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How to run a 3-phase induction motor from 240VAC Over the years, many readers have wanted to run a 3-phase 415V AC induction motor from a single-phase 240V AC supply. It CAN be done, although with some loss of efficiency. This article discusses how to do it. By PETER LAUGHTON W HY WOULD YOU want to run a 3-phase 415VAC induction motor from a single phase 240VAC supply? The short answer is “because a 3-phase supply is not available!” Other answers are that 3-phase motors are typically found on lathes and other pieces of equip­ment and are generally cheaper to buy than equivalent single phase motors. Before we talk about how to do it, let’s look at some of the problems. The first one is that the starting torque is reduced from what it oth- erwise would be. This means that if the motor is connected to a load that needs a large starting torque (like an air compressor that isn’t fitted with an unloading valve), the motor will probably just sit there humming and eventually burn out. In practice, the starting torque is typically reduced by about 20%. My experiments show that some motors are better than others and indeed it is the older types that are usually better than newer ones. This is probably due to the fact that older motors gen- erally have a larger laminated core in the magnetic path and they have more copper in the windings. In other words, older motors are more conservatively designed. Examples of loads that can be successfully started and run are saw benches, band-saws and fans that start up under virtually no-load conditions. Some types of lathes can also be successfully run because they start with no load. Bear in mind that running a 3-phase motor from a single phase supply is far less than optimum because the 3-phase rotat­ing fields will not have the correct 120° relationship to each other. The motor will therefore make more noise, will run hotter than normal and will not produce as much power. Also the pitch and strength of the noise will change ac­ cording to the load on the motor, as the phase vector from the artificially created 3rd phase Fig.1(a) shows the phasor diagram for an ideal 3-phase system. Each phase has a 120° separation from the other two. Fig.1(b) shows the likely phasor relationship with the third phase created by the connection of capacitors across a 3-phase motor with no load. Fig.1(c) is the likely phase diagram when the same motor is under load. These less than ideal phase relation­ships mean that the motor will not be as efficient or produce as much torque and it likely to also produce more noise. 10  Silicon Chip Fig.2: this is how capacitors are connected across a deltaconnected 3-phase motor to artificially produce 3-phase opera­tion. Note that the motor must be capable of deltaconnection 240VAC operation. A 415VAC star connected motor will not have suffi­cient voltage to start and run properly. The capacitors should be rated at 440VAC. changes under load (see Fig.1). This could induce vibrations into a drive under certain condi­tions of load and might possibly cause damage. There are commercial devices which can provide the correct 120 degree spaced phase voltages for a 415VAC motor but we will confine ourselves to the passive solution which just uses high-voltage AC-rated capacitors. WARNING: DANGEROUS VOLTAGES! First, we need to make a few safety comments. We are deal­ ing with mains voltages here, so if you are not a licensed elec­trician, don’t attempt to try any of the ideas presented here. Even when the motor is switched off and disconnected from the 240VAC mains supply, there could still be appreciable voltage left on the capacitors, enough to kill the unsuspecting person. Remember that even if you don’t necessarily have all the leads connected to the motor, the unused ones will still be energised due to induction and transformer effects within its windings and core. I also suggest that you obtain a secondhand motor to experiment with, as you may burn it out if you get the connections wrong. Also be aware that a 3-phase motor, driving a load that still keeps it spinning after the power is removed, such as a drive equipped with a large flywheel, becomes a capacitively-excited induction alternator. Such a spinning motor is capable of killing you with the voltage produced at its terminals, even though it is completely disconnected from the mains supply. As already mentioned, all that is needed to run a 3-phase motor from a 240VAC single phase supply is a few capacitors. But what values? Too much capacitance and we create a leading power factor (which doesn’t usually go down too well with your local electricity supplier), while too little capacitance won’t give a strong enough field when operating under load and the motor will slow down and burn out. How much capacitance do we need? First, we need to briefly review how a 3-phase induction motor works. It has three separate stator windings which are connected in star or delta mode to the three phases of the mains supply. If we are thinking of the star connection, each phase can be regarded as 240VAC, separated by 120°. This is shown in the phasor diagram of Fig.1(a). This crude method of obtaining three phases from a single-phase supply uses a number of capacitors connected as shown in Fig.2, for a delta-connected motor. In effect, we are using the inductance of the stator winding in conjunction with the capaci­tors to provide the desired phase