Inverter Duty Motor Power Factor 1

[ThePunisher]

Hi all,

I noticed based on several projects I dealt with that inverter duty motors' Full Load Power Factors are always lower than standard duty motors.

I would like to query here as to why that is?

Is it because that these motors will have more resistance and leakage reactance values and that they operate at lower speeds that their design would result to a slightly higher losses than their standard duty counterparts?

 

[edison123]

Inverter duty motor windings are more about the type of winding insulation to withstand high dv/dt than about power factor. Lower speed motors will have lower pf due to higher flux leakage regardless of VFD rating.

Muthu

http://www.edison.co.in

 

[ScottyUK]

If you allow for the derating applied to motors in certain VFD applications - notably those for hazardous areas - then the lower power factor may well be the result of the increased magnetising current requirements of the over-sized frame and stator used to achieve a given power output compared to a fixed-speed motor.

 

[itsmoked]

You know that that pf never gets seen by the supply thou right?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Keith - I was going to point that out too. I have no idea why but it doesn't matter anyways. Compare efficiencies if you want to compare a number that matters.

 

[itsmoked]

My point was; that if the VFD motor is run with a VFD then the VFD is going to completely isolate the mains from the motor, pf wise.

Efficiencies.. Excellent point LH.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[ThePunisher]

The low full load power factor question was out of curiosity compared to the standard duty motor.

The next would be this. I got a bit of a character here that plays the idea of using an inverter duty motor for DOL application. When I asked, he consequently toy the idea that if they need one VFD application later, they don’t have to buy an inverter duty motor.

Of course, I find the idea insane and impractical. In addition, with a low PF inverter duty motor DOL, that low PF rating will matter. I asked the question because, the person might say again “well, we can get an inverter duty motor with a higher PF (motor is 4,000 HP by the way).

There is no future plan to operate this motor in VFD, not even in 10 years time.

 

[itsmoked]

4,000... That completely modifies the logic.

If there is any possibility of needing a VFD by any stretch then a VFD motor with some bolted on pf correction might make some sense.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

There are differences in the design, especially with regard to the rotor conductivity and rotor slot geometry, that are often tweaked for "inverter duty" motors (in quotes because there is no official definition of that term, so you have to be careful). In some designs, higher conductivity of the rotor bars (i.e. 100% copper, as opposed to alloys or aluminum) will result in lower losses and better response to step changes in load, which helps improve the overall performance in a VFD + motor combination. With the VFD, torque is capable of being so finitely controllable by the drive that excess torque capability in the motor is generally wasted, so they opt for the other response performance improvements. But that same motor used without a VFD could result in lower slip and lower starting and peak torque capability, albeit higher efficiency. You may end up with a mechanically poorer performing motor if you use it Across-the-Line. But as in all things, it depends greatly on the application. If it's for a centrifugal pump or fan for example, loss of starting torque or peak torque for step change in load response is largely irrelevant. If it's a conveyor, it could be hugely important.


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