Induction motor DOL starting characteristics
Induction motor DOL starting characteristics
(OP)
Good afternoon , A colleague and I have been asked to install a compressor with a 3.3kV 1MW motor as its prime mover. We have been chasing the manufacturer for the typical Starting characteristic. We have been assured that the compressor can only start in an unloaded condition. That would suggest that the starting current would be between 3 and 5 times full load current but for a short time maybe 1 to 2 seconds. The issue being the potential Volt drop that the starting current may induce. Is the suggested starting time accurate or are we potentialy looking at an extended starting period.





RE: Induction motor DOL starting characteristics
Bill
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"Why not the best?"
Jimmy Carter
RE: Induction motor DOL starting characteristics
The starting time depends on the acceleration torque, motor rpm and inertia moment of the motor and the load. See-for instance:
http://www05.abb.com/global/scot/scot259.nsf/verit...
section 4.3.3 Starting characteristics
ts=2*pi()*rpm/60*(JM+J'L)/Tacc ts =starting time [sec]
JM=motor inertia moment [kg*m^2 ] JL=load inertia moment [kg*m^2]
J'L=JL*(rpmLoad/rpmMotor)^2 if there is here a gearbox.
Tacc=Tmotor-Tload acceleration torque[Nm]
Tmotor=0.45*(Ts+Tmax) approx. Ts=start torque Tmax =maximum torque
Tload=kl*Tratedload Tratedload=Tnmot=rated motor torque [usually].
Kl=1/3 for centrifugal compressor or pump.
Let's take an ABB motor HV Cast iron motors 3 KV 1000 KW 600 rpm .See:
http://www05.abb.com/global/scot/scot234.nsf/verit...
JM=140.6 kg.m^2 Tmax/Tn=1.9 Ts/Tn=0.7 Tn=16006 Nm
Tmotor=0.45*(1.9+0.7)*16006=18727 Nm Tload=16006/3=5335 Nm
Tacc=18727-5335=13392 Nm
ts=2*pi()*600/60*(2*140.6)/13392=1.32 sec.
RE: Induction motor DOL starting characteristics
However, the voltage drop remains the same value but for a lessor time. Here, by code, I must use 6 times normal current unless I can justify a lower starting current. I see that the starting current ratio for the ABB motors shown in one of 7anoter4's post run from 5 times to 5.5 times normal current. Nice information, 7anoter4, thanks.
I am not shy about suggesting to a sales rep that if such information is not immediately available then possibly I am dealing with the wrong company. I started with no patience and I don't have much of it left. grin
Bill
--------------------
"Why not the best?"
Jimmy Carter