The Differences between Overload and Overcurrent of the Inverter in the PLC Control Cabinet
In the PLC control cabinet, overload and overcurrent of the inverter are two common faults. Although
both are related to abnormal current, there are significant essential differences. The specific comparisons
are as follows:
I. Core Definitions
| Type | Definition |
| Overload | The load current continuously exceeds the rated value (such as 110% - 150%), but does not reach the instantaneous protection threshold, which belongs to a long-term cumulative fault. |
| Overcurrent | The current rises sharply in an instant (usually several times to dozens of times the rated value), which belongs to a sudden fault. |
II. Core Differences
1. Protection Objects
- **Overload**: Mainly protects the **motor** to prevent the motor from being burned out due to
long-term overheating.
(Example: The wear of the motor bearing leads to an increase in the load, and the current remains
high continuously.)
- **Overcurrent**: Mainly protects the **inverter itself** to prevent internal components (such as IGBT)
from being damaged by the instantaneous large current.
(Example: The short circuit of the motor winding leads to a sudden increase in the current.)
2. Current Characteristics
| Characteristics | Overload | Overcurrent |
| Current Amplitude | Generally 1.1 - 1.5 times the rated value | Can reach more than 5 - 10 times the rated value |
| Rate of Change | The current rises slowly (small di/dt) | The current surges instantaneously (large di/dt) |
| Duration | Lasts for several seconds to dozens of minutes | Instantaneous (usually less than 1 second) |
3. Protection Mechanisms
- **Overload Protection**:
- **Inverse Time Characteristic**: The more the current exceeds the rated value, the shorter the
protection action time.
- Example: When the current is 150% of the rated current, it may trip in 30 seconds; when the
current is 120% of the rated current, it may trip in 5 minutes.
- **Overcurrent Protection**:
- **Instantaneous Trip Characteristic**: Trip immediately when the current exceeds the threshold
(such as 200% of the rated value).
- Example: When the motor is blocked, the current surges to 5 times the rated value instantaneously,
and the inverter triggers the protection within 0.1 seconds.
4. Common Alarm Codes
| Inverter Brand | Overload Code | Overcurrent Code |
| Siemens (MM440) | F0011 (or A0511) | F0001 |
| Mitsubishi | E.OL | E.OC |
| General Inverter | OL (Overload) | OC (Overcurrent) （Overcurrent） |
5. Fault Causes
- **Overload**:
- Excessive mechanical load (such as too tight belt, poor lubrication).
- Mismatch between the motor power and the inverter (a small motor driving a large load).
- Poor heat dissipation caused by long-term low-speed operation.
- **Overcurrent**:
- Short circuit or grounding fault of the motor or circuit.
- Mechanical jamming (such as blockage of the pump impeller).
- Damage to the inverter's inversion module.
6. Handling Methods
- **Overload**:
1. Check whether the mechanical load is normal (such as bearings, belts).
2. Increase the power of the inverter or adjust the parameters of the electronic thermal relay.
3. Improve the heat dissipation of the motor (such as installing an external fan).
- **Overcurrent**:
1. Disconnect the load and use a megohmmeter to detect the insulation of the motor and the circuit.
2. Check whether the output module of the inverter is damaged.
3. Extend the acceleration and deceleration time to avoid instantaneous impact.
III. Summary Comparison Table
| Dimension | Overload | Overcurrent |
| Keywords | Long-term, Cumulative, Motor Protection | Instantaneous, Impact, Inverter Protection |
| Current Multiple | 1.1 - 1.5 times the rated value | More than 5 times the rated value |
| Typical Scenarios | Gradual increase of the load | Short circuit, motor blocking |
| Solution Direction | Optimize load matching and heat dissipation | Check for short-circuit points and hardware faults |
**A Sentence for Memory**:
**Overload is like "boiling a frog in warm water" (slow heating leads to death), and overcurrent is
like "the instant of electric shock" (instantly fatal)**. Understanding the differences between the two can quickly locate faults and avoid misjudgments.
