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How Induction Motors Work Induction motors power many devices, from industrial applications like lathes and mills to domestic fans, pool pumps and more. We describe their clever design and the differences between them. If you’re considering building the VSD described in late 2024, this article will help determine if it suits your induction motor. By Andrew Levido F ig.1 shows a cross-section through the stator of a simplified threephase induction motor. The stator, made of laminated steel sheets (like a transformer core), is equipped with three axial windings, represented by the red, blue and green circles. The direction of the winding (into or out of the page) is indicated by a cross or a dot, respectively. As current flows in a winding, a magnetic field is produced along the dotted axis, shown in the same colour as the winding. The strength and direction of the field depend on the instantaneous magnitude and polarity of the current. At time A, for example, the red phase current is at its positive peak, while the blue and green phase Fig.1: a rotating magnetic field is produced in the stator of a three-phase induction motor by placing the windings at 60° intervals around the rotor. The black arrow is the vector sum of the fields produced by the windings. that rotates smoothly around the stator once for each mains cycle. The rotational speed of the flux vector in this arrangement of windings (called a two-pole configuration) is 50 revolutions per second, assuming 50Hz mains – equivalent to 3000 RPM. This is known as the motor’s synchronous speed. By interleaving additional sets of three windings, lower synchronous speeds can be achieved, such as 1500 RPM for a four-pole motor or 1000 RPM for a six-pole motor. The rotor of an induction motor is also made of laminated steel, as shown in Photo 1. You can just make out a series of longitudinal slots in its surface, into which aluminium bars have been cast to form the windings. These bars are shorted together at each end of the rotor by thick rings of cast aluminium. This type of motor is sometimes referred to as a ‘squirrel cage’ induction motor since the arrangement of bars and rings resembles a cylindrical cage. The rotor bars are skewed slightly to ensure smooth rotation. Without this skew, the motor would exhibit noticeable ‘cogging’ as it rotates, similar to a stepper motor. The rotating stator field induces a current in these rotor bars by transformer action. In this sense, the induction motor can be considered a kind of rotating transformer with shorted secondary turns. When the rotor is stationary, the current induced in the rotor can be huge, as you might expect with a shorted transformer. The rotor currents create their own magnetic field, which interacts with the rotating stator field to produce a strong torque that sets the rotor moving. Initially, the frequency Australia's electronics magazine siliconchip.com.au 48 Silicon Chip currents are half the maximum magnitude and negative. The magnetic fields these currents produce are shown as vectors (arrows indicating magnitude and direction) of the appropriate colour on the diagram. The net magnetic field, which is the sum of the three coloured vectors, is indicated by the heavy black vector. At point B in the waveform (onesixth of a cycle or 60° later), the green phase current will be at its maximum negative excursion, while the red and blue phase currents will be positive with 50% of the maximum magnitude. This results in a net flux vector, shown at B. It works similarly for point C and so on. The three-phase winding therefore produces a net magnetic field vector of the rotor current is the same as that in the stator. However, as the rotor accelerates, the frequency and level of the rotor current begin to drop because the rotational speed of the stator field, seen from the rotor’s perspective, reduces as the rotor ‘catches up’ to it in speed. If the rotor could reach the synchronous speed, the stator field would appear stationary to the rotor. There would be no induced rotor current and consequently, no torque. The induction motor rotor therefore settles down to a speed just a little lower than the synchronous speed where the diminishing torque produced by the rotor-stator field interaction is balanced by the torque required by the load. Fig.2: the operating point of an induction motor is the intersection between the motor’s torque-speed characteristic (red curve) and that of the load. Slip The difference between the synchronous and rotor speeds is known as the slip. Slip can be described as an absolute frequency (the difference between the rotor and stator current frequencies) or as a percentage of the synchronous frequency. The typical slip for an unloaded three-phase induction motor is just a few percent, so the no-load speed of a typical two-pole motor might be 2900 RPM. That would correspond to a slip frequency of about 1.7Hz. The slip is typically 