Direct-Current Motors: Commutation

A Practical Approach to Commutation and Brush Selection

There are many factors that affect commutation. Among these are inductance, brush friction, surface speed of the commutator, brush material, commutator finish, brush pressure, contact film, induced emf, current density, armature balance, and concentricity of the commutator to the shaft.

Perfect linear commutation, as shown in FGR. 67, would occur when the current in the coil under commutation goes from a peak +I through 0 to a peak -I as the brush position changes from full contact on one bar to complete shorting of one coil to full contact on the next bar. This condition would provide a relatively constant current density in the brush.

FGR. 67 Overcommutation, undercommutation, and linear commutation.

Undercommutation ( FGR. 67) occurs when the current stays at +I_PK for most of brush travel and is abruptly changed to -I_PK as the brush leaves the segment.

This effect is a result of the winding inductance trying to maintain the current in the coil during commutation. The current density in the brush varies from low to very high as the brush moves from segment 1 to segment 2.

Overcommutation ( FGR. 67) occurs when the emf induced in the coil during commutation causes the current reversal to take place before the brush contacts the adjacent segment. In this case the current density in the brush is very high initially and drops to very low.

In the cases of both undercommutation and overcommutation, the high brush current densities cause excessive heating and wear.

Considering the list of things that affect commutation, it’s obvious that perfect commutation is impossible. However, there are some things to keep in mind when designing a motor which will tend to minimize commutation problems.

Brush pressure is important. Fgr. 68 shows relative brush and commutator wear verses brush pressure. At low pressures, the wear is mostly electrical in nature because the poor contact results in high-resistance spots with localized heating and arcing. At high pressures, the wear is mostly mechanical and is due to the parts in effect grinding themselves away because of friction losses. For fractional horsepower motors using commonly available brush and commutator materials, the "ideal" range is from 3 to 9 lb/in^2 …

.A good approach to this is to select 6 lb/in2 as a nominal value and design the brush rigging and spring system to maintain as close to 6 lb/in2 over the usable brush length as possible. One can use a constant force spring or, if this is not practical, try to select a spring that starts out with a pressure of 7 to 8 lb/in2 and has a pressure of 4 to 5 lb/in^2 at the end of the usable brush length. In applications where the brushes and commutators are submerged in a liquid, the pressure needs to be increased to overcome hydroplaning. A good range is 10 to 16 lb/in^2 ,with

...being preferred over the usable brush length.

FGR. 68 Brush pressure versus wear.

The next thing to do is to ensure that the machine is commutating in the neutral zone. The neutral zone Wn is defined as the mechanical arc distance between the pole tips.

… where ?pole = pole angle, degrees

Da = outside diameter of armature

P = number of poles

The magnetic neutral zone is located at some angle shifted from the mechanical neutral position. The angle of shift is a function of the armature mmf .This angle can be estimated from experimental data on similar motors. Try 15° to 20° to start. A better way is to make a finite element analysis and estimate the angle from the flux plot with the motor at full load current. Next, consult a manufacturer's catalog and select a standard commutator that is about half the diameter at the armature lamination being utilized. Select a brush material that the manufacturer recommends for the type of application in which the commutation system will be used. Design the brush thickness so that it will cover about one bar pitch of the commutator. Set the brush width to cover as much of the commutator bar as possible, taking into consideration armature end play and the required mechanical and electrical clearances.

Calculate the commutation zone Wc. It’s defined as the circumferential distance the coil under commutation travels before current reversal is complete.

....where tbr = brush thickness

tcin = insulation thickness between bars

Da = outside diameter of armature

P = number of poles

a = number of parallel armature paths

Cb = number of commutator bars

Sac = number of teeth spanned by an armature coil

Nat = number of armature teeth

Dc = finished outside diameter of commutator

Then calculate the commutation-to-neutral zone ratio ?cn.

The result should be less than 1 for good commutation. According to Puchstein (1961), the optimum value is 0.667, but this seems practical only in larger machines.

The precise calculation for this ratio is shown.

