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In an oven circulation fan, heat reaches the motor in three ways: by conduction through the fan shaft and mounting plate, by radiation from the hot oven casing, and through the warmed air drawn in by the motor's own cooling fan. This heat mainly wears the winding insulation, the bearing grease and the cables in the terminal box. A standard three-phase motor is therefore operated outside the hot zone, at a point where it can draw cool air; the air temperature at the motor location matters as much as the power rating.
Rather than repeating the general selection of an oven fan motor, this article focuses on what heat damages in the motor, how to recognise it on site and how the findings translate into motor selection.
Which part of the motor does heat affect, and how?
Not all parts of the motor are affected equally. Knowing where a failure starts makes it easier to tell whether the problem lies in the motor or in the fan arrangement.
| Part | Effect of heat | Symptom seen on site |
|---|---|---|
| Winding insulation | The insulating material ages with temperature and loses its flexibility and dielectric strength. | Falling insulation resistance, eventually a phase-to-frame or inter-turn short circuit |
| Bearing and grease | The grease thins, loses its oil and dries out; the lubricating film weakens. | Increasing noise at the front end shield, grease leakage, bearings failing quickly |
| Shaft and front end shield | Heat coming from the fan side travels along the shaft and reaches the front bearing first. | Front end shield noticeably hotter than the rear one |
| Terminal box and cables | Cable sheaths harden and seals lose their elasticity. | Brittle cable insulation, loosening connections, darkened terminals |
| Fan cover and cooling air | If the air drawn in by the motor is hot, the frame's ability to reject heat drops. | Excessive frame temperature even at normal current, thermal protection tripping |
The key point in the table is this: most symptoms suggest that the motor is overloaded, yet the current may be below the nameplate value. In that case the problem is not a lack of power but heat being carried into the motor from outside.
What does class F insulation tell you, and what does it not?
The three-phase models sold by HEM Motor have class F insulation and IP55 protection. Class F defines the maximum temperature class that the winding materials can withstand (155 °C in the standard). This limit applies to the hottest point inside the winding, not to the air around the motor.
Winding temperature is the sum of two parts: the temperature of the motor's cooling air and the temperature rise the motor produces itself under load. As the air next to the oven gets hotter, the first part grows and, at the same load, the winding approaches its class limit. According to a common engineering rule of thumb, every permanent rise of roughly 10 degrees in winding temperature shortens insulation life markedly, by roughly half. So even if the motor keeps running, its life quietly shrinks at a hot location.
Nameplate values are given for standard ambient conditions. If the air at the motor location is hotter than that, derating or a larger frame may be needed. For this assessment you need to state the ambient temperature with the order; a value measured on site gives a far better result than an estimate. For details of the cooling arrangement, see the article on IC411 and IC416 cooling methods.
How can you tell on site whether heat is reaching the motor?
Before ordering a new motor, a few simple measurements show where the problem comes from. Take them after the oven has reached operating temperature and the motor has run for at least an hour.
- Intake air: Measure the air temperature behind the motor's fan cover, where the air is drawn in. Note how much hotter it is than the workshop in general.
- Front and rear end shield comparison: Use a non-contact thermometer on the front end shield at the shaft side and the rear end shield at the fan side. If the front one is clearly hotter, heat is being conducted through the shaft.
- Current measurement: Measure the current in all three phases both with the oven cold and at operating temperature, and compare it with the nameplate current.
- Frame surface: If there is a clear difference between the side of the frame facing the oven and the opposite side, radiation is involved; a heat shield can be placed in between.
- Visual check: Look for hardened cable sheaths, cracked terminal box seals and grease traces around the bearings.
If the frame and front end shield are still very hot while the current is below the nameplate value, buying a larger motor alone will not solve it. First the heat barrier, the cooling disc on the fan side and the airflow around the motor must be corrected.
