In short. Three-phase and industrial UPS systems serve larger loads, drives and process controls. Key choices are transformer or transformerless design, overload capability, environmental protection, neutral arrangement and how the input behaves on generator supply.
Design choices
| Choice | Consideration |
|---|---|
| Transformer-based | Robust, galvanic isolation, heavier, good for harsh duty |
| Transformerless | Lighter, efficient, needs attention to neutral and fault behaviour |
| Overload capability | Inrush loads and short faults |
| Environmental rating | Dust, heat and humidity |
| Battery system | Voltage, runtime and maintenance. See batteries |
Sizing
kVA = √3 × V × I ÷ 1000, with kW within rating
Check power factor of the load, crest factor and overload. Do not size a UPS for motor starts; separate them. See motor starting.
Neutral and unbalance
Single-phase loads create neutral current and imbalance. Check the UPS output rating per phase and the neutral conductor size. See unbalanced load regulation.
Generator interface
- Size the generator for UPS input including battery recharge.
- Use rectifier walk-in and generator-compatible input.
- Control harmonics with filters or 12-pulse/active input.
Illustrative example
A process plant puts PLCs, instruments and control room on a transformer-based UPS with a maintenance bypass, while motors are fed from a separate bus. The generator is sized for UPS input plus recharge.
Transformer-based or transformerless
| Aspect | Transformer-based | Transformerless |
|---|---|---|
| Isolation | Galvanic | None inherent |
| Size and weight | Larger | Smaller |
| Efficiency | Slightly lower | Higher |
| Overload and fault handling | Robust | Depends on design |
| Neutral handling | Easier with transformer | Needs care |
| Suitability for harsh sites | Often better | Good in clean rooms |
Input and bypass arrangements
Single-feed UPS uses one input for both rectifier and bypass; dual-feed uses separate inputs. Dual-feed improves resilience if the two supplies are independent. Check the bypass line rating, since it must carry the full load, including any fault clearing duty.
Selecting by environment
- Dusty plants: filtered, sealed or positive-pressure rooms.
- High temperature: derated capacity or cooling.
- Vibration: specially mounted units.
- Corrosive air: coated boards or protective enclosures.
Sizing example
A process control room has 40 kW of loads at 0.9 power factor with growth margin of 25 percent: 40 × 1.25 ÷ 0.9 ≈ 55.6 kVA. A 60 kVA class unit, with a check of the kW rating and low power factor behaviour, may be selected. Illustrative only.
Commissioning checks
- Verify phase sequence and wiring.
- Test on load with simulated mains failure.
- Test static bypass transfer.
- Check generator operation and walk-in behaviour.
- Record input harmonics and efficiency.
Maintenance
Plan fan, capacitor and battery replacement intervals. Capacitors age with heat, and the maker will give indicative life. Keep spares of items with long lead times.
The master engineering question
Which loads truly need no-break supply, and which can ride a short generator start?
What to send us
Send load schedule and environment. Send your application details.
Questions engineers ask
Transformer or transformerless?
Depends on isolation, size, efficiency and environment.
Can a UPS run motors?
Not recommended for starts; separate them.
How do I handle imbalance?
Balance loads and check per-phase ratings.
What about heat?
Plan cooling for UPS losses.
Values are illustrative. Use manufacturer data.