There are many advantages of using drives in submersible pump applications. Variable speed drives can offer great energy savings, often in the region of 30 to 50%, in applications where there is a need to vary the velocity, head or flow. This can be highly significant in applications with long running hours. Drives also offer improved process control. By matching pump output flow or pressure directly to the process requirements, small variations can be corrected more accurately than with other types of control. Drives can also help reduce stagnant water storage in pressurised systems.
When using drives in submersible pump systems, there are however a few complications that users need to be aware of, but these can generally be overcome with some careful planning. For instance, motors in borehole pumps can have a motor cable as long as 100 metres and it is often necessary to fit output reactors or sine filters. These are quite effective at reducing the rate of voltage change (du/dt) and the peak motor voltage.
Other potential problems include undesirable network distortion, which is possible if the drive uses a simple diode bridge. This can be avoided if low harmonic drives are used. High rates of voltage change can also lead to radio frequency interference, unless special care is taken at installation. The drive will generally require installation in a clean environment.
Specialist advice should be sought on cable length, voltage spikes, motor insulation and other technical issues before installation.
Reduced speed saves energy
Using a variable speed drive in systems running below maximum capacity saves energy, reduces maintenance and also avoids the problems associated with impeller trim.
Impeller trim may seem equivalent to varying the speed and most pump manufacturers accept a 15% impeller trim, however excessive trims change the hydraulic condition within the pump. This can lead to reduction in efficiency and instability in operation.
When using a drive and running the motor at low speed in systems with a high static head, users have to be careful not to move too close to the shut-off point, as this risks stopping the flow through the pump. This may typically happen to pumps that pump out of a tank which has a variable input. It may be tempting to implement the very simple control philosophy of maintaining a fixed level in the tank, but this will only work if the inflow meets the minimum flow requirements of the pump. If the inflow does not meet the minimum flow requirement, the pump could be severely damaged. The way around this problem is to determine the minimum acceptable running speed of the pump and to use this as the minimum speed setting for the drive.
In some applications, reduced speed can also cause particles to come out of suspension and cause problems.
Drives enable over-speed
Drives are generally used to reduce speed, but the drive may also be used to increase speed if there is sufficient motor power available. However, It is necessary to check the limits for both the pump and the motor.
At speeds above the nominal speed, the torque available will drop as the drive cannot increase its output voltage beyond the input voltage. The motor then becomes progressively under-fluxed. This is known as field weakening and it does not necessarily restrict the ability to run at over-speed, but it needs to be considered.
There will be a change in the noise from both the pump and the motor as the speed is changed. With higher speed, greater noise and potentially greater vibration will occur. If the pump is handling liquid containing abrasive particles, an increase in flow will result in increased abrasion and wear. With higher speed, it is necessary to ensure that the net positive suction head available at the pump is still sufficient to prevent cavitation.
Reverse rotation can cause tripping
Unless a non-return valve is installed in the discharge, reverse rotation may occur on shut-down. When stopping, the column of fluid may pass back through the pump hydraulics and turn the motor into a generator, which can cause the drive to trip. A drive with an active rectifier may be able to feed the power back to the supply network. Alternatively, a non-return valve can be installed.
Avoiding critical speeds and vibration
The risk of the pump or motor reaching a critical speed increases when using a variable speed drive. When running at fixed speed, the chance of the fixed speed coinciding with a critical speed is fairly small. But when the speed varies across a wide range, this likelihood increases dramatically. Lateral critical speeds occur when running speed excitation coincides with one of the rotor’s lateral natural frequencies.
This can be remedied by programming the drive to lock out certain speeds or speed ranges from the continuous operating speed range. It can also be achieved by detuning the excitation frequency, or by reducing excitation levels by improving balance and alignment.
Submersible motors in installations without rigid fixing to a base plate or foundation (e.g. duck foot bend or pipe shaft) may have a higher vibration level. This is also the case for deep well motors, where the absence of a rigid fixing leads to a higher vibration level.
