Q for Sparkies and EE

[theoldwizard1]

How much more efficient is a 1 hp 3 phase motor compared to a 1 hp single phase motor ?

Why am I asking ? Several companies selling "window shaker" AC units have come up with a new twist. "Dual/double inverter" Of course I can find no details of the marketing speak so here is my speculation.

The input (120VAC single phase 60 Hz) is convert to DC (full wave bridge rectifier, small amount of filtering, net 65-70 VDC). This is feed into 2 different 3 phase converters (inverters), one to run the compressor and one to run the fan. (The motors would have to be designed to run on about 50VAC, and the frequency would have a wide range.)

The bonus is no on/off cycling or TXV is required. A microprocessor would read temperatures and adjust compressor speed and fan speed accordingly.

 

[Jim greengo]

Sounds like the same thing they've done to blower motors on newer furnaces.
Replacement parts are going to be expensive,most window units are already throw away.
I'd say it will cause a lot more of them to end up in scrap pile.

 

[californiaHank]

It's not that there is a big difference in the efficiency of 3 phase vs single phase motors, it's how the motor is used. The fact that inverter air conditioners typically use 3 phase motors isn't so important as the fact that they use inverters.

In inverter air conditioners, the frequency of the power fed to the compressor motor is varied, causing it to run at different speeds under different conditions. A traditional air conditioner compressor motor runs directly off your houses's 60Hz AC power, and doesn't change speed, so it needs to start/stop a lot. An inverter air conditioner takes the incoming 60Hz AC, converts it to DC, and then back to AC of varying frequency, and the control circuitry speeds up and slows down the motor to produce different amounts of cooling as required.

Some manufacturers claim up to a 30% improvement in efficiency for their inverter units vs traditional air conditioners with single-speed compressor motors. There are a lot of factors that affect that, so you'll have to do your own research to figure out if the (generally more expensive) inverter units are worth it for you. There are a few other advantages to inverter units - they're generally quieter because they're not always running at full speed, and (at least in theory) they should be more reliable beacuse they don't cycle off/on so often.

 

[DC73]

When comparing one specific 1 phase motor to one specific 3 phase motor, you can estimate the difference in operating cost by comparing the nameplate efficiency rating for each motor.

The efficiency of a motor is calculated by dividing power output by power input.

There are a few reasons why 3 phase motors are more efficient. For one, they have much greater starting torque due to the 3 phases. That means they go from zero rpm to running load much faster than a 1 phase motor. That means the motor inrush current is less and the power consumed during motor startup is less.

3 phase motors are self starting because of the rotating magnetic field. 1 phase motors need a starter winding and something to power it like a capacitor. The starter winding acts as a second phase to get the motor started. This starter winding and the power consumed by the capacitor are something the 3 phase motor doesn't deal with.

DC

 

[theoldwizard1]

Sounds like the same thing they've done to blower motors on newer furnaces.

It is ! The difference is that all of the electronics are done one just a few chips. With low current (?A) and lower voltage (no transformer to boost the voltage back up to "standard" voltage) even the driver MOSFETs are cheaper.

 

[theoldwizard1]

There are a few reasons why 3 phase motors are more efficient. For one, they have much greater starting torque due to the 3 phases. That means they go from zero rpm to running load much faster than a 1 phase motor. That means the motor inrush current is less and the power consumed during motor startup is less.

I have always heard that a single phase motor is "effectively" coasting (or at least not totally contributing to maintain the RPM) after it hits it peak voltage positive of negative voltage of each half cycle.

 

[DC73]

I have always heard that a single phase motor is "effectively" coasting (or at least not totally contributing to maintain the RPM) after it hits it peak voltage positive of negative voltage of each half cycle.

Fighting to remember the particulars on that one but it makes sense that as voltage heads back toward zero from either peak, the motor doesn't get a push from that part of the cycle. In theory, this would also happen during a small(er) portion of the cycle on a 3 phase motor before the next phase picked up the slack so to speak.

That reminded me of this: If I remember correctly, the starter winding and capacitor drop out after the 1 phase motor reaches a certain rpm as they are no longer needed. My brain is wanting to say they kick out around 75% of running rpm.

DC

 

[Bert_]

Fighting to remember the particulars on that one but it makes sense that as voltage heads back toward zero from either peak, the motor doesn't get a push from that part of the cycle. In theory, this would also happen during a small(er) portion of the cycle on a 3 phase motor before the next phase picked up the slack so to speak.

That reminded me of this: If I remember correctly, the starter winding and capacitor drop out after the 1 phase motor reaches a certain rpm as they are no longer needed. My brain is wanting to say they kick out around 75% of running rpm.

DC

Many single phase motors also use a run capacitor in series with the run winding after the start capacitor is dropped out of the circuit. It improves efficiency and lowers running current. Most all compressors and fan motors use a run capacitor.