Why Does a Voltage Stabilizer Trip When a Motor or Compressor Starts?

Have you ever installed a voltage stabilizer only to find that it trips immediately when an air compressor, water pump, refrigeration compressor, or industrial motor starts? While the stabilizer works perfectly under normal loads, it suddenly shuts down the moment the motor begins rotating.

This is one of the most common questions asked by factory owners, HVAC contractors, and equipment installers. In most cases, the stabilizer is not defective—it is simply protecting itself against a temporary electrical condition known as motor starting inrush current.

Understanding why this happens helps you choose the right stabilizer capacity, avoid unnecessary downtime, and improve equipment reliability.

In this guide, we’ll explain:

• Why motors draw extremely high current during startup
• Why voltage stabilizers trip during motor starting
• How voltage drop affects stabilizer protection
• How to correctly size a stabilizer for compressors and motors
• Practical solutions to prevent nuisance trips

What Happens When a Motor Starts?

Unlike resistive loads such as heaters or lighting, induction motors require a very large amount of current to overcome inertia and generate enough starting torque.

During the first few hundred milliseconds after startup, the motor behaves almost like a short circuit.

This temporary surge is called:

• Motor starting current
• Locked rotor current (LRA)
• Inrush current

Depending on motor design, the startup current can reach:

• 3–5× rated current (soft-start motors)
• 5–7× rated current (standard induction motors)
• 6–10× rated current (air compressors)
• 8–12× rated current (heavy industrial equipment)

Although the surge only lasts a fraction of a second, it is large enough to trigger protective circuits inside many voltage stabilizers.

Example:

Motor Rating	Rated Current	Starting Current
5 HP	8 A	48 A
10 HP	16 A	96 A
20 HP	32 A	180–220 A

【img】
Alt: Motor starting current compared with normal running current showing inrush current peak

Why Does the Voltage Stabilizer Trip?

Modern automatic voltage stabilizers include multiple protection functions to safeguard both the stabilizer and connected equipment.

When a motor starts, one or more protection mechanisms may activate.

1. Overload Protection

The most common reason is overload protection.

If the motor’s startup current exceeds the stabilizer’s instantaneous overload capacity, the protection relay disconnects the output.

For example:

• 20 kVA stabilizer rated output: approximately 30 A
• Motor startup demand: 150 A
• Protection system detects excessive current
• Output disconnects immediately

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2. Input Voltage Drop

Large startup current causes a temporary voltage dip in the electrical supply.

For example:

• Normal supply: 400 V
• Motor starts
• Voltage drops to 310 V
• Stabilizer detects undervoltage
• Protection activates

This problem becomes worse when:

• Power lines are long
• Cables are undersized
• Generators have limited capacity
• Transformer impedance is high

【img】
Alt: Voltage dip during compressor startup causing voltage stabilizer protection trip

3. Output Voltage Recovery Delay

Servo voltage stabilizers require a short amount of time to adjust output voltage after detecting a sudden input voltage change.

If the voltage changes faster than the servo motor can compensate, protection may operate before voltage recovery completes.

4. Internal Thermal Protection

Frequent motor starts generate repeated overload conditions.

After multiple starts, transformer windings and power components heat up.

To prevent damage, thermal protection temporarily disconnects the stabilizer.

Why Compressors Cause More Problems Than Ordinary Motors

Air compressors and refrigeration compressors are among the most demanding electrical loads.

Unlike centrifugal pumps, compressors often start under pressure, requiring significantly higher starting torque.

This results in:

• Higher locked rotor current
• Longer startup duration
• Larger voltage sag
• Higher mechanical stress

Typical compressor startup currents:

Equipment	Running Current	Startup Current
Scroll compressor	15 A	80–100 A
Screw compressor	40 A	200–300 A
Piston compressor	25 A	150–220 A

This explains why many voltage stabilizers that work perfectly with lighting or heating loads suddenly trip when connected to compressors.

【img】
Alt: Compressor startup current compared with servo voltage stabilizer rated output

How to Correctly Size a Voltage Stabilizer for Motors

One of the biggest installation mistakes is sizing the stabilizer based only on the motor’s running power.

Instead, engineers should consider:

• Motor starting current
• Power factor
• Starting method
• Duty cycle
• Voltage fluctuation

General Recommendation

Motor Starting Method	Recommended Stabilizer Size
Direct-On-Line (DOL)	2.5–3 × motor rating
Star-Delta	1.8–2 × motor rating
Soft Starter	1.3–1.5 × motor rating
Variable Frequency Drive (VFD)	1.2–1.3 × motor rating

Example:

A 15 kW compressor may require:

• 15 kVA for running
• 40–50 kVA stabilizer if started DOL
• 25–30 kVA with soft starter

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Practical Ways to Prevent Voltage Stabilizer Trips

Choose a Larger Stabilizer

Increasing stabilizer capacity provides greater overload margin during startup.

Use a Soft Starter

Soft starters gradually increase motor voltage, significantly reducing inrush current.

Typical reduction:

• 600% starting current → 250–300%

Install a Variable Frequency Drive (VFD)

A VFD controls acceleration electronically and may reduce startup current close to rated current.

Besides preventing stabilizer trips, VFDs improve energy efficiency and motor lifespan.

Improve Cable Size

Undersized cables increase voltage drop.

Proper conductor sizing minimizes input voltage sag during motor startup.

Check Generator Capacity

If the stabilizer is supplied by a diesel generator, ensure the generator can support motor inrush current without excessive voltage dip.

Servo Stabilizer vs Static Stabilizer for Motor Loads

Feature	Servo Stabilizer	Static Stabilizer
Response speed	Medium	Very fast
Motor starting performance	Good	Excellent
Voltage correction	Mechanical	Electronic
Frequent load changes	Moderate	Excellent
Industrial compressors	Suitable	Highly recommended

Facilities with frequent compressor cycling often benefit from static voltage stabilizers because of their millisecond-level response speed.

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Alt: Comparison between servo voltage stabilizer and static voltage stabilizer for motor startup

Industry Standards and Technical References

Motor starting behavior and voltage quality are well documented by international organizations.

• IEEE Std 3004 – Motor system engineering practices
• IEC 60034 – Rotating electrical machines
• IEC 61000 – Electromagnetic compatibility and voltage quality
• ABB Technical Guide – Motor starting methods
• Schneider Electric – Motor protection and power quality

These references emphasize that high inrush current and temporary voltage dips are normal electrical phenomena that must be considered during system design.

FAQ

Conclusion

A voltage stabilizer usually trips during motor or compressor startup because the equipment draws a very high inrush current, causing either an overload condition or a temporary voltage sag. In most cases, the stabilizer is functioning exactly as designed by protecting itself and the connected load.

The most effective solutions include selecting the correct stabilizer capacity, reducing startup current with a soft starter or VFD, improving power distribution, and ensuring the electrical source can handle transient loads.

If your application involves air compressors, refrigeration systems, pumps, CNC machines, or other heavy motor loads, choosing a stabilizer specifically designed for high inrush current is essential for long-term reliability.

Need help selecting the right voltage stabilizer for motor-driven equipment?
ZHENGXI specializes in industrial servo and static voltage stabilizers engineered for compressors, pumps, HVAC systems, and manufacturing equipment. Contact our technical team for a customized sizing recommendation based on your motor rating, starting method, and site conditions.