El-Sitgeti- Electrical point 4-Pumping, irrigating and sawing

Electrical point 4: Pumping water, irrigating and running a 3-phase 3KW table saw

 

It is now getting more interesting. In this post I describe how to run a 1.5 KW (2HP) submersible pump and a 3KW table saw directly from the solar panel arrays, without connection to batteries, using a variable frequency drive.

Pumping water

Most of the water resources of the farm come from a natural spring located at a high elevation point on the farm. It provides for drinking water as well as irrigation water. The stream of the spring varies over the seasons being obviously lower in summer, but never stops though. The water is saved in a 70m3 water pool, which was built in the year 1938. Since all the farmed land lies at lower elevation than the pool, the crops are irrigated from the water pool by gravity. An irrigation network runs over the farmed area of the land. 

 However, even if this system allows for irrigating without the use of energy, water sill needs to be pumped upwards in some specific cases:

1)   Throughout the winter there is often a surplus of water in which case it is pumped upward in a 20m3 reservoir.

2)   For personal consumption (washing machine, shower, etc…) the water is regularly pumped upward in a PP 1m3 reservoir, in order to provide for sufficient water pressure out of the tap.

These tasks were carried out with a petrol surface pump until recently, which was replaced by an electric 3-phase submerged pump.

The pump

Submersible pumps are most often used for wells and especially deep wells. They are designed is such a way as to fit into tight holes (typically 10-15 cm diameter) and to pump water at high elevations (typically higher than 100 m). Therefore they consist of small diameter cylindrical shaped enclosures fitted with a watertight motor which drives a number of stacked turbines on its axle. The more turbines there are the higher the pressure the pump can reach and thus the higher up it can pump water.

We purchased one such pump from DAB, an Italian manufacturer, and not being from the field I have to say I am very impressed by the technology. It costs a mere 500 euros, it is made completely from stainless steel parts, is incredibly small (see picture), has a rated power of 1.5 KW and can pump water at a rate of 6m3/hour and keeps that rate at elevation up to 90m! It feels to me that these pumps have a very high usefulness/price ratio! They are particularly attractive for our application, as they do not have any problems of air intake, as is often the case with surface pumps and can simply be placed at the bottom of the pool on a simple stand that comes with it.

View of the spring and water pool. The pump lies at the extremity of the pool, one can appreciate how small the pump is. The right picture shows the pump from a closer distance.

 The variable frequency drive

As I have already mentioned in the previous posts, variable frequency drives (VFD) are one of the most important piece of equipment in our installation. Lets explain briefly what they are used for and how we adapt them to our system.

VFDs are used extensively in industry and their use is specific to the drive of 3-phases motors. 3-phases induction motors, by conception, have their speed limited to a single RPM value, which is directly linked to the frequency of the 3-phase mains supply (50 Hz or 60 Hz). This is were VFD come very handy, as they allow controlling the speed of the motor as well as a lot of different parameters such as torque, acceleration and have PID speed control features, load compensation algorithms, etc… In a standard AC system, in order to be able to vary and control the speed of the motor, the VFD rectifies the 3-phase input to a DC supply of around 325V (in the case of a 220V VFD) and then recreates a variable frequency alternating 3-phase voltage to feed the motor with using power IGBTs. The trick is that to drive a motor the VFD does not need to recreate a pure sine wave voltage, rather the output of the VFD is a Pulse width modulated signal of varying frequency. By varying the shape of the signal the VFD controls both the RMS Voltage applied to the motor windings as well as the frequency (see here for a very nice explanation on VFDs).  This is very important as when a velocity lower than the nominal velocity of the motor is required not only the frequency of the 3-phase must be lowered, but also the RMS voltage to avoid the motor from withdrawing more current than it is rated for. The voltage/frequency ratio generally follows a linear law but most VFDs have many different voltage/frequency curves options.

What is the advantage of using VFD in DC photovoltaic system (see here for additional info)?

1)   Similarly to Switched mode power supplies, VFDs’ working principle implies the rectification of the AC mains supply to a DC source used internally to produce the variable frequency 3-phase output. It can, therfore be used with a DC source. In practice it is as simple as connecting the DC BUS to the AC input, or in some brands (e.g. Fuji Electric) to the DC input connection. When the DC source is connected to the AC input it simply passes through the rectifier bridge and comes out of it unchanged (see scheme).

