Motor Efficiency

[mahon]

Hi all,

I have been tasked with trying to calculate the motor efficiency of our processes at work, but i do not know where to start. My boss is under the impression that we are using oversized motors therefore not being as efficient as they could be with smaller motors. We have standard induction motors which are inverter driven. Is it possible to calculate the motor efficiency using the standard maintenance tools such as ammeters and DMM's. And if so how do i go about it.

 

Thank you all for your time and any replies will be gratefully recieved.

 

Mat

[AB2005]

 

Read very important information in this link related with your topic.

Over Sizing of Motors

 

[jOmega]

Hi all,

............. Is it possible to calculate the motor efficiency using the standard maintenance tools such as ammeters and DMM's. And if so how do i go about it.

Mat

 

 

The simple answer is ....... yes! and no! but not practical

 

 

While it is possible to use instrumentation to measure the input power to the motor, ——you would need true RMS reading Ammeter, Voltmeter to record voltage and current draw, you would then still need to know the True Power Factor at which the motor was operating ..... How do you get that piece of information ..... which is further complicated by your source of power not being a true, fundamental sine-wave but rather, the output from an inverter which is rich in harmonics as well.

 

Watt meter if used would have to be true RMS type and would have to possess capability for being compensated for low power factors ...

 

Then how to determine the output power .... Measuring Slip .... is not reliable.

If it were possible, you could install a strain gauge or torque transducer between the output shaft of the motor and the driven load.. This would give you the torque value that the motor is producing, and you would also need to measure actual speed at which the shaft is rotating. From this information the calculation of output power is a simple one.

Strain Gauges ... and the instrumentation to read out the torque and hopefully the speed as well.... are expensive

 

If it is not possible to break the coupling between the motor and driven load for the purpose of installing an interposing straing gauge (torque transducer) ... then the

answer would, in all practicality, be NO.

 

It might be possible to come up with a theoretical calculation based upon knowing the input power to the motor and the losses such as

• [-]stator IR^2

[-]rotor IR^2

[-]bearing friction

[-]windage

[-]iron

[-]stray

For this data, contacting the engineering department of the motor manufacture would be necessary.

 

Also one must be mindful that for many motors, the highest efficiency occurs around the 75% load point and decreases above and below that. That is, efficiency would be less at 100% load than at 75% load.

 

You did not state the character of the machine or the character of the driven load; i.e. variable torque or constant torque.... with the former being of the centrifugal fans and pumps classes. The reason I mention this is that historically, designers of such variable torque machines have historically oversized the motor, and for different reasons; not the least of which is that before inverter drives, the motors were operated at constant frequency and the load was varied by outboard mechanical means. Often, said motors were operated into the service factor and beyond ....ergo the practice of oversizing was born.

 

With the advent of inverters as the variable power source for the motors, the need for oversizing akin thereto— no longer exists and continuing said practice makes for a very inefficient system.

 

Kind regards,

[mahon]

thank you all for your replies, it makes for interesting reading. I will put this forward to my boss and go from there.

 

Thank you again

 

Mat

[jOmega]

HI,

 

[urlEngine efficiency of thermal engines is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work. There are two classifications of thermal engines-

 

1. Internal combustion (gasoline, diesel and gas turbine, i.e., Brayton cycle engines) and
2. External combustion engines (steam piston, steam turbine, and the Stirling cycle engine).

 

Angelinabv:

 

Ok, but how does your posting above.... relate to the original post by mahon on June 14, 2010 where his interest was specific to an ELECTRIC MOTOR (induction type) as driven by an inverter (Adjustable Frequency Drive) ??????

mahon: "We have standard induction motors which are inverter driven. Is it possible to calculate the motor efficiency using the standard maintenance tools such as ammeters and DMM's. And if so how do i go about it."

 

Am I missing something, or did you take a Left Turn .....and head off in another direction.....for whatever reason ?

[marke]

Me thinks that it is an excuse to put a link to page. - called spam by some!!

Links removed.

 

Best regards,

Mark.

[jOmega]

The Engine produces KW and has a short term overload margin that can vary from as low as 120% up to 300%. The governor regulates the speed of the engine to allow for variations in load and minimise variations in output frequency. The engine must produce enough KW to cover the shaft load on the motor, the losses in the motor, the losses in the starter and the losses in the cables.

The Alternator produces KVA and it must produce sufficient current to enable the motor to start. The minimum start current required is dependant on the minimum start torque required by the pump at all speeds to full speed, the aility of the motor to convert amps into torque at all speeds to full speed and the ability of the starter to control the current applied to the motor. The AVR controls the excitation applied to the motor to regulate the voltage and cope with changes in load without variations in voltage.

 

 

Megna73 - Perhaps I missed something, but I sure fail to see how your quoted post above relates to Mahon's original post or the subject of this thread.

 

Perhaps you'd care to explain so we all might better understand the meaning / value of your post.

 

Thanks.

 

jO

[marke]

Hi jO

 

More spam, words were a cut and past from one of my other posts. It was ust an excuse to get a link on the forum. Some people think that this will increase their traffic.

I won't because I will delete them.

 

Best regards,

Mark.

[jOmega]

Hi MarkE.

 

Thanks for keeping an eye on such and acting for the benefit of us all and the integrity of this forum.

 

Kind regards,

 

jO

 

p.s. Happy belated birthday.