Unloaded or Partial Load Motor Efficiency 2

[HEMillet]

What are the best techniques for improving PSC motor part load efficiency? The application load is frictional VS inertial.
Also 3-Phase motors, same type of load?

 

[Marke]

Hello HEMillet
What is a PSC Motor??
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jOmega]

Single Phase .....

Permanent split capacitor....

Permanent split capacitor (PSC) motors do not have a starting switch or a capacitor strictly for starting. Instead, permanent split capacitor motors have a run-type capacitor that is permanently connected in series with the start winding. This makes the start winding an auxiliary winding after the motor reaches running speed.
Because the run capacitor must be designed for continuous use, it cannot provide the short-term "boost" of a starting capacitor. Therefore, starting torque of a PSC motors is low, ranging from 30 to 150 percent of rated load, which makes the motors unsuitable for hard-to-start loads. However, unlike split-phase motors, PSC motors have low starting currents, usually less than 200 percent of rated full-load current, making them excellent for applications with high cycle rates.
Permanent split capacitor motors have several advantages: since they do not require a starting mechanism, they can be designed for easy reversing. They can also be designed for optimum efficiency and high power factors at rated load. They are considered to be the most reliable single-phase motors, primarily because a starting switch is not required.

Permanent split capacitor motors have a wide variety of applications depending on the design. Examples include direct drive fans, blowers with low starting torque requirements and intermittent cycling applications such as adjusting mechanisms, valve actuators, gate operators and garage door openers, many of which also require reversing.

 

[Marke]

JOmega
Thank you, Of course, brain not fully in gear. Not a TLA that I come across very often.

OK, back to the original question, the part load efficiency of small single phase motors can be improved by reducing the terminal voltage. This is the basis on which so many energy saving devices have been manufactured for the last 25 years.
Be careful with expectations though, and look at the efficiency relative to full load efficiency. The Efficiency does not drop as quickly as many people believe. Reducing the voltage will reduce the iron loss of the motor. If the motor is over fluxed, (common in small motors) then the reduction in iron loss is relevent. There will be an increase in work current at reduced voltage, this will increase the copper loss and offset the reduction in iron loss. It is a compromise deal, generally the savings are only significant at very light loads. Triy using a variac to see the effect on your motor in your installation.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jOmega]

HEMillet

Just out of curiosity....

What is the power rating of the PSC motor in your application.

The WW Grainger catalog lists PSC motor ratings from
fractional to 2 HP ...

If your application falls within this range... then my curiosity is to why the concern over a couple of points of efficiency ? Certainly, over the course of a year... what could the savings be ....???? $0.50 ? %1.00 ? $2.00? etc.

Seems the time spent in the investigation of the question is far, far greater than the savings to be realized.

Would you agree?

And what are your alternatives.... ?

PSC are considerably more efficient than Shaded-Pole motors...

 

[Marke]

Hello HEMillet
You commented that if the motor is too big, then the efficiency wil fall. This is not always the case for two reasons:
1. The efficiency of an induction motor is not, as many believe, proportional to the loading on that motor. In may cases, the efficiency of the induction motor does not fall significanlty until the motor loading is very light. Some motors are actually more efficient at 80% load than at 100% load!
2. Larger motors are more efficient than smaller motors. It is posible to oversize a motor for a given load and end up with a higher overall efficiency!

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[rbulsara]

I am with Jomega on this!

 

[UKpete]

Although the efficiency of the capacitor start and cap start/run motors doesn't change much with load, it isn't always the case for the permanent-split capacitor types, see data at:

http://www.brookcrompton.com/pdf-files/2227E-1.1e.pdf

- the efficiency can drop by up to 10% according to the motor rating. The obvious factor here is the value of the permanent capacitor - I dare say this could be optimized for each value of load if you are keen to save running costs. For a 1.5kW motor this could mean maybe US$60/yr per motor for continuous running at half load, but that is an extreme case.

For anything above 2kW (3hp) the 3-phase motor is more likely the better choice assuming 3-phase is available.

 

[HEMillet]

Marke,

I know that PSC motors Efficiency is best for the loading that they were wound for. The application is hermetic HVAC compressors that have a load for rating and a wide range of loadings depending on the system, weather conditions, controlled space set point etc. etc. The need is to improve the efficiency at non optimum loading, normally low loads. To jOmega, the use of psc has many reasons. For single phase operation such as residential, this is the type use almost universally. The sizes can go from 1.5 to 5.5 or greater HP. For Residential Split Systems $$ is everything. Efficiency is a competitive edge. Thanks for everyones input. Need something radically out of the box.

Hank

 

[jbartos]

Suggestion: Typically, the motor efficiency is set by the motor engineer/designer. After the motor is produced, there is very little it can be done about the motor efficiency versus the load curve. Perhaps, if the motor is in very cool place, then the motor fan propeller might be removed.
The reduction of voltage at PSC terminals will require reduction at the motor shaft load. This is obvious since at zero PSC terminal voltage, the motor will not turn.