Tech Topics: Variable Frequency Drives Effective, Affordable
January 4, 2013
They are known by many names: VFDs, variable frequency drives, variable speed drives, adjustable speed drives, inverters, etc. They have been used for many years in the heating and air conditioning industry to automatically adjust the speed of blower fans and circulating pumps as the heating or air conditioning load of the building changes. They are used every day to control the speed of conveyor belts and overhead cranes. And, because VFDs are finding their way into more and more pumping applications, to match pump output to varying system loads, having a general understanding of their function and applications would seem appropriate and timely. This article is the first part of a three part series on VFDs.
VFDs work in conjunction with three-phase alternating current motors. To understand how VFDs work, we first need to know how three-phase motors work. I like to use the analogy of a dog chasing the rabbit in a dog race. You’ve seen pictures of greyhound dog races where an electric rabbit is on the inside rail leading the dogs around the track, always staying a few feet out in front, just out of reach. A three-phase electric motor uses this principle. Instead of a rabbit, the three phase motor has a rotating magnetic field created in the stator, which is the stationary portion of the motor containing the wire windings. These windings form magnetic poles, and when they are energized by three-phase alternating current, a rotating field is set up much like the rabbit on the inside rail.
The dogs are the rotor in our scenario, the rotating part of an electric motor. The rotor is made up of steel laminations that are attracted to the rotating magnetic poles of the stator, and follow it around like the greyhounds follow the rabbit. Just as the greyhounds lag behind the rabbit by a little bit, the rotor lags behind the stators magnetic field. This is called the slip of the motor, and it is an important factor that we will explain in the next section.
The basic speed at which a three-phase motor runs is called its synchronous speed, and is dependent on two factors: the number of poles in the motor, and the frequency of the AC power supplied to the motor. The actual running speed is determined by one additional factor, the slip, as mentioned above. Let’s look at each of these individually.
To determine the basic synchronous speed, multiply the frequency of the AC power in terms of cycles per second or Hertz, by 60 seconds to get to cycles per minute, and divide by half the number of poles in the motor. We divide by half the poles because one complete three phase cycle moves each segment of the rotor past two poles. Therefore, 60 Hertz power will run a two-pole motor at a synchronous speed of 3600 RPM (60 Hz times 60 seconds divided by 2 poles/2 = 3600). A four-pole motor will run at 1800 RPM, and a six-pole motor at 1200 RPM.
The actual running speed is determined by these factors plus the motor slip. In order for a motor to produce torque, there must be a difference between the speed of the rotating field in the motor’s stator and the actual speed of the rotor. This lag is just a few degrees, and results in typical two-pole motor speeds of 3450 RPM, and four-pole speeds of 1750 RPM, etc.
How VFDs Work
The function of a VFD is to change the frequency of the three-phase AC power, either higher or lower, to change the speed of the motor. As the frequency is increased or decreased, the supply voltage is also increased or decreased to maintain a constant volts per Hertz relationship. In addition, VFDs provide other functions: variable acceleration and deceleration rates, torque control characteristics, motor protection functions and much more. In this article, we will focus on the speed changing capability of VFDs. The rest are bells and whistles, all important, but not basic.
A VFD can be controlled by a pressure reading device to maintain a constant pressure in the system, or by a flow measuring device to maintain a constant flow rate, the former by far the most common. As the pump motor changes speed, the output flow rate and pressure capability of the pump changes. It is therefore possible to match the capacity of a pump to a varying system demand. Of course, this can be done without a VFD by using a constant pressure valve, as will be discussed in a future article, but the advantage of using a VFD to control capacity is the dramatic drop in the horsepower required to run a pump at a reduced speed. Horsepower drops as a cube of the speed. Horsepower equates to kilowatt hours and kilowatt hours equate to dollars.
VFDs can also be used to increase the capacity of pumps. When the speed of a pump’s motor is increased, the flow rate increases. There are environmental pumps on the market that run at 400 Hz, spin at over 20,000 RPM and pump lots of water.
In the last few years, manufacturers have begun offering VFDs to run residential pumps. These systems are typically more expensive than pressure switch operated systems, but do offer some selling features.
They allow for the use of a smaller pressure tank. Earlier, we talked about the rule of sizing of pressure tanks, i.e., one gallon of drawdown for one GPM of pump capacity. Using this rule, a 20 gallon drawdown tank is recommended for a 20 GPM pump. If that pump is controlled by a VFD, a much smaller pressure tank will do the job, representing a big cost savings. Remember though that without a larger pressure tank, you have no backup storage capacity. When you have a power failure with a VFD system, you are out of water. When you have a power failure with a standard system, you have the 20 or so gallons of usable water in the pressure tank to get you a cup of coffee or a toilet flushed.
In addition to the cost savings potential, a residential VFD system offers the comfort of constant pressure. Residential VFD systems are controlled by a pressure transducer that feeds a signal to the controller telling it to slow down or speed up the pump to maintain a constant, preset pressure. This can be a real selling point to a customer who is tired of pressure fluctuations in a conventional system. As we mentioned earlier, constant pressure can also be accomplished by using a control valve.
Energy savings. Whether you are dealing with a small residential system, or a large industrial system, the pump must be sized to the worst case flow demand, which most likely only occurs part of the time. In smaller conventional systems, the excess capacity of the pump is used to fill a pressure tank, and the pump cycles on and off as the demand fluctuates.
In larger conventional systems, the excess capacity can be reduced using a control valve to reduce the flow. This would seem total waste of energy but when a control valve limits the flow, it moves the system operating point from Point A to Point B on the pump curve, which reduces the horsepower required to run the pump. The energy savings are not as marked as they are with a VFD system, but they are appreciable. We will take a closer look at control valves after this series on VFDs.
How much energy can be saved with a VFD system? Let’s say you have a one-acre gentleman’s farm with a 1 horsepower pump to handle the outside watering and household usage. A 1 horsepower pump operating at 3450 RPM will draw about 1000 watts. That same motor slowed by a VFD to 1725 RPM will draw 125 watts. Assuming the pump is sized for maximum load, but can satisfy the household load at 1725 RPM, a savings of 875 watts can be realized for each hour of operation. Let’s say this pump runs five hours per day to satisfy the household load. Using my local residential utility rate of $.1 per kWh, you could theoretically save about $18.00 per month running this little residential pump at a reduced speed. Think about the savings possible with a 100 horsepower irrigation pump running 24 hours per day. We will get into more cost savings calculations next month. Once again, these comparisons are made against a conventional system. If the system utilized a control valve the savings verses the VFD system would be less.
With this background, in VFDs Part 2 next month, we take a look at some specific industrial and commercial applications for VFDs. We will look at how changing pump speed affects pump performance in terms of pressure, horsepower, and efficiency, and more on exactly how VFDs work. Till then… ND
If you would like to have a copy of all of my articles for reference or training purposes, they have been compiled into a book, titled The Pump Book, which is available for $20. Send an email to firstname.lastname@example.org requesting the book, and I’ll mail you a copy.