5-10% at full load, so around 2700 RPM for our example. The red curve in Fig.2 shows the torque-speed characteristic of a typical three-phase induction motor. The blue and green dotted lines represent the torque-speed characteristics of two common types of load – a constant-­ torque load, such as a conveyor, and a square-law load, such as a fan. The motor’s operating point is at the intersection of the motor and load curves. The no-load operating point is also shown for reference. This is close to the synchronous speed, since the only load torque on the motor is produced by the rotor friction and windage. The motor’s speed regulation is defined by the slope of the leading edge of the motor’s torque-speed curve. A stable operating point can only occur on the “leading edge” of the torque speed curve, where the torque provided by the motor is decreasing with increasing shaft speed. If the operating point reaches the crest of the curve (the “pull-out” torque), the motor will stall. The motor’s rated siliconchip.com.au Fig.3: a single-phase induction motor stator has just one winding, so it produces a pulsating rather than rotating magnetic field. Photo 1: the windings on an induction motor’s rotor are aluminium bars cast into slots that run the length of the rotor. These bars are short-circuited at each end by aluminium rings (shown here with cooling ‘studs’). The bars are skewed to ensure smooth torque production. Source: https://w.wiki/AxgX full-load torque is therefore somewhere well below this point. Single-phase induction motors Single-phase induction motors work on the same principle but only Australia's electronics magazine have one stator winding, as shown in Fig.3. During the positive half-cycle of the stator current, the field points to the right and increases from zero to some peak at point A before reducing again to zero (point B). In the negative half-cycle, the same thing September 2026  49 happens but in the opposite direction (point C). Thus, a pulsating rather than rotating magnetic field is produced. While a current can still be induced in the rotor winding by transformer action, there is no rotation of either the field vector or the rotor field at a standstill, so no torque is produced on the rotor. If the rotor is moving, however, the pulsating stator field appears to rotate relative to the rotor, and a torque is produced. This torque-speed characteristic is shown in Fig.4. There is zero torque at the origin, but once the rotor is moving, it will accelerate to some operating point, just like in the three-phase example. It can rotate in either direction – the direction of rotation depends on the direction of the initial starting torque. The diagram shows that the size of the starting ‘kick’ required depends on the load type. The fan requires the rotor to be just barely rotating to generate more torque than the load requires. In the case of the constant-torque load, the rotor must be spinning at almost half the synchronous speed before it becomes self-sustaining. Single-phase induction motors use various techniques to generate this starting torque, as shown in Fig.5. Shaded-pole motors (purple curve) use a shorted turn on the stator to distort the magnetic field to create a modest starting torque. You can see that this is enough to get things moving, but it does not provide much low-speed torque, so these motors are usually limited to easyto-start loads like fans. Shaded pole motors are not very efficient (typically no more than 30%), so they are generally used for motors of just a few hundred watts at most. A separate start winding fed via a capacitor can achieve better starting torque. The capacitor introduces some phase shift in the start winding with respect to the run winding, creating a reasonable start torque. In the ‘permanent split capacitor’ (PSC) motor, the start winding and its series capacitor are permanently connected in parallel with the run winding. This is shown on the orange curve in Fig.5. For loads requiring even higher starting torque, like cement mixers, a large start winding current is necessary. This is supplied through a capacitor, as for the PSC motor. However, since the start winding cannot sustain so much current indefinitely, a centrifugal switch is used to switch it out of circuit once the motor reaches about 70% of full speed. This is called a ‘capacitor-start motor’; its torque curve is shown in dark blue. Not shown on the diagram is a variation on this theme: the capacitor start/ run motor, which has two capacitors and a centrifugal switch. At start-up, both capacitors are connected in parallel to drive the start winding with a very high current. When the centrifugal switch Fig.4: the torque-speed characteristic of a single-phase induction motor shows that there is no torque at a standstill, but as the motor rotates, a torque is produced. Thus, an initial ‘kick’ is required to get the motor moving; the direction of the kick determines the direction of rotation. 