When a motor is driven by a VFD at low frequency, it may run much hotter than expected.
Why?
1️⃣ The Motor's Cooling Fan Runs Slower
For a standard induction motor, the cooling fan is usually mounted directly on the motor shaft.
When the VFD reduces the frequency:
Lower Frequency → Lower Motor Speed → Lower Fan Speed → Less Cooling Airflow
For example, at 25 Hz, the motor may run at roughly half its rated speed, while the cooling fan also slows down significantly.
As a result, the motor cannot dissipate heat as effectively.
2️⃣ Low Speed Doesn't Always Mean Low Current
This is another common misunderstanding.
Even when the motor is running slowly, if the mechanical load remains high, the motor may still draw considerable current.
And:
Higher Current → Higher Copper Loss → More Heat
Copper loss is approximately proportional to I²R, so a significant increase in current can cause a substantial increase in heat generation.
3️⃣ V/F Settings Also Matter
At low frequency, incorrect V/F parameters or excessive torque boost can cause the motor to draw excessive current and generate additional heat.
Therefore, proper VFD parameter settings are important.
How Can We Reduce Motor Heating?
- Avoid continuous operation at extremely low speed under heavy load
- Use an external cooling fan when necessary
- Optimize V/F or vector control parameters
- Monitor motor current and temperature
- Select a motor designed for VFD operation for demanding applications
The Key Point
Low frequency itself doesn't necessarily cause motor overheating.
The real problem is usually the combination of:
Low Speed + Reduced Cooling + High Load + High Current
That's why VFD applications need to consider both motor speed and cooling capacity, not just frequency.
Your message must be between 20-3,000 characters!
