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Motor Control Part 1: DC Motors The focus of this new series of articles is to drill down into how various types of motors work and how they are controlled. In this first part, we’ll start by looking at how DC motors work and how we can control them. By Andrew Levido I n this series, we will look at both the power electronics and the control systems involved. As usual, there will be a little bit of theory, although I will try to keep the mathematics to a minimum and provide plenty of practical examples. Electric motors are truly ubiquitous in our lives today. They are so common that we often don’t even give them a thought. There are probably several motors within a few metres of you wherever you are reading this – whether it is the cooling fan in your PC or laptop, the haptic motor in your smartphone, or the pump in your coffee machine. They are all electric motors of some sort or another, and they almost all have some kind of electronics to control their operation. Electrical machines In electrical engineering circles, motors and generators are classified under the fancy title of “electrical machines”. Electrical machines are any devices that convert electrical energy to mechanical energy (motors) or mechanical energy to electrical energy (generators), or both. The mechanical energy is usually, but not always, transferred by means of a rotating shaft. Most practical electric machines achieve the conversion between electrical and mechanical energy using magnetic fields in some shape or form (one exception is the rare electrostatic motor). For the purposes of this article, I am going to assume the reader has an understanding of the fundamentals of magnetics, including the concepts of magnetic flux, flux density, permeability and the relationships between them. I’m also going to assume a basic familiarity with the concept of magnetic equivalent circuits, where magnetomotive force (mmf) is analogous to voltage, flux is analogous to current, and reluctance is analogous to resistance. If you are not familiar with these concepts, they were all covered in the third article in the Power Electronics series, published in the January 2026 issue (siliconchip.au/Article/19557). I am also going to assume a basic understanding of rotational motion, since this is how most electrical machines transfer mechanical energy. Table 1 contains a summary of the key quantities in rotational motion compared to their linear motion equivalents. You will note that we are using radians to describe angles rather than degrees. You can easily convert between radians and degrees by remembering that there are 2π radians and 360° in a circle. Radians are a bit special because they are actually unitless (or dimensionless in some textbooks). An angle in radians is defined as the ratio of an arc length around a circle to its radius – so a length divided by a length. This technically means the units of angular velocity and angular acceleration are inverse seconds and inverse seconds squared (s-1 and s-2) respectively. The same thing applies if you measure angular velocity in revolutions per minute (RPM) – a “revolution” is a unitless number, so RPM has units of inverse minutes. Since you can convert between radians and degrees, that means degrees is not a true unit either (it is a ‘quantity kind’). Generating torque Now that we have the foundations clear, we can begin to build a picture of how a DC motor works. Fig.1 shows a short piece of conductor with length l suspended in a magnetic field. Ignore for a moment how this field comes about – it could be created by Table 1 – linear vs rotational motion quantities Linear Motion Rotational Motion Displacement s m Angular position θ unitless (radians) Distance Velocity v = ∆s ÷ ∆t m/s Angular velocity ω = ∆θ ÷ ∆t radians/s (1/s) Speed Acceleration a = ∆v ÷ ∆t m/s2 Angular acceleration α = ∆ω ÷ ∆t radians/s2 (1/s2) Acceleration I kg·m2 Inertial Mass T = Jα N·m (kg·m2/s2) Force (Newton’s Law) Work (energy) Power Mass Force m kg F = ma N (kg·m/s2) Moment of inertia Torque Work W = Fs J Work W = Tθ J (kg·m2/s2) Power P = Fv W Power P = Tω W name symbol units name symbol units 50 Silicon Chip Australia's electronics magazine siliconchip.com.au an electromagnet or by a permanent magnet. Suffice it to say that it produces a magnetic flux density of B tesla across the gap. If a current of I amps flows in the conductor, it experiences a force of F = B·I·l newtons in a direction perpendicular to both the current and the field. You can use the right-hand palm rule to remember which direction the force will be. If you place the fingers of your right hand together to represent the magnetic field lines flowing from north to south, and extend your thumb at right angles to represent the direction of the current, your palm will ‘push’ in the direction of the force. It may seem odd that there are no constants of proportionality in the formula for force. This is because, until 2019, this formula formed the basis for the definition of the ampere. Despite this the formula for force has not changed The new definition describes the ampere in terms of charge per unit time, specifically the number of elementary (electron) charges carried by a current of 1A in one second, but originally the amp was defined in terms of force. It is not a huge leap to imagine a loop of wire in the gap that can pivot around a central point, like that shown in Fig.2. You will notice that in the cross-section