Dimension according to motor plate data
A variable speed drive is a source of current and must always be selected on the basis of the motor name plate current. Where a catalogue gives kW ratings, these are always on the basis of typical values for conventional motors of 4 pole or 6 pole speed; these should be ignored for the purposes of submersible pumps. The kW rating of a motor is the power that is delivered at the motor shaft and not the power delivered to the motor terminals. The current drawn from the supply is related to the kW rating, the supply voltage, the motor efficiency and the motor power factor.
Drives need better motor insulation
Using a variable speed drive enables gentle starting of the pump system, potentially extending the life of the motor. However, it is important that the motor is suitable for variable speed drive operation. Variable speed operation puts a higher voltage stress on the motor insulation system and it is recommended to have either reinforced insulation system or a filter between the motor and the drive. The rating of the motor insulation is of little help in this context. Motor insulation is generally only defined by the thermal capability, i.e. class B, Class F or Class H. These classes do not define the electrical capabilities of the insulation system. The manufacturer’s advice has to be sought on whether the motor is suitable for variable speed drive operation.
Automatic EMC shield
Good practice installation technique involves ensuring the lowest possible impedance path on the shield connection to ground. In submersible motors, this principle is implemented automatically as the water around the housing provides a perfect EMC shield and equalizes the electric potential of all metal construction elements.
The drive is installed as a piece of wall or floor mounted equipment, with a continuous cable running to the motor, allowing the Faraday Cage to be readily completed. Generally, the cable length is greater than in conventional installations due to the distance between pump and cabinet. To reduce the distance, the drive should be installed as close to the well head as possible.
Unscreened cable should be used as down-hole cable, although this can pose a problem in relation to sensors. However, this can normally be overcome by using a an appropriate filter and having the sensor cables moved as far from the drop cable as possible, putting these cables into steel pilot tubes and using a good screen on the sensors.
Where long cable runs are involved, it is necessary to consider the drive’s filtering capabilities. Manufacturers recommend specific conducted emissions filters, based on tests with maximum lengths of cables. Longer cables will reduce the performance of the filter.
Two basic forms of output filtering can be utilised; du/dt filters will reduce the rate of change of the voltage pulses and this will reduce the effects of long cables, although there will still be a finite limit. Sinus (or sinusoidal) filters are designed to produce a completely sinusoidal waveform, which has no distance limitation.
Remote monitoring reduces maintenance costs
It is difficult to get access to submersible pump motors once they have been installed and inspection may not be possible. The motors are often equipped with sensors to enable remote condition monitoring, which must operate without disturbance from the drive.
Monitoring of motor winding temperatures is particularly important, as ageing and lifetime are closely related to temperature. Operating at 10° C above the thermal class temperature will halve the winding lifetime.
Sensors that are robust in relation to electro-magnetic interference are PTC temperature sensors; thermal contact temperature sensors (bi-metal); and float switches with mechanical contacts. In general, these sensors do not need separate screened sensor cables. Even combined power and control cables are possible for small drive systems.
Critical sensors which need some attention include all conductivity electrodes and Pt 100 RTD thermometers. These sensors always need a separate control cable. Special monitoring relays with electronic filters may be necessary to suppress nuisance tripping. Combined power and sensor cables are not possible.
Vibration sensors can be problematic to use in a variable speed drive installation. These sensors need a separate screened control cable for themselves, as well as screened power cables. Special digital filters may be needed to eliminate high harmonics from the drive.
Generator supply in critical applications
Many borehole pumps are used in rural areas or in applications where reliability of the supply is of high importance, and are fed by generators either permanently or as a backup. Special care must be taken in this case. Maximum allowable voltage dip at starting and continuous run must be specified by the drive manufacturer. It is also necessary to keep an eye on the harmonic content, as generators might be an additional source of, or be susceptible to, harmonics. The presence of harmonics must also be considered when dimensioning feeder cables.
Generator frequency may not be of great importance since the input voltage waveform is rectified to obtain an approximately constant DC voltage source.
These are the main factors to take into consideration installing a submersible pump system with variable speed drives, but it is wise to always consult the drive and motor manufacturers when planning an installation.
Considering the long running hours that many submersible pump systems have, installing a variable speed drive is normally an effort that is well worth making.