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Associate

Join Date: Mar 2012
Posts: 42

Medium Voltage (3.3 kV) Pump Motors

04/02/2012 3:47 AM

For low voltage (400 V), I normally specify DOL starters for motors below 10 - 15 kW and assisted starters such as star delta for higher kW's so as to limit the affect of the in-rush current on the utility grid.

My question is: what are the guidelines for the limit on DOL starers for 3.3 kV motors for water pumps?

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Pathfinder Tags: Medium Voltage Starters
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Power-User

Join Date: Nov 2011
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#1

Re: Medium Voltage (3.3 kV) Pump Motors

04/02/2012 6:02 AM
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Associate

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Posts: 42
#4
In reply to #1

Re: Medium Voltage (3.3 kV) Pump Motors

04/02/2012 10:17 AM

Thank you for providing the links for information on 3 phase starting systems. I am actually quite familiar with these systems.

However, my question was: what is the limit for DOL starters on 3.3 kV motors.

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Guru
Engineering Fields - Electrical Engineering - New Member United Kingdom - Member - New Member Engineering Fields - Electromechanical Engineering - New Member

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#2

Re: Medium Voltage (3.3 kV) Pump Motors

04/02/2012 7:45 AM

It's purely down to the capacity of your supply. 400V motors I've quite happily used DOL for 300HP, on 3.3KV 800HP. I wouldn't even consider star delta in most circumstances.

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Associate

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Posts: 42
#5
In reply to #2

Re: Medium Voltage (3.3 kV) Pump Motors

04/02/2012 10:24 AM

Thank you for your time to respond to my question.

I fully agree with you that it depends on the capacity of the supply. However, where the supply is restricted and the utility company insists that above 10 kW motors shall be provided with assisted starters, then we need to be inovative.

By simple proportion, the current for a 10 kW motor on 400 V will be the same as for a 82.5 kW motor on 3.3 kV. So, where there is a restriction, I suppose, we could go up to 82.5 kW on 3.3 kV motors or not?

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Guru

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#6
In reply to #5

Re: Medium Voltage (3.3 kV) Pump Motors

04/02/2012 11:01 AM

You cannot have an 82.5kW motor in 3.3kV Voltage rating. Vide Cl. 8.1.1 of IS 325, the minimum rated output for a motor to be at 3.3kV is 100kW.

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Power-User

Join Date: Nov 2011
Posts: 131
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#9
In reply to #5

Re: Medium Voltage (3.3 kV) Pump Motors

04/03/2012 1:32 AM

Smooth starting of motor is guided mainly by two things - voltage drop during starting and inertia of the mechanical ssystem with which the motor is coupled.

In a weak power system with low fault level when the motor is started, due to flow of high starting current (typically 6 to 7 times) there will be appreciable voltage drop at the motor terminal due to combined effect of high source impedence (due to low short circuit level), transformer impedence, cable impedence etc. The problem will further aggrevate if the load inertia (G D square value) is high. Because of less voltage available across the motor terminal it will develop less torque ( torque being proportional to voltage square) which will not be adequate to accelerate the motor. Hence accelerating torque will be negative ( load torque being more than the torque developed by the motor) and the motor will stall.

For assisted starting it is common practice to use either star-delta starter/ reactor starter or electronic soft starter to reduce voltage during starting. However, if load inertia is high as explained above you have to use VFD which can maintain same torque by reducing both voltage and frequency ( maintaining v/f constant).

In your case as it is pump application, load inertia will be less. Also power supply is fed at 3.3 kV, so available short circuit level presumably will be adequate. With these two considerations the DOL starting should not be a problem. There is no such thumb rule for DOL starting vis-a-vis assisted starting. Decision should be based on above considerations on case to case basis.

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Associate

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#10
In reply to #9

Re: Medium Voltage (3.3 kV) Pump Motors

04/03/2012 2:07 AM

Thank you for your time in providing an excellent explanation for motor starting.

I have one more question: in what ranges would you put low fault level and high fault level?

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Power-User

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#12
In reply to #10

Re: Medium Voltage (3.3 kV) Pump Motors

04/05/2012 12:10 AM

I reiterate that this has to be determined on case to case basis. Low fault level in the power system may be adequate for starting a pump. But it may pose problem for starting a fan ( with high inertia). Hence 'low' and 'high' are relative terms - there is no rule of thumb.

Thanks.

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Guru

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#3

Re: Medium Voltage (3.3 kV) Pump Motors

04/02/2012 10:14 AM

It is not always that you must go for star-delta starters for motors of capacity 10kW and above. Only when the voltage drop during DOL starting of such a motor is going to affect any neighbouring motor or any neighbouring customer conneceted to the same LV bus, you opt for such a starting. If there are no worries about such voltage drop affecting other loads, then one need not go for any method reduced voltage starting. One can very well go with direct-on-line starter. In my experience, there are instances where I have designed DOL even for a 300HP motor and star-delta starter for a 10HP motor.

If such an effect is anticipated, then even a 3.3kV motor needs to be started with reduced voltage starting, albeit not with star-delta, but with a soft starter.

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Guru
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#7

Re: Medium Voltage (3.3 kV) Pump Motors

04/02/2012 12:03 PM

The mistake you are making is that you are focusing on inrush* current. It is not starting current alone that is the determining factor, it is APPARENT POWER that it represents for starting a motor. So when, in your 400V systems, the utility requires reduced voltage starting at 10-15kW that is because they know that the kVA represented by that motor power is going to tax the transformer feeding it to the point of causing an unacceptable voltage drop in the entire system.

When you move to MV motors, they are not fed from the same transformer, ostensibly they are being fed from a much larger transformer. Therefore you cannot equivocate the LV starting requirements to the MV starting requirements, they are apples and oranges.

So back to the original responses, there is no "right" answer that can be universally applied, it is completely dependent upon the requirements estanblished by YOUR local utility and for them, those requirements will be based on what THEIR system can provide in terms of starting kVA without adversely affecting the voltage.

* Side note: you are making a very common mistake when discussing motor starting current by referring to it as "inrush" current. Inrush current is the very brief current flow that establishes the magnetic fields in the stator windings before the rotor interracts with it to provide back EMF. It only lasts a few cycles at most and is much higher than the typical 600% "Starting Current" peak of a Design B motor.

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Associate

Join Date: Mar 2012
Posts: 42
#8
In reply to #7

Re: Medium Voltage (3.3 kV) Pump Motors

04/03/2012 1:08 AM

Thanks for that excellent response. It is the starting kVA (apparent power) which is the deciding factor and not just the current.

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Guru

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#11
In reply to #8

Re: Medium Voltage (3.3 kV) Pump Motors

04/03/2012 11:29 AM

You can get the opinion of the utility(service provider) whether the strength of the agreed capacity can handle the starting/inrush currents.

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pnaban
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#13
In reply to #8

Re: Medium Voltage (3.3 kV) Pump Motors

04/17/2012 10:50 AM

Mohamed ,

I rarely have seen 6 wires from the motor on that voltage level which is first requirement to think about star - delta starting.
Second three MV breakers are robust and expensive for starting that motor. If I were you I should buy soft starter and probably solve all problems. .

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