Why is there an inverter on an air conditioner?

[winnie]

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

That makes good sense.

If you are using a standard compressor you are going to be pretty limited in frequency range, both maximum speed and minimum speed. The maximum speed is set by mechanical limits; the minimum speed will likely be set by cooling.

If you are using a compressor specifically designed for VFD use, then it will have a pretty wide operating range.

-Jonathan

 

[garbo]

I would prefer an inverter ( VFD ) on my mini spots for reversal reasons. To save energy, quieter whenever running less then full speed, motors last longer & could require a smaller KW backup generator. On the hundreds of drives that I PM'ed , troubleshoot or replaced only found a few that had maximum Hertz set to 70 & 72 Hertz but never actually saw them operating that fast. The DC Buss voltage on VFD'S supply power to IGBT'S that can fire at least to 5,000 Hertz. Most drives the output frequency & voltage are proportional. So a 240 volt AC motor at 50 % speed would have 120 volts & 30 Hertz drive output.

 

[Besoeker3]

Not the 1500 Hz which you used

Wrong, sunshine.

 

[ggunn]

There are AC to AC converters that have a DC stage between the AC stages (rectify and invert) as well as DC to DC converters that have an AC stage between the DC stages (invert and rectify).

 

[Jraef]

The use of inverter drives in appliances now is based mostly on increasing energy efficiency. In centrifugal machines, like pumps, fans and some types of compressors (scroll), the energy used by the motor varies at the CUBE of the speed change. So if you can slow a motor down to 1/2 speed, the motor consumes 1/8th of the energy. In a refrigeration system, the amount of refrigerant being circulated has to to with the necessary cooling at any given moment. Without an inverter drive, i.e. the compressor is at a fixed full speed, if you needed less cooling, you had a valve that throttles the coolant, but the motor still goes full tilt boogie. The energy consumed DOES reduce because of the lower flow through the valve, but not as much as if you were to eliminate the valve and slow the motor down instead to reduce the flow. So the longer you can run art reduced speed, the more energy you save compared to an older designed system. This is now being done in refrigerators, washing machines, even new heat-pump based dryers.

In addition, using an inverter means that you use a 3 phase motor, even though the source is single phase, because the inverter just uses the line source as a "raw material" to create DC that then gets "inverted" back to pseudo AC. The motor runs more smoothly, lasts longer (there is no more centrifugal switch or capacitors in the motor) and is quieter than a single phase motor. Yes, the motor must be specially designed to run on an inverter drive, but for an OEM making millions of mini-splits per year, that's irrelevant.

 

[caribconsult]

Wow! Talk about some explanations! Very nice, guys. So it appears the current passes through a rectifier first, where it is converted to DC, then an inverter where it reverts to AC but the frequency (cps) can be varied to control the compressor, instead of just on/off. Basically the same concept as used on diesel-electric locomotives. Thanks to everyone who took the time to explain this.

 

[garbo]

The use of inverter drives in appliances now is based mostly on increasing energy efficiency. In centrifugal machines, like pumps, fans and some types of compressors (scroll), the energy used by the motor varies at the CUBE of the speed change. So if you can slow a motor down to 1/2 speed, the motor consumes 1/8th of the energy. In a refrigeration system, the amount of refrigerant being circulated has to to with the necessary cooling at any given moment. Without an inverter drive, i.e. the compressor is at a fixed full speed, if you needed less cooling, you had a valve that throttles the coolant, but the motor still goes full tilt boogie. The energy consumed DOES reduce because of the lower flow through the valve, but not as much as if you were to eliminate the valve and slow the motor down instead to reduce the flow. So the longer you can run art reduced speed, the more energy you save compared to an older designed system. This is now being done in refrigerators, washing machines, even new heat-pump based dryers.

In addition, using an inverter means that you use a 3 phase motor, even though the source is single phase, because the inverter just uses the line source as a "raw material" to create DC that then gets "inverted" back to pseudo AC. The motor runs more smoothly, lasts longer (there is no more centrifugal switch or capacitors in the motor) and is quieter than a single phase motor. Yes, the motor must be specially designed to run on an inverter drive, but for an OEM making millions of mini-splits per year, that's irrelevant.

