1. Affinity Laws and Pump Motor Power
The centrifugal pump affinity laws describe the relationship between pump speed, flow rate, head (pressure), and shaft power. These laws are fundamental to understanding why VFD speed control saves so much energy on centrifugal pump drives, and why the motor must be sized for the worst-case operating point on the pump curve rather than the average duty point.
The cubic power law is the reason why VFD speed control on centrifugal pumps delivers large energy savings. A pump running at 80% of rated speed delivers 80% of rated flow but consumes only 80³ = 51.2% of rated power. For a 22 kW pump motor running at 80% speed for 6,000 hours per year at $0.13/kWh, the annual saving compared to throttle-valve control at full speed is approximately 22 × (1–0.512) × 6,000 × 0.13 = $8,400 per year.
2. Motor Power Sizing for Pump Drives
The motor shaft power required to drive a centrifugal pump is determined by the pump hydraulic power (flow rate × head × fluid density) divided by the pump hydraulic efficiency at the design duty point. Add a service factor to account for operation away from the best efficiency point (BEP) and for end-of-life impeller wear.
P = (0.05 × 40 × 1,000 × 9.81) ÷ (1,000 × 0.72) = 27.25 kW
With 1.15 service factor: 27.25 × 1.15 = 31.3 kW
→ Select Y2 series 37 kW, 4-pole, IE3
| Pump Application | Typical Pump η | Service Factor | Typical Motor Range | Y2 Pole |
|---|---|---|---|---|
| Building water supply | 0.65–0.78 | 1.10 | 0.75–30 kW | 2P or 4P |
| HVAC chilled water | 0.70–0.82 | 1.10–1.15 | 3.0–75 kW | 2P or 4P |
| Industrial process water | 0.68–0.80 | 1.15 | 1.5–55 kW | 2P or 4P |
| Irrigation / water transfer | 0.62–0.75 | 1.15 | 2.2–90 kW | 2P or 4P |
| Slurry / waste water pump | 0.55–0.70 | 1.25 | 2.2–75 kW | 4P |
3. Starting Method: DOL, Star-Delta, and Soft-Start
The starting method determines how much starting current the motor draws from the supply and how much torque is available during acceleration. For centrifugal pumps, the choice is usually straightforward because the pump load at zero speed is near zero (centrifugal pumps start unloaded against a closed valve), meaning any starting method that provides sufficient torque to overcome bearing friction and accelerate the impeller will work.
Starting current 6–7.5× rated. Suitable for pump motors up to approximately 11 kW on a stiff supply where the voltage dip during starting is below 10% of rated. Above 11 kW, check with the supply authority whether DOL starting is permitted. DOL provides maximum starting torque — more than adequate for a centrifugal pump starting against a closed valve. Simple and low-cost starter; no maintenance beyond the contactor.
Reduces starting current to 33% of DOL value and starting torque to 33% of DOL value. Suitable for centrifugal pumps because the load torque at start is low. Requires a 6-terminal motor (standard on Y2 series) and a star-delta timer relay. The switchover from star to delta at approximately 80% of synchronous speed causes a torque and current transient — ensure discharge valve is closed or partially closed during star-delta switchover to avoid water hammer.
Soft-starter provides a smooth, adjustable torque ramp that eliminates both the starting current spike and the water hammer from rapid valve-open. VFD provides the same smooth start plus variable speed operation — the combination of smooth start and speed control makes the VFD-plus-YVF2-motor the preferred specification for pump systems above 7.5 kW where flow control is required. See the Korea Ever-Power YVF2 VFD pump drive article for the full VFD pump application guide.
4. Duty Classification for Pump Service
Most centrifugal pump applications fall into S1 (continuous duty) — the pump runs continuously at approximately constant load for the full working shift or 24 hours per day. This is the most demanding duty class thermally, and the Y2 nameplate power is the maximum continuous output at 40°C ambient.
Building water supply circulator, chilled water pump in HVAC, process cooling water, boiler feed pump, fire pump on test run, irrigation main pump. Motor runs continuously at rated load. IE3 efficiency choice has the greatest financial impact in this duty class — a 22 kW IE3 pump motor saves approximately $500 to $900 per year in electricity compared to an IE1 motor at the same duty.
