Variable frequency drives (VFDs) are one upgrade we regularly implement to improve energy efficiency in schools. In a previous blog post, we covered how VFDs in schools can benefit HVAC systems by saving more than just electricity.
By controlling the electricity going into a motor, VFDs turn any motor into a variable speed motor. This has the biggest benefit for single-speed motors, but VFDs can also save energy for dual or multi-speed motors.
There are two main benefits of a VFD:
- They reduce the wear-and-tear on a motor by reducing motor’s speed or torque when not required. This means less maintenance and a longer life span for the motor.
- They save energy.
The second benefit is the interesting one. With the help of some simple math, we can see just how much energy can be saved.
Let’s use the example of a water pump filling a bucket. Assume the pump can fill a bucket in one hour.
Now let’s add a VFD into the equation. Let’s use the VFD to cut the power input by half so now it will take twice as long to fill the bucket. Over two hours, the same amount of water will be moved into the bucket.
But how much power was consumed? This is the interesting part.
When the flow rate is cut in half, the pressure is reduced to a quarter of the original pressure. And the power consumed then falls to one eighth.
Although it took twice as long to fill the bucket, the power consumption for the entire process was one quarter compared to filling the bucket in one hour. This math is explained by the Affinity Laws – a set of formulas that explain the relationship between shaft speeds, flow rates and power.
The Affinity Laws explain the following:
- Shaft speed is proportional to the flow, or ∆ speed = ∆ flow
- Pressure is proportional to the square of the shaft speed, or ∆ pressure = (∆ speed)2
- Power is proportional to the cube of the shaft speed, or ∆ power = (∆ speed)3
A more down-to-earth example of the Affinity Laws is trying to walk through a pool when the water level is nearly up to your head. It requires considerable more energy to attempt to run through the pool than to just walk through the pool.
This is why VFDs are a powerful device for energy efficiency. If it is no problem to wait an extra hour to fill the bucket, you will save three quarters of the energy.
Want that in plain English? Call us and we will explain.