Submersible Pump Control – VFD Notes

dv/dt

The typical submersible motor is wound in plastic insulated wire with a relatively low maximum voltage rating. When used with a Variable Frequency Drive, there is a danger of high transient voltages (generated by the drive) causing an insulation breakdown in the motor. The amplitude of the voltage transients caused by the PWM waveform of the drive, increase with the length of the cable and approximately double with 150M of screened cable. Reducing the dv/dt of the output waveform by the addition of reactors and/or filters will reduce the potential for motor insulation failure.

Minimum Head Pressure

Within the motor, there are carbon thrust washers that keep the stationary rotor away from the housing. When the motor and pump are operating with pressure and flow, the rotor is lifted away from the thrust washer. If the motor operates at reduced pressure, the thrust washer will become worn very quickly causing the rotor to drop and commonly causing the impellors to wear also. The pump must operate above a minimum pressure at all times. In low lift installations, where there is a significant pipe fill time at very little pressure at the pump, it may be necessary to fit a pressure sustaining valve or controlled valve to provide an artificial head on the pump. Deep well pumps are pressurised by the column of water above the pump.

Minimum Flow

The induction motor has two major sources of heat. The Iron loss is significant on a submersible motor and is constant and independent of pump load. The copper loss is also significant but is proportional to the current squared. At low flow, the copper loss is significantly less than at high flow. Because the iron loss is cooling at low flow, there is a minimum velocity of water that must be flowing over the case of the motor at all times that the motor is operating. The velocity of the water is a function of the flow and the gap between the motor casing and the well casing.

Minimum Height of water above the pump

As the pump is drawing water into it’s intake, there is the potential for vortexes to rise up to the surface of the water. This allows air to be drawn down into the pump from above. In order to prevent this and subsequent damage to the pump, the water level above the pump must be maintained at a sufficient level.

Acceleration

In order to prevent thrust washer damage and motor over heating, it is important that the motor reaches pumping speed such that the minimum flow is achieved, within two seconds of starting. The time for acceleration from zero speed to minimum flow speed must be two seconds or less.

Deceleration

The maximum deceleration speed between minimum flow and zero speed must also be two seconds or less.

Minimum Speed

The minimum speed setting of the VFD should be set above the speed required to ensure minimum flow under all conditions, and it must also be low enough to ensure that the system will not trip on over pressure at minimum flow. If the system is operating with sleep mode, the minimum speed setting must be low enough to allow the system to reduce the speed and flow. If the minimum speed is too high, the PID systems will not be able to control the pressure and flow correctly and will not cascade additional pumps in, or go to sleep on low demand. 8. Maximum speed. : The maximum speed must be high enough to enable the pump to fulfil the pressure and flow requirements, but low enough to prevent the motor from overloading.

Maximum Speed