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Servo presses can recover significant energy during the return stroke through regenerative braking. Understanding and optimizing this energy recovery can reduce operating costs by 20-40%.
## How Regenerative Energy Works
During the return stroke (BDC to TDC), the servo motor decelerates the slide. This deceleration generates electrical energy that flows back into the DC bus of the drive.
This regenerative energy can be:
1. **Dissipated** in a braking resistor (wasted as heat)
2. **Shared** with other drives on the same DC bus
3. **Returned to the grid** via an active front end (AFE) unit
4. **Stored** in a capacitor bank or battery system
## DC Bus Sharing (Most Common)
If multiple servo axes share a common DC bus (typical in multi-axis presses), regenerative energy from one axis can be used by another axis that is accelerating simultaneously.
Example: On a 4-point servo press, when two axes are decelerating (returning), their regenerative energy powers the other two axes that are accelerating (forming). Net energy consumption is significantly reduced.
## Active Front End (AFE)
An AFE unit replaces the standard diode rectifier with an IGBT bridge that can both draw power from and return power to the AC supply.
Benefits:
- Returns regenerative energy to grid (reduces energy bill)
- Maintains near-unity power factor
- Reduces harmonic distortion
Cost: AFE units cost 30-50% more than standard rectifiers, but payback period is typically 2-4 years on high-SPM presses.
## Measuring Regenerative Energy
To quantify regenerative energy on your press:
1. Install a bidirectional power meter on the press supply
2. Run press at production SPM for 1 hour
3. Record total energy consumed and total energy returned
4. Regenerative fraction = returned / (consumed + returned)
Typical values: 15-35% regenerative fraction for servo presses vs 0% for mechanical presses.