Servo motors 101: Commutation and alignment

The main difference in servicing permanent magnet brushless (PMBL) motors versus other kinds of motors comes down to correctly aligning the position of the feedback devices relative to the rotor magnet and stator windings so that the drive system can correctly commutate the excitation currents to the armature winding. This article explains what commutation is and why this alignment is necessary for correct commutation.

How magnets produce force and torque

Magnets always have two poles, north and south, and lines of magnetic flux flowing from north to south. This flux will always try to take the path of least resistance (which is increased with distance). In fact, a force will be applied to the magnet and surrounding objects to move them into a position that reduces the flux path resistance, that's what happens when you move a permanent magnet close to an iron or steel object. 

This force is what allows motors to generate torque. Torque is a force applied to a radius for a rotating object, multiplied by the length of the lever arm, this is why when you tighten a bolt with a socket wrench, the further away from the bolt you apply the force to the handle, the more torque will be applied. You can verify this with a torque wrench.

Motors typically use electromagnets, produced by winding coils around a soft iron core and applying current to them, to produce torque by the interactions of the magnetic fields within the motor. The compass pointer is similar to a motor rotor while the horseshoe magnet is similar to a motor stator field. This would represent a 2 pole motor because there are 2 magnetic poles on the compass pointer. For PMBL motors the fields are produced by windings on the stator and permanent magnets on the rotor while Brush type DC Motors will have windings on the rotor and permanent magnets or a field winding on the stator. Each type has its advantages that the motor designer takes into account depending on the application.

Commutation

If we apply current to the windings in figure 2 by closing the switch, the rotor will move to align with the path of least resistance of the magnetic flux and will stay there. This could be used as an actuator, but it doesn't look much like a motor that would be rotating continuously until we remove the current from the winding and not lock up in just one position. 

If we add another phase to our stator winding, we can get the motion going. In Figure 3A, we apply a voltage on phase A, the compass pointer will move and stay aligned with the generated magnetic field and flux. If we then remove voltage on phase A and apply voltage on phase B the pointer will move 90° clockwise (1/4 of a turn) as shown in figure 3B.

Figure 3C shows that, if we apply a voltage on phase A with a reversed polarity, this will also reverse the direction of the magnetic fields, which will move the pointer another 90° clockwise. Finally, if we apply a reversed polarity voltage on phase B, we will move another 90°. Applying the original voltage on phase A will cause the pointer to go back to its original position, as in figure 3A, completing a full turn. We could keep repeating this sequence to keep our 2 phase motor rotating.

We have just designed a 2-pole 2-phase stepper motor which we can keep rotating in 4 different positions. For simplicity in the drawings, this concept has been shown for a 2 phase motor. Most modern
servo motors are 3 phases, but the principle is the same. Commercial stepper motors typically have 50 poles and 200 locations in one revolution.

The differences between stepper motors and servo motors are beyond the scope of this article, but in general, servo motors need to run smoothly at high rotational speeds (measured in RPM) and provide considerable torque over their speed range. This means that servo motors don't need to stop before the voltage switches to a new phase. The voltage must switch while the rotor is at certain positions such that magnetic fields are providing maximum torque. This switching is called commutation. 

Conventional brushed motors incorporate an ingenious mechanical apparatus using brushes and a commutator to achieve this. As the rotor turns, the commutator turns, thereby changing the windings to which the right voltage is applied, never to a winding that is already in the position of the path of least resistance of magnetic flux. It's similar to a dog chasing its tail. The rotor is never allowed to get to the stable position or rest. Again, this switching is called commutation.

Brushes and commutators usually are high-maintenance, that's why PMBL motors are becoming more and more commonly used. However, we still require the commutation process in order to keep brushless motors moving, which now is achieved by a driver that is electronically switching the voltage from winding to winding just as we did manually in our 2 phase winding and compass pointer example. We want the motor to keep running smoothly like we are used to with a DC brushed motor. To accomplish that, we must know the position of the rotor magnets relative to the stator windings so that we can always apply the voltage to the right winding to get the maximum torque. This requires us to add a new device to our system: the feedback.

The feedback device reports the position of the rotor magnets to the drive so that it can apply voltage to the winding that will provide maximum torque in its current position.

Feedback devices commonly used with PMBL servo motors are Hall-effect switches, resolvers,
incremental encoders, and serial encoders. Often they can be used for positioning as well as for
commutation, but they must be present for electronic commutation.

The job of the feedback device is to synchronize the voltage applied to the windings with the position of the rotor magnets. Exactly how these are synchronized is up to the motor designer, and will vary greatly among different manufacturers. The basic idea is always the same, but the details can differ considerably.

Feedback alignment

Our automobile engines use a distributor to synchronize the crankshaft position and the spark plug ignition voltage to each cylinder. Maximum torque results when the distributor provides ignition to a cylinder when the fuel mixture is fully compressed nearly in position for the downstroke. We know that the engine will perform poorly or not at all if this timing is not adjusted correctly

Something similar happens with PMBL motors. The position feedback device synchronizes the relative rotor magnet and stator winding positions to the current provided by the drive, getting maximum torque. The motor will perform poorly or not at all if this timing is not adjusted correctly. Improper feedback alignment on the motor can cause it to run backward, sometimes resulting in a hazardous runaway condition. Correct alignment is absolutely essential for the proper operation of PMBL servo motors.

So far, we have been talking from the perspective of a motor designer, but in a way, they are the ones making the rules, they know how the feedback device must be aligned because they were the ones to set it up that way in the first place. Technicians on the other hand must be able to check and correct feedback alignment for the motor to run at peak performance.

During troubleshooting, a technician must be able to perform the following checks: 

1. Is the feedback device functional?
2. Is the feedback device aligned per manufacturer specifications?
3. Is the feedback device wired correctly?
4. Are the armature leads wired correctly?

Besides that, the alignment can be lost during the repair if the feedback is removed for troubleshooting, bearing replacement, or encoder replacement. Of course, feedback alignment is only one of the many steps in the process, but it is one that is absolutely crucial. Without the proper tools, this essential task is difficult if not impossible.