50 Silicon Chip Australia's electronics magazine opens, one of the capacitors is disconnected, and the start winding current is reduced to a level that can be sustained indefinitely. Motors with a centrifugal switch are usually not suitable for use with a variable speed drive (VSD), such as the design described in the November & December 2024 issues (siliconchip. au/Series/430). If the motor is run at less than full speed, the centrifugal switch may never open and the start winding can burn out. If the speed of such a motor is to be controlled, the range of possible speeds may need to be limited to those above which the centrifugal switch opens (more on this later). Speed control For many types of motor, such as DC or universal types, varying the speed can be as simple as reducing the voltage. However, that is not very effective for induction motors; Fig.6 shows why. Varying the voltage fed to an induction motor gives a very limited speed control range and poor torque at lower speeds, which is worse for constant torque loads. That makes sense because the synchronous speed is locked to the mains frequency. On the other hand, varying the supply frequency gives a very wide range of speed control and pretty good torque over that range. It should be noted here that we must also reduce the voltage with the frequency to avoid saturating the motor. It turns out we have to reduce the voltage more-or-less linearly with frequency, so a 50Hz 230V motor running at 25Hz requires the application of about 115V. The exception is at very low frequencies, when the fixed voltage drop across the motor winding resistance(s) means we may need to boost the voltage slightly to produce the same flux density and therefore torque. So ideally, an induction motor speed controller should produce a sinusoidal voltage that can vary in frequency from something less than 1Hz to 50Hz at a voltage between 0V and 230V RMS. Most modern solid-state induction motor controllers (including the one described in 2024) approximate that using pulse-width modulation (PWM) to synthesise sinewave(s) from a DC bus derived by rectifying the mains. siliconchip.com.au Controlling a motor with a centrifugal switch While this should be approached with caution, it is possible to control the speed of an induction motor with a centrifugal switch with some provisos. The first approach is to limit the range of the speed control potentiometer so that the motor will always ramp up to the speed range where the start winding is switched out. This could be done by placing a trimpot in series with the track of the speed control pot, between it and ground. This pot would be adjusted so that the centrifugal switch disconnects the start winding even with the speed control pot at its minimum setting. That will, of course, restrict the range of speeds you can run the motor at, but it will at least allow some control, and as long as it’s adjusted properly, there should be no risk of the winding burning out. You will need to check that the minimum speed is sufficient under load, though. The other approach only works if you can access the wiring to the centrifugal switch and involves some extra hardware. Usually, the switch is switching in an external capacitor, so you will usually be able to access the wiring. In this case, you could use a mainsrated relay instead of the centrifugal switch to energise the start winding. The wiring and insulation would need to be done safely for mains voltages. The relay then needs a control circuit, likely a microcontroller. This microcontroller would monitor the motor speed (eg, by measuring the frequency of a reduced voltage version of the AC waveform powering the motor). The microcontroller would energise the start winding when it first detects power being applied to the motor, and it would switch it off once it reached a sufficient speed, or after a set timeout (eg, 30 seconds). The microcontroller should measure the time the start winding is energised, multiply that by a safety factor (eg, five), and refuse to re-energise the start winding for that many seconds after it’s switched off, to give it time to cool down. Note that in this case, if the motor was set to run at a low speed, it’s possible it could stall when the start winding switches off. That’s something the operator would need to be SC aware of. siliconchip.com.au Fig.5: the torque-speed curves of a single-phase induction motor varies depending on its type. Shaded-pole motors are the simplest but are really only suitable for fans. Capacitor start is used when a motor needs lots of starting torque. Fig.6: reducing the voltage is not a very effective way of controlling the speed of an induction motor since its synchronous speed is locked to the mains frequency. Fig.7: varying the mains frequency is far more effective – giving a very wide speed control range. Usually, a VSD will vary both the voltage and frequency applied to the motor together. Australia's electronics magazine September 2026  51