at the top, the wire on the left has a dot in the centre and that on the right has a cross. The convention (yes, another one) is that the dot represents a current coming out of the page and a cross represents a current going in. You can think of the dot as the point of an arrow coming toward you, and the cross as the fletching at the end of a departing the arrow, if that helps. Each wire in the loop will experience a force in the direction shown by the blue vectors. These forces, acting over the distance from the conductors to the pivot, produce a torque around the pivot. If the wire loops are free to rotate, they will tend to move clockwise until both conductors are outside the magnetic field. If the rotating loop has a meaningful moment of inertia, the left-hand conductor will continue to rotate past the 12 o’clock position and back into the field on the opposite side. Unless we changed the direction of the current, the conductors would experience a force in the opposite siliconchip.com.au direction and be pushed back toward the ‘neutral’ vertical position. Fortunately, it is pretty easy to switch the direction of the current using a commutator and brushes, as shown at the bottom of the figure. The commutator rotates with the shaft and has (in this case) two conductive segments on its surface, each connected to an end of the conductive loop. The brushes are fixed and slide across the surface of the commutator as it rotates, ensuring the current is always maintained in the right direction to produce pulses of torque in the clockwise direction whenever the loop is in the magnetic field. If the polarity of the voltage source connected to the brushes was reversed, the torque pulses would be in the opposite direction. Fig.1: a conductor of length l carrying a current I in a magnetic field with flux density B will experience a force F in a direction perpendicular to both the field and the current. Improvements This simple motor would work, but there are a few things we can do to improve it. The torque is given by the expression T = 2F·r, so we could increase the torque by increasing the force experienced by the conductors or by increasing the radius of the motor. One way to increase the force (from F = B·I·l) would be to increase the length of the motor. The torque is therefore proportional to both the motor’s length and its radius, so its volume, which explains why bigger motors have more torque and therefore more power. But what if we want to optimise the torque for a motor with a fixed volume? We could try to increase the magnetic field density, but we can only take this so far, since the core material will saturate at some point. We are left with only two choices: increase the current through the loop or increase the number of loops, which have the same outcome in practice. If the loop shown in Fig.2 was made up of many turns of wire, the force developed for a given current would be multiplied by the number of turns, since the current loops through the field that many times. This is equivalent to having a single coil and increasing the current by the same factor. If we are going to increase the number of conductors in the field, we could also do it by adding more loops and more commutator sections, as shown at the top of Fig.3. Here, we have four loops in total, which would require eight commutator segments. Australia's electronics magazine Fig.2: a loop of wire that can pivot around a central axis will experience a torque proportional to the forces acting on the conductors and the radius. If the direction of the current is reversed every 180°, continuous rotation is possible. Fig.3: the efficiency of the simple motor in Fig.2 can be improved by increasing the number of conductors subject to the magnetic field, and by reducing the air gap, by filling most of the space with core material. October 2026  51 At any given time, three loops are energised and one is open-circuited. This arrangement will produce a smoother torque, since there are now eight overlapping pulses of torque in each rotation instead of just two. I mentioned above that the upper limit on flux density is dictated by the saturation of the core material. This is true, but it does not speak to how easy (or difficult) it is to create this level of flux density in the first place. The amount of mmf required to create a particular level of flux is proportional to the reluctance of the circuit. This is analogous to the amount of voltage required to push a particular current around an electrical circuit, being proportional to the electrical resistance of the circuit. The reluctance of air is four or more orders of magnitude higher than the reluctance of the transformer steel used in the core. This means that the reluctance of the air gap largely determines the amount of mmf required to produce a given magnetic flux density. The reluctance of the gap is proportional to its length and inversely proportional to its cross-sectional area. The cross-sectional area of the gap is fixed by the motor’s size, so the only way to reduce the reluctance of the gap is to reduce its length. This is why we usually fill the gap with a rotating section of core material and embed the conductors in slots, as shown at the bottom of Fig.3. This rotating core, together with the windings and commutator, is known as the armature. This is just one possible DC motor configuration – some others are described in the accompanying panel. DC motor model We have seen that a current flowing through