Pressure booster pumps in building services that start and stop based on demand signal (pressure switch or flow sensor) operate in S3 intermittent duty. The motor runs for periods of 1 to 10 minutes, then rests. At a 40% on-time duty factor, the motor can operate at up to 115% of its S1 nameplate power without exceeding thermal limits. However, frequent starting also generates heat from starting losses — confirm the motor’s maximum starts per hour limit in the Y2 data sheet.
Fire pump motor note: fire pump motors must be selected for locked-rotor (stall) duty — the pump may be required to run against a closed valve (zero flow) at full speed for a period, which causes the pump to heat the circulating water. The motor must remain operational and not trip on overload during this condition. Korea Ever-Power Y2 series motors with Class F insulation meet the thermal requirements for fire pump service under the locked-rotor condition duration specified in EN 12845 and NFPA 20.
5. IP Rating for Pump Room Environments
Clean to moderately dusty indoor pump rooms with ambient humidity below 90% RH, no direct water spray. Standard for building services chilled water pumps, HVAC circulating pumps, and industrial process water pumps in enclosed plant rooms. Y2 series standard specification at IP54.
Outdoor pump stations exposed to rain, or indoor pump rooms subject to hose-down cleaning or water spray from pipe leaks. Water treatment plant pump stations, irrigation pump houses, and industrial process plant pump areas specify IP55. Y2 series with IP55 option or YB2 series for hazardous area outdoor installation.
Chemical plant, oil and gas facility, and solvent handling pump stations in Zone 1 or Zone 2 classified areas require explosion-proof motors. Korea Ever-Power YB2 series Ex d IIB T4 IP55 is the standard specification. Gas group must be matched to the specific materials in the classified area — IIA for propane/butane, IIB for ethylene, IIC for hydrogen or acetylene.
6. Korea Ever-Power Y2 Series for Pump Drives
The Y2 series is the standard motor for centrifugal pump drives in the Korea Ever-Power range. For fixed-speed pump drives, the Y2 series delivers IE3 efficiency at a competitive price across the complete 0.18 to 200 kW range. Centrifugal pumps are direct-coupled to the motor shaft via a flexible coupling — no gearbox is required because the pump and motor operate at the same speed (matched at 1,450 rpm or 2,900 rpm for 4-pole and 2-pole motors at 50 Hz). The complete Y2 motor range is available in the three-phase motor product section.
For pump systems requiring variable flow control, Korea Ever-Power recommends the YVF2 inverter-duty motor paired with a VFD. The YVF2 provides IC416 forced ventilation for full rated torque at reduced speeds, Class H insulation to withstand VFD voltage spikes, and PTC thermistor protection for the most demanding pump applications. Please contact the technical team for VFD pump drive sizing support.
| Power range | 0.18 – 200 kW |
| Typical poles | 2P (2,900 rpm) or 4P (1,450 rpm) |
| Efficiency | IE3 standard |
| Protection | IP54 standard; IP55 option |
| Duty | S1 continuous |
| Starting torque | 2.0–2.8× rated (adequate for pump) |
| Coupling type | Direct flexible coupling (no gearbox) |
| Certification | CE, ISO 9001, IEC 60034 |
7. Applications by Pump Type
Pressure booster sets, domestic water supply pumps, and fire fighting jockey pumps. Y2 2-pole (2,900 rpm) motors in 0.75 to 7.5 kW range, direct-coupled to multistage vertical centrifugal pumps. S3 duty for pressure booster, S1 for primary supply. IP54 for indoor plant room.
Primary and secondary chilled water pump sets in central plant HVAC systems. Y2 4-pole 3.0 to 75 kW, VFD controlled for variable flow in primary-secondary or variable primary systems. YVF2 inverter-duty motors for all VFD-controlled pump drives.
Cooling water circulation, heat exchanger service pumps, reactor cooling loops. Y2 4-pole 2.2 to 55 kW, S1 continuous. Where process fluid is corrosive, specify stainless pump with standard Y2 motor — the motor itself does not contact the fluid.
Agricultural irrigation, water supply transfer, and reservoir filling pumps. Y2 4-pole in 5.5 to 90 kW, often operated seasonally with extended downtime — follow storage maintenance procedure. IP55 for outdoor installation. Star-delta or soft-start for long pipe runs to prevent water hammer.
8. Frequently Asked Questions
Edited by Cxm
