Motor Start Up Voltage Drop

[innersurffer]

I have a concern with voltage drop.

We have a 750' of number 8's with 10HP, 3Ph, 480Vac, Squirrel Cage Motor with an FLA of 14

2.81% Voltage Drop at full load FLA of 14Amps
17.31% Voltage drop at locked rotor FLA of 83Amps

I tried discussing with Baldor (the motor manufacture) if the pump will be able to get over the momentary locked rotor voltage drop.

At locked rotor the FLA puts the voltage at 397.

Will this be sufficient to get this pump going? :-?

Any help will be greatly appreciated.

 

[winnie]

It is impossible to answer without knowing what sort of pump is involved.

The way to answer this question is to compare the torque/speed curve of the motor, as adjusted for voltage drop with the torque/speed curve of the load. As long as the torque output of the motor is greater than the torque consumed by the load at any given speed, the motor will accelerate.

The torque output of the motor at any given speed will scale as the square of the voltage. Say, for example, that the nominal locked rotor torque of this motor is 75 foot pounds with 480V supply. Then with your reduced voltage the motor will develop about 51 foot pounds. As long as the 51 food pounds is greater than the starting torque of the load, the motor will accelerate from zero speed. (Note: I've just made up the motor torque numbers; you will need to use the real numbers for your particular motor.)

You need to check at several different speeds, to make sure that you don't hit some point that is below full operating speed, where the motor output torque and the load torque are equivalent. If this happens, then you will not accelerate to full speed, and the motor will quickly overheat.

Many types of pump have a torque which scales as the square of the speed, and will have no problem starting with the voltage drop. But if you have a pump with constant torque requirements, you will need to be careful.

-Jon

 

[bob]

Winne's post has the essential information you need. This site has some infomation you may want to read http://findarticles.com/p/articles/mi_m0BPR/is_10_22/ai_n15766075/
My calculation using a Pf of 0.50 gave a VD of 11% or 52 volts which would leave you 428 volts.

 

[innersurffer]

The application is for a lift station. It makes sense about the torque and speed curve of the motor. I will get with the manufacture and find out this info. Thanks again for pointing me in the right direction.

 

[iwire]

Is this just a typical centrifugal pump?

If so I have little doubt it will start rolling.

 

[Hameedulla-Ekhlas]

I have a concern with voltage drop.

We have a 750' of number 8's with 10HP, 3Ph, 480Vac, Squirrel Cage Motor with an FLA of 14

2.81% Voltage Drop at full load FLA of 14Amps
17.31% Voltage drop at locked rotor FLA of 83Amps

.

Maximum voltage drop for locked rotro is 15% not 17.31%. So, 17.31% drop is high

There are some points to be considered.

Among other things, three electrical parameter are absolutely crucial to the successful starting and running of any motor.



1- The starting(locked rotor) voltage drop at the controller(15% maximum drop allowed).

2- The service factor drop 115% drop

3- load current voltage drop at the motor 5% load