One of the advantages of the PowerFlex 525 component class drive is that it can engage in laod sharing through the droop method.  Droop load sharing is a simple and cost effective way of controlling multi-drive applications where close coordination of movement is required.  Below is a quick overview of the advantages and disadvantages of this method of load sharing and a parameter overview as well.  This is followed by an excerpt from the DRIVES-WP001 document, which describes load sharing in more detail.

· Simple
· No extra wiring for interconnection
· High performance drive not required
· No runaway condition with load loss

· Poor speed regulation
· Limited speed range
· Sharing of load not precise

From DRIVES-WP001:

Load sharing is a term used by many to describe a system where multiple drive and motor sets are coupled and used to run one mechanical load. In the strictest sense, load-sharing means that the amount of torque applied to the load from each motor is prescribed and carried out by each drive and motor set.

Droop is the simplest form of load sharing to set up and, therefore, the least precise and less flexible. The precision of this control is dependent on three factors, the drives control algorithm, the motor characteristics, and the type of load being controlled.

Since a volts/hertz drive does not have the ability to run in “torque mode”, a more loose interpretation of the term “load sharing” is sometimes used. Load sharing on a volts/hertz drive is much less controllable and to a large extent dependent on the motor and type of load coupling. Since a loaded induction motor has “slip” as an inherent characteristic, load sharing is a
natural response to loading.

When two motors are coupled to the same load each will slip according to its portion of load, forcing load to be picked up by the other motor. In this regard, a high slip motor is better for load sharing applications. However in a general sense, the slip of a motor is representative of its efficiency rating. High slip motors tend to have high starting torque but have low efficiency. Though high efficiency motors will slip when loaded, they may not have enough slip at full load to achieve a reasonable degree of load sharing. For this reason many drives offer a feature called “Droop”.

The Droop feature enhances the natural slip of the motor by reducing the output frequency in proportion to load current. Ideally it is only the torque-producing component of the load current that determines the droop. The amount of droop can usually be programmed in the drive in hertz or % speed.


The master and follower drive(s) receive the same speed reference and no interconnection of the drives is required. The speed of the motors will be dependent on the load and amount of droop that is programmed at full load current.