The use of inverter drives in appliances now is based mostly on increasing energy efficiency. In centrifugal machines, like pumps, fans and some types of compressors (scroll), the energy used by the motor varies at the CUBE of the speed change. So if you can slow a motor down to 1/2 speed, the motor consumes 1/8th of the energy. In a refrigeration system, the amount of refrigerant being circulated has to to with the necessary cooling at any given moment. Without an inverter drive, i.e. the compressor is at a fixed full speed, if you needed less cooling, you had a valve that throttles the coolant, but the motor still goes full tilt boogie. The energy consumed DOES reduce because of the lower flow through the valve, but not as much as if you were to eliminate the valve and slow the motor down instead to reduce the flow. So the longer you can run art reduced speed, the more energy you save compared to an older designed system. This is now being done in refrigerators, washing machines, even new heat-pump based dryers.

In addition, using an inverter means that you use a 3 phase motor, even though the source is single phase, because the inverter just uses the line source as a "raw material" to create DC that then gets "inverted" back to pseudo AC. The motor runs more smoothly, lasts longer (there is no more centrifugal switch or capacitors in the motor) and is quieter than a single phase motor. Yes, the motor must be specially designed to run on an inverter drive, but for an OEM making millions of mini-splits per year, that's irrelevant.

I have replaced old motor starters with VFD'S that had plain Jane NEMA 2 to 250 HP frame motors ( not designed for drives ) and ten years later were still running. Would only go with drive rated motors if replacing bad motors or new install. I have been reading about bearing failing with some motors that are controlled by VFD'S for over twenty years but never came across a single bearing case.

 

[kwired]

I have replaced old motor starters with VFD'S that had plain Jane NEMA 2 to 250 HP frame motors ( not designed for drives ) and ten years later were still running. Would only go with drive rated motors if replacing bad motors or new install. I have been reading about bearing failing with some motors that are controlled by VFD'S for over twenty years but never came across a single bearing case.

Do you mean a bearing failure that you can attribute to having a VFD?

I see bearing failures all the time and most not even on VFD driven motors.

 

[PaulMmn]

There are AC to AC converters that have a DC stage between the AC stages (rectify and invert) as well as DC to DC converters that have an AC stage between the DC stages (invert and rectify).

Does converting to DC in the AC to AC eliminate the need for a transformer?

 

[winnie]

The main reason for going through the intermediate DC is to permit changes in output frequency to control the speed of the motor

VFDs can also produce an output voltage lower than the input voltage; so that when you operate your 480V 60Hz motor at 20Hz to slow it down, you also lower the voltage to 160V so you don't oversaturate the machine.

Most VFDs do not have a transformer to achieve this voltage reduction, but they eliminate the transformer in a sneaky way: they use the inductance of the motor being supplied to filter the high frequency pulsing of the output waveform. So you still need the inductive energy storage provided by a magnetic coil for the system to work.

If you were to take an ordinary motor control VFD, supply it with 480V, tell it to output 240V 60Hz, and then connected a resistive load, that load would 'see' a high frequency chopped ~700V, not the desired synthesized waveform.

-Jonathan

-Jonathan

 

[don_resqcapt19]

I have replaced old motor starters with VFD'S that had plain Jane NEMA 2 to 250 HP frame motors ( not designed for drives ) and ten years later were still running. Would only go with drive rated motors if replacing bad motors or new install. I have been reading about bearing failing with some motors that are controlled by VFD'S for over twenty years but never came across a single bearing case.

That was my experience to with converting existing motor starters to VFDs in industrial plants.
The only issue we had was the control circuits that were installed with the motor conductors. Originally we added relays to interface the existing 120 volt Class 1 control circuits to the low voltage control in the VFDs.
Later, we could buy an add on control interface card to install in the VFD so we could use the existing control circuits.

As time went on, and the drives got smaller, we had issues with the control circuit not being able to stop the drive because of inductance from the motor leads, so we went back to using relays. Not idea of what they were changing in the drive internals, but it happened as the decreased the physical size of the drives.

For all new installations we used separate conduits for the motor leads and the control circuits, but for the existing motors, it was more cost effective to add the parts in the MCC room to make the drives work with the control conductors in the same conduit.

