If pressure does not build up even though the motor has been renewed, either the new motor is not driving the pump at the correct speed and in the correct direction, or the problem was in the pump or the pipework from the start. Two quick motor-related checks come first, followed by the pump and the suction side. What looked like a faulty old motor may in fact have been the symptom of another problem.

DRG-100L-2 3 kW 3000 rpm cast iron frame DRG three-phase motor – pump pressure not rising

Two checks on the motor side

Speed match

Compare the speed on the old motor's nameplate with that of the new motor. If a 1450 rpm motor has been fitted to a pump that should run at 2900 rpm, the pump will deliver about a quarter of its head. Even if the power rating is the same, this problem arises when the number of poles differs.

Direction of rotation

A centrifugal pump turning backwards still delivers some water, but the pressure stays noticeably low. The arrow on the pump casing should be compared with the direction of shaft rotation. Changing the direction requires work on the connections and must be done by qualified electrical personnel.

Pump and suction side

  • Worn impeller or wear ring: As internal leakage increases, the pump recirculates water within itself; however new the motor is, pressure drops.
  • Air in the suction line: Air entering through a loose union, a cracked pipe or the gland causes pressure to fluctuate.
  • Blocked strainer or foot valve: The pump cannot draw enough water, and cavitation noise may be heard.
  • Water level: If the well or tank level has dropped, the suction lift has increased.

Pipework and measurement

A throttled valve, a fouled pipe or a line added later can also reduce pressure. Measuring with a pressure gauge close to the pump outlet shows whether the losses are in the pump or in the line. If motor current has fallen well below its usual value, the pump may be delivering less water than expected; in that case the problem is most likely hydraulic.

The relationship between the pump's operating point and the motor is explained on the centrifugal pump motors page. If you have identified a speed mismatch, you can compare cast iron and aluminium frame models of the correct speed in the shop.