Multi-Voltage capability for pump test station

The testing station is based on a 400Amp, 11kV soft start fitted with special electronic potential transformers that automatically identify the incoming voltage to assist with swift and accurate set up for each test.

The design brief, which was agreed with SoftStart UK in March, required that the test station accepts 3.3kV, 5kV, 6.6kV, 10kV and 11kV incoming voltages from five different transformer feeders. This enables the station to test different voltage motors from 30A to 400A.

"The smallest motor the station can test is 155kW at 3.3kV,” explains SoftStart UK’s managing director Stuart Harvey. "The largest is 6.4MW at 11KV. So it has the capability to test some very large pumps, such as would be used on city-wide water distribution networks, for major crop irrigation systems, flood defences and in oilfield installations.”

Shock loads

Starting pumps of even the smallest sizes associated with the test station can raise several issues. There can be damaging mechanical shock loads transmitted through the pump and drive system; pressure pulses – often experienced as water hammer – can be sent reverberating around the pipe network, and harmonic disturbances can be induced into the electrical mains.

"Larger pumps are usually installed with a soft start to get over these problems,” explains Harvey. "So it is fitting that the test station is also based on a soft start.”

A soft start is an electrical device used in conjunction with electric motors, and as the name implies it functions to smooth out shock and surges that may be experienced when a motor is started up. It works by temporarily reducing the load and torque in the power train during the start-up phase. It then gradually increases the current and voltage supply from zero to full power. This has the effect of containing the electrical surge caused as the motor’s powerful internal electromagnets are energised.

The effect of this is to reduce the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network. It not only ensures smooth, shock-free motor starting, but it also helps reduce breakdowns and maintenance requirements, which can be a major consideration on remote pumps. Ultimately it extends the life of the system.

Harvey again: "Smaller soft starts are usually solid state devices and are configured to control all three phases of the supply, although in some cases it is adequate to control just two phases.

"Larger medium voltage soft starts use silicon-controlled rectifiers (SCRs) in a series/parallel connection to share the heavy currents and are preferred for applications like pumps, compressors and fans etc, when full torque is not necessarily required at start up.”

Accurate metering

However the pump test station uses the soft start in a different and novel way. The voltage measurement transformers are configured in such a way that they provide very accurate metering of the voltage, as does the current transformer. They thus provide usable test data, voltage readings and power profiles during the start up phase of each pump’s drive motor.

From this data a comprehensive starting profile can be mapped out and used to develop a fully documented technical manual for each pump and its motor. "This documentation is very important, as it enables the pumps to be set up and optimised. It is the main point of reference throughout the pumps’ working life (which on a capital pump is likely to be several decades),” says Harvey.

Building the test station took five months and was treated as a priority by SoftStart UK’s engineering team, to meet an August delivery.