Variable Frequency Drives
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.Three-phase electric motors are like dogs chasing a rabbit. 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. A three-phase electric motor works the same. 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 that contains 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 (see Figure 1).
The dogs are the rotor in our scenario, the rotating part of an electric motor. It is made up of steel laminations that are attracted to the rotating magnetic poles of the stator, and follow it around like the greyhounds following the rabbit. Just as the greyhounds lag behind the rabbit by a little bit, the rotor lags behind the stator's magnetic field. This is called the slip of the motor, and it is an important factor, which we will explain in the next section. The basic speed at which a three-phase motor runs is called its synchronous speed, and it 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, the 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 the rotor past two poles. Therefore, 60-hertz power will run a two-pole motor at a synchronous speed of 3,600 rpm ((60 Hz X 60 sec.)/(2 poles X 1⁄2) = 3,600 rpm). A four-pole motor will run at 1,800 rpm, and a six-pole motor at 1,200 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 3,450 rpm, and four-pole speeds of 1,750 rpm, etc.
How VFDs WorkThe 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 also is 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 our attention on the speed changing capability of VFDs. The rest is bells and whistles - all important, but not basic. As the pump motor changes speed, the output flow rate and pressure capability of the pump changes. It therefore is 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 discussed last month, but the advantage of using a VFD to control capacity is the dramatic drop in the horse power 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 equates to dollars. VFDs also are used to increase the capacity of pumps. When the speed of the 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 a ton of water.
Residential ApplicationsIn the last few years, manufacturers have begun offering VFDs to run residential pumps. These systems typically are more expensive than pressure switch operated systems but do offer some selling features.
1. They virtually eliminate the need for a pressure tank. Earlier, we talked about the rule of sizing of pressure tanks, i.e., 1 gallon of drawdown for 1 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 1- or 2-gallon 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 gallons or so of usable water in the pressure tank to get you a cup of coffee or a toilet flushed.
2. In addition to the cost savings potential, a residential VFD system offers the comfort of constant pressure. 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 last month, constant pressure also can be accomplished by using a control valve.
Energy SavingsWhether 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 occurs only 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 is reduced using a control valve to reduce the flow. This would seem like a total waste of energy, but when a control valve limits the flow, it moves the system operating point to the left of 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. As to how much energy can be saved with a VFD system, here is an example. Let's say you have a 1-acre gentleman's farm with a 1-HP pump to handle the outside watering and household usage. A 1-HP pump operating at 3,450 rpm will draw about 1,000 watts. That same motor slowed by a VFD to 1,725 rpm will draw 125 watts. Assuming the pump is sized for maximum load, but can satisfy the household load at 1,725 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 $0.15 per kilowatt-hour, you would save about $18 per month running this little residential pump at a reduced speed. Think about the savings possible with a 100-HP irrigation pump running 24 hours per day. We will get into more cost-savings calculations next time. Once again, these comparisons are made against a conventional system. If the system utilized a control valve, the savings versus the VFD system would not be as great.
Next month, with this background, we will 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. 'Til then ….