750hp, 460vac Motor Starting At Star-delta

[nightfox1925]

I have encountered a plant vendor offering their plant design with a distribution voltage of 480VAC, 60Hz. They have submitted SLDs in their bid and specified the maximum plant load to be 3MW and specified 2 x 1.5MW Co-generation Steam Turbine Generators rated at 480VAC with no utility interconnection. What rattles me a bit is that there is a cane shredder motor rated at 750HP, 460Vac connected to a star-delta starter and some couple of 200HP motor driving some fans for the cooling towers.

 

Is a cane shredder considered a high inertia load and the Y-D starter may not be able to provide enough driving torque for the shredder? What type of starter is recommended to reduce the starting current and maintain required torque so as to drive the load and at the same time maintain voltage drop levels and avoid overloading the STGs during starting? Is a soft starter going to be sufficient. The cane shredder shreds sugar can baggase after sugar cane is cut and chopped. Help please..

[Jeremy]

I'm inclined to agree with your feelings of being rattled, I wouldn't describe a cane shredder as being a high inertia load, more medium. I would be more inclined to use a VF or more probably a soft start type starter for the application. Direct on Line at these sort of loads is quite demanding on plant life, increased maintenance, mainly with contactors and a decreased motor life due to the high starting currents involved. Admittedly DOL is cheaper initialy and considerably more simple for fault diagnosis, I feel that an increased initial spend would prove worthwhile in the longer term.

I have encountered a plant vendor offering their plant design with a distribution voltage of 480VAC, 60Hz. They have submitted SLDs in their bid and specified the maximum plant load to be 3MW and specified 2 x 1.5MW Co-generation Steam Turbine Generators rated at 480VAC with no utility interconnection. What rattles me a bit is that there is a cane shredder motor rated at 750HP, 460Vac connected to a star-delta starter and some couple of 200HP motor driving some fans for the cooling towers.

 

Is a cane shredder considered a high inertia load and the Y-D starter may not be able to provide enough driving torque for the shredder? What type of starter is recommended to reduce the starting current and maintain required torque so as to drive the load and at the same time maintain voltage drop levels and avoid overloading the STGs during starting? Is a soft starter going to be sufficient. The cane shredder shreds sugar can baggase after sugar cane is cut and chopped. Help please..

 

[jraef]

Depending on your process, a cane shredder is essentially just a hammer mill and is typically started with no material in the hopper. That being the case, the starting inertia is relatively low compared to the running HP requirement. In fact, a lot of hammer mills use two smaller motors for convenience and are often started with only one motor on a soft starter first, then the 2nd motor is added in DOL once the mill is at full speed. That means that we know it can be started with reduced torque from 1/2 the connected HP.

 

So from that standpoint, a Y-Delta starter should work, I just think they are the nastiest way to start a motor, often even WORSE than DOL! A solid state soft starter is a much more smooth and forgiving method and usually comes with other benefits such as advanced motor protection features, communications to industrial networks etc. so that you can someday, if not now, connect it to a plant-wide control system. Adding those things to Y-Delta starters pushes their cost closer to being the same as SS, so why not avoid the risks of damaging your machine from Y-Delta torque shock?