the armature conductors, switched to be in the right direction by the commutator and brushes, creates a continuous torque. This torque is proportional to the armature current multiplied by a torque constant km, such that T = km·I. This torque constant wraps up the physical dimensions of the motor, the winding arrangement and the field flux density into a single handy number. There is another thing happening at the same time. Faraday’s Law tells us that a voltage is induced in a conductor 52 Silicon Chip Fig.4: the equivalent circuit of a DC motor and the two equations to the right are all that is necessary to characterise any DC motor. If the rotation speed is such that the backEMF exceeds the armature voltage, the current reverses and the motor acts as a generator. that is moving in a magnetic field, and that is certainly what is happening in the DC motor as it rotates. A voltage is therefore induced in the armature conductors in a direction that opposes the voltage applied to the motor’s terminals. This voltage is called the “backEMF” and it is proportional to the speed of the shaft by a speed constant ke such that Eb = ke·ω. These two constants let us construct the very useful DC motor model shown in Fig.4. The model consists of the armature resistance Ra, which takes into account the winding resistance as well as the equivalent resistance of the commutator and brushes, the armature inductance La and the back-EMF source Eb. The formulae to the right relate the electrical quantities to the motor’s torque and speed. Now for the really odd thing: for any given DC motor, the constants ke and km are actually identical. How can this be, given that the torque constant has units of Newton-metres per ampere (Nm/A) and the speed constant has units of volts per radian per second, or volt-seconds (Vs)? These units actually both describe work (energy) per amp, just in different forms. The proof of this is shown in the grey box below the equivalent circuit in Fig.4, if you are interested. Australia's electronics magazine This blew my mind when I first learned it back in the day. You often see different figures for these two constants in motor data sheets, but this will be because one or the other is expressed in some different terms. The speed constant is often given in terms of volts per RPM, for example, and the torque constant may be given in gram-cm (or worse, ounceinches) per ampere. If you convert them to Nm/A and Vs, you will find they are equal. For steady-state operation, we can ignore the inductance, although you may need to consider it under transient conditions, along with the rotor’s moment of inertia. You cannot, however, ignore the motor’s armature resistance, which should be provided in the data sheet. If your motor has carbon brushes, you may not be able to accurately measure the armature resistance from the motor terminals when it is stationary. If your motor has so-called ‘precious metal’ brushes (usually plated with some alloy of silver or palladium), you may have better luck. Nevertheless, I recommend using the value given in the motor’s data sheet. Motors with precious metal brushes are most suitable for light-load, high-performance applications, while those using carbon brushes are most suited for heavy-duty, high-current applications. The brushes on a DC motor in a power tool will almost always be carbon, for example, while those in a model train motor will probably be precious metal types. We can use the DC motor model to understand how the motor will behave under various conditions. If the nominal motor voltage Vn is applied to the motor while the rotor is stationary, the back-EMF will be zero, so the current is limited only by the armature resistance. This stall current, or ‘locked rotor’ current Is will be Vn ÷ Ra, and the corresponding stall or starting torque will be Ts = km·Is. This current and torque will be quite high, and the motor will overheat very quickly if these circumstances are allowed to persist. Fortunately, the back-EMF begins to rise as the rotor accelerates and the current drops as the voltage across the armature resistance decreases. If there was no shaft load and the motor was completely lossless, the current would drop to zero when the motor reached siliconchip.com.au the speed where the back-EMF exactly balanced the terminal voltage. Of course, this does not happen in reality; instead, an unloaded motor reaches an equilibrium speed, called the no-load speed, where the current drops to the level necessary to produce just enough torque to overcome the friction and windage. Torque-speed curve We can plot the torque-speed characteristic of the DC motor as shown in the middle of Fig.4. I have labelled the vertical axis with both shaft torque and armature current, seeing as these are proportional to each other. The red line is the nominal voltage torque-speed characteristic, given by the relationship Ts = km·(V – ke·ω) ÷ Ra. The blue line is the same characteristic at half of the voltage, and the green line is the torque-speed characteristic with zero terminal voltage. There is one very interesting thing to note here. If the shaft’s angular velocity is high enough that the back-EMF exceeds the applied armature voltage, the current flow will reverse and the motor acts as a generator. This can happen when you reduce the voltage on a motor with a high-inertia load, and the resulting reversed torque acts as a brake, slowing it rapidly. That’s the big picture, but it can be a bit misleading