DrDrreeeaaa
Electrical
- Apr 25, 2008
- 266
Hi All
So I have a client with several three phase induction motors ranging from 220kW to 335kW. The motors are 2 pole star connected and were built in around 1985. They are started DOL at the moment. The client's supply authority has read him the riot act and he has asked us to design a replacement MCC for him that will include soft starters for the existing motors.
The motors are driving blowers which I consider to be 'high inertia' loads (I am not sure if this classification is relatively correct, but anyway). A DOL start on these things ranges from 8 to 15 seconds for the 220kW to 335kW respectively.
Now, the issue I have is determining the best process to specify the new soft starters. My technical design requirement for the reduced voltage start was going to be a current limit start at %300-%400 FLA but I haven't based this on any quantifiable process (i.e. relating the desired voltage dip to the motor starting current).
The client has zero information on his plant and there is no documentation for the existing motors (speed-torque curves, etc.). All I have is the existing plant in front of me and whatever observations I can make from that.
I have gathered some very rough 'rules of thumb' from technical references around the place which, when summarized, basically say that, for a constant load inertia, 50% starting voltage gives rise to %400 start time. So, considering 50% of the DOL start voltage (i.e. %400 current limit, assuming %800 DOL start current) will lead to worst case 60 second start time (400% of the worst case DOL start time of 15 seconds). I have two problems with this:
1) The customer has specified AB SMC flex starters which have a maximum start time of 30s (plus a 10s stall time) before tripping out
2) The process/reasoning I have applied, summarized above, I dont believe would stand up to any engineering scrutiny.
3) This doesn't give me any idea of wether or not the motor will be able to apply the required initial torque to start the load at the reduced voltage
So, can anyone give me any quantifiable tips, processes, calculation references etc. that I can use to extrapolate data from the plant in front of me in order to better specify the soft starter and guarantee it's performance at the required reduced voltage. Keeping in mind I have no technical data on the motors, loads, etc.
Maybe an approximation/rule of thumb based on measured DOL start currents and durations? I am pretty desperate as you maybe can tell.
In an ideal world, what information would you ask for to deliver this solution, assuming any information was available?
So I have a client with several three phase induction motors ranging from 220kW to 335kW. The motors are 2 pole star connected and were built in around 1985. They are started DOL at the moment. The client's supply authority has read him the riot act and he has asked us to design a replacement MCC for him that will include soft starters for the existing motors.
The motors are driving blowers which I consider to be 'high inertia' loads (I am not sure if this classification is relatively correct, but anyway). A DOL start on these things ranges from 8 to 15 seconds for the 220kW to 335kW respectively.
Now, the issue I have is determining the best process to specify the new soft starters. My technical design requirement for the reduced voltage start was going to be a current limit start at %300-%400 FLA but I haven't based this on any quantifiable process (i.e. relating the desired voltage dip to the motor starting current).
The client has zero information on his plant and there is no documentation for the existing motors (speed-torque curves, etc.). All I have is the existing plant in front of me and whatever observations I can make from that.
I have gathered some very rough 'rules of thumb' from technical references around the place which, when summarized, basically say that, for a constant load inertia, 50% starting voltage gives rise to %400 start time. So, considering 50% of the DOL start voltage (i.e. %400 current limit, assuming %800 DOL start current) will lead to worst case 60 second start time (400% of the worst case DOL start time of 15 seconds). I have two problems with this:
1) The customer has specified AB SMC flex starters which have a maximum start time of 30s (plus a 10s stall time) before tripping out
2) The process/reasoning I have applied, summarized above, I dont believe would stand up to any engineering scrutiny.
3) This doesn't give me any idea of wether or not the motor will be able to apply the required initial torque to start the load at the reduced voltage
So, can anyone give me any quantifiable tips, processes, calculation references etc. that I can use to extrapolate data from the plant in front of me in order to better specify the soft starter and guarantee it's performance at the required reduced voltage. Keeping in mind I have no technical data on the motors, loads, etc.
Maybe an approximation/rule of thumb based on measured DOL start currents and durations? I am pretty desperate as you maybe can tell.
In an ideal world, what information would you ask for to deliver this solution, assuming any information was available?