A 7.5 kW motor on a VFD was tripping on thermal overload every two weeks. The motor was rated for 40°C ambient. The application was a screw conveyor running at 30 Hz. The motor was running at 15 Hz for 10 minutes, then ramping to 30 Hz. The VFD was set to 10 kHz carrier. The thermal overload was set to motor nameplate current. The motor kept overheating. The customer thought the VFD was undersized. It wasn’t. The problem was cooling.
Why VFD motors overheat at low speed
A TEFC motor has a fan on the shaft that blows air over the cooling fins. At full speed (1500 RPM), the fan moves enough air to keep the motor cool at rated load. At 30 Hz (900 RPM), the fan moves half the air. The motor can only deliver about 60% of rated torque without overheating. At 15 Hz (450 RPM), the fan moves a quarter of the air. The motor delivers 25% of rated torque without overheating. The screw conveyor was asking for 80% torque at 15 Hz. The motor was cooking itself.
The VFD’s thermal model should catch this. But most VFDs use a simple I²t model that assumes full fan cooling at all speeds. They don’t derate the motor for reduced fan speed. The motor overheats before the VFD trips. The customer sees the motor’s internal thermal switch open. The VFD says “all normal.” The motor says “I’m burning up.”
What fixed it
Three changes. First, the VFD was set to 4-pole motor with a separate cooling fan. The existing motor didn’t have a separate fan — it relied on the shaft fan. We switched to a force-ventilated motor (TEBC) with a 230V blower that runs continuously regardless of shaft speed. The blower moves enough air at any speed to keep the motor cool at rated torque. The motor didn’t overheat again. The TEBC motor costs 30% more than TEFC but it’s the right motor for low-speed continuous duty.
Second, the VFD was set to a motor thermal model that accounts for fan speed. Most modern VFDs have a “motor overload compensation” parameter that derates the current limit as speed drops. Set it to match the motor’s low-speed torque curve. The VFD now limits current at 15 Hz to 60% of rated instead of 100%. The motor isn’t asked to deliver more than it can cool.
Third, the carrier frequency was dropped from 10 kHz to 4 kHz. Higher carrier frequency causes more switching losses in the motor. At 10 kHz, the motor runs hotter than at 4 kHz. The tradeoff is audible noise — 4 kHz is in the audible range, so the motor whines. For a screw conveyor in a plant, the whine is acceptable. For a quiet office environment, use 8 kHz and a TEBC motor. The carrier frequency shouldn’t exceed 8 kHz on standard motors without derating.
The derating table every VFD user should know
| Speed (% of base) | TEFC motor torque (% of rated) | TEBC motor torque (% of rated) |
|---|---|---|
| 100% | 100% | 100% |
| 75% | 85% | 100% |
| 50% | 60% | 100% |
| 25% | 30% | 100% |
| 10% | 10% | 90% |
If your application runs continuously below 50% speed, use a TEBC (force-ventilated) motor. If you can’t change the motor, derate the VFD current limit according to the table. Don’t ask a TEFC motor to deliver rated torque at half speed — it will cook itself.
TEFC motors don’t cool well at low speed. The screw conveyor overheating wasn’t a VFD problem — the motor was delivering torque at 15 Hz with no fan cooling. Use TEBC motors for low-speed continuous duty, set the VFD derating, and don’t run carrier frequency above 8 kHz on standard motors. The motor’s thermal switch isn’t being dramatic.
