Booster (electric power)
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A Booster was a motor-generator (MG) set used for voltage regulation in direct current (DC) electrical power circuits. The development of alternating current and solid-state devices has rendered it obsolete. Boosters were made in various configurations to suit different applications.
In the days of direct current mains, voltage drop along the line was a problem so line boosters were used to correct it. Suppose that the mains voltage was 110 V. Houses near the power station would receive 110 volts but those remote from the power station might receive only 100 V so a line booster would be inserted at an appropriate point to "boost" the voltage. It consisted of a motor, connected in parallel with the mains, driving a generator, in series with the mains. The motor ran at the depleted mains voltage of 100 V and the generator added another 10 V to restore the voltage to 110 V. This was an inefficient system and was made obsolete by the development of alternating current mains, which allowed for high-voltage distribution and voltage regulation by transformers. Although there are many different water pumping applications, most tend to fall into three basic categories -- constant pressure, constant flow, and variable flow. This tutorial is the first in a series that will investigate each of the three. Because design philosophies differ, some booster system designers may not agree with some of the contents of this tutorial. I, however, believe them to be valid as they are based on my almost twenty years experience in packaged, booster system design and fabrication. All booster pumps and booster systems take advantage of the “additive” pressure rule that applies to series pump operation, regardless of whether their source is another pump, a municipal water line, or and elevated tank. And, as its name implies, the constant pressure booster not only adds pressure to the incoming water (boosts), but also provides constant (or nearly constant) pressure at its discharge. These systems can be as simple as a jet pump boosting the domestic water supply to a home or as complex as a quadraplex (four pump) system servicing a manufacturing plant, high rise building, or a subdivision. But, regardless of the application they all operate in much the same way with the single exception that complexity tends to increase with the number of pumps in the system. In the past, booster systems relied on hydraulic valves and electromechanical devices to maintain constant pressure. Today we see a mix of those older technologies and the variable frequency drive (VFD), a device that can electronically vary the speed of an electric motor and, thus, achieve the same result. (If you are not familiar with the VFD.Since both technologies are in use today we will investigate how each achieves its goal of providing constant pressure at varying flows.Before we enter the realm of constant pressure, lets take a look at a typical booster that utilizes a hydropneumatic tank to store water, under pressure, for use during periods of low or no demand. In this system (seen to the right) demand causes the tank to initiate flow and a pressure switch starts the pump when pressure drops to some minimum. When demand disappears, the tank is repressurized and the switch turns the pump off when pressure reaches some predetermined maximum. In between, it provides some intermediate pressure that is dependent upon demand and the capability of the pump. A check valve prevents the higher pressure water from returning to the source when the pump is not operating. Now, this type of booster operates exactly like a domestic water well system except that, in this example, it is used to boost the existing pressure of some water source rather than drawing water from a well. It is known as a differential pressure booster because the pressure switch is set to start the pump at a certain pressure (say 50 PSI) and then stop it at a higher pressure (say 65 PSI). This differential allows a properly sized tank to accumulate pumpage and provide some, predetermined, minimum run time for the pump. As you can see, it is not a constant pressure system as the pressure of the flowing water ranges between the on and off points. These booster systems can be perfectly adequate in many applications but in others a constant discharge pressure is more desirable.PRV Controlled Constant Pressure Systems The figure to the left is that of a simple, simplex (one pump) constant pressure booster. It consists of pump that runs continuously, a check valve mounted on its suction (or discharge), a pressure reducing valve (PRV) mounted on its discharge, and a control panel. Often a low volume bypass line is installed between the discharge and the suction so that some water will flow through the pump during periods of zero demand. If the pump must withstand long periods of operation at or very near shut off head, the water temperature within the pump case can rise to an unacceptable level. When this possibility arises a solenoid valve, controlled by a temperature sensor, can be used to purge water to a drain when the temperature reaches a certain level.As fresh water enters the pump case, temperature drops and the valve closes. The PRV maintains a constant down stream pressure by continuously varying its discharge orifice via a preset spring or a small control valve. The latter provides more precise control at higher flows. The orifice closes completely when demand is zero and downstream pressure is maintained. The check valve simply protects the source supply from contamination by the higher pressure system water when the source is shut down for maintenance.In some applications, the desired flow range may be too great for a simplex booster. In these applications a duplex (two pump), triplex (three pump), or even a quadraplex (four pump) system can be employed. These designs are