The Difference between an ‘AC’ Submersible Pump and a ‘DC’ Solar Submersible Pump

With an AC pump and motor, the most important factor to consider is the duration of pump operation. When an AC power source is being used it places restrictions on the running time of the pump; which is why running a smaller pump for longer would be the wiser choice. Once we have arrived at a total flow (Q) requirement vs Head (H) we can then determine our hourly flow rate in m³/h. From this we can select an appropriate pump.

For example:

3.5m³/h @ 60m TDH x 10hr = 35m³ per day:

5-12 Pump Assembly c/w 1.1kw Motor

In AC this pump will require 1.1kw’s of power running 10 hours per day.

Lorentz uses a brushless, electronically communicated DC motor to power all the pumps in the PS2 range. These are known as a Lorentz ECDrive.

The main benefit of these motors is the ability to achieve very high efficiencies at most speed ranges (better than brushed DC or AC) for this power level. Lorentz motors are water filled and lubricated (no oil), and all electronics are contained within the controller (not the motor), unlike many other motors on the market.

This makes it much more unlikely for water to cause damage to internal motor components. Although it does mean 3-core and earth motor cable is required (for 3 phases), compared to most other DC motors which required 2-core.

There is a slight difference between the motors used for centrifugal and Helical Rotor pumps. The motors compatible with the centrifugal ends use a spline shaft to connect to the pump, where the motors compatible with the helical rotor pumps screw onto the pump.

When we consider a DC Solar Pump, all the previous conditions need to be considered, along with knowing the amount of solar irradiation that is available at the pump site. A common misconception is that the sun is up on average 10-12 hours per day, so shouldn’t my pump run for that length of time?

This is where the Lorentz Compass Program for solar pump selection becomes an invaluable tool. When sizing using Compass, the following criteria are automatically taken into account.

• Geographical location (allows correct orientation and tilt angle of solar array).
• Calculated the temperature coefficients of the selected solar panels.
• Average solar irradiation based on data collected from the NASA Langley Research Centre for Atmospheric Sciences Data Centre Power Project.
• Dirt & panel losses.

Now if we take the same pump and apply the duty to Compass we achieve the following:

As can be seen in the above chart, we do not have 10 hours of sufficient irradiation to run the pump at maximum flow. This loss of irradiation in the early morning and late afternoon is compensated by running the pump at a higher speed than its’ AC equivalent. This allows us to achieve a slightly improved daily average.

Next we need to compare the efficiencies of a standard 240v AC motor when compared to the PWM Brushless DC Motor.

• 1kw 240v Cap Start/Cap Run 4” AC Motor is on average 69% efficient.
• 230vac 60hz three phase the efficiency improves to 74%
• 7kw PWM Brushless DC Motor is on average 93% efficient.

If we were to convert the AC Motor with one of our leading competitors solar controllers, the advantages of the DC Brushless motor become significantly clearer.

The following example was the result of laboratory testing of the Lorentz DC Brushless pumping system against two of our leading AC pump competitors.

So where to from here?

1. Look at the Numbers.

Sometimes the only way to make the difference between AC & DC clear is to look at the data. The charts above compare a leading global pump solution to the Lorentz PS2 DC system under the same conditions with identical Solar input. The competitors pump, motor & controller are approximately 15% cheaper than the equivalent Lorentz. If you want to deliver the same amount of water in real world conditions, it becomes 40% more expensive. The efficiency of the PS2 with the Brushless DC motor means the system starts earlier in the day, pumps more during the day and then stops later. This makes a significant difference to the people, animals or crops that use the water.

2. Compare like with like.

When you compare systems, look at the water that is pumped for your investment. Comparing motor size, solar panel sizes or theoretical maximums are not good indicators of how much water you will be able to pump. When making comparisons you need to look at the whole system cost. Adding extra panel, racking, cabling and labour to get competitors’ products to match Lorentz performance gets expensive. As previously demonstrated, Lorentz Compass will accurately simulate & design a system that will give you the water you need when you need it.

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