Chuck Yung
EASA Senior Technical Support Specialist
This is an update to articles I wrote starting in 1999 and subsequently used as part of the foundation for “Fundamentals of DC: Operation and Repair Tips” (FDC). In FDC, we included various popular methods for some tasks. This article takes a “best practice” approach, sharing the preferred method for these tasks.
Many service centers are unable to perform full-load testing of large DC machines. The good news is that there are specific tests that can be performed on a DC motor to verify the integrity of windings, the correct relative polarity of armature to interpoles, and proper running performance. This article explains specific tests that can help ensure a motor operates properly when the motor is installed.
This article will cover brush spacing, how to confirm the relative polarity of interpoles and armature, checking compound field polarity, setting brush neutral and some specific tests that are useful on the test panel. Each of these tests can be performed with the motor assembled.
Brush Spacing
Unequal brush spacing can lead to sparking at the brushes. To measure brush spacing, it is critical that the brushes first be fully seated. Wrap a strip of paper around the commutator and mark where the ends cross, then mark where the toe of each brush meets the paper. Remove the paper and lay it flat, then use a machinist’s scale to measure the distance between the marks. Brush spacing should be equal within 3/64” (1.2 mm) variation. For motors with several brushes per post, repeat this check for the brush path closest to the risers and the path furthest from the risers. If the spacing requires adjustment, look at the relative position of the marks to determine the easiest way to correct it.
Setting Brush Neutral
When an armature coil is rotated through a magnetic field, voltage is induced into the armature coil. Since the fields are connected in alternate polarity, the polarity of the voltage induced into the armature coil reverses each time the coil passes from one field to the next. For optimum commutation, the brush should short the armature leads at the exact place where the armature coil polarity reverses (Figure 1).
That position is called the brush neutral. If the brushes are not set in the neutral position, sparking at the brushes will occur. The further from neutral the brushes are, the worse the sparking will be.
Pro Tip: When a DC machine is disassembled, always follow these steps:
- Indelibly mark the orientation of the brush rigging and end bracket. A Sharpie or metal marker works well for this. Be sure the marks are exactly in line.
- Use aluminum “pigeon tags” to label the brush posts and leads attached to each.
Brush Neutral
There are several ways to verify or set the brush neutral position. The most reliable of them is the AC induced voltage method. For those familiar with the DC inductive kick method, think of the AC test as a continuous inductive kick.
Consider the fields and armature as a transformer. When AC voltage is applied to the fields, voltage is induced into the armature. At the exact brush neutral position, no voltage will be induced into the armature coil. The further off neutral the brushes are, the greater the voltage that will be measured between adjacent brush posts.
Note: For compound-wound machines (i.e., those having a shunt and series field winding), the series field must be disconnected from the armature leads before checking neutral. The reason for this is that when AC voltage is applied to shunt fields, the series fields act as the secondary of a transformer. If a series lead and armature lead are connected, the auto-transformer effect would affect the voltmeter reading. Again, brushes must be fully seated to obtain best results!
Pro Tip: Always make sure that brush posts of the same polarity are connected by an equalizing jumper (Figure 2). The main purpose of these jumpers is to cause all brushes to carry the same current, yielding uniform rates of brush wear. A side benefit is that brush neutral will be more accurately obtained. Without them, setting neutral of a 4-pole machine can yield different results if checking between the 12:00 to 3:00 posts than between the 12:00 and 9:00 posts.
Apply 120V AC to the shunt field leads, and connect a digital AC voltmeter to adjacent brush posts. Shift the brush rigging until the lowest voltage reading possible is obtained. When the brushes are in the correct neutral position, the voltage across adjacent brush posts should be less than 0.01 volts AC (10 millivolts). If the voltage reading exceeds 0.01 volts AC, check for other items that could affect neutral such as:
- Uneven brush spacing
- Brush spacing around the circumference of the commutator should be equal (within 3/64” or 1.2 mm).
- Brushes that are not fully seated
- If the brushes are only partially seated, there may be unequal distances between brushes. Additionally, as the brushes “wear in,” the neutral setting will shift and cause sparking.
- A spacing problem with the shunt fields or interpoles. A spacing problem with field poles or interpoles could occur if the bolt holes in the frame are oversized or if someone drilled and tapped new holes in a field pole after breaking a bolt.
- The same 3/64” (1.2 mm) spacing tolerance applies to field poles and interpoles.
After the neutral has been set, mark the location of the brush rigging to identify the neutral position. If the new setting varies from the original location, investigate the cause.
Pro Tip: For larger machines (over approximately 1000 hp (750 kW)), some manufacturers use a “working neutral” instead of fine-tuning the interpole strength. (See the Theory section of FDC for more information.)
Note: One method some use is to run the motor in both clockwise (CW) and counterclockwise (CCW) directions with exactly the same field current and armature voltage. The speed should be the same in both directions, within 1% or 3 rpm, whichever is greater. This test requires precise field current settings and is only reliable when testing the motor under load.
Interpole Polarity
This should more correctly be referred to as the relative polarity of interpoles and armature.
Interpole polarity should be such that the interpoles oppose the magnetic flux of the armature (Figure 3). If the interpole polarity
relative to that of the armature is incorrect, severe sparking at the brushes typically results. In some cases, reversed interpole polarity can lead to flashover when the motor is loaded.
This is the most common error made by repairers. Any of these common repairs can result in the polarity of the armature relative to that of the interpoles to be reversed: the brush rigging is removed for cleaning; the interpoles or armature may have been rewound; the interpole leads might have been replaced.
Low voltage AC, typically 20-30 volts, can be applied to the armature and interpole (Figure 4) circuit to verify the correct relative polarity of the armature and interpoles.
The voltage is applied to two adjacent brushholders (opposite polarity), and the output voltage is measured between the A1 and A2 leads in the terminal box. The output voltage should be less than the input voltage if the interpole polarity is correct. The principle at work here is that the interpole opposition of armature flux is like a “buck” autotransformer; correct polarity results in lower combined armature and interpole circuit output voltage. The typical output voltage of correct polarity interpoles is about 1/2 to 2/3 of the input voltage. If the output voltage is higher than the input, reverse the interpole leads at the brushholders. If the voltages are the same, either the interpoles are misconnected, or an equal number of them are opposing each other, or the armature is connected for the wrong "plex." Therefore, check for incorrect polarity. If the voltage is significantly lower than expected, there are two possibilities:
- The interpoles might have been connected for too few circuits (e.g., series rather than series-parallel), or
- Motors with compensating (pole-face) windings will typically develop a very low output voltage.
A second method to confirm the relative polarity is correct is to shift the brush rigging off neutral – in either direction – and apply DC to only the A-leads. Repeating – for this test, the shunt fields are not energized. Raise the armature circuit voltage only enough to cause rotation. The armature should follow the direction the brush rigging was shifted. If the armature rotates in the direction opposite the brush rigging move, exchange the leads at the brush posts.
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