Locked rotor

[Manoj]

In locked rotor test we have achieved two different type locked rotor current of different same hp motor. so, please tell me what is the better motor, Less current or high current for submersible motor.

Manoj Maheshwari

 

[tom baker]

A locked rotor test is not a test I have ever seen done. What type of pump is it? How do you perform the test?

I would recommend looking at the overall efficiency. Look at the pump curve, and what is the sweet spot, the submersibles I have worked on have a BEP, Best Efficiency Point.
Compare the current to the pumping rate, preferably after the drawdown has stabilized.
Often centrifugal pumps are incorrectly sized or the system head changes, so they end up pumping off the curve.

 

[Manoj]

tell me which motor is better less current or high current in locked rotor test.

 

[tom baker]

Please describe how the locked rotor test is done. As I pointed out that may not be the best evaluation of a motors efficiency. Locked rotor current would be related to break away torque. This is not an area I am familiar with

 

[Besoeker3]

In locked rotor test we have achieved two different type locked rotor current of different same hp motor. so, please tell me what is the better motor, Less current or high current for submersible motor.

Manoj Maheshwari

Assume this is a three phase rotor cage motors?
If so, I have quite a lot of experience of these. In all cases, the locked rotor was just one parameter so I'm not you mean by two different values.
Care to elucidate?

 

[Manoj]

we have locked rotor of motor then gave power to motor. Panel show the current and power of locked rotor.

 

[retirede]

Locked rotor current is not a direct indicator of what constitutes a superior machine. If that’s what you’re getting at.

For example, a motor designed for high starting torque will generally have a higher locked rotor current than a motor with standard starting torque - all else being equal. That doesn’t mean that a high starting torque motor is superior to the other one. If the application doesn’t require the higher starting torque, it’s a waste of money.

 

[tom baker]

Locked rotor current is not a direct indicator of what constitutes a superior machine. If that’s what you’re getting at.

For example, a motor designed for high starting torque will generally have a higher locked rotor current than a motor with standard starting torque - all else being equal. That doesn’t mean that a high starting torque motor is superior to the other one. If the application doesn’t require the higher starting torque, it’s a waste of money.

Thats what I was getting at, I was going to mention NEMA designs but didn't think the OP would use that standard

 

[tom baker]

And submersibles are generally pretty easy starting, unless there is a very high head on the motor.

 

[Besoeker3]

we have locked rotor of motor then gave power to motor. Panel show the current and power of locked rotor.

Not all the motors are the same. This is one at locked rotor:

Current PU 5.75
Torque PU 2.4

 

[Julius Right]

From installation point of view the less start current will be better since the voltage drop will be less and you don't need to increase the cable section in order to reduce the impedance. Depends on the installation conditions-if it is strong enough to withstand the current the less start current is better.
In any case if the source-transformer, generator or else-is large and the motor is small the start current is not important.
However, from the starting diagram, the difference in start current it could be due a small difference in supply voltage or voltage total drop [it could be the difference in transformer or supply load or else].

 

[tom baker]

Again, look at the pump curve and the peak efficiency point for the different motors and pumps

 

[kwired]

Locked rotor current is not a direct indicator of what constitutes a superior machine. If that’s what you’re getting at.

For example, a motor designed for high starting torque will generally have a higher locked rotor current than a motor with standard starting torque - all else being equal. That doesn’t mean that a high starting torque motor is superior to the other one. If the application doesn’t require the higher starting torque, it’s a waste of money.

I agree, each motor in OP may have situations that it is better for vs the other motor. This locked rotor current is only one piece of a puzzle.

 

[drktmplr12]

not enough information to say which is better. in any case, like everything, it depends on the application.

 

[bwat]

The question is like asking which car is better when all you know is the length of the car.

 

[Besoeker3]

Again, look at the pump curve and the peak efficiency point for the different motors and pumps

Yes but the question was about locked rotor current, not peak efficiency. That said, I think Manoj is maybe at a different tangent.

 

[kwired]

The question is like asking which car is better when all you know is the length of the car.

Or displacement of the engine. Other factors can make a difference and it still depends on what results you are prioritizing.

 

[gar]

210225-1456 EST

Manoj:

See https://en.wikipedia.org/wiki/wound_rotor-motor

The above is not a very good discussion, but does provide some information. In this discussion external resistors are used with a low resistance wound rotor to provide essentially a constant motor except for the ability to adjust rotor resistance during operation.

In an Internet search I am not coming up with what I consider a good simplified discussion on induction motor rotor resistance.

.

 

[Besoeker3]

210225-1456 EST

Manoj:

See https://en.wikipedia.org/wiki/wound_rotor-motor

The above is not a very good discussion, but does provide some information. In this discussion external resistors are used with a low resistance wound rotor to provide essentially a constant motor except for the ability to adjust rotor resistance during operation.

In an Internet search I am not coming up with what I consider a good simplified discussion on induction motor rotor resistance.

.

Again, I have quite a lot of experience on wound rotor motors. Many are in the field of Static Kramers and some are simple variable resistance stages. But the topic here is Manoj discussing the locked rotor - not rotor resistance.

 

[gar]

210225-2206 EST

Besoeker3:

Rotor resistance is very much a part of the question on locked rotor.

For everything else held constant on an induction motor the amount of rotor resistance will determine locked rotor current, and this also influences the locked rotor torque.

.