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A booster set is a pump and tank arrangement that keeps the pressure in the water network of a building or plant within a fixed range. When a tap is opened the pressure drops and the pressure switch starts the pump; when the pressure reaches the upper value the pump stops. This simple operating logic can force the motor to start dozens or even hundreds of times a day. When a booster pump motor burns out or needs to be renewed, two issues come to the fore: whether the motor is operating in an arrangement suited to these frequent starts, and whether the new motor will fit the pump body exactly at the flange. This article covers both issues with steps that can be applied on site.
First identify the pump type: standard motor or special-shaft motor?
The pumps used in booster sets fall into different groups according to how they are connected to the motor. This distinction directly determines whether a replacement can be made with a standard motor.
- Pumps working with a standard flanged motor: The pump has its own bearings and shaft, or the motor is connected to the pump adaptor with a standard IEC flange and shaft end. Most vertical multistage pumps and coupled horizontal pumps are in this group. In this case a standard three-phase motor can be used once flange and shaft compatibility has been confirmed.
- Close-coupled (monobloc) pumps: The pump impeller is fitted directly onto the motor shaft, and the gland or mechanical seal runs on the motor shaft. On some of these pumps the motor shaft has a length, diameter or thread specific to the pump manufacturer. Such a motor is not equivalent to a standard motor; a special shaft is needed, and the replacement must be enquired about separately with the motor nameplate and pump information.
The easiest way to find out which group the pump belongs to is to look at the connection between pump and motor before removing the motor. If an adaptor and a coupling are visible in between, replacement with a standard motor is most probably possible. If the impeller casing is connected directly to the motor's front end shield with no coupling in between, it is a close-coupled design.
How do frequent starts strain the motor?
When started direct-on-line, an asynchronous motor draws several times its rated current at the moment of starting. Even though this current lasts a very short time, it generates heat in the winding. If the motor does not run long enough, or stand still long enough, between two starts to dissipate this heat, its temperature rises a little more with every cycle. In booster sets this build-up often progresses unnoticed, and in the end the winding insulation ages.
Another effect of frequent starting is mechanical. With every start the coupling, shaft and bearings receive a sudden torque shock; the contacts of the pressure switch and the contactor also wear each time. For this reason the problem with a booster motor often stems not from the motor itself, but from how often the system starts it.
Common causes that increase the number of starts
- Loss of tank pre-charge pressure: In a membrane tank the pressure on the air side decreases over time. The tank can no longer store water and the pump starts with every small draw-off.
- Ruptured membrane: The tank fills completely with water and no longer acts as a buffer; the pump runs and stops at very short intervals.
- Narrow pressure switch differential: If the lower and upper pressure values are very close to each other, the pump runs in short cycles.
- Leaks in the system: A cistern or valve that leaks continuously makes the pump start at regular intervals, even at night.
- Check valve failure: When the pump stops, water flows back, the pressure drops quickly and the pump starts again.
Checking these points before fitting the new motor prevents the same fault from recurring within a short time. If the motor has burned out and the tank pre-charge or membrane is faulty, the new motor will face similar conditions.
What measures can be considered for frequent starting?
The most effective way to solve the frequent starting problem is to reduce the number of starts on the system side. On the motor side, the following approaches are assessed:
- Tank capacity and pre-charge: Setting the pre-charge correctly in relation to the lower value of the pressure switch and, if necessary, increasing the tank volume directly reduces the number of starts.
- Setting the pressure differential: Leaving a reasonable range between lower and upper pressure allows the pump to run longer in each cycle.
- Constant pressure control with a frequency inverter: Instead of stopping and starting the pump, the drive adjusts its speed to demand; the starting shock and the number of starts decrease noticeably. Our three-phase series are suitable for speed control with a frequency inverter; for long periods of full-torque operation at low speed an external fan may be required, and if the cable between drive and motor is long a dv/dt or sine filter is considered; these conditions are stated with the order. Details are in the question on inverter operation.
- Correct thermal protection: The thermal relay must be set according to the rated current on the motor nameplate. A relay set higher does not protect the motor during frequent starts.
In a system with a high number of starts per hour, state the existing and planned operating pattern clearly in the order; the suitability of the motor for this condition is assessed together.