often referred to as lead / lag systems because the additional pumps (lag pump(s)) are brought on line as demand exceeds the capacity of the lead or primary pump. Multi-pump boosters take advantage of series operation to boost pressure and also benefit from the rules of parallel pump operation to increase flow. Duplex systems will utilize either two pumps of the same size or a small, continuous run “jockey” pump and a main pump. The configuration depends upon the flow range required. The purpose of the duplex design is to reduce power consumption during periods of varying demand as compared to a simplex system of the same capacity. When two pumps of the same size are used, a Cycle Limiting PRVhydropneumatic tank may be incorporated if periods of no demand are anticipated. Such a system is seen to the left. In this case, the lead pump is controlled in the same manner as the tank based, simplex system we saw before.The lag pump is controlled by a flow switch or a pressure switch that is set at a slightly lower pressure than that of the primary pump. Either will bring the second pump on line just as the primary pump approaches its maxim flow (or minimum pressure). Depending upon the flow rate of the pumps a single, discharge header mounted, PRV or individual pump mounted PRV’s may be used. In some cases a single PRV, capable of handling the flow of both pumps, may not be able to maintain constant pressure at lower flows. Depending upon the sophistication of the controls, the lag pump may always be the same physical pump or, it can be alternated to the lead pump position after each stop cycle thus evening out usage. Delay timers are also usually employed to protect both pumps from short cycling. A bypass may also be required depending upon the pump design.In duplex systems utilizing a continuous run jockey pump, hydropneumatic tanks are usually not employed. The jockey is sized to handle “off hours” periods of low demand and a pressure switch brings the main pump on line as demand increases. A bypass and thermal purge are usually installed on the jockey pump and a delayoff timer protects the main pump from short cycling. In this configuration, the pumps cannot be alternated from a lag to a lead position due to their differing capacitiesTriplex systems (shown to the right) consist of three pumps of the same capacity or a jockey and two lag pumps, depending upon the flow required. Jockey based systems usually do not incorporate a tank since the jockey is sized to handle low demand. As with the duplex booster, the triplex design is used to provide a broad capacity range and, at the same time, use electrical power effectively. They are sequenced by pressure switches (or flow switches) that bring the next pump (Pump 2 or Pump 3) on line just as the previous pump approaches its maximum flow. Pumps 2 and 3 are shut down in a reverse order and are protected from short cycling by delay-off timers. The parallel flow characteristics of the triplex are similar to that of the duplex booster except that there is the potential for three pumps operating at the same time. Once again, a thermal purge system protects the jockey from over heating and delay timers protect the main pumps from short cycling.Some triplex systems incorporate more sophisticated sequencing controls and will shut down the jockey when pump 2 comes on line. If additional flow is needed, the jockey is brought back on line. If pump 3 comes on line, the jockey is again shut down and pumps 2 and 3 provide flow. (During extreme flow conditions, all three pumps will be on line.) When demand decreases, pump 3 shuts down and the jockey is brought back on line. Pump 2 is then shut down and the jockey, once again, carries the load. Can you imagine the number of relays required for this type of sequencing prior to the advent of programmable logic controller (PLC)? I can, because I used to design them! The advent of VFD control has replaced many triplex applications with duplex systems and we will discuss these a little later. We will not get into quadraplex systems but, after reviewing the triplex system, I suspect that you can appreciate the opportunity to mix various pump sizes and the potential complexity of the logic required to sequence them.Pump Selection (PRV Operation) Depending upon its design, a pump will produce a flat, moderate, or steep head / capacity curve. All are used in PRV controlled, constant pressure booster systems, however, a “flatter” curve is often preferred. Why? Because the flat curve offers the greatest potential for power savings as demand declines and the pump progresses towards shut off. The power required at any point on the head / capacity curve is function of both flow and head (see the HP equation we stated earlier). Flat curves exhibit a much lower head rise at shut off and therefore consume less power than steeper ones. In duplex and triplex boosters incorporating identical pumps, these flat curves can be accommodated by use of a flow switch to control the lag pump(s).When a hydropneumatic tank is installed in a constant pressure booster system, a pressure switch is normally used to control the pump(s) and a slightly steeper curve will be required to assure accurate on / off control. Although more sensitive pressure switches are available, many flat curves do not offer the differential pressure necessary to pressurize the tank to a level that will provide even a few seconds of flow during periods when the pump(s) are off line. Duplex systems that incorporate a continuous run jockey pump typically use a pressure switch to control the lag pump. Again, it is desirable for the lag pump to produce a flat curve, however, the jockey is usually chosen with a steeper curve so that the pressure switch can, consistently, bring the main pump on and off line. In triplex systems, flow switches can be used to control lag pumps that produce flat curves or those pumps can be sized with