Energy Savings with Variable Speed Drives – Pumps

3-Phase Hot Or Cold Water Pump

Typically installed in many applications operating a continuous process and used for cooling/heating or supply purposes. When a pump motor is connected directly to the mains power supply it can only run at one speed, maximum. Traditionally a valve is then fitted into the pipe work to reduce the flow to the required level. The valve can be on the inlet (suction) side or on the outlet side of the pump and is normally closed by some percentage to control the flow rate.

Example (Before)

  • Pump Motor: 15Kw
  • Duty: 24 Hrs, 7 Days, 51 Weeks
  • Control: On / Off
  • Flow Control: Valve Position: 20% Closed
  • Average Cost Of Electricity: 10P/Kwhr

Annual cost of operating


= 15 x (24 x 7 x 51) x 0.1

= £12,852 per year

Example (After)

  • Install Variable Speed Drive (VSD) between manual motor starter and motor.
  • Reduce output frequency until flow from pump is required level of 80% (same as previous level when valve was installed and partly closed).
  • Valve can be removed (unless required for maintenance).

Cold Water Pump Example

Using Affinity Law 3, Power ∞ Speed²

Reduce motor speed (VSD output frequency Hz) to 80% ie from 50Hz to 40Hz

The same as 80% x 80% x 80% or 0.8 x 0.8 x 0.8 . . . = approx 50%

So the new operating power for the pump will be 15KW x 0.5 = 7.5KW

SO… The new annual cost of operating this cold water pump


= 7.5 x (24 x 7 x 51) x 0.1

£6,426 per year

a 50% energy saving!!


For simplicity let’s keep the numbers easy . . .

Cost of VSD = say £1000

Cost of Installation = say £500

Total Investment = £1500

Payback time is Investment ÷ Savings made

= £1500 / £6426 = 0.23 Year ie PAYBACK TIME IS LESS THAN 3 MONTHS!

After less than 3 months the VSD will have paid for itself and the energy savings will result in much lower electricity bills.

And it works just as well with fans!

Energy saving Variable Speed Drives for 3-phase motors

  • Single phase input 0.4kW – 2.2KW
  • Three phase input 0.4KW – 375KW