Calculate the full load current of the motor. Then calculate the current density in the brush.

This value should be less than the value recommended by the brush supplier for the type of brush selected.

If the current density is not acceptable, select a brush material with a higher rating, or increase the brush thickness until it’s acceptable. If the brush thickness is increased, the commutation-to-neutral zone ratio must be recalculated. If it’s now greater than 1, increase the commutator diameter until it’s less than 1. Note that changing the brush area will change the brush pressure. Because of this, it will be necessary to recheck the spring to ensure that the pressure remains in the preferred range.

Other tips on improving commutation include reducing the motor speed to as small a value as possible and keeping the coil inductances low. When designing the brush rigging, keep 0.002 to 0.003 in total side-to-side clearance and top-to-bottom clearance between the brush and its guide. Less clearance may cause binding problems and may not allow the brush to remain in contact with the commutator. Try to extend the brush guide to within 0.020 in of the commutator surface if practical. This will minimize brush movement, which will improve contact and reduce arcing. Balance the armature to its ISO 1941 requirement. Keep the shaft straight and the bearings lined up and concentric to the field core.

Brush Dust and Slot Packing. As the commutator and brushes move relative to each other, they wear, and fine particles of dust are released into the motor. Depending on the brush materials used, this dust may contain copper and silver, but it’s mostly carbon. In any event, the material is conductive.

As the particles are dislodged from the brushes and the commutator, they are moved to the edge of the commutator slots and forced down into them. If the surface speed of the commutator is sufficient, the tangential forces will throw the particles from the slots out into the surrounding area. This is the desired effect.

If the surface speed is too slow, or a contaminant, such as oil, is present, the particles will be forced into the slots until the slots are packed with the material. Since the material is conductive, the effect of this packing is to short the bars together, creating one or more shorted coils. This adversely affects motor performance and life. To avoid this situation, keep oils away from the commutator by using slingers and anti wetting agents. Run the motor at sufficient speed to throw the particles from the slots. The actual armature minimum speed required depends on the mass of the particles and the commutator diameter, but an old rule of thumb is to keep the speed above 1500 rpm.

Excessive dust buildup can cause other problems as well, such as shorting the brush rigging to the end frames. This can be minimized by designing the brush rigging to trap the dust or by isolating the surrounding area with insulating materials.

Commutation System Design

A lot of effort goes into designing a motor that has good commutation. Good com mutation saves on brush wear and reduces sparking. In order to calculate commutation parameters, we need to know the following:

FGR. 69 Commutator construction.

Cb = number of bars on the commutator

Dc = outside diameter of commutator

tbr = brush thickness (thickness which is tangential to the commutator)

Tcin = insulation thickness between commutator bars

a = number of current paths in armature

P = number of poles

Nat = number of armature teeth

Sac = number of teeth spanned by a coil

Dc = commutator diameter Width of the commutation zone

Wc and the neutral zone Wn:

The figure of merit for the commutation-to-neutral zone ratio should be less than 1.0 for good commutation.

Flat commutators are commonly used where long brushes are required for long motor life. Flat commutators allow the length of the brush to extend the motor length instead of the motor diameter. Fgr. 71 shows a flat commutator and the dimensions used to calculate commutation parameters. The following dimensions (which can be substituted into the commutation parameters just given) need to be determined for the commutator and corresponding brush:

FGR. 70 Round commutator dimensions.

abr, flat = angle which the brush encompasses Dbr, path = outside diameter of the path the brush travels on the commutator dbr, path = inside diameter of the path the brush travels on the commutator tcin = thickness of the insulation between commutator bars

The following formulas calculate the values to be used in the commutation zone formula.

Geometric mean diameter of the brush path, to replace the diameter of the commutator, in inches:

FGR. 71 Flat commutator dimensions.

The thickness of the brush for a flat commutator is the arc distance across the brush at the radius of the brush path, in inches:

Commutation is the process of switching the direction of current in a coil. If the direction of the coil were not switched, the motor would not turn. Fgr. 72 shows a current-carrying coil rotating in a magnetic field. The reaction of the flux from the magnets (or from the field poles in the universal motor) and the flux from the armature coil sides causes the coil to rotate until a steady-state position is reached. The direction of current in the coil needs to be reversed in order to keep the coil turning.