slightly steeper curves for pressure switch operation. VFD Controlled Constant Pressure Systems The variable frequency drive (VFD) changes a motor’s rotational speed by increasing or decreasing the frequency of the AC current that supplies it The beauty of the VFD is, that by changing a motor’s speed and therefore that of the pump, the system takes full advantage of the laws of affinity. These laws state that flow varies directly with the rotational speed, head varies as the square of a change in speed, and power varies as the cube of that speed change. For example, if you reduce pump rotational speed by one half -- flow is halved, head is reduced by three quarters, and power is reduced by seven eighths. Instead of a pressure or flow switch, a pressure transducer is used to control the drive and, ultimately, the output of the pump. The transducer is an electronic device that converts pressure into a small current (4-20 ma) that can be used by the VFD to monitor system pressure. Depending upon the “feed back” from the transducer, the drive will either increase or decrease frequency, in 0.1 hz increments, to maintain constant pressure regardless of the flow (a little later you will see that a one hz change can make quite a difference!). Just how well such asystem can maintain constant pressure depends upon the drive itself, the accuracy of the transducer, and rate of change in flow demand. Huge dips or increases in demand may require a few seconds for the drive to restabilize pressure (the same is true for PRV controlled systems). I would show you a picture of a VFD operated pressure booster but it would be redundant because, on the exterior, they look just like the boosters we have seen earlier. The only exception is a PRV is not required. They may be continuous run systems or their controls may incorporate the logic necessary to accommodate a small hydropneumatic tank for periods of no demand. Often their controls will include a “bypass starter” so that the pump(s) can be operated manually if the drive malfunctions. Although simplex systems are quite straight forward, the logic required to sequence multipump systems can get a little complex. Fortunately the PLC (programmable logic controller) has reduced this complexity substantially. We will take a look at duplex and triplex systems a little laterThe striking difference between this illustration and that of the PRV controlled simplex system is the lower power required at intermediate flow points. Both the VFD and PRV controlled units require the same power at maxim flow. But, at 100 GPM the power required drops from 5.3 to 3.7 HP and at 50 GPM, it is reduced from 4.3 to 2.5 HP. A similar reduction would be seen at every point on the system curve.
Again in the days of direct current mains, power stations often had large lead-acid batteries for load balancing. These supplemented the steam-powered generators during peak periods and were re-charged off-peak. Sometimes one cell in the battery would become "sick" (faulty, reduced capacity) and a "milking booster" would be used to give it an additional charge and restore it to health. The milking booster was so-called because it "milked" the healthy cells in the battery to give an extra charge to the faulty one. The motor side of the booster was connected across the whole battery but the generator side was connected only across the faulty cell. During discharge periods the booster supplemented the output of the faulty cell.
Before solid-state technology became available, reversible boosters were sometimes used for speed control in DC electric locomotives. To avoid confusion, it should be explained that it is the electrical output of the booster that is reversible, not the direction of rotation.
The motor of the MG set was connected in parallel with the supply, usually at 600 volts, and was mechanically coupled, via a shaft with a heavy flywheel, to the generator. The generator was connected in series with the supply and the traction motors, and its output could be varied between +600 volts, through zero, to -600 volts by adjusting switches and resistors in the field circuit. This allowed the generator voltage to either oppose, or supplement, the line voltage. The net output voltage could therefore be varied smoothly between zero and 1,200 volts as follows:
- Generator producing maximum opposing voltage, net output zero volts
- Generator producing zero volts, net output 600 volts
- Generator producing maximum supplementary voltage, net output 1,200 volts
To match the 1,200 volt output, the locomotive would have three 400 volt traction motors connected in series. Later locomotives had two 600 volt motors in series.
When the locomotive was working at full power, half the energy came through the MG set and the other half came directly from the supply. This meant that the power rating of the MG set needed to be only half the rating of the traction motors. Thus there was a saving in weight and cost compared to the Ward Leonard system, in which the MG set had to be equal in power rating to the traction motors.
If the power supply to the locomotive was interrupted (e.g. because of a gap in the third rail at a junction) the flywheel would power the MG set for a short period to bridge the gap. During this period, the motor of the MG set would temporarily run as a generator. It was this system that was used in the design of British Rail classes 70, 71 and 74 (Class 73 does not utilise booster equipment).
Some types of London Underground stock (e.g. London Underground O Stock) were fitted with Metadynes. These were four-brush electrical machines which differed from the reversible boosters described above.
- Elliott, T. C., Electric Accumulator Manual, George Newnes Ltd, London, 1948, page 29
- Cooper, B. K., Electric Trains and Locomotives, Leonard Hill Ltd, London, 1954, pp 35–38
- Cooper, B. K., Electric Trains and Locomotives, Leonard Hill Ltd, London, 1954, page 38