DC Electrics - DC Motors — Page 110, Lesson 107

I want to walk you through a key concept in DC motors: back EMF. As the conductor — the wire — moves inside the magnetic field of the motor, it cuts through magnetic flux. That movement induces an electromotive force, or EMF, in that conductor. And from Lenz's law, we know that this induced EMF will always oppose the rate of change of magnetic flux that produced it. So here's what happens: the rotating part of the motor generates its own voltage, and that voltage opposes the rotation itself. In other words, the induced voltage pushes back against the supply voltage that's driving the motor. That's why we call it the back EMF — it's an EMF that works against the applied voltage. Now, the back EMF is proportional to motor speed. The faster the motor spins, the larger the back EMF becomes. But it can never equal or exceed the supply input voltage — it's always a bit less. The difference between the applied EMF from the supply and the back EMF is what allows current to flow through the conductor and keep producing motion. If the back EMF ever matched the supply voltage exactly, no current would flow, and the motor would stop turning. Let's talk about a practical problem this creates. When a motor first starts, it isn't spinning yet, so there's no back EMF. That means the only thing limiting the initial current is the very low resistance of the armature windings. That initial starting current can be extremely high — potentially damaging. So some motors include a slow start resistor in the circuit. This resistor is connected in series with the armature when the motor is first started, to reduce that initial surge of current before the back EMF has had time to build up. Once the motor is turning and the back EMF is established, a switch — either a centrifugal switch that responds to speed, or a time switch — bypasses the resistor, allowing full current to flow to the armature. You can see this arrangement in Figure 7.4, the slow start resistor circuit. Now, let's move to commutation. The simplest form of DC motor has a single loop of wire that can rotate freely between the poles of a permanent magnet. A connection from the DC supply — say, a battery — is made to the loop through brushes that ride on a commutator. The commutator in this simple case has two segments, and each segment is connected to one end of the loop. Figure 7.2a shows an example of this type of motor. But a single-loop DC motor can't handle heavy loads. To get a large mechanical output with smooth running, we make the same improvements that are used in a DC generator. That means we use a laminated iron core carrying a number of armature coils, and a corresponding number of commutator segments — not just two. And the magnetic field is produced by an electromagnet, not just a permanent magnet. So the basic principle of commutation remains, but the construction becomes much more robust for real aircraft-grade applications.