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YS Circuit Solutions YS Circuits Solution provides expert PCB repair, basic electronics training, and PLC automation services for students and professionals.

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27/09/2026

THREE-PHASE MOTOR BUT CURRENT IS NOT BALANCED?

If one phase is drawing noticeably more or less current than the other two, don't ignore it.

Possible causes include:

• Loose motor terminal
• Damaged cable
• Unbalanced supply
• Motor winding problem
• Poor connection
• Incorrect motor connection
• VFD output-stage issue
• Mechanical or magnetic imbalance

Compare all three phase currents under the same operating condition.

If the imbalance changes significantly with load, that information can help narrow down the problem.

Never judge a three-phase motor by checking only one phase.

25/09/2026

A VFD can show output voltage while the motor current remains almost zero.

That doesn't automatically mean the VFD is healthy or the motor is healthy.

Possible causes include:

• Motor cable open circuit
• Loose motor terminal
• Broken conductor
• Motor winding open
• Incorrect output measurement method
• VFD output stage problem
• Motor not actually receiving a complete 3-phase supply

Important point:

VFD output is PWM, not the same as a normal fixed-frequency AC waveform.

So measuring VFD output with an ordinary multimeter can sometimes give misleading readings.

Check the motor terminals, cable continuity, winding resistance and VFD output properly before replacing the drive.

24/09/2026

A VFD can reach the commanded frequency while the actual motor speed still doesn't behave as expected.

Possible reasons include:

• Excessive mechanical load
• Motor slip
• Incorrect motor parameters
• Wrong motor nameplate data
• Torque limitation
• Mechanical resistance
• Incorrect control mode
• Speed feedback problem in closed-loop systems

One important point:

Frequency and motor RPM are related, but they are not always exactly the same.

Before blaming the VFD, compare:

Commanded frequency → Output frequency → Motor current → Actual RPM → Mechanical load

That comparison can tell you where the problem is.

23/09/2026

A motor can run perfectly with light load and start showing problems as soon as the machine load increases.

Possible causes include:

• Mechanical overload
• Incorrect motor parameters
• Voltage drop under load
• Motor winding problem
• Loose motor terminals
• Cable voltage drop
• Poor VFD tuning
• Bearing or gearbox resistance
• Insufficient motor torque

The important part is to check what changes when the load increases.

Compare:

Light load → motor current + voltage + frequency

Heavy load → motor current + voltage + frequency

The difference between these two conditions can quickly point toward the real problem.

Don't troubleshoot only at no-load condition.

22/09/2026

A VFD can trip on overcurrent during acceleration even when the motor size matches the drive.

The problem is often related to how much current the motor needs to accelerate the actual load.

Common causes include:

• Acceleration time too short
• Heavy starting load
• Mechanical jam or excessive friction
• Incorrect motor parameters
• Incorrect VFD control mode
• Motor cable or connection problem
• Motor already rotating against the commanded direction
• Excessive load inertia

A simple but important check:

Increase the acceleration time temporarily and observe the motor current.

If the trip disappears, don't immediately assume the VFD is faulty. Investigate the acceleration demand and mechanical load.

Troubleshoot the cause — don't just increase the current limit.

21/09/2026

A VFD can show RUN status while the motor is not actually delivering the expected mechanical output.

This can happen when the drive is running but the motor is being affected by:

• Excessive mechanical load
• Motor brake not releasing
• Coupling or gearbox problem
• Incorrect motor parameters
• Low-speed torque limitation
• Current limit activation
• Motor phase or cable problem
• Mechanical jamming

One important point: don’t assume the VFD is faulty just because the motor stops.

Check the actual output frequency, motor current, DC bus condition and mechanical load at the moment the problem occurs.

Good troubleshooting means checking what the system is actually doing under load — not only what it shows at idle.

20/09/2026

The VFD runs normally with a light load.

But when the machine becomes heavily loaded…

⚠️ DC BUS UNDERVOLTAGE.

This fault doesn't always mean the VFD itself is defective.

The incoming supply may be dropping when the motor demands more power.

Possible causes include:

🔹 Low incoming voltage
🔹 Voltage drop in long cables
🔹 Loose or overheated terminals
🔹 Weak contactor or isolator connection
🔹 Supply imbalance
🔹 Transformer capacity problem
🔹 Heavy load causing excessive current demand
🔹 DC bus capacitor problems inside the VFD

The important clue is WHEN the fault happens.

If the VFD trips only when the motor is heavily loaded, measure the input voltage during that exact condition.

Don't check the supply only when the motor is stopped.

A supply can look perfectly normal with no load and drop significantly when the machine starts working.

Good troubleshooting means measuring the voltage under the actual operating condition.

19/09/2026

The motor is running normally.

Current looks normal.
No fault is showing.

But there is a noticeable high-pitched whining sound.

This doesn't always mean the motor bearings are failing.

With a VFD-driven motor, the sound can sometimes come from the high-frequency switching used to control the motor.

Possible causes include:

🔹 VFD carrier frequency
🔹 Motor electromagnetic noise
🔹 Resonance in the motor or machine
🔹 Motor construction
🔹 Cable and motor interaction
🔹 Bearing or mechanical problems
🔹 Loose motor components

One useful clue is whether the sound changes when you change the VFD frequency or carrier-frequency setting.

If the sound changes with the VFD operating conditions, investigate the electrical side before assuming it's purely mechanical.

But don't ignore mechanical inspection either.

A strange sound is a symptom — not a diagnosis.

18/09/2026

Speed command is 100% steady, but the VFD output frequency drops under load? ⚙️⚡ Before swapping out your speed reference wiring or PLC card, check the motor current!

When heavy mechanical load hits, the VFD's internal Current Limit & Stall Protection activates to fold back output frequency—preventing an instant overcurrent trip and protecting the power module! 🔬🛠️

📍 YS Circuit Solutions – Karachi, Pakistan

📞 WhatsApp: +92-346-2438301

Address

HOUSE # 303 Sector 13/f Orangi Town
Karachi
75800

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Sunday 12:00 - 05:00

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