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Pump frequency control applications note
**Application Note on Pump Frequency Control**
**Date: December 12, 2005**
In recent years, frequency control in water supply systems has seen rapid development. However, in practical applications, there is still a significant amount of blindness, leading to unsatisfactory energy-saving results. This paper discusses some simple viewpoints regarding the use of frequency control for energy-efficient pumping, aiming to provide useful insights.
### 1. Frequency Control and Pump Energy Saving
Pump energy consumption is closely related to the reasonable adjustment of operating conditions. These adjustments can be categorized into two types: modifying the pipeline characteristic curve (e.g., valve throttling) or adjusting the pump characteristic curve (e.g., changing pump speed or impeller cutting). In terms of energy efficiency, altering the pump’s performance curve is far more effective than changing the pipeline characteristics [1]. As such, adjusting the pump's performance curve has become the primary method for energy saving in pumps. Frequency control offers clear advantages in both modifying the pump’s performance curve and enabling automatic control, making it widely adopted. However, it is important to note that many factors influence the energy-saving effectiveness of frequency control. Blind application may lead to adverse outcomes.
### 2. Factors Affecting the Frequency Control Range
Generally, reducing the pump speed is the main approach. When using frequency control, the original design parameters of the pump and motor are significantly altered. Additional factors, such as the pipeline characteristic curve, parallel operation with constant-speed pumps, etc., also affect the speed range. If the speed is out of the appropriate range, energy conservation goals may not be achieved. Therefore, frequency conversion speed regulation cannot be unlimited. It is generally recommended that the frequency-controlled speed should not be lower than 50% of the rated speed, ideally between 75% and 100%, and this should be determined based on actual calculations.
#### 2.1 Pump Process Characteristics and Speed Range
Theoretically, the efficient area of the pump speed lies within the parabolic middle area OA1A2 under similar conditions (see Figure 1). In reality, if the pump speed is too low, its efficiency drops sharply. As a result, the high-speed pump’s operational range is limited to PA1A2 [2]. If the operating point exceeds this area, energy savings are no longer guaranteed. The figure H0B represents the pipeline characteristic curve, and the CB section becomes the effective range for high-speed operation. To simplify calculations, it is assumed that point C lies on curve OA1, so the efficiency at points C and A1 is theoretically equal. Point C then becomes the left endpoint of the pump’s performance curve at its minimum speed.
From this, the minimum speed can be calculated as follows:
- Since C and A1 are in similar conditions, by the law of proportion:
$$
\left(\frac{Q_C}{Q_1}\right)^2 = \frac{H_C}{H_1}
$$
- Point C lies on the curve $ H = H_0 + S \cdot Q^2 $:
$$
H_C = H_0 + S \cdot Q_C^2
$$
- Solving these equations gives:
$$
H_C = \frac{H_1 \cdot H_0}{H_1 - S \cdot Q_1^2}, \quad Q_C = Q_1 \cdot \sqrt{\frac{H_0}{H_1 - S \cdot Q_1^2}}
$$
- Using the law of proportion again:
$$
n_{\text{min}} = n_0 \cdot \sqrt{\frac{H_0}{H_1 - S \cdot Q_1^2}}
$$
#### 2.2 Impact of Constant-Speed Pumps on Speed Range
In practice, water supply systems often involve multiple pumps operating in parallel. Due to high investment costs, it is not feasible to speed up all pumps, so variable-speed and constant-speed pumps are typically used together. In such systems, it is crucial to ensure that both types of pumps operate in their high-efficiency zones. At this stage, the constant-speed pump significantly affects the speed regulation range of the variable-speed pump running in parallel [2].
There are two main scenarios:
- **Same Type of Pump**: Running them side by side provides flexibility but limits the effective speed range.
- **Different Models of Pump**: If the head at the rated speed of the variable-speed pump matches the head of the constant-speed pump, the maximum speed range can be achieved. However, they must not run in parallel after switching.
#### 2.3 Effect of Motor Efficiency on Speed Range
Under similar working conditions, the shaft power is proportional to the cube of the speed (N ∠n³). As the speed decreases, the shaft power drops sharply. However, if the motor output power deviates significantly from the rated power or the operating frequency is too high, the motor efficiency will decline rapidly, affecting the overall efficiency of the pump unit. Additionally, continuous low-speed operation of the motor may cause overheating due to insufficient air circulation, posing risks to safe operation.
