Sizing a UPS for motors and inductive loads
A UPS rated for 10 kW of servers is not rated for a 10 kW motor. The difference is not marketing margin — it is the shape of the current the load draws in the first 200 milliseconds, and whether the inverter can survive it.
The short answer
Size on starting current, not running current, and expect to oversize by a factor of 1.5 to 3 depending on the starting method. Use a transformer-based UPS with an output isolation transformer for anything with a contactor or a direct-on-line motor. A transformerless rack UPS will transfer to bypass on the first start and then keep doing it.
Three things a motor does to a UPS
Inrush. At the instant of start, a squirrel-cage motor draws 5–7 times its full-load current for 100 ms to several seconds, decaying as it accelerates. A UPS sees this as a near short circuit. If the inverter's current limit is reached, the output voltage collapses, the load drops, and a protected transfer to bypass follows.
Low displacement power factor. A lightly loaded induction motor runs at 0.3 to 0.5 power factor. UPS output stages are rated in kW at a specified PF — often 0.9 or 1.0. A 10 kVA / 9 kW UPS cannot deliver 9 kW into a 0.4 PF load; it can deliver 10 kVA, which is only 4 kW of real power. The rest circulates as reactive current and heats the inverter.
Regenerative and transient voltage. When a motor is switched off, the collapsing magnetic field produces a voltage spike at the UPS output. Multiple motors switching at once multiply the effect, and the UPS may report output overvoltage.
What the UPS does about it
A line-frequency UPS with a transformer handles all three better. The transformer's leakage inductance limits the rate of rise of fault current, its magnetising reserve absorbs short transients, and the oversized inverter stage in an industrial unit is usually designed for 125% continuous, 150% for a minute and 200%+ for a few seconds.
A high-frequency rack UPS is designed for the opposite load profile: a constant, near-unity power factor draw with a small holdup requirement. Its overload window is narrow — often 110% for a minute — because the whole design is optimised for efficiency and density.
How to size, step by step
- List every motor: kW, starting method (DOL, star-delta, soft starter, VFD), and starting current from the datasheet.
- Take the largest motor's starting kVA and add the running kVA of everything else that is on at the same time.
- Add 20–30% for growth and temperature derating.
- Check the resulting figure against the UPS overload curve for the duration of the start, not just the steady-state rating.
A worked example: a 7.5 kW pump started direct-on-line draws about 6 × 15 A at 400 V for 1.5 s, which is roughly 62 kVA of apparent power for that period. A 20 kVA UPS will not ride through it however the marketing describes the overload rating. A 40 kVA transformer-based unit with a 150%/60 s overload window will.
Starting methods change the arithmetic
| Starting method | Starting current | UPS oversize factor |
|---|---|---|
| Direct on line | 5–7 × FLC | 2.5–3 × motor kW |
| Star-delta | 2–3 × FLC | 1.5–2 × |
| Soft starter | 2–3 × FLC, ramped | 1.5–2 × |
| VFD | limited to ~1.1 × FLC | 1.1–1.3 × |
A VFD is the single most effective way to make a motor compatible with a UPS, because its rectifier front end limits the current the UPS ever sees. Where the drive is fed from the UPS, check that the drive's input filter does not present a capacitive load the UPS dislikes at start-up — some drives draw a charging current spike when first energised.
Power factor correction
Do not connect power factor correction capacitors downstream of the UPS. A capacitor bank on the UPS output can form a resonant circuit with the inverter's output filter, and at light load the leading power factor can push some UPS models into overvoltage shutdown. Keep PFC at the mains side of the distribution, upstream of the UPS input.
When you cannot oversize
If the UPS must feed a mixed load — servers plus a motor — and the budget does not stretch to full oversizing, three practical options exist:
- Move the motor to a separate circuit fed directly from the mains, protected by a motor-rated surge device and a small dedicated UPS only for its control circuit.
- Fit a soft starter on the motor and keep the UPS sized for the reduced inrush.
- Use a flywheel or a transformer-based unit with a documented high overload window, and verify with a real start test during commissioning rather than trusting the curve.
FAQ
Can a UPS run an air conditioner? Small inverter-driven air conditioners can run on a suitably sized online UPS, but compressor start current on a fixed-speed unit usually forces a 3× oversize. For comfort cooling it is almost always cheaper to leave the AC off the UPS and protect only the control electronics.
Why does my UPS beep and switch to bypass when the pump starts? Output current exceeded the inverter limit, or output voltage dropped below the transfer threshold. Both are inrush. The UPS is behaving correctly; the sizing is wrong.
Is a "motor load" UPS different hardware? Not necessarily. The difference is the transformer, the inverter current rating and the overload firmware settings. Industrial ranges are built for it; IT ranges are not.
Does low power factor damage the UPS? It does not damage it instantly, but it derates usable real power and raises inverter current and temperature for the same delivered kW, which shortens capacitor and fan life.
See the GX industrial-frequency range for motor-heavy sites, the H6-10K with external isolation transformer for mixed loads in a smaller footprint, and the industrial three-phase range above 30 kVA.