3.3kv auto transformer starter vs star delta

[power2020]

We have to design a starting circuit for 350kw pump motor. My client is asking for auto transformer starting, with 2pole contactor on star side of auto transformer.
However, as per my opinion after considering voltage drop calculations we can very well manage with normal star delta starting (it is 3.3kv system, moreover). Now my questions.

1) the main difference between the auto transfomer starting & the normal Star- delta starting.
2) If it is Auto transfomer starting.. what is the best option, whether it is 2 pole or 3 pole contactor, to keep at star point of the auto transformer? why?

i appreciate your inputs.

Thomos

 

[dpc]

Autotransformer starting provides highest starting torque per line ampere due to transformer action. Also has adjustable taps on the autoxfmr to allow different starting voltages at the motor.

Wye-delta starter will provide a starting torque that is only 25% to 30% of normal, so is used only for loads with very low starting torque requirements.

I'm guessing you are not in the US. Wye-delta RV starting is not very common in the US, actually.

Most attractive option would be a solid-state reduced voltage starter, if you need reduced voltage starting at all. The solid-state starter will probably be less expensive than the autotransformer starter.

 

[Marke]

A star/delta starter has ownly two possibilities, star connection and delta connection. In many cases, there is not sufficient torque available in star to accelerate the machine to full speed, so the transition to delta occurs at part speed. If you step to the delta connection (full voltage) at less than 90% speed, you are stepping to DOL start current and you have achieved nothing from the use of the star / delta starter.
Additionally, the star / delta starter is an open transition starter, so at the transition form star to delta, there is a very high current and torque transient that is significantly higher than any DOL transients. - you are worse off than DOL!!

The auto transformer starter gives a number of start voltage options, so the starter can be configured to provide sufficient current onto the motor to develop sufficient torque to accelerate the motor to full speed.
In many cases, this requires the 80% tap to get through the final acceleration to full speed, so the best result is achieved by the use of a two step auto transformer starter rather than a single step. - The autotransformer starter must be configured as a closed transition rather than open for any real benefit. (You must switch the star point of the transformer.)

Consider a solid state soft starter, it may be a better solution depending on your application.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

I never recommend Star (Wye) Delta starting, Marke has covered most of the issues I have with it. I have seen that transition torque transient break a shaft on a 500HP motor, also warp the fan blades on a 700HP compressor to where they hit the guard. Closed Transition starting will take care of some of the problems, but the additional cost makes it more expensive than solid state.

Solid State soft starters at 3.3kV will not necessarilly be cheaper than Autotransformer, but they will defnitely be smaller so if you have a floor space issue, that will help. Generally MV Soft Starter pricing is at par with RVAT, maybe a litle more. One benefit is that most brands available have all of the protection capabilities of a decent Motor Protection Relay already built-in, so that can help with the comparison cost if you were inclined to use one on the RVAT. There are other benefits to Solid State, especially if you are running a pump.

The only benefit to RVAT starting is the issue raised by dpc; that the transformer action provides more torque per line amp than any other method. So if you have a limited supply, RVAT starting sometimes becomes your only choice.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
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[power2020]

Thanks for you inputs.
Our induction motor (centrifugal pump load) rating is 380kW, 3.3kV. We would like to connect a capacitor bank across terminal box for power factor improvement. Motor will be started with an auto-transformer. pl. discuss pros & cons of the above configuration.

 

[jraef]

I can't think of any cons, other than it seems a bit over-kill, but that's your issue, you know more about the application. On a pump, Solid State would also offer you the benefit of "Soft Stopping" the motor, gradually reducing the output torque so as to avoid creating water hammer and thereby reducing strain on your piping system, valves and flanges. You can't do that with RVAT or any other electro-mechanical system. If that holds no value to you however, then RVAT would be fine.

 

[power2020]

If i connect a capacitor at motor terminal for power factor improvement(remember this is a auto transformer assisted starter, 3.3kv 280kw), Does somebody think any resonance problem occur? as this is mixed of inducatnce & capacitance, do i need to take any special precautions.
if yes what should i?

Thnaks a lot

 

[ScottyUK]

Your OP said it was a 350kW machine, now it is 280kW? Both these motors are small for a typical 3.3kV system. Is this a particularly weak supply where starting current is a major problem? For comparison, we routinely start 2000HP drives off the 3,3kV network at our plant. I know you said that the client is asking for it, but is it necessary for the duty?


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Sometimes I only open my mouth to swap feet...

 

[power2020]

Hi ScottyUK,
We have both types of motors..
I know these are small motors for 3.3kv systems.
Since there are number of motors, Client is insisting.
We have 280kw, & 380kw motors with auto transformer assiting.
For 280Kw motor rated for 0.85power factor.
KVA=330
KW=280
KVAR oborbed by motor for unity PF= 174KVAR
KVAR obsrobed by motor to operate at 0.95PF = 148KVAR

so for 280Kw motor rated at 0.85PF, we need to add a capacitor bank of 174-148= 26KVAR to bring the PF to 0.95.

Is this correct?

What about resonance while motor is under transient conditions?

thanks.