Why should I install a VSD?
Motors that are used to operate fans, pumps, and air compressors typically operate at a constant load. To control air or fluid flow, you can either mechanically throttle the motor—for example, a valve or damper—or you can electronically reduce the speed or frequency of the motor using a variable speed drive (VSD) or a variable frequency drive (VFD).
When you mechanically limit the output of a motor in these applications, the motor is still operating at full load and consuming the same amount of electricity, even though the amount of work that you get from it is being reduced.
However, when you electronically limit a motor, the resulting power consumption of the motor can be drastically reduced, benefiting from favorable scaling offered by fan and pump affinity laws:
Which can be rearranged to make the calculations easier when you’re trying to calculate the new demand of your motor:
So, if your existing motor is operating at 90% loading and has an electric demand of 5 kW, reducing the load down to 50% will reduce the motor’s kW down to 0.86 kW:
This is an 82% savings on electric load over the existing case! So yes, adding VSDs or VFDs to existing motors has the potential to save you a lot of energy at your facility.
VSDs on Pumps
Many facilities are interested in replacing their constant speed pumps with either a two-speed pump or a VSD controlled pump. But which one fits best? To demonstrate the difference between a dual speed pump and a pump with a VSD, the graph below displays a demand curve over a 20 hour period. Both are providing the same total flow of water (in gallons), but the two-speed pump is cycling between high/low and on/off as a way to control the overall flow, whereas the variable speed pump adjusts its flow automatically according to the system’s demand.
The Short Answer
Two speed pumps are appropriate if you have large amounts of storage available or your water demands are fairly constant or predictably at a ‘low’ or ‘high’ flow periods. If you have a varied water demand or little to no buffer storage, a pump equipped with a VSD may be the best solution for you.
The Long Answer
The type of pump and controls that are most effective for your system depends on your water demand profile. For example, conventional pump stations control their pumps by using high and low level tank controls. When the tank water level reaches its low set point, the pumps initiate full speed operation to fill the holding tank until the water level reaches its maximum set point. If there is enough water storage, the pumps may only operate for a few hours a day or intermittently throughout the day. In this case, the best option may be a two speed pump. The benefit of operating the pumps is found by working with pump suppliers to determine the make and model with best fit impellor size to find pump curves that maximize the efficiency of the two proposed flowrates with your system curve. The idea is to operate at the best efficiency points (BEP) of the pump versus the best flowrate for your facility, which is possible if the required flowrates are predictable and there is enough storage room as a buffer.
A pump station that serves a receiving tank with a low amount of storage, or that has demands that range widely may benefit from a VSD-controlled pump. With a small storage buffer, the pumps have to react quickly to meet demands or slow down to prevent overflow conditions. The VSD would prevent excessive on/off cycling of the pump motor and decrease power demand (through the affinity fan laws) by automatically varying its flow to meet the requirements of the system.
VSDs on Air Compressors
A lot of the compressed air projects that we look at involve the replacement of a constant speed air compressor with a new comparable VSD unit. But how do you know if your facility would benefit from a similar replacement project?
The Short Answer
Again, it depends on your compressed air demands. If your existing air compressor is consistently loaded and your compressed air loads are fairly high and don’t vary much throughout the day/week, then you’re probably better off sticking with your constant speed unit. If your existing air compressor is frequently switching between being loaded and unloaded, this could indicate that your compressed air loads have a lot of variation throughout the day/week, and switching to a VSD unit may be beneficial to you. We usually put a data logger on the air compressor in question to see what it does when no one’s watching.
The Long Answer
VSD air compressors do save you energy—but only up to a certain point. Below I’ve graphed the average electric demand for two air compressors: one 200 HP VSD compressor, and one 200 HP constant speed compressor. They both have the same max flow rate (1,115 CFM), but the constant speed compressor must switch between operating at full load and then idling to meet any compressed air demands that are less than the maximum rated flow, whereas the VSD compressor is capable of easily modulating itself to meet lower compressed air demands. As you can see, at very high loads, the VSD air compressor actually consumes a little bit more energy than the constant speed air compressor. This is why if your compressed air loads are already near the peak flow capacity of the air compressor, you’re better off sticking with your constant speed unit.
Many facilities choose to have a constant speed air compressor as their base-loaded unit, with an additional VSD unit being used to meet their swing demand. This arrangement capitalizes on the advantages that are offered by both the constant speed and VSD units and has the potential to save them a significant amount of energy.
Energy efficiency projects such as VSD installations are some of the easiest ways to reduce your facility’s overall energy consumption. To date, ANTARES has performed over 100 energy audits for a wide range of commercial and industrial clients. The results of our studies have helped clients to identify areas of their facility that could benefit from efficiency projects, and help them prioritize their project implementation.