Rotary Valve Motor — Thermal Duty
Two objections come up whenever a rotary valve is put on demand-driven pull control: that pulsing it short-cycles the motor, and that slowing it on an inverter instead would be kinder. Both describe something real, but they belong to different regions of the same chart. This puts all three duties on one motor and shows where each of them actually lands.
Rotary valve motor — thermal duty
| Duty | Average loss | Steady winding temp | Of rated rise |
|---|
Use the nameplate. A four-pole IE3 motor of this rating is near 0.80; smaller frames are markedly worse. Expect the chart to barely move: a more efficient motor loses less heat but sheds it through a correspondingly higher thermal resistance, so it still reaches its rated rise at rated load. Efficiency shows up in the rated loss quoted below, and in how much a start costs.
Duty A — pulsed
Duty C — turned down
Model assumptions
How much worse the frame sheds heat with the shaft fan barely turning, as a multiple of its fan-cooled value. Held as a ratio rather than an absolute K/W so it still describes the same frame when the rating or efficiency changes. Currently 1.28 K/W against 0.43 K/W fan-cooled.
Reading the verdicts
- Percentages are of the motor’s rated temperature rise, not of the distance to the Class F limit. A motor at rated load is at 100% by definition — that is its design point, not spare capacity.
- Standards basis: EN/IEC 60034-1 Table 7, resistance method — Class B 80 K, Class F 105 K, Class H 125 K at 40 °C maximum ambient. Class F insulation built to a Class B rise is the common industrial specification, giving 25 K of margin.
- Insulation life is roughly 20,000 hours at the class maximum, and halves for every 10 °C above it.
- Single-body model: it reports an average winding temperature and understates the hotspot, which flatters duty C rather than duty A.