because the motor is only rated to operate continuously in a very small area of this graph. I have put some numbers on the chart to demonstrate. I am using data for a relatively large and expensive DC motor about the size of a beer can (65mm in diameter and 125mm long). It is a 24V motor with a nominal speed of 3200RPM (335rad/s). Its stall current is 34A and its stall torque is 2.0Nm. In this condition, the power dissipation in the motor is 816W. The no-load speed of the motor is 387rad/s, at which point it draws 0.45A. A closer look at the data shows that its shaft power is limited to 90W continuously and its electrical input power is limited to 120W. This input power limit puts upper and lower bounds on the continuous current at ±5.0A and therefore a limit on the continuous torque of around ±0.3Nm. The continuous operating region is therefore limited to the space between the two horizontal lines. I have reproduced the torque/speed siliconchip.com.au characteristics of this motor in Fig.5 on the left, focusing on the continuous operating region while motoring. The blue lines represent the motor’s torquespeed characteristic for increasing steps of armature voltage. The red dot is the point where the motor is at its maximum continuous power (24V at 5A = 120W). The black and green lines represent the torque-speed characteristics of two possible loads: a constant-torque load, like a hoist, and a square-law load, like a fan. The dots represent the operating points for the loads at each armature voltage. The main takeaway is that you can control the speed of a DC motor by controlling the armature voltage, but the speed regulation will not be perfect – the actual operating point will depend on the load. Field weakening The motor I used in the example above is a permanent-magnet type, so the value of ke (and km) that relates back-EMF to rotor speed is fixed. It turns out that ke is inversely proportional to the field strength, which makes sense when you consider that the back-EMF is produced by the movement of the armature conductors in the magnetic field and reducing the field strength should reduce the back-EMF. The result is the slightly counterintuitive idea that reducing the field excitation increases the shaft speed for a given armature voltage – but at the expense of torque. This is obviously only possible in wound-field motors where you can reduce the field strength by reducing the field current. By doing this, we can increase the shaft speed of a woundfield DC motor beyond the nominal speed or ‘base speed’ suggested by the nominal voltage/nominal current limit indicated by the red dot in Fig.5. The graph on the right shows how this works. With full flux, the motor can operate at any speed up to the base speed, and any torque (current) up to its rated value by an appropriate choice of the armature voltage. This full-flux region of operation is indicated by the blue torque-speed lines in the chart on the right of Fig.5 and is often referred to as the ‘constant torque’ region. In this context, ‘constant torque’ signifies that the maximum continuous torque is constant. You can increase the speed beyond the base speed by weakening the field. This field-weakening region is indicated in the figure by the green torquespeed lines. Each line represents a step reduction in the field current. This region is also known as the ‘constant power’ region. It is ‘constant power’ in the sense that the continuous power can’t exceed the motor’s maximum rated power. Since power is torque times speed, increasing the speed at a constant power means the torque is reduced. Motor diagrams Before we move on to look at controlling DC motors, I want to show you a typical DC motor operating chart that you will find in most motor data sheets (see Fig.6). These charts can be a bit tricky to read, but have pretty much everything you need on them. This one is for a RS-555PH motor from the Japanese manufacturer Mabuchi. Unlike the torque-speed curves, these graphs have torque on the horizontal axis and speed, current, power and efficiency plotted on the vertical axis. The no-load point is the extreme left of the horizontal axis (zero shaft torque), where you can read the no-load current from the y-intercept of the curve marked I (~150mA) and the no-load speed from the Fig.5: for a wound-field motor, you can reduce the field strength to increase the rotor speed beyond the base speed. Torque falls off quickly in this mode because the motor is operating at a constant power. Australia's electronics magazine October 2026  53 y-intercept of the speed curve marked N (~5500RPM). The stall or locked rotor torque is at the extreme right at around 200mNm. You can read the stall current (a little over 10A) by projecting up to the I curve and across to the vertical axis. The last two curves show the motor power (marked P) and efficiency (marked with the Greek letter eta, η). It’s worth noting that the rated continuous torque of this motor is around 37mNm and the rated current is 2.45A, so continuous operation will be confined to a narrow slice of this graph that I have shaded in cyan. It’s really easy to burn out a DC motor if you don’t control the current carefully! Motor controllers DC motor drives, especially those intended for industrial applications, tend to have a pretty standard control scheme. The diagram at the top of Fig.7 shows the block diagram of a typical DC motor controller. It consists of two nested control loops feeding a modulator; the latter including the power electronics. We’ll look at the modulator in a moment; we will concentrate on the control loops first. The inner loop is a current/torque control loop. This accepts a current demand coming out of the speed controller and compares it to the measured armature current to produce an error signal that is applied to the current controller. This is normally a proportional-­ integral (PI) controller, which can reduce the error to zero. The outer speed control loop accepts a desired speed setpoint and compares it with the rotor speed, as measured by a tachogenerator, or more Fig.6: this extract from a typical motor data sheet shows the curves used by most manufacturers to characterise motor performance. The added shaded area shows the region where continuous operation is possible. 