Motivation for VSD usage.

For your consideration:

To fully understand the use of Variable Speed Drives in Irrigation Systems one has to have a good understanding of the ‘Total Process Requirement’ starting with the seasonal water demand of the crop, as well as irrigation schedules insofar as supplementary / continuous demand. Working backwards from the crop, the irrigation ‘machine’ needs to be understood as well as varying performance with regards to changing pressures and flows to be able to design control Points to ensure that the demands are actually met (and logged if necessary). Only from this, pump and pipeline hardware has to cater for start up and shut down to be achieved in such a way as to limit the effects of surges and water hammer.

Traditionally, the control of flows and pressures was achieved through the use of control valves with the motor driving the pump running at the machine nominal speed. This is the speed of the motor derived from the number of poles and the mains frequency. Typically, a 2 or 4 pole motor connected to the mains supply in South Africa would produce a shaft speed of around 2950 RPM (2P) and 1460 RPM (4P) and the irrigation engineer would use this in selecting the correct pump to meet the head and flow requirement.

Starting the motor proved challenging and typically a pump motor would be started against a closed valve using either Direct On Line (DOL), Star Delta or more recently an Electronic Soft Starter.

The rapid speed increase from standstill and the resultant torque transfer from motor to pumped medium also proved challenging and the use of valves meant that often a pump could only be started by having a skilled plant Operator present or by employing the use of expensive pressure operated valves that require some maintenance when used on ‘raw’ water typically found in irrigation systems.

The arrival of Variable Speed Drive (VSD) technology meant that it was possible for pumping plant designers to become more ‘flexible’ in their choice of pumps as they were not tied into the design factor of the fixed speed motor. The ability to vary the shaft speed from somewhere below nominal motor speed to that above nominal motor speed allows for a pump to be operated within a large operating ‘envelope’ using a full size impellor from the outset. This of course eliminates impellor modifications and associated costs thereof.

Further to this, the fact that a VSD is able to produce full motor torque from zero speed means that the motor (pump) can be started and the speed increased to suit system requirements without the use of control valves. Typically, the ‘ramp’ or acceleration rate can be matched with the water column increase in velocity and that would see a non return valve open slowly with no water hammer at all. The stop ramp can be controlled in a similar way. Starting a pump like this means that if properly set up, the motor will never require more current to start up than is required to produce the process result, be this pressure or flow (unlike a fixed speed starter that requires between 300% to 800% of motor nominal current to achieve a start).

This eliminates the need for control / start up valves as well as allowing for ‘operator less’ starting on timed schedules or remote control.

All field instrumentation and controls including emergency stop can be connected directly to the drive inputs with no additional relays or interface devices.

Condition and operational signals can be displayed on the touch screen panel. Typically we show process  pressure, current, Eskom tariffs, motor load, motor speed and drive temperature.

Understanding that the ‘process’ of all pumping plant is either of a flow or pressure nature, and considering  the above point where the field instrumentation signals can be ‘fed’ directly into the drive, the use of the on board PID (process) controller means that constant flow and or pressure can be achieved.

 

Estimated savings on startup

As can be seen from the graph above, different starting methods produces peaks in current consumption during startup that varies from 300% to 800% of the full load current of the motor.

With a VSD starter there is no peak on startup if programmed correctly resulting in zero maximum demand charge (depending on the Eskom tariff structure.)

 

Herewith the estimated maximum demand charge of a 90KW motor on Eskom WEPS, Mini flex, Mega flex or Ruraflex.

90KW STARTUP:    VSD vs. STAR DELTA STARTER

90KW = 112,5 KVA TRANSFORMER LOAD ON A VSD STARTER

90KW = 168,8 KVA TRANSFORMER LOAD ON A STAR DELTA STARTER

Estimated maximum demand = 56,25KVA  on the star delta starter, resulting in a Reactive energy charge (Max demand) of R499.50  per month.

 

Eskom T.O.U (time of use) Tariffs.

Our standard Delta vsd starter range incorporates the Eskom WEPS, Mini flex, Mega flex and ruraflex t.o.u. (TIME OF USE). Although t.o.u. is not always possible on certain irrigation systems – huge savings can be obtained by managing the t.o.u. see the percentage difference below.

With all of the above mentioned – The average payback period of a VSD starter varies between 9 to 14 months.

Should you have any more questions, please do not hesitate to contact me.

Looking forward to your approval.

 

Regards

Nico Grobler

Power Factor Systems

082 458 7429