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.
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.
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!