 

[Jraef]

AC induction motors produce torque based on the designed ratio of voltage and frequency, aka the V/Hz ratio. So long as you maintain that ratio, you can (in theory) make a motor produce full torque at any speed. So for example a motor designed to be used at 460V 60Hz has a V/Hz ratio of 460/60 = 7.67:1 and if I apply 230V 30Hz to it, it will turn at half the speed, but still supply 100% of the rated torque. By artificially recreating the AC going to the motor, we can change the speed by varying the Hz, then at the same time change the effective voltage by changing the pattern of pulses that make up the artificial AC, referred to as “Pulse Width Modulation” or PWM. So no transformer needed, assuming the goal is to LOWER the speed (and voltage).

A VFD can also RAISE the output frequency to be higher than the base frequency, but it cannot raise the voltage going to the motor beyond what the input voltage is. So when you increase the frequency above the motor design frequency, but the voltage stays the same, you are DECREASING that V/Hz ratio and losing torque. Some people do it knowing the consequences, it all depends on what you need from the motor, you just need to understand those consequences.

There are lots and lots of subtleties in that scenario that get complex very fast, I just wanted to give a brief overview for those who indicated they didn’t fully understand the basics. I discovered VFDs when they were first coming into the market 30 years ago and decided to shift my career focus around them. 30 years later I’m still learning new stuff all the time.

 

[ggunn]

Does converting to DC in the AC to AC eliminate the need for a transformer?

The main benefit is the ability to change the frequency of the AC output, which you cannot do with a transformer.

 

[caribconsult]

OK, I get the idea of variable speed compressor use,but why do you need to first convert the AC to DC, then back to AC at a different frequency, when there are AC motors that will do this on AC just using a room dimmer...no inverter, no rectifier, variable speed available. Why don't AC units use the same simple design? It seems all this rectifying and inverting is a lot more than necessary if you want to vary the speed of the compressor motor.

 

[LarryFine]

OK, I get the idea of variable speed compressor use,but why do you need to first convert the AC to DC, then back to AC at a different frequency, when there are AC motors that will do this on AC just using a room dimmer...no inverter, no rectifier, variable speed available. Why don't AC units use the same simple design? It seems all this rectifying and inverting is a lot more than necessary if you want to vary the speed of the compressor motor.

In a word, torque. Those speed reducers (only) work by reducing power.

It's easier to manipulate DC with electronics, and invert back to AC last.

 

[winnie]

Efficiency.

Any AC motor controlled by supply voltage alone will be low power, inefficient, or have something like mechanical commutation (horrible for life).

 

[caribconsult]

OK! Thanks so much, you folks, for such explicit answers to my question.

 

[Jraef]

OK, I get the idea of variable speed compressor use,but why do you need to first convert the AC to DC, then back to AC at a different frequency, when there are AC motors that will do this on AC just using a room dimmer...no inverter, no rectifier, variable speed available. Why don't AC units use the same simple design? It seems all this rectifying and inverting is a lot more than necessary if you want to vary the speed of the compressor motor.

In an AC induction motor, varying the voltage alone also results in a loss of peak torque at the SQUARE of the change in voltage. So at 1/2 voltage for example, you end up with 1/4 of the motor’s available peak torque. That then directly equates to the motor HP being 1/4 of rated. On SOME LOADS, such as centrifugal fans, that loss of torque results in a loss of speed, but then at the lower speed, the fan NEEDS less HP by the CUBE of the speed change, so it works out. But that’s about the only place where that is true.

 

[kwired]

In a word, torque. Those speed reducers (only) work by reducing power.

It's easier to manipulate DC with electronics, and invert back to AC last.

Yes and a compressor needs higher torque type motor to drive it. Those power reducing methods are more suitable for low torque loads like fans. Still not as efficient as a VFC either but depending on the application still have some instances where they are maybe they are ok way to go.

 

[caribconsult]

Yes and a compressor needs higher torque type motor to drive it. Those power reducing methods are more suitable for low torque loads like fans. Still not as efficient as a VFC either but depending on the application still have some instances where they are maybe they are ok way to go.

Thank you very much for your clear explanation. I Had no idea the torque issue was the central factor, and that AC-> DC->AC was a lot morre efficient was to gain speed control via frequency manipulation for high load applications, like an A/C compressor. Does all this back and forth result in much wastage?