CONTROL OF WOUND ROTOR MOTOR
Posted 06 February 2005 - 04:03 AM
Duty is shiploader luffing winch.
The motor is about 20 yo.
We have had some incident regarding the luff controls and am looking to change the way that it operators to reduce the risks.
The WR motor rotor can be connected to 5 external stage resistor banks.
To go UP ie against the dead weight of the machine - one resistor is shorted and the motor started with 4 resisitors in line, these are dropped out as the speed gathers to full speed.
For the LUFF DOWN motion, we have 2 options.
SLOW SPEED down - this is done by PLUGGING the motor ie all 5 resistors are in series and the motor started in the UP direction ie the motor is trying to go UP but runs BACKWARDS.
FAST SPEED - 2 resistors are shorted and the motor started in the DOWN direction and 2 more resisotrs are shorted for the FULL FAST speed down ie there is ONE resistor still in series with the rotor winding.
A SLOW down speed is required for the luff down movement to provide good control when loading a ship.
The probelm is that the control system calls up a UP control for the luff down motion...this can be an issue when the full upper luff up limit sw is reached and activated.
We would like to remove the PLUG up setup for the SLOW speed down and have a normal run DOWN but still slow speed.
I am trying to come to grips with the wound rotor motor and the speed/torque characteristics.
I rang a man in TECO who told me that the motor will still go to full speed when resistors are in...this is not true. He could not help me further, hence, this email.
From the stuff than I can get my hands on:
A WR motor can/should develop a high starting torque and low stator currents with the resistors in series.
A WR motor is not good at regenerative braking cf squirrel cage induction motors.ie overhauling scenarios.
I have seen sev. articles on the torque from a WR motor at high rotor resistances...ll seem to vary.
What confuses me is:
With all 5 stages in series..the motor can NOT generate enough torque to lift the shiploader > it runs backwards. This goes against item 1 as above....I have yet to measure the resistance of each stage. The only reason for this could be that the resistance is such that the current is that low that the torque (locked rotor) for the WR motor ius not enough to overcome the load. It should be pointed out that the load will vary depending upon the shiploader shuttle extension...further out, more laod on the motor.
For the luff down, I am concefned with over running of the motor and runaway situation...this my be why the last resistor is not shorted for the FAST speed luff down. Can U pls comment on the ability of WR motors to handle overhauling loads.
Further, how can I have a SLOW speed down with the motor actually running (connnected) to run down rather than trying to go UP???
Many thanks for yr help.
Posted 14 February 2005 - 10:04 AM
I think that this question was quite well answered on Eng-tips. The solution using stator control as well as rotor control with a complete closed loop system certainly seems to be a good solution.
I believe the best solution is a closed loop vector drive on a standard cage motor (or a wound rotor motor with the rings peramently shorted).
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