On industrial automation sites, the two most common protective trips of a variable frequency drive (VFD / frequency converter) are overvoltage (OV) and overload (OL). The former means the DC-bus voltage exceeds the safe threshold; the latter means the output current exceeds the rated capability for a sustained period. They have different causes and different protection logic but are often conflated, causing on-site personnel to keep troubleshooting while trips recur. Based on international standards and publicly available engineering documents from major VFD manufacturers, this article explains the causes of OV and OL, the drive's internal optimized protection logic, and on-site diagnosis and parameter-optimization recommendations, helping engineers reduce unnecessary downtime while keeping equipment safe.

1 OV Overvoltage: Causes and Diagnosis

1.1 What Is DC-Bus Overvoltage

A VFD first rectifies AC into DC, stores it in DC-bus capacitors, and then outputs variable-frequency power through the inverter. After three-phase rectification, the DC-bus voltage is approximately 1.35 times the line voltage; for example, a 400 V supply produces a bus voltage of about 540 V under load. To prevent damage to the bus capacitors and power devices, when the bus voltage exceeds the overvoltage threshold the drive's protection operates and outputs an OV trip. For 3AC 400 V-class drives, the DC overvoltage threshold is typically about 820 V DC. The threshold varies by manufacturer and voltage class and is usually fixed in hardware: it cannot be modified and cannot be masked.

1.2 Three Main Causes of Overvoltage

First, the supply-side voltage is too high. Grid fluctuation, power-factor-correction capacitor banks switching in, and switching overvoltages on the supply side can raise the DC bus directly. Second, load-energy regeneration, the most common cause. When a high-inertia load decelerates quickly, a potential-energy (hoisting/lowering) load moves, or the load suddenly drops, the motor is dragged by the load and enters generating mode; energy flows back along the motor into the DC bus and the bus voltage rises rapidly. Third, hardware and wiring problems, such as a faulty DC-voltage sensing circuit, output cable or motor ground faults, or loose terminals, can cause false or genuine overvoltage.

2 OL Overload: Definition and Causes

2.1 What Is Overload Protection

Overload means the drive's output current exceeds the rated value and lasts for a period. Drives commonly use electronic thermal overload protection (I²t), integrating the product of current squared and time to model the thermal buildup of the motor and power devices; the operating time is inversely proportional to the current (inverse-time characteristic): the higher the current, the shorter the allowed running time. This differs from short-circuit protection and instantaneous overcurrent protection; it mainly guards against overheating damage from long-term thermal accumulation.

2.2 Light Duty vs. Heavy Duty and Overload Capability

Drives are typically classified by overload capability into normal duty (ND) and heavy duty (HD). A typical definition is: light duty allows about 110% of rated current for 60 s (on a 300 s cycle); heavy duty allows a higher overload (e.g., 150%). Parameters differ among manufacturers; refer to the specific model's nameplate and manual. Overload capability directly affects sizing: heavy-duty applications with frequent overloads should use a heavy-duty model with lower rated power, or a larger-capacity model, to avoid prolonged overload that triggers OL trips.

2.3 Common Causes of Overload

Mechanical load too high, stalling, or jamming; acceleration/deceleration time too short, causing excessive starting and accelerating current; mismatch between VFD and motor sizing; poor motor cooling or inadequate cooling during prolonged low-frequency operation; and improper parameter settings (for example, wrong rated current or electronic thermal overload threshold) can all trigger overload protection.

3 The Drive's Optimized Protection Logic

3.1 Optimized Protection Logic on the Overvoltage Side

At the software level there are “overvoltage stall prevention” and the “maximum voltage controller”. The drive continuously monitors the DC-bus voltage; when it approaches the overvoltage threshold, the drive automatically extends the deceleration ramp, temporarily slows down, or reduces the output frequency, so the regenerative energy is absorbed within the motor and system and the bus voltage stops rising; after the bus returns to normal, the original setting is restored. This function is available in both V/F and vector control.

At the hardware level there is the “braking chopper plus braking resistor”: when the bus voltage exceeds the set value, the braking chopper's power device conducts and dissipates the regenerative energy as heat through the braking resistor, bringing the bus voltage down. It should be emphasized that the braking resistor's role is to dissipate the regenerative energy fed back to the DC side by the load inertia, not to directly produce braking force.

For applications with frequent braking and high energy-recovery value, a regenerative unit / active front end (AFE) can feed the regenerative energy back to the grid for higher efficiency. Multiple drives can also be interconnected on a common DC bus, so regenerative energy from one braking drive is absorbed by other units that are motoring, reducing braking-resistor requirements.

3.2 Optimized Protection Logic on the Overload Side

Electronic thermal overload (I²t) inverse-time protection: continuously monitors current and models thermal buildup; when the set threshold is exceeded, it acts along the inverse-time curve. This is the core of overload protection.

Stall prevention and current limiting: when the output current exceeds the stall-prevention level, the drive automatically lowers the output frequency or limits current so the current returns to an allowed range, preventing an overcurrent trip; after the current recovers, the set frequency is restored.

Underload detection: alarms or stops when the load stays below the set value for a long time, indicating mechanical or process anomalies such as belt slip or idling. Overload pre-alarm: gives an early warning before the trip threshold is reached, allowing the process side to respond in time and reducing sudden stops.

4 On-Site Diagnosis and Optimization Recommendations

OV and OL have different causes; it is recommended to distinguish the fault nature first, as shown in Table 1.

Table 1 OV vs. OL Troubleshooting Comparison

For parameter optimization: for OV, extend the deceleration ramp appropriately and enable the maximum voltage controller / overvoltage stall prevention; for frequent deceleration or potential-energy loads, configure a braking unit with a braking resistor, and verify the resistor's minimum resistance and braking power per the manufacturer. For OL, check the VFD rated current and motor rated current settings, check whether the accel/decel time is too short, confirm the electronic thermal overload threshold matches the motor nameplate (self-cooled motors need a lower allowed current at lower frequencies), and check the installation environment and cooling.

5 Conclusion

OV overvoltage and OL overload are two essentially different but common trips that can both be significantly mitigated by optimized protection logic. Understanding the source of DC-bus voltage and the integration principle of electronic thermal overload is the basis for correct sizing, commissioning, and troubleshooting. On site, systematically investigate the four dimensions of supply, load, hardware, and parameters, and work with manufacturer manuals and the IEC 61800 series standards to reduce unnecessary downtime while ensuring equipment safety, improving line availability.

6 References

[1] IEC 61800-5-1:2007+A1:2016, Adjustable speed electrical power drive systems – Part 5-1: Safety requirements – Electrical, thermal and energy. IEC Webstore.

[2] Siemens Industry Online Support. How to deal with fault F0002 (DC-link overvoltage) of MM4 series frequency converters. 2015-11.

[3] ABB. Electrical Braking, Technical Guide No. 8. 2010.

[4] Mitsubishi Electric. FREQROL-F800 Series, Protective Functions.

[5] GB/T 12668.1-2002, Adjustable speed electrical power drive systems – Part 1: General requirements – Rating specifications for low voltage adjustable speed d.c. power drive systems. Standardization Administration of China.

[6] Schneider Electric. ATV630 heavy-duty (HD) and normal-duty (ND) overload capability. 2020.