Star Delta Starter

Hint: has it got anything do with the power available within the circuit, voltage drop, and the circuit's resilience to motors starting on it?

u giving me or asking hint?

Er, what part of the <...?...> is difficult to understand, please?

Think it through.

non english speaker here dear thats why asked "intent" of "?".

Starting current is already pretty low on MV/HV motors. (And, can you just imagine how big a Y-Δ MV/HV starter would be?)

Quite.

i saw a 4MW ,11KV compressor motor being started across the line. the whole 11KV busbar tripped on undervoltage. the guys then changed tapping of the 220/11KV transformer that fed the 11KV substation to bring voltage to 11.5KV. so starting current is not that low if equipment is large

4 MW motor at 11KV starting across the line are not applicable as the starting current are too high. Basically for these 4MW motor at 11KV, we use liquid starters to start. Motor below 400KW at 11 KV are aceptable to start across the line as the starting current are low.

yes , big cabinets reserved for four contactors for close transition will make star-delta very difficult and noneconomical.agreed

Star (wye) - delta starting is basically a scam to begin with. It works to a much lesser degree than most people realize and adds more problems than it solves. But it will technically satisfy requirements for reduced voltage starting because we collectively agree to ignore the evidence to the contrary and the negative consequences when used on low voltage motors, because people want a cheap solution. That's all it really offers compared to all alternatives.

Once you get into the world of MV motors, you have entered the realm of critical path machinery; meaning that which, it not functioning, generally means your entire operation shuts down and the economic risks to the company or entity border on catastrophic. So the little ignored details in LV systems such as severe torque spikes on transition, voltage dips because the transition current spike is equal to or greater than Across-the-Line starting current anyway, the rapid destruction of contacts in the contactors and the extreme mechanical stresses imparted onto the test of the drive train components, all add up to allow real engineers to take precedence over cost cutting bean counters.

If you need to truly reduce starting current and/or stating torque on a MV motor, use a solid state soft starter. It likely will cost you the same at MV compared to a star-delta starter anyway.

soft starter also does not maintain V/f=K ,i think. only option left is VFD.

You should understand the V/f control is only used because the frequency is variable.

On a variable speed drive, if volts were not reduced in proportion to Hz, the motor would be overfluxed - saturation, high current, strain on electronics.

If volts not increased with Hz, torque at 150% speed would fall to 100/150 rated - constant power behaviour.

A usefull "rule" is that for transfos at a constant frequency "the primary voltage determines the magnetic flux". Also, for a given inductance, current is proportional to frequency and flux is proportional to current.

With generators bolted to main step-up or unit auxiliary transformers, transfo overfluxing would occur below normal speed if V/Hz control was not applied. This causes mighty humming, mechanical vibration stress in transfo, tripping of differential relays and transient voltages going in/out of saturation. Note that on big generators, excitation comes on/off at about half speed with correct V/Hz ratio then, and at higher speeds.

Star/Delta starters do not vary frequency so V/Hz is not "in the picture".

Maintaining the V/Hz ratio is what a VFD does to MAINTAIN torque in the system.

Reduced Voltage Starting does change the V/Hz ratio, which reduces the torque, which is one of the REASONS to use it. That's why you cannot do it for long though.

normally in industry, when motor is started on reduce voltage , the low starting torque is handled by closing discharge valve of centrifugal pump,damper of air handling unit etc so that mechanical starting load imposed on motor shaft is low enough to prevent motor stalling.

We have at our factory five 2/ 0,5 MW 6kV motor for fan operation which has 3 switches to achieve pole scheme Y / YY .Last 30 years system for starting and motor works well and recent idea about their replacement with soft starter we refused .

Perhaps the problem is that they are really rare , bearing in mind that the higher consumption of copper for making windings and a small low cos φ at low speed .

Your Y-YY situation is different, that's a 2 speed motor. You likely always start it in low speed but that's not the same as a Y-delta start scheme.