Motor stalling limit

[Power0020]

I am carrying out a motor starting study and would like to know if any reference or standard states the allowable voltage drop on system during starting of other motors?

Stalling of running motors isn't easy to predict with different load c/c's and inertia. any figure of permitted voltage dip and duration?

 

[jraef]

No, because it is almost totally load dependent and based on the selection criteria for the motor in the first place. In other words the motor can be selected to allow for a severe voltage drop without stalling if the designer anticipates this as a potential problem, so without knowing that, you are left with assumptions.

But if you know the load torque requirements in detail, you can get the Break-Down torque values of the motor on its torque-speed curve, below which you are beginning to risk stalling. Then you can predict that since running torque will follow voltage drop in a fairly linear fashion, so you can plot out the point at which voltage drop will affect THAT load. This will not be applicable for every load in your facility, so to be of value on predicting the overall effect of starting a large motor, the only safe bet is to do them all and look at the worst case, because anything that tolerates the VD better than that will be fine.

All that said, the generally accepted value is -10% voltage (NEMA standards). In other words motors are supposed to be designed to provide rated torque at +-10% of nominal (design) voltage, then utilities are supposed to maintain +-5%, which leaves some fudge factor for line to load voltage drop.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Parchie]

What jraef said, +/-10% can also be seen on CBEMA curve for steady state voltage drop limits.

Also, there is an old paper by D. A. Lentz regarding "Equation that help determine the biggest motor that can be connected to utility lines". It says that the maximum allowable inrush current approximately equals the maximum allowable voltage flicker X the available fault current at the POC. That said, if you are allowed a VD of 5%, your motor should only be sized to draw = 5% X available fault current at the point of connection.

On CBEMA limits, the CBEMA curve shows that large "flicker" voltage does no damage to equipment if they doesn't persist for long; i.e. 500% for 0.01 cycle, or 200% for 1 msec, or 20% for 3 msec to 0.5 seconds!

 

[LionelHutz]

In practice I doubt actually stalling motors due to a low voltage ever becomes much of a concern. Most motors have a breakdown torque that is at least 2X their rated torque which means that the voltage has to dip down below 71% of nominal before the voltage drop is enough to potentially cause a motor to stall. At 70% voltage, you're likely going to have control circuit issues causing motors and other systems to drop-out.

There are utility standards for the service +/- voltage tolerances based on your location. There is also the typical +/-10% motor voltage tolerance. Control systems can start to be a problem at -15% to -20%.

 

[jraef]

Also, there is an old paper by D. A. Lentz regarding "Equation that help determine the biggest motor that can be connected to utility lines". It says that the maximum allowable inrush current approximately equals the maximum allowable voltage flicker X the available fault current at the POC. That said, if you are allowed a VD of 5%, your motor should only be sized to draw = 5% X available fault current at the point of connection.

Wow, I have never heard that before. Do the folks at SKM and ETAP know about this? It would be a serious threat to their TMS software sales if it became more widely known!

It was apparently published in 1965, I found a scanned copy on line, here is the link. Very interesting read.

http://files.engineering.com/downlo...2b27-40ae-9b57-e42347e2ae42&file=Scann001.PDF

Given that it was uploaded to the Engineering.com upload site, I wonder if this was discussed in this forum some time in the past and I missed it?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[7anoter4]

I think LionelHutz is right. “Control systems can start to be a problem at -15% to -20%.”
Nema limits are 85% to 110% for control.
According to IEC 60034-12 minimum breakdown torque will be 1.6 times the rated for rated voltage. That means at 80% the motor will stop.
To be in safety zone of curve Torque/ speed 90% is the limit for motor- in operation-terminal voltage-usually.
From experience-as in some short-circuit cases-the motor in operation could withstand 70% voltage for 1 sec [or even 0 volt in ATS for maximum 10 cycles], but it is risky, of course. It depends on how the motor is loaded and on voltage dip duration. The motor starting process could be even 12 sec. [or more] long.