54 Silicon Chip Australia's electronics magazine likely these days, by a digital encoder. The speed error is applied to a speed controller to produce the current/ torque setpoint. The speed controller can be as simple or complex as the application demands. A fan application may be able to get away with a very simple speed controller, while a mine winder or rolling mill will probably incorporate an advanced digital controller. The role of the current control loop is to keep the motor current within the safe operating area to avoid operating the motor outside its continuous ratings for any length of time, and to protect the semiconductors in the modulator. We have seen how important this is, since the locked-rotor current of a DC motor can be 20-50 times its continuous rating. The current control loop is generally fast, typically having a bandwidth in the order of the armature’s L/R time constant. The speed control loop is slower, with a bandwidth dictated by the moment of inertia of the motor’s rotor and the load. The speed control loop is normally in saturation until the motor’s speed gets quite close to the setpoint, so the maximum torque is applied to accelerate or decelerate the load. In low-cost or small drives, it is possible to do away with the need for a speed sensor if you can tolerate a little bit of speed error. In this case, the shaft speed is estimated from the motor voltage and current using a technique known as ‘voltage feedback with IR compensation’. The current feedback is used to estimate the voltage drop across the armature resistance, which is then subtracted from the terminal voltage to get an approximation of the backEMF, which is proportional to shaft speed. You can also sense back-EMF directly in some very specific circumstances that I will demonstrate below. Of course, you can run your DC motor in an open-loop configuration if you don’t care about speed regulation. Still, you should provide some sort of current limit to keep the motor in its continuous operating region. There are many variations of this basic control strategy, two of which are shown in Fig.7. The first is an example where the controller is driving multiple motors that are mechanically coupled. This happens quite a lot in siliconchip.com.au Fig.7: the basic control scheme for driving a DC motor is at the top, plus two variants; one for driving coupled motors and one for servo drives. ▶ Fig.8: the four possible combinations of voltage and current polarity represent four quadrants. Quadrants I and II correspond to driving and braking in one direction of rotation, while quadrants III and IV represent braking and driving in the other. industrial applications, like conveyors and rolling mills. Each motor has its own current/ torque control loop fed by a common speed control loop. This arrangement forces the motors to share the load evenly. The next example is the servo-drive. Servos are capable of precise position control and are used extensively in robotics and automation. The servo controller is identical to the speed controller described above, but has an additional position control loop wrapped around it. The position is measured by some position sensor, like a potentiometer or an absolute digital encoder. You will sometimes see servo controllers implemented without the intermediate speed control loop, such as in hobby servos. Unless the system is very well damped, this approach is prone to overshoot and ringing when a step change in position is requested. Hobby servos have plenty of damping built in, thanks to their multi-element gear train and relatively high friction. Operating quadrants I mentioned above that the DC motor will act as a generator (ie, the armature current will reverse) and power will be siliconchip.com.au exported if the back-EMF exceeds the applied armature voltage. The direction of rotation can also be switched by reversing the polarity of the armature voltage. We therefore have four possible combinations of armature voltage and current polarity, as shown in Fig.8. If this seems familiar, I used a very similar diagram in the AC-to-DC Converter article from the Power Electronics series in the February 2026 issue (siliconchip.au/Article/19657). The quadrants are denoted by Roman numerals counterclockwise from the top right. In quadrants I and III (shaded green), the voltage and the current have the same sign, so positive power is supplied and the motor is driving the load. In quadrants II and IV, the voltage and the current are of opposite polarities, so ‘negative power’ is ‘supplied’ and the motor is being driven by the load. I have called this “braking” since the motor torque acts as a brake on the shaft. Thyristor modulators The ‘meat and potatoes’ of a DC motor drive is the modulator, which consists of the power electronics and its drive circuitry. The classic industrial DC motor controller uses a Australia's electronics magazine thyristor bridge, as shown in Fig.9. I won’t cover the analysis of this topology here, as I did so in the AC-to-DC Converter article mentioned above. The converter can be driven by a single- or three-phase mains supply. I have shown the armature voltage and current waveforms for a single-phase version to the right of the circuit diagram. You will recall that due to the inductive nature of the load, this converter can produce a positive or negative average output voltage, although the current can only ever be positive. This is technically a two-quadrant converter (quadrants I and IV). However, this combination is not really all that useful in motor applications, as the drive can only produce a braking torque if the shaft is rotating in the opposite direction than it rotates when being driven. Active deceleration of the load is not possible, so this two-quadrant circuit is effectively a one-quadrant motor drive. You can achieve four-quadrant operation with this converter if you add a mechanism to reverse the field. This gives you rotation in both directions, but it is difficult to transition smoothly between driving and braking because field reversal is not instantaneous. Field windings tend to have a high inductance, so changing the direction October 2026  55 of the current has to be done with care and takes appreciable time. It is better to use a true four-quadrant converter, as shown at the bottom of Fig.9. This is effectively two two-quadrant converters connected to the motor with opposing polarity. With this circuit, the armature current can reverse smoothly and so the transition from driving to braking and vice versa is much better. This is the arrangement generally used in applications like hoists, mine winders, ski lifts and the like, which require large, high-voltage motors and where the torque can reverse frequently. There is a limit to the braking performance of this type of drive circuit. When driven by the load, the motor terminal voltage must remain lower than the peak voltage of the mains by some margin to ensure the voltage across the thyristors can reverse, to allow them to switch off. ‘Chopper’ modulators Fig.9: the classic two-quadrant and four-quadrant controlled rectifier circuits are commonly used for driving kilowatt or megawatt scale DC motors in industrial applications. The two-quadrant variation cannot provide a braking torque in the direction of rotation, so it is effectively a one-quadrant circuit in motor drive applications. Fig.10: this diagram, extracted from the DRV8874 data sheet, shows that it contains everything necessary to control two motors in two quadrants or one motor in four quadrants. It includes motor current sensing and limiting, making it very easy to implement a safe and effective DC motor drive. 56 Silicon Chip Australia's electronics magazine You can also drive DC motors from a DC source using a ‘chopper’ type drive as shown in Fig.11. If you think the one-quadrant modulator looks a lot like a buck converter, you would be right. That is exactly what it is. If you consider the armature inductance to be the filter inductance and the armature resistance to be the load, and take into account the back-EMF, all of the equations describing a buck converter apply. If the motor current is continuous, the motor voltage is a square wave with an average voltage determined by the duty cycle, as shown in the top chart. If the current is discontinuous, the output voltage has a stepped shape. The motor’s terminal voltage during the zero-current portion of the cycle will be its back-EMF. If you can be sure to always stay in discontinuous current mode, you can sample this voltage to get direct feedback of the motor’s speed. This is generally possible only with small motors that have low inductance and operate over a limited load range. The buck converter is a one-­quadrant drive because neither the motor voltage nor the current can reverse. If the freewheeling diode is replaced with an active switch, as shown in the middle of the figure, this circuit becomes a two-quadrant drive. It behaves like a buck converter in quadrant I, with siliconchip.com.au Q1 as the switch and Q2’s body diode as the freewheeling diode. If you squint, you might be able to see that in quadrant II, it behaves like a boost converter, with the motor’s back-EMF as the voltage source, Q2 as the switch and Q1’s body diode as the output diode. By appropriately driving the two Mosfets (or IGBTs), you can produce a positive or negative motor current, but only with a positive voltage (ie, operating in quadrants I and II). This circuit can therefore provide both driving and braking in one direction of rotation. If you want four-quadrant operation, you have to resort to an H-bridge type driver, like the one at the bottom of the figure. You can think of this as two two-quadrant converters, one connected to each motor terminal. This can produce a driving or a braking torque in either direction of rotation. One thing you need to pay attention to in the two- and four-quadrant circuits shown here is the ability of the source to absorb the energy fed back during braking. It may not be a concern if your source is bidirectional, like a rechargeable battery, but it may well be a problem if your power is coming from a one-quadrant source