Mike Howell, PE
EASA Technical Support Specialist
Unlike their AC counterparts, DC machines do not have rotating magnetic fields. Rather, there are fixed magnetic field axes for the field (direct axis) and armature (quadrature axis). Even though the armature is rotating, the magnetic field axis in the armature is fixed thanks to commutation, which allows the direction of current in an armature conductor to change as it passes from the region under one main field pole to the next.
This is shown in Figure 1 where all armature conductors on the right side of the armature axis (marked ×) have current flowing into the page and all armature conductors on the left side of the armature axis (marked ●) have current flowing out of the page. If the armature was rotating counterclockwise, the conductor shown at position “a” would experience a current reversal in moving to position “b.” The armature axis shown also coincides with the mechanical neutral plane as it is directly between the N and S main field pole. Ideally, as a conductor moves from “a” to “b,” the coil will be shorted or bypassed by the brush at a point where it has zero volts across it, preventing damaging currents and sparking at the brushes. Unfortunately, ideal situations rarely exist in practice.
To demonstrate the function of interpoles and compensating windings, we will use the DC machine modeled in Figure 2. It is a 2-pole machine and includes armature, main field, interpole and compensating windings. Practical machines with all these winding elements generally have 4 or more poles. Several simulations will be presented with different combinations of the windings energized and the resulting magnetic field.
Armature Reaction
Current in the armature conductors produces a magnetic field around the armature conductors that distorts the magnetic field produced by the field winding. The distortion increases with increasing load (armature current) and is called armature reaction. Two problems arise due to armature reaction in DC machines.
The first problem is a shift in the magnetic neutral plane position. The magnetic neutral plane, what we commonly refer to as brush neutral or just neutral position, is the position where armature conductors are exactly parallel to the main field flux such that the induced voltage in the coils is zero. When the magnetic neutral plane shifts due to armature reaction, it shifts in the direction of rotation for a generator and opposite the direction of rotation for a motor. The problem with this shift is that the brushes will now short out conductors with a voltage across them and this will result in sparking at the brushes. Shifting the brushes to a “working neutral” is only helpful if the load is constant because the magnetic neutral plane shift angle will change if the armature current changes.
The second problem due to armature reaction is called flux weakening or field weakening. The flux density in most main field poles is near the saturation point for the pole lamination steel. This means that any decrease in magnetomotive force (MMF), or ampere-turns, will cause a larger decrease in flux than the same increase in MMF would increase the flux. For example, a 10% reduction in MMF might result in a 12% reduction in flux whereas a 10% increase in MMF might only result in a 5% increase in flux. When the armature current is significant, the resulting distortion adds MMF to one side of the pole and subtracts it from the other side. This causes a small increase in flux on the additive side and a larger decrease in flux on the subtractive side. The net result is that the total flux per pole is decreased. In generators, this reduces the generated voltage for a given load. For motors, the field weakening increases speed. If the motor load increases with speed, the effect can be significant because the increasing load will lead to more field weakening and a cycle would ensue that could lead to runaway.
Figure 3 shows the model simulation of the magnetic field produced by the armature winding alone, the field winding alone, and by the armature + field windings. This clearly demonstrates both the magnetic neutral plane shift and the flux weakening of the main poles.
L di/dt Voltages
Aside from armature reaction, another significant effect related to commutation is the so-called L di/dt voltage or voltage of self-induction. The armature coil undergoing commutation has an inductance. The voltage (V) across an inductor is equal to the inductance of the coil (L) multiplied by change in current (di) divided by the change in time (dt). That is, V = L di/dt. Since the coil will pass through the commutation zone in a very short time (milliseconds) and the change in current will be twice the load current (because of distance from positive value to negative value), even a small value of inductance can result in a significant generated voltage, sometimes referred to as an inductive kick, that causes sparking at the brushes.
How Can Interpoles Help?
If any voltage that would otherwise be present in conductors undergoing commutation can be reduced to zero, sparking at the brushes due to that voltage would be eliminated. The interpoles (or commutating poles) are placed directly across the air gap from the conductors being commutated and connected in series with the armature winding but with opposite polarity. Additionally, with respect to rotation direction, the interpoles must have the same polarity as the next upcoming main pole in a generator and the same polarity as the previous main pole in a motor. If the interpoles are properly designed and/or adjusted, brush sparking resulting from L di/dt voltages or magnetic neutral plane shift voltages can be eliminated. Unlike the main field poles, the interpoles are typically designed to operate well below magnetic saturation so that their strength varies linearly with load.
Figure 4 shows the model simulation of the magnetic field produced by the armature + interpoles with correct polarity, the armature + interpoles with reversed interpole polarity, and the armature + interpoles + fields. Note that for the armature + interpoles with correct polarity that the resulting magnetic field is eliminated from the interpolar region completely. With reversed interpole polarity, the amount of flux present in the interpolar region is even greater than it would be without interpoles; this is evident when severe sparking occurs when applying load to a motor misconnected in this fashion. With the armature + interpoles + fields, note that the resulting field is still distorted but the interpoles do eliminate flux in the interpolar region.
So, the flux weakening problem due to field distortion at the main poles is not corrected by the interpoles. For most small and medium-sized, general-purpose motors, this condition and its results are just accepted. For larger DC machines with severe duty cycles, the flux weakening is a serious problem if not addressed. An example of this might be a large rolling mill where the machine is frequently and rapidly reversed. These heavy surges can lead to flashovers between positive and negative brushes due to severe field distortion.
How Can Compensating Windings Help?
Compensating winding conductors are placed in slots in the main pole face with polarity opposing the armature conductors across from that pole. Like the interpoles, the compensating windings are in series with the armature. If properly designed, the compensating winding MMF is equal and opposite to the armature MMF, eliminating the field distortion.
Figure 5 shows the model simulation of the magnetic field produced by the armature + interpoles + fields + compensating windings. Now there is no flux in the interpolar region, and the field distortion is eliminated. Thus, the effects of armature reaction and L di/dt voltages have been mitigated.
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