Load sharing using asynchronous motors
To answer your 3 quesions


Droop can be used in many cases, including yours. But there are more elegant solutions to your problems. (Taking the pitfalls from the other answers in account
Droop is the opposite setting of a proportional term (gain). On most (drive, engine, generator) systems the setting is in percentage. This percentage is the amount of speed (rpm, Hz) that would be dropped in case of 100% load. So instead of using a gain, we set a desired result.
See my use case below.


Example for droop

We have a motor we want to drive. It's nominal frequency is 60 Hz and its nominal power is 100 Kw. Because it's necessary in our plant, we set the droop to 2.5%. (Let's assume full load is reached at nominal speed.) 

We give a speed setpoint of 60 Hz. The motor loads up to 100%. The actual speed you are going to end up with is 60-2.5%=60-1.5=58.5 Hz.

In parallel operation this would result in the following: if one of the coupled motors would pick up more power for whatever reason, its resulting speed would drop. At that point all the other motors would take it's load. But because they are drooped as well, they will drop along. So this can be tricky in an automated system where you need guaranteed speed.

This way all the power will be distributed near-equal. It is possible in some drive situations, where oscillating power shifts happen. (Like diesel generator plant). Increasing droop fixes this problem and is a cheap workaround to fix cheap regulators. 

Use case

Last year I commissioned a diesel-electric ship where we used drives in a electric propulsion system, where we connected two 1000 kw drives in parallel on a single motor with a double set of windings. First we set them up in droop mode. But because we needed a guaranteed speed for our DP systems we changed the settings to master / follower.

Master / follower mode (answers your 3rd question) is timed over a fibre optic bus communication loop, and if the master fails, they all fail. This can be good or bad, depending on your application. But basically, only the master needs to handle external I/O like:


Start / stop commands
Safety stop system
Speed input


So multiple drives act as a single unit and only one unit to interface with. Also the regulation is also very accurate, because speed set-point results truly in that speed (if not exceeding other limits, like torque). Whereas droop is dependent on the actual load. Also the load sharing is completely handled by the drives.

We used the vacon nxp drives. If you purchase them with the parallel running application, you can switch freely between droop and master / follower settings. Depending on your power demand: you can use stand alone units, common DC units or water cooled units. All from the same controller (NXP) range.