like a DC-DC converter or a rectifier-­filter. In that case, you can use a capacitor bank to temporarily store the regenerated energy. When driving the motor, the capacitor voltage will be equal to the supply voltage. When the motor is regenerating, the capacitor will charge to some higher voltage. As long as you size the capacitor correctly to limit the voltage rise to something acceptable, this is a good way to manage the regenerative energy, because it can be re-used when the motor is driven again. Sometimes, this approach is just not enough, so a ‘braking resistor’ can be switched in to absorb some or all of the regenerated energy. This energy is obviously lost as heat. Getting practical If you are driving smallish motors (up to a few amps continuous rating), you can get plenty of low-cost chips that implement most of the power electronics of a two-quadrant (halfbridge) or four-quadrant (full-bridge) chopper type controllers that do a lot of the hard work for you. siliconchip.com.au Fig.11: so-called ‘chopper’ style motor drives look like the standard DC-DC converters you may be familiar with. In contrast to the two-quadrant thyristor circuit in Fig.9, the circuit in the middle provides useful two-quadrant operation. The DRV8874 from TI is one such controller I used in a recent project. This chip (Fig.10) has two Mosfet halfbridges capable of switching currents up to 6A. The motor supply voltage can be anywhere from 4.5V to 37V. The half-bridges can be operated separately, to control two single-direction motors in a two-quadrant arrangement, or together to drive one motor in all four quadrants. The chip includes the charge pump and level-shifters necessary to drive the high-side Mosfets, as well as comprehensive safety features including undervoltage lockout on the main supply and the charge pump voltage, overcurrent and over-temperature protection. The logic inputs support 1.8V, 3.3V and 5V logic levels. One very nice feature of this chip is that the lower Mosfet source current is sensed internally and a proportional current (1mA/A) appears at the IPROPI pin. This is really only meaningful for the H-bridge configuration, since the currents from the two half-bridges are summed. An external resistor to ground converts this to a voltage. Australia's electronics magazine Better still, by providing a current reference voltage at the Vref pin, the chip can regulate the Mosfet current to not exceed some desired level. Two regulation modes are available: a cycle-by-cycle mode that works like a typical current-mode controller, or a fixed-off-time mode that holds the Mosfets off for a short period whenever the current exceeds the threshold. The latter has the advantage that it will regulate current even if the PWM frequency is very low or the duty cycle is 100%. As we have seen, current limiting is very important for DC motors, so this feature means the current control loop is effectively done for you. This chip costs just $4.51 in one-off quantities, which is a bargain in my book. There is an even cheaper 3.5A version, the DRV8876, that costs just $2.80. I am out of space, so that’s all I can fit in this month. Next time, I will take a look at externally commutated DC motors, like brushless and stepper motors. ...see overleaf for more October 2026  57 DC Motor Configurations There are literally dozens of DC motor configurations and not enough space to describe them all. I have chosen to describe just a few of the more common ones. Fig.a shows three woundfield motors with the field windings shown in yellow. In all of these figures, the grey areas represent the magnetic path, usually made of laminations of transformer steel. The rotor windings (in blue) are sunk into slots in the rotor steel so that the air gap can be reduced to an absolute minimum. Although it is not shown here, the slots in the rotor do not normally run straight down the length of the rotor. Instead, they are ‘skewed’ or twisted slightly around the motor’s axis. This helps minimise the torque ripple or ‘cogging’ produced as the windings are energised and de-energised as they rotate. Adding more field pole pairs helps to increase the available torque because more of the armature current loops are intersected by the magnetic field. The maximum power of a given-sized motor is fixed, so increasing the torque in this way usually comes at the expense of speed, since power is the product of speed times torque. As you would imagine, every time the brushes slide off a commutator segment, there is the potential for an arc as the commutation is effectively interrupting an inductive circuit. In very small motors, and especially in permanent magnet motors, the inductance is small and a level of sparking can be tolerated. In larger motors, this can be a problem since the sparking can erode the commutator and brushes. Larger motors therefore often have small ‘interpoles’ on the stator positioned between the main field poles. Their purpose is to induce a voltage in the coil undergoing commutation, in such a direction that it speeds up the reversal of current, thereby limiting sparking. The required induced voltage is proportional to the current being commutated, and to the rotor speed, which can be achieved by wiring the interpoles in series with the armature. Fig.b shows two permanent magnet DC motor configurations. In both cases, two-pole field excitation is produced by a pair of permanent magnets attached to the motor’s housing, which serves as the return path for the