Speed Droop

Droop is a common electrical technique for controlling load. It is a stable method of reducing current if the current increases. Recently I have used droop for controlling the current on a synchronous generator. When the reactive current gets excessive, voltage is reduced, resulting in a lower reactive current. This is particularly useful when several generators are connected in parallel as in a generating station.

In web handling drives we also run into the situation of excessive load if one roller pulls and another drive drags with both drives running with excessive load or torque. The web is used to transfer power between the two rollers.

Adding droop to each drive is the simplest form of load control. We define droop as reducing the speed reference by a percentage based on 100% torque on the motor. Droop is usually set at 5%. That means if a roller is commanded to run at 1000 MPM and torque runs to 100%, the speed reference will be reduced by 5% or 50 MPM. This will automatically reduce the torque to a stable level below 100%. Droop is particularly attractive when two adjacent drives tend to run at either +100% or -100%. Droop will automatically let them find stable running conditions.

The reason droop is so simple and cost-effective is that no wiring or signals need to be passed between drives to share load. Everything that happens is controlled by signals within the drive itself namely its own torque and speed.

Unfortunately, droop has an unfortunate side effect. The speed is not controlled and we often need to control speed accurately. Droop results in stable loads during speed changes, but affects the precision of the inertia compensation or speed matching needed to coordinate speed changes between drives. Two examples are nipped soft rollers and two drum winders. Other more complex methods of load sharing are needed in these cases.