WHY 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.

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.

Before we talk about how to do it, let’s look at some of the problems.

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.

The first one is that the starting torque is reduced from what it otherwise 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 generally 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.