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On greenhouse and poultry house fans, the propeller speed must be kept, because the propeller diameter and blade angle were chosen for a particular speed: if the speed rises the motor is quickly overloaded, and if it falls the air exchange needed by the animals or plants is not achieved. When the motor is replaced, the number of poles, the pulley ratio and the direction of rotation are kept the same as before, as well as the power. This article explains why and how the propeller speed is preserved when a burnt-out fan motor is replaced.
Two drive arrangements on greenhouse and poultry house fans
How the motor is selected depends on how the fan drives the propeller. Two arrangements are common on site:
- Belt-driven box fans: A large-diameter propeller is driven by pulley and belt. The motor is usually fixed to a small bracket on top of or beside the fan housing. The propeller speed is much lower than the motor speed and is set by the pulley ratio.
- Direct-drive axial fans: The propeller is mounted directly on the motor shaft. The propeller speed equals the motor's rated speed; the motor is usually fixed by flange or feet to a support in the middle of the fan ring.
In both arrangements the motor runs in the air the fan moves. In poultry houses this air carries dust, feathers, moisture and ammonia; in greenhouses there is high humidity and, at times, spraying vapour. That is why ambient conditions are the second most important topic after speed.
What happens if the speed changes? Fan laws in practice
For the same fan and the same building, the air moved by the propeller changes roughly in proportion to speed, the pressure it creates with the square of speed and the shaft power roughly with the cube of speed. In practice this means:
| Change | Airflow | Motor load | Result on site |
|---|---|---|---|
| Speed rises by about 10% | Rises by about 10% | Rises by about one third | Motor overheats, thermal relay trips; noise increases |
| Speed falls by about 10% | Falls by about 10% | Falls markedly | Insufficient ventilation in summer heat, heat stress |
| 2 poles instead of 4 (direct drive) | Tends to roughly double | Rises several times | Motor burns out; propeller may exceed its permitted speed |
| 6 poles instead of 4 (direct drive) | Falls by about one third | Drops below half | Fan appears to run but air exchange is not enough |
The ratios in the table are approximate results of the fan laws; actual values vary with the fan and the building. But the direction is always the same: a small rise in speed loads the motor disproportionately. In a poultry house, the number of fans and their stages are calculated for stocking density and the need on the hottest day; even one fan turning more slowly can upset the balance of staged ventilation.
How is speed preserved on belt-driven fans?
On a belt-driven box fan, the propeller speed results from the motor's rated speed and the ratio of the pulley diameters. Points to watch when replacing:
- Keep the number of poles: If the old motor was 4-pole, the new motor must also be 4-pole. Fitting a motor with a different number of poles and compensating by changing the pulley is possible, but it must be confirmed by calculation that the new ratio gives the speed specified by the fan manufacturer.
- Look at the rated speed: The speed on the nameplate is not synchronous but the speed under load. For example, the 4-pole 1.1 kW model shows 1,445 rpm and the 0.75 kW model 1,420 rpm. This small difference can change the propeller speed by a few percent; that is another reason why replacing at the same power is preferred.
- Move the pulley to the new shaft: The bore and keyway of the motor pulley must fit the new motor's shaft. Shaft dimensions vary with the frame; the values are in the 2026 technical and product catalogue.
- Tension the belt correctly: An overtight belt loads the motor's front bearing, while a loose belt slips and lowers the propeller speed. Belt slip is a common reason why ventilation stays insufficient despite a new motor.
What the motor must handle on direct-drive fans
When the propeller is mounted directly on the shaft, the motor carries not only the torque but also the propeller's weight and thrust. Therefore:
- The shaft diameter and length of the new motor must suit the propeller hub, and the hub must be fitted at its previous position on the shaft. If the propeller's position relative to the ring changes, the tip clearance is disturbed and airflow drops.
- Whether the fan runs horizontally or inclined determines the motor's mounting position. Mounting arrangements are offered according to the frame; the flange size is confirmed with the order.
- The propeller's direction of rotation must match the arrow on the fan housing. On a three-phase motor the direction is reversed by swapping two phases. A propeller turning backwards moves much less air, but it may go unnoticed because the motor appears to be running.
- A propeller with broken or dust-coated blades is unbalanced. Clean the propeller before fitting the new motor and replace it if damaged; otherwise the vibration passes to the new motor's bearings.
