Protect Your Motors: Understanding the Thermal Risks of Low-Frequency Operation

Protect Your Motors: Understanding the Thermal Risks of Low-Frequency Operation

Introduction

Induction motors are the backbone of industrial operations, known for their efficiency and reliability. However, operating below a certain minimum frequency — typically 30 Hz — can lead to overheating and premature failure. This article explains the technical reasons, including formulas and data, behind maintaining the 30 Hz safe operating limit.

1. Cooling Efficiency Reduces at Low Frequency

A standard TEFC (Totally Enclosed Fan-Cooled) motor uses a shaft-mounted fan. Fan speed and cooling air flow decrease directly with frequency.

Relationship:

N ∝ f

Since air flow depends on fan speed cubed:

Q ∝ N³

At 30 Hz (60 % of rated speed):

Q = (0.6)³ = 0.216 → 21.6 % of rated cooling air flow

This means only one-fifth of the normal cooling capacity is available, while losses remain significant — causing heat buildup.

2. Magnetic Flux Increases if Voltage Is Not Reduced

Magnetic flux (Φ) is proportional to the ratio of voltage to frequency:

Φ ∝ V / f

If voltage is not reduced in proportion to frequency, the magnetic core saturates, increasing iron losses and current draw.

Example for a 400 V, 50 Hz motor:

V_required = (30 / 50) × 400 = 240 V

If the voltage remains 400 V at 30 Hz, flux increases by about 67 %, causing excess magnetizing current and heating.

3. Copper Losses and Slip Increase

To maintain torque at low speed, the motor draws more current, which raises copper losses in both stator and rotor windings:

P_cu = I² × R

For example, if current increases from 10 A to 12 A:

(12² / 10²) = 1.44 → 44 % higher copper loss

At low cooling airflow, this additional loss quickly leads to overheating.

4. Practical Test Data (Example)

Below 30 Hz, the motor temperature can exceed 100 °C — beyond insulation class limits — leading to thermal aging and failure.

5. Recommended Practice

• Maintain at least 30 Hz for continuous operation under load.
• Use an external or forced cooling fan if running below 30 Hz for long periods.
• Always maintain a constant V/f ratio with a properly configured VFD.
• Use inverter-duty motors for wide-speed applications.

Conclusion

Maintaining a minimum operating frequency of 30 Hz ensures a safe balance between motor cooling, magnetic flux, and copper losses. Continuous operation below this limit without additional cooling can cause rapid temperature rise, insulation breakdown, and a shorter motor lifespan.