2) The VFD voltage working limits are very wide and most VFD handle input voltages from DCV200 to DCV400, adapting automatically the output with respect to the input voltage. This is particularly interesting for our system as the DC BUS varies during the day depending on various factors such as weather conditions, load on the BUS, etc…

3)   One of the most important features of the VFD is the starting acceleration ramp, which allows increasing slowly the speed of the motor on start up reducing to a great extent the current surge associated with the motor's start up. This feature is absolutely vital to off-grid PV systems, as it allows running motors of a nominal power virtually similar to the PV system nominal output. This would be absolutely impossible in a conventional PV system since the current surge generated upon motor start up can easily be 10 times its rated current and can easily exceed this value if the motor is under constant load (e.g fridges, we have recorded current surges 20 times the motor rated power in some cases).  It would therefore be difficult or most probably impossible to have a 1KW AC motor running on an AC inverter with less than 5KW nominal power without it disconnecting during motor start up due to over-current detection! Additionally, the start up speed ramp induces less stress on the motor mechanical parts and therefore increases its longevity very significantly. 

4)   The second most important feature is the ability to vary the frequency, and therefore the speed of the motor while in use. Reducing the speed, even slightly, induces a very significant reduction in power consumption and therefore allows for the use of the motor under non-optimum conditions, and allows adapting the consumption to external factors. This implies that motors can be used without the need for batteries, as would be the case in conventional PV systems. 

5)   Virtually all VFDs have a low voltage control board, allowing controlling their parameters (such as frequency, turn ON and OFF, etc…) with external relays or switches, or programmable logic boards.

6)   Last but not least they are pretty economical, and their lifetime is around 10 000 hours of continuous use.

The set up

As mentioned above, the pump is placed in the water pool. Therefore we built a pretty little stone structure besides the pool that hosts an electrical cupboard which contains the necessary equipment to run the pump, and a table saw (see picture). Indeed we use the same VFD to run a table saw for cutting wood lumber, when the pump is not in use.  

View of the stone structure and the electrical cupboard with the VFD on the left and the modified electrical box on the right.

The saw being the equipment having the highest power rating (3KW) we chose a VFD with a slightly higher rated power. We acquired one from Fuji-Electric (Frenic Multi model, 4KW) for about 380 euros. It has an incredible amount of functions, but we only used a few of them. Much cheaper alternatives are available, e.g. Chinese brand 3KW VFD from ebay for about 150 € (new)!

Similarly to previous electricity points we modified an electrical box with a voltmeter/ammeter and switches.  The VFD is directly fed with the DC BUS wired through a fuse. The output of the VFD is wired through a conventional 3-phases switch, which allows selecting which equipment between the pump and the saw is being used. The saw is connected to the VFD output via a conventional three-phases plug.

VFD control

It is worth mentioning that for a 350V DC BUS voltage, only VFDs with 3-phase input of 200V (manufactured for north America), and VFDs with single phase input of 200V (manufactured for Europe) will work. They will provide a maximum value of 220V RMS output (the output voltage is always referred to as between phases). This voltage output is compatible with 220V/380V 3-phases motors, however the winding of the motor will require to be wired in a delta configuration. In the case of the pump which has a sealed connection box, the rated voltage is specified on order (3-phases 230V in our case) and the pumps comes ready to be plugged in.
 
The VFD is controlled via a contol box for the saw and by a timer placed in the electrical cupboard for the pump. We modified the original saw’s control box and placed and ON/OFF switch, an emergency switch and a mini reset switch (bought from RS components). All the switches are connected to the main control box via a RJ-45 (internet) cable. The switches actuate relays whose output is directly connected to the VFD control board. 

 

View of the saw and close up view of the control box.

The control box includes as well a timer (timer prototyping board acquired from ebay: 6€!), a switch which allows selecting the frequency control mode –automatic or manual– and a potentiometer which allows varying the frequency when the latter mode is selected (see pictures below). Selecting the automatic mode with the switch activates a VFD built-in frequency control method, which basically adjusts the frequency linearly with the DC BUS input value. The function allows reducing the speed of the saw or the pump automatically in case of a sudden decrease of the DC BUS voltage, possibly due to an increase in the motor’s load, or to a temporary decrease in solar irradiation.  Although the manual mode has no real usefulness for the saw, it is very useful for the pump when it is used for direct irrigation as it allows varying the flow of water and the pressure on the irrigation lines. It also allows pumping water in cloudy conditions by adjusting the frequency as to maintain the highest possible voltage and current having a simple look at the voltmeter/ammeter. This is equivalent of finding the MPP manually!

View of the VFD control box (right), and close up view of the modified electrical box (left).