What Is an Inductive Load? 5 Things to Check When Choosing a UPS

When choosing a UPS, it is easy to look at the load rating and stop there: "The equipment is 3 kW, so a 5 kVA UPS should be enough."

That may work for simple loads, but motors, pumps, compressors and fans are different.

A 3 kW motor does not behave like a 3 kW heater. The starting current can be several times higher than the normal running current, and an undersized UPS may overload during startup.

Here are five things to check before choosing a UPS for an inductive load.

What is an inductive load?

An inductive load usually contains coils or windings and uses a magnetic field to operate.

Common examples include:

• Motors

• Pumps

• Fans

• Air-conditioning compressors

• Transformers

The current of an inductive load normally lags the voltage and includes reactive power. That is why 3 kW of motor load and 3 kW of resistive load can require very different UPS capacities.

1. Starting current

Starting current is one of the first things to check.

A motor started directly from the line can draw several times its rated current. The actual value depends on the motor and its starting method.

If the UPS cannot provide enough short-term current, it may go into overload, transfer to bypass or fail to start the motor.

Common ways to reduce starting current include:

• Star-delta starting: reduces starting current but also reduces starting torque.

• Soft starter: controls the starting process and limits current.

• VFD: provides controlled acceleration, but the VFD's input characteristics also need to be considered.

Before selecting the UPS, check:

Motor power + starting method + starting current

2. Power factor

Motor load should not be sized by kW alone.

For example, a 3 kW motor with a power factor of 0.8 requires approximately:

3 ÷ 0.8 = 3.75 kVA

That is the approximate running apparent power. Starting demand and other loads still need to be considered.

Also, don't confuse motor power factor with UPS output power factor. They describe different things and both can affect UPS sizing.

3. UPS overload capability

UPS overload capability is designed for short-term load changes. It does not mean the UPS is automatically suitable for starting a motor.

Different UPS models have different overload curves. Some may support 125%, 150% or higher loads for a limited time, but the actual limits depend on the manufacturer and model.

If every motor startup pushes the UPS into its overload range, the system is not properly sized.

Do not use the UPS overload rating as a substitute for motor starting capacity.

4. Peak current and waveform

kVA is not the only electrical parameter that matters.

Some industrial loads, especially equipment using soft starters, VFDs or other power electronics, can produce non-linear current waveforms.

When selecting a UPS, check:

• Crest factor

• Output waveform

• Current harmonics

• Compatibility with VFDs and other non-linear loads

A UPS that works well with office computers is not necessarily suitable for continuous motor loads.

5. Voltage drop during motor starting

There is another issue that is easy to overlook.

A motor may start successfully while other equipment connected to the same UPS is affected by the voltage drop during startup.

For example, if a UPS supplies both a pump and a PLC, the pump's starting current can cause a temporary output voltage drop. The PLC, sensors or communication equipment may restart before the pump has even finished starting.

For mixed industrial loads, check the UPS output voltage response during motor startup.

How to choose a UPS for an inductive load

Don't simply use a rule such as "motor power × 3."

At a minimum, collect these parameters:

If the motor starting data is not available, confirm it with the motor or equipment manufacturer before finalizing the UPS capacity.

How TAFENG specifies UPS for inductive loads

At TAFENG, under the BLAZING POWER brand, we size UPS systems for motors and pumps from the starting data, not the nameplate kW.

For heavy or continuous industrial loads, we offer low-frequency online UPS systems with built-in isolation transformers for applications that require higher short-term overload capability, electrical isolation or voltage conversion. For sites where voltage regulation is needed without battery backup, our SBW three-phase voltage stabilizers are another option. See also our notes on UPS transfer time for industrial and motor loads, where contactor drop-out and voltage sags are covered in more detail.

Send us the motor power, power factor, starting method and starting current, and we can confirm the UPS capacity and configuration for the application.

Final takeaway

When a UPS is used with motors, pumps or compressors, the nameplate kW is only part of the picture.

Starting current, power factor, kVA, overload capability and peak current all need to be considered.

For inductive loads, checking the starting method and starting current before selecting the UPS is often more useful than simply buying a larger UPS.

Frequently Asked Questions

Can a UPS start a motor directly?

It depends on the motor's starting current and the UPS's short-term current capability. A direct-on-line (DOL) motor can draw several times its rated current at startup, so check the starting current and UPS overload curve before assuming it will start.

Why does kVA matter more than kW for a motor?

A motor draws reactive power, so its apparent power (kVA) is higher than its real power (kW). A 3 kW motor at 0.8 PF needs about 3.75 kVA during normal operation, before starting demand is considered.

Do I need a UPS compatible with a VFD?

If the motor is driven by a VFD or soft starter, the load may produce harmonics and a higher crest factor. Check the UPS crest factor, output waveform and compatibility with the drive before selecting the system.

What UPS type is best for pumps and compressors?

An online double-conversion UPS provides regulated output and no transfer time during normal operation, but the UPS still needs to be sized for the motor's starting current. For heavy industrial loads, a low-frequency online UPS is often the right choice.