### 3. Pipeline Characteristic Curve and Energy-Saving Effect
Although changing the pump’s performance curve is the main method for energy saving, the energy-saving effects of speed regulation vary greatly depending on the pipeline characteristic curve. For clarity, Figure 2 is used here. In three water supply systems with the same design conditions (i.e., the maximum design point A), the same pump type is used, but the pipeline characteristics differ:
- â‘ $ H = H_1 + S_1 \cdot Q^2 $ (Hâ‚€ = Hâ‚)
- â‘¡ $ H = H_2 + S_2 \cdot Q^2 $ (Hâ‚€ = Hâ‚‚, Hâ‚ > Hâ‚‚)
- ③ $ H = S_3 \cdot Q^2 $ (H₀ = H₃ = 0)
If valve throttling is used, the flow QB corresponds to point B, with the shaft power NB. If speed regulation is applied, the flow QB corresponds to points C, D, E, with speeds nâ‚, nâ‚‚, n₃ and corresponding shaft powers NC, ND, NE. Since N ∠Q·H, the power satisfies NB > NC > ND > NE.
In systems with the pipeline characteristic curve $ H = H_0 + S \cdot Q^2 $, the smaller the Hâ‚€, the better the energy-saving effect. Conversely, when Hâ‚€ is large enough, the efficiency of the motor and the speed control system itself may cause the use of variable frequency speed regulation to not save energy, or even increase energy waste.
### 4. Comparison of Two Types of Speed Control Water Supply Energy Saving Effects
In water supply systems, variable frequency speed control typically involves two types of water supply: variable frequency constant pressure variable flow and variable frequency variable pressure variable flow. The former is more widely used, while the latter is technically more reasonable, though more complex to implement. It represents the future direction of pump frequency control technology.
#### 4.1 Variable Frequency Constant Pressure (Variable Flow) Water Supply
Constant pressure water supply ensures that the pump maintains the same head regardless of flow changes, matching the design head. Using valve throttling leads to wasted head (ΔH = Hâ‚ - H₃ = ΔHâ‚ + ΔHâ‚‚). With constant pressure water supply, the speed adjusts automatically to nâ‚, and the operating point moves to Bâ‚ (see Figure 3). This allows for continuous flow adjustment, keeping the operating point on the line H = Hâ‚‚. Setting a pressure control value at the pump outlet makes this mode simple and easy to implement. However, it only saves ΔHâ‚ without considering ΔHâ‚‚, making it less economical, especially in systems with large pipe resistance and steep pipeline curves.
#### 4.2 Variable Frequency (AC) Water Supply
This method follows the same principle as constant pressure water supply but uses different pressure settings. Instead of maintaining a fixed head, it moves along the pipeline characteristic curve (see Figure 3). When the flow decreases from Qâ‚‚ → Qâ‚, the speed adjusts to nâ‚‚, and the operating point moves to Bâ‚‚. At this point, the shaft power is less than that of constant pressure water supply, theoretically avoiding head waste and offering better energy efficiency.
However, this system essentially operates as a constant pressure system. It has two common forms:
- **Flow-based Head Determination**: Measuring the flow Q and using it to determine the pump head H via H = H₀ + S · Q². While theoretically possible, it is complicated in practice, especially in municipal water networks where accurate pipeline characteristic curves are hard to obtain.
- **Most Unfavorable Point Pressure**: Setting the most unfavorable point pressure Hm and adjusting the pump accordingly. This method faces challenges due to signal transmission and network fluctuations, making implementation difficult.
### 5. Conclusion
- Frequency control is a widely used energy-saving technology for pumps, but it has strict application conditions. It should not be blindly applied to any water supply system; instead, specific measures must be tailored to the actual situation.
- Frequency control is more suitable for systems with unstable flow, frequent changes, and large amplitude flows, where the proportion of pipeline loss in total head is larger.
- It is ideal for systems with stable flow, a single operating point, and a higher proportion of static lift in the total head.
- Variable frequency pressure water supply is superior to constant pressure water supply in terms of energy efficiency.