They can, but only if the two pumps are different from each other and each motor has been selected for the needs of its own pump. If two pumps of the same model take turns, their motors must also be kept at the same power and speed; otherwise flow and pressure change every time the pumps switch over, and the lower-rated side may not cope with the load.

DRG-160M-6 7.5 kW 1000 rpm DRG three-phase motor with aluminium frame – motor power in alternating pumps

Why should the motor be the same if the pump is the same?

The power absorbed by a pump depends on the impeller diameter, the speed and the system curve; the motor simply supplies that power. If a smaller motor is fitted to the same pump, its rated power falls below what the pump needs and the motor is overloaded. Fitting a larger motor does not harm the system; it is merely an unnecessary investment. A difference in speed is even more critical: 1500 and 3000 rpm motors produce completely different heads on the same pump.

When does a different power rating make sense?

  • Layouts that use a small pump for low night-time demand and a large pump for high daytime demand
  • Systems where it is considered sufficient for the standby pump to meet only the minimum demand in an emergency
  • Older installations where the pumps come from different makers and have different impeller sizes

In these layouts each motor is selected for the operating point at which its own pump draws the highest power.

The possibility of running together

Pumps intended to alternate may run at the same time because of a fault or a control error. When two pumps with different characteristics run in parallel, the weaker one delivers very little water against the pressure generated by the stronger one and may overheat. Check whether the control logic prevents this.

From a spare-parts point of view

Matched motors make it easier to use one as a spare for the other if one fails. To verify the power figures against the pump data you can use the pump and fan motor power calculator and then review suitable models in the pump motors family.