PF OF THREE PHASE MOTOR

[jimbo123]

I though as a rule of thumb when taking a amp reading on motors if the motor has amp draw of 50% or less that it is not efficient and a smaller motor should be sized. Is this correct ? How would one determine when a motor is oversized for a application ?

 

[kwired]

I though as a rule of thumb when taking a amp reading on motors if the motor has amp draw of 50% or less that it is not efficient and a smaller motor should be sized. Is this correct ? How would one determine when a motor is oversized for a application ?

Other factors need consideration, like is the load able to be increased during "normal operation", or sometimes a motor is derated because of frequent starting, reversing, plugging, etc.

Energy delivered to the load is same no matter what but an oversized motor very likely will have more losses than one sized closer to the load.

 

[Besoeker]

I though as a rule of thumb when taking a amp reading on motors if the motor has amp draw of 50% or less that it is not efficient and a smaller motor should be sized. Is this correct ? How would one determine when a motor is oversized for a application ?

Not based on a single reading.
The question is rather too general to come up with a definitive answer.
The load may be cyclic requiring the full load rating at some points in the cycle and little at others.
On the other hand, continuous steady state operation at 50% of rated current would indicate an oversized motor and less than optimal efficiency.

 

[charlie b]

From the title of the thread, I have to ask whether you know that the PF of a motor has nothing to do with how heavily it is loaded during normal operation. The PF does vary with motor loading, but if a motor is only loaded to 50% of its capacity, that does not tell you that the PF is 50%. Forgive me if I have belabored a point that you already understood, but since you did put PF in the title of the thread, I thought I would probe a bit.

 

[jimbo123]

I take readings on motors for the company and it seems like it is for nothing . I have very limited knowledge about checking on motors and I ask these questions to gain some knowledge on this forum about this subject. I picked up 50% somewhere as limit of a oversized motor and poor pf. Poor PF. to me means a motor oversized for the job, wasted power. And yes I would guess it would be more than one reading to determine such.
Thanks for the help here.

 

[topgone]

From the title of the thread, I have to ask whether you know that the PF of a motor has nothing to do with how heavily it is loaded during normal operation. The PF does vary with motor loading, but if a motor is only loaded to 50% of its capacity, that does not tell you that the PF is 50%. Forgive me if I have belabored a point that you already understood, but since you did put PF in the title of the thread, I thought I would probe a bit.

Honestly, there is a load range of a motor where the PF is indicative of how much the motor is loaded. Beyond a 50% load on this specific motor below, the PF change is not a good indication of the motor load. Please see below:

 

[GoldDigger]

Poor PF by itself just means a high reactive (inductive) component in the current through the motor.
Unless you are paying a penalty for low overall power factor on your service, the reactive power will not be costing you much.
Only the I squared R losses in the wiring and the motor itself.
But you may have initially paid more for the motor and associated wiring than you needed to. Changing the motor now will not affect that.


Sent from my XT1080 using Tapatalk

 

[Besoeker]

but if a motor is only loaded to 50% of its capacity,

OP had it as 50% current, not 50% capacity.

 

[kwired]

I take readings on motors for the company and it seems like it is for nothing . I have very limited knowledge about checking on motors and I ask these questions to gain some knowledge on this forum about this subject. I picked up 50% somewhere as limit of a oversized motor and poor pf. Poor PF. to me means a motor oversized for the job, wasted power. And yes I would guess it would be more than one reading to determine such.
Thanks for the help here.

Do some research on the term "power factor". You are not really very close to it's meaning in most professional opinions. It is more of misuse of a term that you likely don't have much understanding of. Do a little reading then come back here with questions if you have them on "power factor". What you are asking about is simply overall "efficiency".

 

[Besoeker]

From the title of the thread, I have to ask whether you know that the PF of a motor has nothing to do with how heavily it is loaded during normal operation. The PF does vary with motor loading..........

Seems like contradictory statements......

Yes, the PF does vary with loading. Here are some figures from a project we did a few years ago. The application was variable speed operation on pumps and, as is usual, we had to provide guaranteed performance figures at the bid stage with serious financial pain if we got them wrong. The first step, what you see here, is modeling the motor from fixed speed data to get equivalent circuit values. Those are then then used to determine under variable speed operation - but that's a different topic.



A little background. The motors are 625kW 4-pole 50Hz hence the nominal 1500 rpm.
I sometimes sweat and fret over these calculations. Rock and a hard place.

 

[Jraef]

I take readings on motors for the company and it seems like it is for nothing . I have very limited knowledge about checking on motors and I ask these questions to gain some knowledge on this forum about this subject. I picked up 50% somewhere as limit of a oversized motor and poor pf. Poor PF. to me means a motor oversized for the job, wasted power. And yes I would guess it would be more than one reading to determine such.
Thanks for the help here.

Let's start with this statement. It may seem like "nothing" to you if you are only being asked to do the work of taking the readings. But for someone like a maintenance planner, this could be vital information, if only as a baseline and later for comparison AGAINST that baseline. So for example if the Mtc planner knows the flow of a pump, and over 3 years he sees that at a given flow the motor power requirement is increasing, that can tell him that the bearings are wearing out. Or if it's a conveyor and the power requirement is decreasing without there being a decrease in load, it could mean a belt is slipping.

As to the motor size vs consumption, unless you are the machine designer you may not be in the loop on all of the issues that go into selecting a motor. One issue is acceleration torque for example. It could be that the load requires a very specific acceleration time, as provided by the peak torque capability of a specific motor size. Once running it may no longer need all of that power however. If you then decide arbitrarily that the motor is "over sized" based on that and put in a smaller motor, it could be that there is some unforeseen consequence of reducing the size; either the motor stalling and being unable to accelerate, or taking too long and having it affect something else, like load timing or lubrication. It's a great idea to pay attention and bring it up to someone who CAN evaluate all those issues, but it's not good to ASSume.

 

[JoeStillman]

Besoeker shows that lightly loaded motors are less efficient, so the losses would be slightly lower for a properly sized motor. But even if a smaller motor could do the job under all circumstances, it may not make sense to replace it. The present value of the energy you expect to save has to be worth more than the cost to replace the motor. In Besoeker's case, he was analyzing some pretty big motors that probably run 24/7. For smaller motors, even a big increase in percent efficiency doesn't amount to a lot of energy.

It's like the head usher was saying to the rest of the volunteers at church yesterday; "Big changes are coming next year. We're going to double your pay!" (2 x 0=0.)