WHAT DETERMINES STARTING CURRENT
WHAT DETERMINES STARTING CURRENT
(OP)
I have a 4.5MW 11kV 3ph 50HZ squirrel cage electric motor driving a gas compressor on an offshore platform. If the onboard GT is down the compressor will be run from a sub sea cable, I am told we cannot start the motor with the SS cable because the starting current is too high. Our specification called for a start current of 450%FLC.
Is the starting current on the motor influenced by the load it sees, i.e. if the compressors is depressurised for a re-start will the unit be more likely to start. Requests to vendors and in house electrical engineers have produced the following: The start current is given as a percentage of FLC and is related to the design power of the motor, when the power is determined the start current cannot be changed. One criteria to determine the motor power is the start-up torque on the shaft. This implies to me that the motor is sized to start with the compressor pressurised or not and once that decision has been made it the start current won't change. Am I right?
Is the starting current on the motor influenced by the load it sees, i.e. if the compressors is depressurised for a re-start will the unit be more likely to start. Requests to vendors and in house electrical engineers have produced the following: The start current is given as a percentage of FLC and is related to the design power of the motor, when the power is determined the start current cannot be changed. One criteria to determine the motor power is the start-up torque on the shaft. This implies to me that the motor is sized to start with the compressor pressurised or not and once that decision has been made it the start current won't change. Am I right?





RE: WHAT DETERMINES STARTING CURRENT
This big current causes a voltage drop on the cable. So the voltage at the motor terminals is reduced.
The motor torque is proportional to the squared terminal voltage. So a vltg decrease of 10% produces a torque decrease of 20%! If the voltage drop is too big, the motor could stall.
Concerning the cable, the starting current is more or less constant during the starting period, which usually lasts some secs for a compressor. Probably the starting current will not significantly heat the cable, but it must be verified.
RE: WHAT DETERMINES STARTING CURRENT
I can add the following to what Alex68 has said above.
The starting current is not affected by the load connected to the motor shaft but the starting duration is certainly affected.
To illustrate the point, if the compressor is depressurised, the torque demand on the motor is less and thus the accelerating torque available is correspondingly greater resulting in motor accelerating to full speed in a shorter time.
It can also be said that the torque being proportional to Square of V, the chances of motor accelerating to full speed (so to say successful start), even with higher voltage drop, are better if the motor is started with compressor depressurised than otherwise. This is because at lower terminal voltage, though the available torque from the motor is less it may still be adequate for a successful start.
RE: WHAT DETERMINES STARTING CURRENT
Under full voltage starting, the current is independant of the load. The start current is initially equal to Locked Rotor Current and remains high until the motor is at 85 - 90% full speed when the current begins to fall quickly.
Reducing the voltage during start with reduce the current (proportional to voltage reduction) and the torque (proportional to voltage reduction squared). The driven load determines the minimum starting torque required, and the rotor design determines the minimum current required to achieve that torque. see http://www.lmphotonics.com/m_control.htm and http://www.lmphotonics.com/m_start.htm
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: WHAT DETERMINES STARTING CURRENT
RE: WHAT DETERMINES STARTING CURRENT
http://www.joliet-equipment.com/glossary.htm
for definitions.
RE: WHAT DETERMINES STARTING CURRENT
RE: WHAT DETERMINES STARTING CURRENT
I remember reading that motors are designed to push leakage reactances into saturation during starting. I never understood what that meant. Wouldn't a saturated leakage reactance allow a higher current than an unsaturated leakage reactance?
RE: WHAT DETERMINES STARTING CURRENT
RE: WHAT DETERMINES STARTING CURRENT
RE: WHAT DETERMINES STARTING CURRENT