FGR. 72 Coil rotating in magnetic field.

Fgr. 73 shows the magnetic field resulting from the stator winding being energized.

Fgr. 74 shows a rotating armature with two coils. Coil 1 has end points on commutator bars C1 and C1 which are about to be commutated by Brush 1.The cur rent in the coils is in a steady state at this point. Fgr. 75 shows the brushes immediately before they will short out their respective coils. Fgr. 76 shows the coils being short-circuited by the brushes. The current direction in the coils is in the process of being switched. Note that the coil sides (which are into the paper) are now moving parallel to the magnetic flux lines shown in FGR. 73. This is beneficial because a short-circuited coil moving across a magnetic field will cause voltages to be induced across the ends of the coil (which are the commutator bars). Fgr. 77 shows the position of the brushes after the coil has been commutated. The current in this coil is now in the opposite direction in a steady state.

Example. A universal motor with a C-shaped lamination is produced to run at 240 V ac. The motor currently has the following attributes:

FGR. 73 Magnetic field with only stator energized.

FGR. 74 Armature coils with steady-state current.

FGR. 75 Coils just about to be commutated.

FGR. 77 Coils with new current direction in steady state.

FGR. 76 Coils being commutated (shorted by brush).

Where S = motor speed, rpm

Pa = number of armature paths

tcom = thickness of one commutator bar

Tpca = turns per armature coil being commutated

Kst = 1.0 if Sac is full pitch

Kst = 0.5 if Sac is less than a full pitch IL = line current

In ac operation there is a transformer voltage in the armature, in volts per coil:

Epc should be less than or equal to 5 to 7 V.

This is difficult to achieve in small high speed motors.

Flashover and Ring Fire

Under steady-state conditions, the field and armature fluxes and currents are relatively constant. Commutation voltages can be calculated using the previously described formulas, and a suitable commutation system can be devised. If, however, the motor is expected to see rapid load changes or voltage reversal, the steady-state commutation system may no longer be suitable. Sudden load changes that cause the motor to rapidly accelerate or decelerate will cause transformer voltages to be induced in the windings.

There are two components to these voltages. There is a voltage etae caused by changing field distortion armature flux. It’s largest near the pole tips and falls to zero under the pole. Since the commutated coil is nearest the pole tip, this voltage shows up at the brushes. If it’s high enough, the voltage exceeds the insulation dielectric value between bars nearest the brush. This will result in arcing or flashover between bars. This voltage has the value

...where Tpc = turns per coil

P = poles

Pa = armature paths

dfa/dt = time rate of change of armature flux at the pole tips

The second is a voltage etap induced in the armature coils under a main pole. This results from a change in armature mmf caused by rapidly changing armature current as the load changes. Its value is:

[...] where dfc /dt is the rate of change in flux in a coil under a pole with respect to time.

These voltages are superimposed and seen at all bars. If these voltages are high enough, the dielectric between all bars may break down, resulting in arcing or flashover between all bars. This phenomenon is sometimes referred to as ring fire. It should be pointed out here that the magnitude and polarity of these voltages is dependent on whether the load is increasing or decreasing and at what rate.

Flashover and ring fire may be observed on starting some motors yet have very little effect on their life because it’s intermittent and not seen under normal operation.

It’s obvious from the formulas that the best way to control flashover on motors with variable loads is to limit Tpc. This usually means more armature slots and larger commutators with more bars. For a more detailed explanation of flashover, see Puchstein (1961) and Gray (1926).

Flashover and ring fire may be observed on starting some motors yet have very little effect on their life because it’s intermittent and not seen under normal operation.

It’s obvious from the formulas that the best way to control flashover on motors with variable loads is to limit Tpc. This usually means more armature slots and larger commutators with more bars. For a more detailed explanation of flashover, see Puchstein (1961) and Gray (1926).