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We need to replace an old Ward Leonard system based DC motor speed control system. The existing system is a constant torque/tension system. The main parts of the systems are:
a) Wire Rope
b) Cable Drum (to wound wire)
c) Reduction Gear coupled with Drum at one end and DC Motor at the other
d) DC Motor to Drive Cable Drum through Reduction Gear
e) DC Generator (to generate Variable Armature Voltages for DC motor]
f) AC Motor (works as prime mover for DC Generator)

The tension on wire rope is generated via DC motor whose speed/torque is controlled through DC generator, which is driven by AC motor. A strain gauge is installed on reduction gear to measure the actual tension. The DC motor has permanent excitation (separately excited), however the DC generator excitation (field) is controlled and it depends upon the desired torque. There are two solutions:
a) One probable solution is to remove the Ward Leonard system and direct controlling of DC motor via DC drive. But this solution requires a large DC drive. The DC motor rating is 220KW, 440V. One more important modification is that the existing system utilizes strain gauges to measure torque. Unfortunately the sensors are defective and could not be repaired. We need a solution that will eliminate the requirement of external sensor. As motor torque is directly proportional to its armature current, we can achieve the desire result by controlling the armature current.

b) Another solution is to use the existing Ward Leonard motor-generator set and control the field current of DC Generator by DC drive which monitors the armature current of DC motor to ensure constant torque. This solution requires a small drive system since max field current of DC generator is 30A @ 220 V DC.

We need expert advice on above mentioned solution and recommendation of requisite hardware (SIMOREG Drives, etc.)

Thee above equipment is installed onboard a ship so Marine Grade equipment will be preferred.

Your reply will be highly appreciated. If you need further info, please contact us.

Could anyone also suggest the implication of replacement of torque sensor? If I don't use torque sensor, then does it affect the system performance/safety?

Regards,

Dear Mr. Malik,

I think you may use constant torque AC drive with AC induction motor for complete replacement of the system. This will be a costly affair but can resolve all issues at once. I would recommend to search ABB/YASKAWA/ALLEN BRADLEY drives for correct solution.

Regards.