Example of Variable Torque Load

Fans, pumps, and compressors are absolute representatives of variable torque loads, and they occupy the vast majority of industrial frequency converter applications.

1. Fan

Example: Cooling tower fans, factory/building supply/exhaust fans, boiler induced/supply fans, tunnel ventilation fans.

Working principle: The fan overcomes air resistance to do work. According to the law of similarity in fluid mechanics (the proportionality law of fans):

The air volume (Q) is proportional to the rotational speed (n): Q ∝ n

Pressure (P) is proportional to the square of rotational speed: P ∝ n ²

Shaft power (kW) is proportional to the cube of rotational speed: kW ∝ n ³

For example, if a fan requires 100% power at 100% speed. When the speed demand drops to 80%:

The air volume has decreased to 80%.

The power consumption has decreased to (0.8) ³=0.512, which is 51.2% of the rated power.

Energy saving effect: up to nearly 49%! This is precisely why frequency converters have enormous energy-saving potential in fan applications.

2. Pumps (centrifugal pumps)

Examples: circulating water pumps for central air conditioning, water supply pumps for water plants, cooling water pumps for factories, and sewage pumps.

Working principle: It is completely similar to a fan and follows the proportional law of a centrifugal pump. Similarly:

Flow rate ∝ speed

Head (pressure) ∝ Speed ²

Shaft power ∝ speed ³

Scenario: During non full load operation (such as at night, winter, or when production load is reduced), reducing the pump speed through a frequency converter to regulate flow or pressure instead of throttling through valves/baffles can achieve significant energy savings.

3. Compressor (centrifugal)

Example: Centrifugal chillers for large central air conditioners and centrifugal compressors for factory processes.

Working principle: As a centrifugal machine, the cubic relationship between power and speed still holds. Controlling the outlet pressure or flow rate by adjusting the speed is much more efficient than the traditional “loading/unloading” control method.

Summary of Key Characteristics of Variable Torque Load:

1. Torque characteristics: When starting, a low starting torque is required (usually lower than the rated torque), and the operating torque increases sharply with the increase of speed (torque is approximately proportional to the square of speed: T ∝ n ²).
2. Power characteristics: The power is proportional to the cube of the rotational speed (P ∝ n ³). This is its most core mathematical feature and also the root of energy conservation.
3. Load curve: In the torque speed coordinate system, it presents a parabolic shape.
4. Energy saving principle: Since power is directly proportional to the cube of rotational speed, even a slight decrease in rotational speed can result in a significant reduction in power. The frequency converter matches the actual demand by adjusting the speed, avoiding the huge energy waste caused by throttling through baffles and valves during full speed operation at the power frequency.
5. Selection of frequency converter: For this type of load, a universal V/F control mode frequency converter is usually sufficient to meet the requirements, as it does not require high torque accuracy and dynamic response at low speeds. Of course, choosing a frequency converter with energy-saving optimization or dedicated pump/fan mode would be more effective.