Wye-Delta Starting Under Load

[Coll]

I have been asked to size a generator to start two 75HP motors at star-delta(wye-delta), starting on-load and at the same time.

The machine is a wood-shredder so I assume that there is a high starting torque requirement.

It seems to me though that a star delta start (with open transition) might not be suitable for starting under load.

Any comments or advice will be much appreciated.

Colin

 

[waross]

The cost of a soft starter will be offset by the saving in generator cost. You will be able to use a much smaller generator with a soft start. You may be able to cut the generator size in half with the use of soft starting.
respectfully

 

[LionelHutz]

The wood shredder (chipper??) is probably an inertia load which requires little power to keep it running once at full speed. If so, then the starters will work OK because they will be able to transition at full speed. If not, then you need to better define the load.

If the motors are fully loaded when full speed is reached then the Y-D starter will likely reduce the current very little compared to a full voltage starter. They will have to transition at a lower speed where the full voltage current will still be quite high.

Once you really understand the load and the motor starting requirements then you can start to size the genset.

 

[baxtersdad]

Wye-Delta starting is primarily used to start high inertia loads. A loaded wood shredder would probably qualify.. Problem is that the motor torque is reduced to 1/3 of locked rotor torque when compared to direct on-line start. If the load torque requirement exceeds this value then you have a problem. Motor current is limited to 2x FLA for the starting period so your combined load would be ~400 amps at 480V. That is a pretty big surge current for your generator.

Soft starters are ok if your start period is brief. You get about the same torque as you would with Y-D. I haven't seen many soft starters that would handle an extended acceleration period without significant oversizing. So the questions are how quickly does the load need to reach normal speed and what is the available overload capacity of the generator?

Looks like it will take one big CAT.

 

[badservo]

I don't know if this helps but we have a shredder with 2 60hp motors, we star-delta start first motor, then dol second motor (motors mechanically connected), all with NO LOAD from wood (empty hopper). We have a 200kw gen. that can run the shredder and also runs 30 to 40kw of other load at the same time. Our shredder uses a hydraulic ram to feed the wood and load sensing on main motors to control ram advance, the start-stop action of the advancing ram causes a rather large load fluctuation for the generator and causes lights to dim and brighten slightly.
My opinion, USE A LARGER GENERATOR.

 

[Coll]

Guys - Many thanks for the input,

Colin