Industrial UPS Protection against Regenerative Energy and High Inrush Currents
Managing regenerative loads (such as cranes, industrial elevators, overhead cranes, or machinery equipped with braking inverters) and motor inrush currents represents one of the most complex technical challenges when protecting a Heavy-Duty Industrial UPS.
If not properly managed, these phenomena cause overvoltages on the direct current (DC) bus or electrical/thermal overloads, forcing the UPS into trip/alarm mode or static bypass.
1. Protection Against Regenerative Loads (Reverse Power Flow)
When an electric motor is suddenly braked or slowed down by an inverter or variable speed drive (VSD), the motor temporarily acts as a generator, feeding energy back toward the UPS network. Traditional UPS units are not designed to absorb reverse power and direct it onto the DC bus, causing overvoltages and triggering inverter alarms or shutdowns.
To protect the UPS from this phenomenon, three main engineering solutions are applied:
A. Braking Choppers and Dynamic Braking Resistors
How it works: A braking circuit (chopper) is connected to the DC bus of the UPS or the drive upstream of the load. When the voltage on the DC bus exceeds a safety threshold due to energy feedback, the chopper redirects the excess energy to a resistor bank (braking resistor), which dissipates it as heat.
When to use it: It is the standard solution for regenerative loads with short or intermittent braking cycles (e.g., cranes, hoists).
B. Blocking Diodes and Line Separation
How it works: Power diodes are connected in series on the supply line to physically prevent reverse current flow from the drive/load back into the UPS output.
When to use it: When the regenerated energy is already managed locally by the drive (e.g., via the drive's internal DC bus) and the UPS needs to be isolated from reverse currents.
C. Regenerative Units (Active Front End - AFE on the Drive)
How it works: The load inverter utilizes AFE technology capable of feeding braking energy back into the main electrical grid. If the UPS is bi-directional or integrated into an industrial microgrid, this energy is redistributed to other factory loads.
2. Protection and Management of Motor Inrush Currents
During Direct-On-Line (DOL) starting, an induction motor can draw an inrush current equal to 6 to 8 times its nominal current (I= 6\8times Inom) for several seconds. This peak risks saturating the UPS transformer or triggering overcurrent protections.
Here is how to neutralize the impact of inrush currents on the UPS:
A. Soft Starting Systems (Soft Starters and VFDs/Inverters)
Soft Starter: Reduces the voltage applied to the motor during startup, limiting the inrush current to approximately 2.5\3 times the nominal current.
Inverter / VFD (Variable Frequency Drive): The best solution. It regulates both frequency and voltage, keeping the inrush current close to or equal to the nominal current (1.1\1.5times Inom).
B. Engineering Sizing and Crest Factor
If the motor start must remain Direct-On-Line (DOL), the UPS must be oversized taking into account the overload/crest factor.
UPS Inverter Capacity: A true Industrial UPS (e.g., featuring SCR thyristor rectifiers/inverters or heavy-duty IGBTs with an isolation transformer) can withstand 150% of the load for 1 minute and 200% for several cycles/seconds.
C. Sequential Load Starting (Step Loading / Load Shedding)
If the facility features multiple motors, it is essential to program a timed and sequential startup of the various motors via PLC, rather than restarting them all simultaneously after a power outage.
This prevents inrush currents from adding up, creating an unsustainable peak for the UPS system.
D. Dedicated Output Isolation Transformer
The integrated output galvanic transformer in an industrial UPS provides high intrinsic impedance that dampens current peaks and protects inverter semiconductors from high inductive currents and voltage fluctuations caused by motor starts.
Managing inductive peaks and reverse power flow requires a custom engineering design to prevent plant downtime and protect your operational continuity.
Our engineers are at your disposal to analyze your facility's electrical specifications and configure the Industrial UPS best suited to your operational needs.
