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23 November, 2020

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Nine tips for using VSDs with submersible pumps

11 August, 2020

Using a VSD to control submersible pump systems raises several important issues. ABB’s Martin Richardson examines them.

If you are using a VSD to control a motor powering a submersible pump, there are certain issues that you need to remember:

1. Reducing speed saves energy  Using a VSD in a system running below maximum capacity saves energy, cuts maintenance, and avoids impeller trim challenges. Excessive trims can change the pump’s hydraulic condition, reducing efficiency and causing instability in operation.

2.  Over-speed  If there is sufficient motor power available, a VSD can increase speed to help during storms, but the operational limits for the pump and the motor must be checked. For instance, at higher speeds, you need to ensure that the net positive suction head available at the pump is still sufficient to prevent cavitation. 

3.  Reverse rotation can cause tripping  When stopping, the column of fluid may pass back through the pump hydraulics and turn the motor into a generator, causing a VSD to trip. A regenerative drive can feed the power back into the supply network. Alternatively, a non-return valve can be installed.

4.  Avoid critical speeds and vibration  A VSD increases the risk of a pump or motor reaching a critical speed at which the mechanics vibrate or resonate. You can avoid this by programming the VSD to lock out certain speeds or speed ranges from the continuous operating speed range.

5. Dimension according to the motor nameplate  A VSD is a source of current and must always be selected using the motor nameplate current. Where a catalogue gives kW ratings, these are based on typical values for conventional 4-pole or 6-pole speed motors. For submersible pumps, ignore them. 

6.  VSDs need good motor insulation  Variable speed puts a higher voltage stress on the motor insulation system, so either a reinforced insulation system or a filter between the motor and the drive is recommended. 

7. Automatic EMC shield  Good installation sees the lowest impedance path on the shield connection to ground, which for submersible motors occurs automatically because the surrounding water acts as an EMC shield, equalising the electric potential of all metal elements. However, selection and installation of cabling and junction box earthing must reduce noise and interference with other equipment. 

8.  Remote monitoring reduces maintenance costs  Motors equipped with sensors allow remote condition monitoring, which must operate without disturbance from the VSD. Monitoring of motor winding temperatures is particularly important, because ageing and lifetime are related to temperature. 

9.  Generator supply in critical applications  Borehole pumps are used where supply reliability is critical and use generators, either permanently or as backup. The VSD manufacturer must advise of the maximum voltage dip at starting and when running continuously. Generators can produce, or be susceptible to, harmonics, which must be considered when dimensioning the feeder cable and the generator itself. 




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