Chuck Yung 
EASA Technical Support Specialist 

Have you ever wondered about the purpose of the shims found under the interpoles in most DC machines? Those shims are used by the manufac­turer to adjust the interpole strength. If they are lost, left out or mixed up, the result will be a DC motor or generator that arcs – especially when loaded.

What interpoles do 
DC machines require interpoles to provide a magnetic flux equal, but opposite, to the armature flux. This controls distortion of the field flux through the load range, thus prevent­ing arcing at the brushes. See Figure 1. 

Interpole strength is approximately 1.2 x the ampere-turns per pole of the armature. However, that rough estimate requires fine-tuning. Most manufacturers accomplish this by adjusting the airgap (the physical distance, or gap) between the interpole and armature.

Because there is an inverse square relationship between magnetic force and distance, the effect of an incre­mental change in airgap diminishes quickly as the airgap is increased. Simply put, a 0.010” (0.25 mm) change in airgap has far greater impact for an airgap of 0.040” (1 mm) than a 0.120” (3 mm) airgap; each subsequent increase in airgap has a diminishing effect. It becomes impractical to make large changes in the airgap between the interpole and armature. 

Use non-magnetic material 
The solution is to work from the interface between the interpole and the frame. After all, the flux path passes through the armature, airgap, interpole and frame to the next interpole and back through the armature. (See Figure 2 for the field flux path.) To accomplish this, a non­magnetic shim material is used. material most often used is brass or non-magnetic stainless steel. Substi­tutes such as insulation deteriorate and should never be used. 

The total airgap affecting an interpole is the sum of the physical airgap between the interpole and armature, plus the total thickness of non-magnetic shim(s) beneath that interpole. Given the square relation­ship described above, a non-magnetic shim of a given thickness has a far greater impact on the interpole strength than the same increase in airgap between the armature and interpole. [For example, increasing from an airgap of zero to 0.010” (0.25 mm) is a larger incremental change than an increase from 0.040 to 0.050” (1 mm to 1.25 mm).] 

We know that an iron core yields a much stronger field than an air core, so it should be no surprise that the interpole strength is affected by the amount of iron within the interpole coil. Adding ferrous shims beneath an interpole does two things, both of which strengthen the interpole: It reduces the physical distance between the interpole and armature, and it adds ferrous material to the interpole iron. 

If one of the shims is non-magnetic, the position of the non-magnetic shim relative to the other shims affects the interpole strength. Place the non­magnetic shim against the frame, and all the ferrous shims add functional mass to the interpole. Move the non­magnetic shim closer to the interpole, and the ferrous shims bypassed by doing so no longer act as part of the interpole core. 

Figure 3. The position of the non-magnetic shim (in blue above) on the left permits the ferrous shims to contribute to the interpole core. 

Position of shims 
The position of the interpole shims is critically important to the perfor­mance of the machine. If shims are left out, or replaced in no particular sequence, the interpole strength changes. See Figure 3. The result is arcing at the brush associated with that interpole. When a machine has selective arcing at only certain brush posts, that is often is caused by irregular brush spacing around the commutator. If the spacing is correct, incorrect interpole strength caused by improper interpole shimming may be the problem. 

Before a DC machine leaves the factory, a black band test is performed to confirm interpole strength. This test requires a separate power supply to alternately “buck” and “boost” the interpole strength. Interpole shims are added or removed to fine-tune the interpole strength. On larger machines, a final adjustment is sometimes made by shifting the brush neutral position. 

In those cases, clearly visible marks on the brush rigging and end bracket identify the required position. It is not unheard of to adjust neutral using the preferred AC method, only to have a motor arc when fully loaded. 

Rate of brush wear 
When a DC machine arcs under load, one additional clue may be the rate of brush wear. If the negative brushes wear faster, weak spring tension is suspected. If the positive brushes wear faster, suspect weak interpoles. Remove the brushes and inspect the surface that rides against the commutator for telltale evidence of arcing. 

The arcing caused by distortion of the field flux depends on two factors: Direction of rotation and whether the interpoles are too strong or too weak. See Figure 4. If the interpoles are too weak, the field flux distorts in the direction opposite the armature rotation. If they are too strong, it distorts with the rotation. If arcing occurs on the leading edge of the brush, the interpoles are too weak; if on the trailing edge, the interpoles are too strong. To strengthen the interpoles, add a ferrous shim or move the non-magnetic shim closer to the frame. 

Perform black band test 
When a manufacturer load-tests a DC machine, it is standard to do so at 25% increments from 25% load through 150% load. Assuming the neutral position is adjusted for the expected full-load condition, it is still possible for arcing to occur when a customer loads the machine beyond that percent load. 

It may be necessary to perform a black band test at the increased load rating to eliminate arcing caused by field flux distortion. 

You can see how much extra work is created by careless placement of interpole shims. When a repair requires interpole removal, keep the shims in the correct sequence. Bolt or tie them to the correct frame position, in the order in which they were removed. 

Equally important: If there are non­magnetic shims, the bolts must also be non-magnetic. Expect the fasteners to be either non-magnetic stainless steel or brass. If the interpole bolts are carbon steel, someone has replaced them with the wrong material, and that should be corrected. 

Categories: DC motors, Interpoles

Tags: Interpoles

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