Recycling energy – performance evaluation of regenerative drives

Regenerative drives are widely adopted in modern machine tool, robots and other industrial machinery equipped variable speed electric motors. What is so special about them, then? Today we take a closer look into this topic, including some practical examples

What is a regenerative drive?

Simply speaking, a regenerative drive is an electro-mechanical system that is able to convert electrical energy from the source (such as network) to mechanical energy (to propel the rotor of the electric motor) and, vice versa, can convert kinetic energy of the rotor to electrical one and return it to the source. Conventional drives can only perform an uni-directional conversion: from electrical to mechanical. If it is needed to brake/stop the motor a restive load is attached to the motor outputs in order to dissipate mechanical energy and turn it into heat.

non-regen-conv

Conventional inverter for variable frequency drive [Source]

An inverter, even without regen, does save energy as it allows to adjust the speed of the motor with respect to the demand. Often it can bring significant saving in hydraulic system, such as high pressure pumps or chiller pumps where load may vary. In systems such as controlled axes or spindles in machine tools or robots variable frequency drive (VFD) is a must. Energy regeneration however is rather an option but it may serve additional energy savings. Moreover it eliminates also need for braking choppers (resistors) which emit a considerable amount of heat that has to be removed one way or the other. If you look at an example schematic instead of simple rectifier, regenerative converter has a set of bi-directional switches which in practice send power back to the motor side exceeds the voltage on the network side, which happens typically during breaking.

How does it work?

To maintain continuous control under all load situations, an inverter has to not only drive a motor that is moving a load but also shed excess energy when the load is driving the motor (typically during deceleration). As this generated energy is in the form of electricity, it is often converted and dissipated as heat via braking resistors. However, a specially designed regenerative drive, such as Mitsubishi’s Regenerative A701 drive, controls the load under all conditions and sheds the excess energy by converting the kinetic energy into electricity and injecting it safely into the mains or sharing it with other drives by connecting them together. The whole process is tightly controlled by drive control system, so that the currents and transmitted power is withing prescribed motorl and electronics limits (e.g. S1 or S6 regions). Either way, energy will be saved and electricity costs will be reduced

The basic requirements of a soft start-up and stop can be programmed into a regenerative drive. Furthermore, running speeds can be reduced when demand is lower, thereby saving additional energy, or the motor can even be stopped if there is temporarily zero demand.

Machining process examples

I performed a few test’s on two machine types: a CNC lathe (processes 1 and 2) and a flexible, multi-spindle transfer machine (process 3). Machines worked on demo pieces, representative for each type. Spindles active power was measured using our proprietary fast sampling power meter. Lets see how much of energy was saved in regeneration phases, that is while spindle was breaking after completing each feature of the work pieces.

Process 1 – Turning and milling part

Demo product:

Percentual saving in overall lathe spindle energy expenditure
Spindle 15.7%

Process 2 – Only turning

Percentual saving in overall lathe spindle energy expenditure
Spindle 110.4%

Process 3 – Flexible transfer machine with 2 spindles

A sample gas valve part. This process was composed mostly with drilling and milling operations. Outlets and inlets were finished with treading.

Multicenter results

Percentual saving in overall spindles energy expenditure
Spindle 179.9%
Spindle 261.8%

Summing it up…

Amount of energy saved with respect to overall energy required for the process by the spindle varies significantly between two machine types. For lathe, which has longer periods of constant speed cutting and generally less features per work piece (5-8), savings were ranging from around 5-10%. In transfer type of machine, which performs multiple features (15-20) per work-piece, with numerous tool changes requiring spindle full stop, following completion of each feature, savings were much grates. They reach levels of around 60-80%, which is a significant portion of all delivered electrical power. Regeneration, which in the past was an extra paid option, now has become a standard feature of all modern NC drives, due to dicsussed savings but also it allows skipping the restive break in the machine electrical design.

Read more about energy savings in spindle acceleration/deceleration in my newest journal publication: Electric load management in spindle run-up and run-down for multi-spindle machine tools via optimal power-torque trajectories and peak load synchronization [http://dx.doi.org/10.1007/s00170-017-1341-7]