How real electric motors work

John Storey

 

6. Brushless DC motors

Instead of having the magnets on the stationary casing and the windings on the rotor, we could put the magnets on the rotor and the windings on the stator. That way, we won't need brushes at all because the winding is stationary. However, now we need to find a way to switch the current through the windings at the right moment to ensure the torque on the rotor is always in the same direction. In a conventional motor, this happens automatically as the commutator acts as mechanical switch. With a brushless motor, we need some way to sense the position of the rotor, and then electronically switch the current so it's going the right way through the right winding.

Brushless motors are found in computer hard drives, CD and DVD players, and in anything else where efficiency and reliability are more important than price. As the cost of electronics continues to come down, perhaps one day all DC motors will be built this way.

Advantages:
• No brushes
• Simple
• Efficient
• Windings are attached to the casing, and easier to cool.

Disadvantages:
• Requires complex drive electronics

In fact, brushes are bad news. True, they're a clever way to ensure that, as the rotor turns, the current is automatically switched around the windings to ensure the motor keeps turning. However, everything else about them is bad: they are noisy, create friction, generate electrical interference (because of the sparking) and reduce efficiency (because there will always be a voltage drop across the brushes). Not only that, but they eventually wear out. With modern electronics, we can instead sense the position of the rotor (for example, with a Hall-effect device), then switch the current with, for example, a MOSFET transistor.

This is a fan that spent most of its life inside a computer keeping the microprocessor cool. It runs off 12 volts DC and has a brushless motor, as it thoughtfully explains with large friendly letters on the label.

As promised, the magnets are on the rotor (with fan blades attached) in a ring around the outside of the hub. By “feeling” them by using a small compass as a probe, we find that there are four poles, running N-S-N-S around the ring.

The “stator”, in the centre, has four small coils with shaped pole pieces to create a strong magnetic field next to the rotor. Depending on which director the current flows through each coil, it will attract or repel a north pole. So, all we have to do is to keep switching the direction of current flow through the coils in synchronisation with the rotation of the magnets, and we’ll keep exerting a torque that keeps the fan turning.

Now we’ve peeled the label off and can see the electronics that does the switching. It consists of a single integrated circuit and a few small capacitors, so it’s actually not all that complex! If we google the part number of the chip (LB1962M), we find it is a “Fan motor single phase full-wave driver”, which I guess is reassuring.

But how does the motor know the exact moment that the magnet has passed one pole, and therefore that it’s time to reverse the current flow? There are three techniques commonly used:

• Hall-effect sensors. This is a neat, non-contact way of knowing where the magnets are.
• Back EMF. This is even neater. We don’t use sensors at all, but use the fact that the magnet moving past the coil will induce a voltage in it, and use this voltage to tell us where the magnet is.
• Don’t bother. For the ultimate minimalist approach, just keep switching the coils in sequence and assume the rotor will keep up. For motors with a small load that is well defined (eg, a fan), this works pretty well.

 

 

 

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