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Join Date: Dec 2006
Posts: 88

Capacitor bank for Motor drivers

06/18/2007 2:07 AM

Dear Sir,

In our factory we have 65 HP DC motors with VSD drivers. But some time VSD does fail to operate the motor due to dip in voltage in the system. Can I use a capacitor bank to keep the voltage at steady state for proper operation of the driver? Who can I select the capacitor bank size?

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Active Contributor

Join Date: Jun 2007
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#1

Re: Capacitor bank for Motor drivers

06/18/2007 3:08 AM

Shunt Capacitor banks usually used for power factor improvement/ elimination of harmonics from the supply system. Voltage dip in the supply system usually arises due to load fluctuations in the grid but still using of capacitor banks will save your power and stabilize your supply system.

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#2
In reply to #1

Re: Capacitor bank for Motor drivers

06/18/2007 6:54 AM

Dear Sir,

Could you please guide on selecting capacitor bank size to avoid voltage dips.

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Active Contributor

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#3
In reply to #2

Re: Capacitor bank for Motor drivers

06/18/2007 7:45 AM

Selection of capacitor banks is most easy providing that the existing power factor and KVAR absorbed by your system are measured.capacitor banks will be selected interms of reactive power (KVARs) which can be directly derived from the power tables.

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Guru

Join Date: May 2007
Location: Geelong, Australia
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#4

Re: Capacitor bank for Motor drivers

06/19/2007 1:59 AM

Yes you can in principle, many VSD's have terminals that are connected to their internal DC bus. The energy stored in a capacitor is .5*C*V^2 (half C V squared). The usable energy is in the difference between the normal bus voltage and the minimum voltage necessary for the VSD to function. Plugging some numbers in shows that you'll need a huge amount of capacitance to provide even a few seconds of protection. Jeff

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#5
In reply to #4

Re: Capacitor bank for Motor drivers

06/19/2007 2:27 AM

Dear Sir,

Thank You very much for your help.

The Problem we are facing is malfuctioning of the 60 HP DC motor driver (415 V AC input) even during a small dip in the voltage. What can we do to maintain steady voltage for motor driver?

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Guru

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#6
In reply to #5

Re: Capacitor bank for Motor drivers

06/19/2007 2:37 AM

What sort of dip, is it dropping all the way to zero? How long does it drop for?

Also, check if the VSD monitors the input voltage, it may be faulting when it sees the power is failing. What brand and model is the drive? Thanks Jeff

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United States - Member - Florida Engineering Fields - Power Engineering - Power Systems Studies, Arc Flash, Power Quality, Generation Safety - ESD - Arc Flash Hazard & Training

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#7

Re: Capacitor bank for Motor drivers

06/19/2007 6:58 AM

Be Careful here!

You may have other issues in your power system. Can you install a power quality monitor nearby with date stamping? You need to know what is going on in your system and make corrections to your system. Are you familiar with the ITI curve? Your drive should be able to ride through anything inside this envelope. If it does not check with the drive vendor on adjustments you can make to the drive to handle the ride through. Change or installing capacitors without understanding the problem can lead to even more troubles with harmonics.

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#8

Re: Capacitor bank for Motor drivers

06/20/2007 3:47 PM

************** Quote *********************

... DC motors with VSD drivers ...

**********************************

This requires DC Filter Capacitor Bank of thousands µF to filter Ripples in rectifier O/P

And

support In-Rush current taken by a motor @ start; if it is not thru a "Soft-Starter" .

If the the motors are at different loctios & you have a single-DC-Power-Source then it will be better to have individual Capacitor Banks close to the motor-start-gear.

adam #1 & #3

************ Quote ****************

Shunt Capacitor banks usually used for power factor improvement/ elimination of harmonics from the supply system.

**********************************

The Question is " ... DC Filter Capacitor Bank ...

You are refering to Power-Line [AC Ditribution system about Power-Factor-Correction the Capacitor-banks connected on to off-set the Over-all-Lagging Loads; corrected by introducing leading PF load.

This does not increase the load on Power-Source but cancells some of the Lagging-load & causes to increase PF.

More over these "Capacitor-banks " do not supprt the power lines in decreasing Dips in power-lines except the purpose described by you ie improving of PF for example 0.5 towards 0.7 or better [towards UNITY PF which is an ideal but never attainable]

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#9
In reply to #8

Re: Capacitor bank for Motor drivers

06/21/2007 3:14 AM

it is clear from the question that the motor is a DC motor and not the power supply, in other words the DC is obtained by introducing rectifier units to supply the motor, inwhich case shunt capacitors could be used to filter the AC upstream of the rectifier unit leading to improvement of power factor (lagging) and stabiliziation of power system. Certainly capacitor banks do not decrease the power Dips which might arise due to load fluctuations.

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#10

Re: Capacitor bank for Motor drivers

07/03/2007 9:11 AM

Try Elspec products:

The world fastest power factor correction system- the Equalizer

http://www.elspec-ltd.com/default.asp?V_DOC_ID=1865

Power quality analysis- the Elspec G4400 –provides you with the extraordinary ability to precisely determine why a power event occurred and what caused it. (Continues onboard logging of all power parameters for a year+ at 1,024 samples per cycle resolution)

http://www.elspec-ltd.com/default.asp?V_DOC_ID=2239

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