flux. The motor on the left is typical of high-performance motors and has a slot-wound rotor. The motor on the right is exemplary of a whole family of low-cost motors that have an odd number of ‘salient’ rotor poles. While these salient-pole motors are sometimes designated ‘toy’ motors, they can have very good performance and are often used in battery-operated power tools and other high-demand applications. A salient pole is a magnetic field pole that projects outwards from the rotor toward the stator, creating a non-­ uniform air gap. This type of pole is used in low-cost motors because the armature conductors can be wound directly onto the rotor core by automated machinery. In the case of slot-wound motors, the coils have to be formed first, then inserted into the rotor slots in an interleaved fashion that is difficult to automate. All the motors described so far have a rotor core made from laminated transformer steel. This is great for reducing the air gap, but does mean the rotor inertia is relatively high, limiting the dynamic performance of the motor. Fig.c shows a ‘coreless’ DC motor which, as the name suggests, does not have a rotating magnetic core. Instead, cylindrical permanent magnet poles are fixed in the centre of the motor and the windings rotate about them. A magnetically permeable housing provided the flux return path. The rotating coils are impregnated with varnish or epoxy to form a self-supporting cylinder. The exploded view, extracted from a publication by the German precision motor manufacturer Faulhaber, shows the key components. This particular model has sintered bearings pressed into the ends of the magnet, which has an axial hole in it for the motor shaft. Coreless motors have very low torque ripple because the rotor is comprised of many interleaved turns of skew-wound copper, and very low rotor inertia due to the low rotating mass. They are relatively expensive, but are commonly used in high-performance applications, including servodrives, robotics and medical equipment. Field windings Before the advent of low-cost power Fig.a: the wound-field motor on the left has two field poles, while the other two have four. The motor on the right also has interpoles, which help eliminate commutation sparking. 58 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.c: for very high dynamic performance, it is hard to go past coreless DC motors. These have an armature that is a self-supporting hollow cylinder of conductors that rotates in the air gap between the fixed permanent magnet core and the motor housing. The exploded diagram is taken from a publication by Faulhaber. electronics, it was common practice to power the armature and the field of wound-field DC motors from a common (often fixed) DC supply. You could do this in two different ways, as shown at the top and middle of Fig.d. I have shown a separately excited DC motor at the bottom for completeness. In general, shunt-wound motors are better for constant-speed applications and series-wound motors are better for constant-torque applications, but neither configuration is used as often as it once was, with a couple of notable exceptions. There are three main reasons why: 1. A separately excited motor with the right controller can do anything a series or shunt machine can do and a lot more besides. The reduced cost of power electronics means we can easily control the field separately. 2. Permanent magnet DC motors have more-or-less taken over in the sub-kilowatt range as magnetic materials have improved. No field winding to worry about! 3. Series and shunt machines are hard to reverse. You can’t just switch the polarity of the DC supply to reverse them because that reverses both the armature and field at the same time, so torque is still created in the same Fig.d: wound-field motors can be configured with the field winding in parallel with the armature, in series with it, or powered separately. Universal motors, which can operate from AC or DC, are series-wound DC motors. direction. You have to use relays or contactors to reverse either the field or the armature polarity. I mentioned that there were a couple of exceptions where series-wound motors are still used. The first is in railway traction, where the very high starting torque of series-wound DC motors is important to get heavy rolling stock moving. Having said that, I observe that induction motors with advanced inverter drives are starting to make inroads into this application because of their lower purchase and maintenance costs. The other area where you still see series-wound brushed DC motors is the ‘universal’ motors used in corded power tools and some home appliances. These are actually DC motors that take advantage of the polarity agnosticism mentioned to run directly off the AC mains. The rapidly changing polarity of the mains switches the direction of the current in the armature and field at the same time, leaving the torque direction unchanged from one half-cycle to the next. Universal motors exhibit a bit of 100Hz torque ripple due to the zero crossings in the current waveform, but that isn’t usually a problem for an application like a power drill or a washing SC machine. Fig.b: many smaller DC motors use permanent magnets to provide the field excitation. The motor on the left has a slot-wound rotor, while the one on the right has salient poles. The latter are easier to wind and so are often seen on low-cost motors. siliconchip.com.au Australia's electronics magazine October 2026  59