shifts. Strictly speaking, the amount of capacitance required varies with load because the inductive reactance of the motor varies as the speed of the motor varies. This is because of the varying “slip”. To explain further, the speed of the rotating magnetic fields in a 4-pole motor is 1500 RPM and 3000 RPM for a 2-pole motor, etc. This is the so-called “synchronous speed”. But the actual rotor speed isn’t constant, as it varies with load and even at “no-load” is always less than the synchronous field speed due to the stator windings. The April 2000  11 Fig.3: this is a delta-connected 3-phase motor. Each winding has 240VAC applied to it. Most new 3-phase motors can be run in this mode, as detailed on their nameplate. difference between the two is called “slip” and it typically varies from 2 % to 10 % or more in specially designed motors. For example, a motor rated at 1440 RPM will have a synchronous speed of 1500 RPM and the slip in this case is 4%. As the motor is loaded, the slip increases; ie, the rotor runs slower and slower until it eventually stalls. This change of speed with load affects the back-emf of the rotor and is reflect­ed in the stator inductive reactance and is why the amount of capacitance needed varies according to load. Some commercial units use thyristors to switch in different capacitors but this is really beyond our aim of doing things simply. Note that, of necessity, the above explanation is much simplified. How do we work out the inductive reactance of the windings to allow us to provide the same amount of capacitive reactance in order to give the correct phase shifts? There are several ways. One is by measurement. You can use an AC ammeter and excite the winding from a low voltage AC sup­ply. You can then calculate the reactance from Ohms Law, having measured the voltage and current flow through the windings. This gives a starting point for experimentation. You can also take full load current and volt ratings from the motor’s name-plate and use those to calculate the impedance of the windings. Once again, this only gives an approximate figure. Generally though, the calculation is not critical and the range of tolerances in capacitors is greater than the error anyway. For instance, say you want to use a small motor on a saw­bench. It is rated at 1.1kW, 4.1A, 240VAC (delta-connected) at 2870 RPM (ie, 4.3% slip relative to 3000 RPM). We can use these figures to calculate the inductive reac­tance of the windings, using the following formula: Reactance = √[W2 - (VA)2] = √[(1100)2 - (240 x 4.1)2] This gives a result of 492Ω. We then calculate the value of capacitance to give the same reactance, using the formula: Capacitance = 1/(2π.f.Xc) where f is 50Hz and Xc is 492Ω. The result is 6.47µF. The voltage rating should be at least 440VAC and the capacitor must be rated for continuous duty. Motor-start capacitors are not suitable as they are only rated for a short duty cycle, typically several seconds. Oil-filled motor-run capacitors should be suitable. We now have to connect capacitors to the motor to create a rotating magnetic field. In fact, we only create an unbalanced field and let the motor’s Fig.4: a starting switch and extra capacitors will provide more initial torque from the motor but the additional capacitors must be switched out when the motor comes up to speed. 12  Silicon Chip rotor produce a moving field as it turns. How do we unbalance the field? We connect the capacitors in the ratio C to 2C, as shown in the diagram of Fig.2. This creates our unbalanced field. But this will only work from a 415VAC 2-phase supply which is not practical when we only have a 240VAC single-phase supply! How can we run a 415VAC motor from 240VAC? Fortunately most new small 3-phase motors (rated up to 3.7kW or 5 HP) are now designed to work anywhere in the world, from 60Hz supplies at 220/240VAC (as in America) to 50Hz, 380VAC to 440VAC supplies (as in Europe and Australia). So the solution is to connect one of these motors to run in “delta” rather than “star” mode. This is shown in Fig.3. Note that the capacitors don’t have to be connected right at the motor terminals but should be reasonably close to reduce the effects of lead resistance. To reverse the rotation, it is simply a matter of changing any two connections to the motor, as in reversing a standard 3-phase motor. Improving the starting torque The usual way to do this is to switch in more capacitors at starting and disconnect them when the motor is up to speed, to prevent the power factor problems above (see Fig.4). The switch could be the motor’s inbuilt centrifugal throw-out switch or even a manually-operated toggle switch. What about operating a bigger 3-phase motor? Once you have the 3-phase field from a small motor, you can start a larger motor after the small one is running, as the rotat­ing field is real and available at the small motor’s terminals. No extra capacitance is needed as the already running motor supplies the field. Note that there are limits set by your local supply author­ ity on the size of the motor you can start on the domestic power grid. The idea presented above also allows you to run 3-phase motors from a single phase petrol or diesel generator but it really gives the generator a workout during the starting period, so be careful or you may damage the genset. I can successfully start and run the 1.1kW 3-phase motor described above (on a sawbench) from a 5kVA, 240VAC SC single-phase diesel genset.