A question about the Induction Motor's startup curves, PF and LF

[majesus]

I am trying to grasp motor performance and startup characteristics correctly:

The induction motor's starting characteristic plot is shown here:

http://www.lmphotonics.com/m_control.htm

Here are some concepts that I'm trying to understand
Concept 1)
The PF, Current and Torque characteristics are independant of the motor loading. Ie, if the motor was not loaded, 100% loaded or overloaded, all the curves' magnitude will still be the same. However, if the motor was overloaded (assuming terminal voltage is constant and the motor could still start), then the I, Torque and pf curves will just be stretched out more to the left, ie take longer to reach the full load operating point.

Concept 2)
This plot is only acceptable for start up study. The reason I say this is because the power factor (dashed line) is shown to increase as speed increases, even after the break down torque. However, when a motor is at its running speed and fully loaded, (say a 400 hp motor, 100% load with a pf of 89.) Then if the motor was loaded to 125%, its torque and power factor will also increase, but its speed will decrease. As shown here in the performance table of a typical 400 hp motor:

http://www.reliance.com/pdf/pdf/aced/W06349-A-A001.pdf

What's confusing is in the start up characteristic plot, as the speed decreases, the pf also decreases. That's the opposite. Why?

 

[jraef]

The dashed line on Marke's plot (lmphotonics.com) is not pf, it is typical load torque requirements. it also is not plotting HP, so you cannot make any judgments from that on overloading issues.

The reliance chart is showing 2 separate operating conditions. The upper plots are showing running conditions plotted against HP on the horiz. axis, the lower shows starting conditions plotted against speed on the horiz. axis. Don't confuse the two, they do not mix.

 

[majesus]

Wow, Perfect, thank you very much.

I am curious to see how the power factor curve would look like at starting condition. Is there a plot that shows that?

 

[dpc]

I don't have a curve, but at locked rotor, the power factor of a starting motor is extremely low, maybe 0.15. As it speeds up the change in power factor is inversely proportional to the slip, I would guess. If you look at the motor equivalent circuit, the rotor resistance is a minimum at locked rotor when slip = 1 and increases as the slip decreases.

 

[majesus]

Makes sense.

In Marke's plot

http://www.lmphotonics.com/m_control.htm

The motor operating point will be where the Load Torque Curve intersects the motor's start torque?

 

[LionelHutz]

A note about point 1. The lmphotonics curve shows the torque and current vs speed. This curve will never change no matter what load is on the motor shaft. If the load is greater than the torque the motor produces at 0 speed then the motor never accelerates. To expand further, create a load torque vs speed curve and if it ever goes above the motor torque vs speed curve the motor will quit accelerating at the speed where the curves cross.

The power factor peaks somewhere around the motor loaded full-speed operating point. The power factor curve looks a little like the torque curve. It starts low, say 0.2, and then rises up towards the loaded pf, say 0.89, at rated operating speed and then falls back down as the motor goes above operating speed, or the area where the motor operates when not loaded. Of course, values given are examples, not fixed numbers.