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Brelvi
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Join Date: May 2009
Posts: 12

Bank Capacitor or Soft Starters

11/21/2011 2:30 AM

At one of our Water Pumping Station, we have motors of 3.3 kV & 6.6 kV, 750 kW & 1330 kW. Their Capacitor banks are defective and we are planning to replace them.

Someone suggests that instead of replacing the Capacitor banks, it will be more beneficial, economical and technically viable to induct the Soft Starters in the system.

What our experts say in this regard, we will be extremely thankful to all who suggests.
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SolarEagle
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#1

Re: Bank Capacitor or Soft Starters

11/21/2011 2:48 AM

This should help...

http://en.wikipedia.org/wiki/Motor_soft_starter

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PWSlack
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#2

Re: Bank Capacitor or Soft Starters

11/21/2011 3:15 AM

It depends upon the tariff for electricity. Is it "peak demand"? Is there a penalty for poor power factor?

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electricalexpert65
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#3

Re: Bank Capacitor or Soft Starters

11/21/2011 6:25 AM

I am doubtful, whether softstarters can replace power factor improvement capacitors. Their purposes are different.

One might argue that the (displacement) power factor of semi-conductor devices is close to unity and as such no cpacitors are required when they are in circuit.

But, one must also remember that in soft starters, post starting the semiconductors are by-passed.

Alternately, if one wants to keep the semiconductors in circuot post-start also to gain the advantage of a better power factor, the watt losses in the thyristor sof the soft starter would be huge and it would definitley offset any monetary advantge gained on pf improvement.

So, please undertake a thorough analysis.
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marcot
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#7
In reply to #3

Re: Bank Capacitor or Soft Starters

11/22/2011 9:23 AM

Allow me to add a few details to your comment as you are giving a false impression on the effect of the soft starter on a motor.

Soft starters do not improve the power factor of the motor once running and probably not even when starting. At 100% firing, the PF is only function of the motor and its load.

During the start, the voltage reduction is done by delaying the firing angle which causes a lagging power factor in a resistive load. I don't see how it would improve the power factor of a motor. The reduced voltage might improve the motor's power factor but this voltage is lagging from the main voltage. The net effect will be a bad power factor anyway.

The soft starter also produces a lot of harmonics during the start.

Regards.

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JRaef
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#4

Re: Bank Capacitor or Soft Starters

11/21/2011 9:19 AM

As the others have said, it will totally depend on the purpose of the capacitors. If they are for power factor correction, a solid state soft starter will serve no purpose in that regard whatsoever. However if they are being used in Capacitor Assisted Starting, then a solid state soft starter will vastly improve the situation. Start by investigating the real purpose of the capacitors, you need to know from whence you begin.

One thing to also consider however is using VFDs on those pumps. If they are centrifugal (quadratic) pumps and during the course of operation you are reducing the flow from those pumps in any way, using VFDs instead will save a lot of energy. They usually save enough to pay for themselves in short order, but the payback period will vary by the amount of reduced flow operating time. If however these pumps always run at full capacity, do not even consider this option.

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Bazzer Englander
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#5

Re: Bank Capacitor or Soft Starters

11/22/2011 4:41 AM

I would think that soft start would be beneficial, the instant start provided by caps puts a lot of strain on the impeller shaft & the impeller itself.

Bazzer.

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Snel
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#6

Re: Bank Capacitor or Soft Starters

11/22/2011 5:58 AM

As stated before, soft starters will make sense for replacing starting capacitors only.

In fact the power factor is even worse while a soft starter is ramping up. Power factor correction capacitors must be switched off during this period or else the soft starter will be damaged.

Variable frequency drives would be useful if you have an advantage in modulating the flow or pressure of water. But VFDs are much more expensive than soft starters in this power range.

With such high powers and not being a low voltage application you need assistance to plan carefully and go deep in your calculations before taking a decison.

brgds

Snel.
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Bazzer Englander (1); electricalexpert65 (1); JRaef (1); marcot (1); PWSlack (1); Snel (1); SolarEagle (1)

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