to control the compressor moter maks sense. But you don't use an inverter to do that, it requires a rectifier. Somebody here have the answer?

 

[LarryFine]

Think of a VFD. Fixed AC frequency in, variable AC frequency out.

 

[don_resqcapt19]

They use variable speed drives, and a VFD first converts the AC to DC and then back to AC at a different frequency. The use of VFDs in refrigeration and cooling equipment is being driven by appliance energy standards.
VFDs are often called inverters.

 

[LarryFine]

Exactly. There's no reason to maintain the 60 Hz.

 

[Fred B]

Exactly. There's no reason to maintain the 60 Hz.

Not sure so maybe someone can answer. Do these VFD both reduce and increase the Hz from the static 60Hz of our gid system or just one direction. ie. ranging from say 30Hz to 120Hz to both slow down and speed up the motor?

 

[hbiss]

It's really quite simple. The inverter or VFD is used to control the speed of the compressor and condenser fan. Instead of running full blast and turning on and off, there is a controller that monitors temperature, set point and other parameters like super heat and sub cooling within the system and adjusts the speed for maximum efficiency.

Not sure so maybe someone can answer. Do these VFD both reduce and increase the Hz from the static 60Hz of our gid system or just one direction. ie. ranging from say 30Hz to 120Hz to both slow down and speed up the motor?

60Hz is max.

-Hal

 

[Dennis Alwon]

I believe the fans are all DC on those units

 

[Besoeker3]

60Hz is max.

-Hal

They can be any frequency.

 

[LarryFine]

They can be any frequency.

Absolutely.

 

[LarryFine]

Not sure so maybe someone can answer. Do these VFD both reduce and increase the Hz from the static 60Hz of our gid system or just one direction. ie. ranging from say 30Hz to 120Hz to both slow down and speed up the motor?

Either direction. Not all motors are designed for 60 Hz.

A motor and drive can be designed to work together.

 

[wwhitney]

Absolutely.

Certainly if you get the right motor, and your VFD has the correct voltage supply going to it. But if you have a motor rated for a particular voltage and frequency, it presumably would be unwise to operate it at a higher voltage and frequency, while it would generally be OK to operate at a lower voltage and frequency.

So in practice, 60 Hz is often the maximum.

Cheers, Wayne

 

[Besoeker3]

Certainly if you get the right motor, and your VFD has the correct voltage supply going to it. But if you have a motor rated for a particular voltage and frequency, it presumably would be unwise to operate it at a higher voltage and frequency, while it would generally be OK to operate at a lower voltage and frequency.

So in practice, 60 Hz is often the maximum.

Cheers, Wayne

We used a number of 1500 Hz drives. These were used for aircraft applications. There were a few higher frequency units.

 

[jaggedben]

... Is it really a rectifier and just using the wrong name? ...

Well, really in most VFDs sold as units it's both. That is, a rectifier and an inverter. So you're kind of right that usually 'inverter' is a partially misleading name; it's more than just that. But note that conceptually the 'VFD' part is really just the inverter because that part could also be fed from a straight up DC source like a battery. Another notable application where this is true is modern diesel railway locomotives, where the diesel generator produces straight DC and large inverters (variable frequency) control the rotation of each AC traction motor. So there is no rectifier there.

 

[LarryFine]

 

[winnie]

The VFDs and compressor motors in minisplits are specifically designed together, and because you get more power out of a motor at higher speed, operation above 60Hz is quite common.

The repair manual for the 9000 BTU Fujitsu minisplit in my bedroom says that the compressor frequency ranges from 10 rps to 76 rps. I don't know why they use 'rps' rather than Hz; perhaps the compressor motor is 4 pole and the frequency range is 20-152. Or perhaps it was just translation.

In any case, for the OP's specific application 60Hz is not even a common limit.

Also relevant to the OP: there are minisplits out there designed for connecting solar panels to the DC bus via a DC-DC converter.

 

[Besoeker3]

The VFDs and compressor motors in minisplits are specifically designed together, and because you get more power out of a motor at higher speed, operation above 60Hz is quite common.

So what higher speed is your the higher you want be above 60Hz

 

[winnie]

So what higher speed is your the higher you want be above 60Hz

Not the 1500 Hz which you used

As I said, I poked at the repair manual for units installed in my home. The compressor speed appears to go up to 91 'rotations per second' for larger units. I would not be surprised by variable speed compressors for residential use going up to 120 Hz. I would be very surprised if they went above 400 Hz.

 

[kwired]

It's really quite simple. The inverter or VFD is used to control the speed of the compressor and condenser fan. Instead of running full blast and turning on and off, there is a controller that monitors temperature, set point and other parameters like super heat and sub cooling within the system and adjusts the speed for maximum efficiency.


60Hz is max.

-Hal

Many times yes, but the design of VFD's makes them easily capable of higher output frequency. I also believe you need a motor designed for the application if you are going to go much higher than the 50 or 60 Hz that is pretty standard though.

I know that higher frequency is desirable for some aircraft or even on ships, subs, etc. because you don't need as much iron core at those higher frequencies and that reduces overall weight of the equipment. IIRC 400Hz is a common frequency for those applications.

 

[don_resqcapt19]

Many times yes, but the design of VFD's makes them easily capable of higher output frequency. I also believe you need a motor designed for the application if you are going to go much higher than the 50 or 60 Hz that is pretty standard though.

I know that higher frequency is desirable for some aircraft or even on ships, subs, etc. because you don't need as much iron core at those higher frequencies and that reduces overall weight of the equipment. IIRC 400Hz is a common frequency for those applications.

One of the issues with higher speeds needing specially designed motors is the centrifugal force that tries to tear the motor apart. I think that force changes with the square of the change in speed, but physics classes were a very long time ago.

 

[hbiss]

I was thinking of Carrier packaged units that use standard off the shelf (usually scroll and 3 phase) compressors. Those are not going to be able to handle over 60Hz. Yes, smaller units that are designed as a system with their own compressors could use anything.

-Hal