We added a Variable Frequency Drive (VFD) to our main pump motor this spring. So, you may ask, what is a VFD – and why should I care?
Background
First, a little background. The “main pump motor” is a large motor in the pump house (located on the western edge of the pool deck) that drives a pump that circulates water from the pool through the filter system. This is quite a large, power-hungry 15-horsepower (HP) pump.
The pump is required to circulate at least 580 gallons per minute. According to the county health code, water in our 203,500-gallon pool must be circulated at least four times daily.
In the past, the motor was controlled by two buttons – a START and a STOP. The motor would immediately spring to life following a push of the STOP button with a big ker-chunk. This action put mechanical stress on various systems due to the sudden in-rush of water pressure.
The more complex control issue (for the staff) was maintaining proper flow. If there is too little flow, we violate the health code. If there is too much flow, we violate the maximum flow rate for the sand filters (compromising their ability to filter). This flow rate was maintained by partially closing a large valve on the pump’s output. Unfortunately, as conditions changed (the strainer and filters clogged), this valve had to be regularly repositioned to maintain the desired flow rate.
At this point, you might suspect that restricting the pump’s output is a relatively inefficient way of reducing the flow rate – and you’d be right. Ignoring some of the complexities of exactly how inefficient this is, we know there is a more efficient way to do it. This is where the VFD comes in.
What is a VFD?
A VFD controls a motor by varying the amount of power supplied to the motor. If you recall physics class – you may remember the term “alternating current” or AC. Our motor operates on AC, which is nominally 60 Hz (cycles per second) per the utility. The term “variable frequency” in VFD implies that this frequency can be varied. By reducing the frequency to 55 Hz, 50 Hz, etc., the speed of the motor can be reduced proportionally. This has an exponential impact on power consumption. Evidence has shown that lowering the motor frequency to 50 Hz can reduce power consumption by roughly 50%.
Energy Savings
This is probably a good point to jump in and reveal that the club has saved over $1,000 in three months of operation with the VFD (as of August 2022) – a staggering figure. While we can’t attribute all this to the VFD, we know that the main pump motor accounts for most of our electrical consumption.
In May, we saw a 37% reduction. Much of this was due to the significantly reduced flow (below 600 GPM) permitted due to a lack of pool occupancy. In June and July, consumption was reduced by 21% each month.
The 2022 line (green) in the chart above falls well below the prior year (orange). Note that a portion of the difference in April can be attributed to starting the pool a week later in 2022 than in 2021.
Why did we save this much? Read on.
Closed-Loop Control
In the controls engineering world, the term “closed-loop control” refers to a system where the controlled device (in this case, the main pump motor) is controlled via feedback that is affected by said device.
When the automation system was first implemented over a year ago, an electronic flow meter was added to the output of the filter system. This flow meter provides flow rate feedback in gallons per minute (GPM). Using this data, an electronic controller can instruct the VFD to speed up or slow down the motor to maintain the desired flow rate.
By slowing down the motor, we can reduce power consumption significantly. A figure of 50 Hz was mentioned above – this is typically where we operate in “ideal” conditions. Therefore, we can often operate the pump at roughly half the power it might otherwise consume. This all happens automatically.
Design, Build, and Installation
The VFD control system was designed, built, and installed over 6+ months. So far, the performance has exceeded expectations. A gallery of the project progression from February 2022 through May follows:
Yaskawa Variable Frequency Drive
Utility Power Issues
If you’ve read this far – perhaps you’re interested in diving a little deeper. A recent issue with the electrical utility presented us with an interesting challenge and consequences.
So far, 2022 has been a year with many electrical supply issues. We’ve lost power multiple times. On July 8th, we lost power on the day of a swim meet. Power was restored shortly before the meet was scheduled to start – but there was an issue.
The manager contacted Building & Grounds to indicate that the VFD would only operate momentarily before indicating “PF” – a phase failure. This is where we need to jump back into a discussion of our power supply and alternating current (AC).
Large motors consume enough power that a single electrical feed (like is supplied to the home) is insufficient. Let’s substitute the word “phase” for feed. Our motor operates on three-phase power – a topic best discussed in this Wikipedia article. Each phase ideally supplies 1/3 of the needed power to the VFD. Any imbalance in these phases can cause an issue – which is what happened here. After speaking with DTE, we were told that the lineworkers had to temporarily connect one phase to a different source, resulting in a significant imbalance.
More details to follow.
