The most common signs are unusual heating of the motor frame, a measured current that stays above the nameplate value, and the overload protection tripping at intervals. These may be accompanied by a drop in speed, longer starting and a burning smell from the winding. Since the same symptoms can also come from supply problems, do not conclude that the motor lacks power without taking measurements.

DRG-132S-4 5.5 kW 1500 rpm DRG three-phase motor with aluminium frame – signs of an underpowered pump motor

Symptoms seen on site

  • The motor frame becoming too hot to touch for long while the pump is running, with warm air coming from the fan as well
  • The protection tripping when the circuit is fully open, especially in hot weather
  • Increased slip in the induction motor; when measured, the speed is clearly below the nameplate speed
  • Discoloured paint, a smell of insulation, bearing grease seeping out

What raises power demand in a cooling circuit

In a centrifugal pump, absorbed power usually rises as flow increases. When a new heat exchanger or machine is connected, a throttling valve is opened or the pipework is shortened, the operating point moves to the right on the pump curve and the motor is loaded more heavily. Adding antifreeze to the water changes the fluid properties and can also raise the power requirement. Fitting a larger impeller has the same effect.

Distinguishing lack of power from a supply problem

Low voltage, phase imbalance or a loose connection also push the current up. For this reason the voltage and current of all three phases should be measured by qualified electrical personnel, while recording the pump discharge pressure and, if possible, the flow at the same time. Possible fault sources are listed in the article on induction motor fault symptoms.

When deciding on the right power

Once the pump's actual operating point has been determined, the motor is selected by adding a reasonable margin to the shaft power at that point; instead of a motor running at its limit, the next size up can be considered. The criteria are covered on the cooling water pump motor page, and you can compare power ratings in the online sales list.