The Advantages and Disadvantages of VFDs
Variable Frequency Drives (VFDs) can modernize a pump station and allow operators to fine-tune their control scheme. Many utilities that take advantage of VFDs insist on having them while some utilities remain skeptical and subscribe to the mantra “if it ain’t broke, don’t fix it.” For some pump stations, that’s the right attitude. However, even if you aren't using VFDs, you should be aware of their capabilities. In this post we'll explore some of the advantages and disadvantages of VFDs.
What is a VFD?
Before we get too far, let’s discuss exactly what a VFD does. A VFD is a type of switchgear used to control motors. A control signal tells a drive when to supply or remove power from a motor. Usually, drives take in 1 or 3 phase alternating current (AC) and convert it to direct current (DC) internally. When powering a motor, VFDs actually send a chopped up DC signal that mimics an AC signal. This is called pulse width modulation (PWM). The upside to PWM is that, since a drive’s internal bus is disconnected from the power supply, it doesn’t have to send full load voltage and amps to the motor. By scaling the PWM signal, it can send scaled power to the motor. This fills the same role as a transmission system in a car. It allows the motor to run at different speeds instead of just the supply frequency (60 Hz in the US).
Potential for Energy Reduction
VFDs can be used to save energy at a pump station. However, if not used in combination with intelligent pump management tools like the Dynamic Pump Optimizer, they're often used in ways that increase energy usage. VFDs are a double edged sword in this way and in order to fully understand why, we're going to cover saving energy with VFDs in our next blog post.
Potential for Extended Pump Life
VFDs can be used to extend pump life. However, they're often used in ways that damage pumps because the operators lack the tools to see where their pumps are operating on their tested curves. You can read about the importance of pump testing, how it relates to extending pump life, and how VFDs fit into the equation in this previous blog post.
Adaptable Pump Curve
Pump performance changes as speed changes. How the pump reacts is governed by a set of equations called the affinity laws. Pump flow changes linearly with speed and pump head (or pressure) changes with the square of speed. This means you essentially get a different pump for each speed the VFD allows you to operate. You can read more about this concept in a previous blog post. VFDs allow operators to react to changes in system conditions by getting the pump you need, when you need it.
Simplify Pump Selection
VFDs can also simplify the engineer’s pump selection. Instead of potentially having a pump for peak day and a different pump for low flow periods, you can find a pump that can serve both purposes with the increased flexibility of operation.
Closed Loop Control
A VFD on a pump is a valuable tool in closed loop control. Many utilities run their pumps in simple on/off control loops that cycle pumps based on tank levels. In control theory, this is called “bang-bang” control because of the abrupt switching between states. However, if a pump station needs to hit a target flow or pressure, a closed loop control scheme can take advantage of a VFDs speed to hit a setpoint more precisely. Often, these are implemented with PID loops. Without a VFD, a utility attempting to hit an exact setpoint might need to modulate a valve. While this may work, the valve introduces losses making operation incredibly inefficient. Going back to my car analogy, it’s like pushing the accelerator to the floor and braking hard enough to get the speed you want.
Mitigate Pressure Transients
When a pump starts or stops, it changes the velocity of the water in the piping
system. Any time you change the velocity of water in a pipe, it sets up pressure
waves that echo until friction dissipates them. The magnitude of these events
are proportional to the rate of the velocity change. VFDs can adjust the speed
of a pump in discrete increments at a programmable rate to drastically slow the
change of velocity in the pipe system and resulting in a lower amplitude of the
The traditional way to mitigate pressure transients on a fixed speed pump is a pump control valve (PCV). A PCV is an actuated valve that sits on the discharge of the pump and is configured to open slowly after the pump has been running for a while and close slowly a few minutes before the pump shuts off. While this does reduce transients, PCVs waste a lot of energy (all that transient control is just friction loss through the valve) and they keep the pump running in really bad places while opening or closing. This can accelerate pump wear which was discussed in a previous blog post.
It is worth noting that the absolute worst pressure transient a pump station normally sees is the one that results from a power loss. The motors will lose power almost instantaneously and the pumps will stop very, very quickly. Whether you have VFDs or fixed speed pumps, it doesn’t matter for this particular source of transient.
Controlling Inrush Currents
When a fixed speed motor starts, there’s a large electrical surge from the motor overcoming the inertia of its stationary shaft. The magnitude of this draw might be several times the normal full load current of the motor. These high loads can damage the motor and any upstream electrical equipment. It also makes the sizing of current protection equipment (like circuit breakers) difficult. By ramping up speeds slowly, VFDs reduce the startup motor load and drastically reduce resultant inrush currents.
Controlling Full Load Amps
Sometimes a motor will operate very close to its rated Full Load Amps (FLA). This means it is running at design capacity. Pulling additional power could overload and damage the motor. Many VFDs have a configurable output current. The drive will actively reduce the speed to keep the motor amps below the configured level to protect the motor.
Phase Loss Protection
VFDs can keep a pump and motor running through a phase loss. Again, the DC signal sent to the motor is removed from the AC supply, so as long as the motor isn’t near full speed (and full power), the drive can continue feeding it power. This feature is also configurable. Upon phase loss, many drives fault and stop sending power by default.
Simplified Instrumentation/Additional Monitoring
Most VFDs speak several fieldbus protocols. Often, these protocols are serial communications that can be daisy chained from one drive to the next. When installed correctly, these connections are highly reliable and allow each pump to be started, stopped, and speed controlled through a three-wire connection. By expanding the scope of the communications, all sorts of other interesting data can be pulled from the drives such as voltages, currents, power consumption, fault codes, and various control parameters.
Ability to Oversize Pumps
When a VFD is installed alongside a new pump, the selected pump can be significantly oversized for the design point because a VFD can slow down the pump while still operating at a high efficiency point. When the pump begins to show signs of wear and loses capacity, a VFD can be used to speed up the pump and still meet demand.
Inverter Duty Motors are Required
With traditional AC motors, pulse width modulation produces increased stress on motor winding and greater magnetic noise. These can break down a motor’s insulation and lead to motor failure. Inverter duty motors have improved insulation and other technologies that prevent the modulated signal and its noise from prematurely wearing the motor.
Ground Rings, Insulated Bearings
Pulse width modulation can induce currents on the motor shaft. Without sufficient grounding and insulation, these currents will go to ground through the bearings of the pump and/or motor. This phenomenon is called Electrical Discharge Machining and will pit or flute the bearings, quickly leading to motor failure. Adding a grounding ring and insulated bearings to the motor will prevent these discharges and prevent motor damage.
A PWM VFD has a rectifier circuit that converts the AC current to DC current. This circuit has a non-linear (non-sinusoidal) current draw that creates distortion on the AC supply line. VFDs, particularly larger horsepower drives, need harmonic filters on their line side so this distortion isn’t transmitted to other (potentially sensitive) electrical equipment.
VFDs are more sensitive to cold and heat than across the line starters. Drives need climate control and may need to be oversized to cope with intense heat. Also, NEMA enclosures are used because VFDs need to be kept in a relatively dust free environment.
Increased Operational Complexity
While many VFD user interfaces are getting simpler and better, they’re still more complicated than across the line starters. Some VFDs have poor documentation and complicated settings that require a bit of a learning curve.
While VFDs are fairly inexpensive for small pumps, VFDs are more expensive than across the line starters. For large pump stations and water plants, this cost might not move the needle, but for large lift stations and small utilities, it could definitely be an important factor.