Tom Bishop, P.E.
EASA Technical Support Specialist
There are many tests that can be performed on a DC motor to verify the integrity of windings, correct coil polarities and proper running performance. What we will address in this article are a select few simple tests that can help assure a motor operates properly when the customer applies it. Our intent is not to oversimplify and suggest that performing these tests alone is all that is required for an effective repair. Rather, the intent is to highlight some tests that give a maximum return for the time invested in testing.
The tests we will cover are drop testing fields, checking interpole polarity, checking compound field polarity, brushholder spacing, setting neutral and two-way run testing. All of these tests can usually be performed with the motor assembled, although in some cases the end bracket on the commutator end may need to be removed to access the field lead connections.
Drop testing fields (shunt, series, interpoles)
Shorted shunt fields can cause the motor to operate erratically, vary in speed, or may cause armature current and/or speed to increase. Shorted series fields may cause the motor speed to vary more than normal; and shorted interpoles often cause sparking (sometimes severe) at the brushes. The most common test to detect shorted fields is the drop test.
The drop test can be performed using AC or DC voltage. For an AC drop test, the tolerance is that the coils be balanced within 10% of the average. For the DC drop test, the tolerance is reduced to +/- 5% of the average. The drawback to the DC drop test is that it relies on Ohms Law to detect shorted turns. With a tolerance of +/- 5%, a coil with 1000 turns could have a short between two turns less than 50 turns apart, and still be within tolerance (50/1000 = 0.05, or 5%).
The AC drop test is more suitable than the DC drop test in probing for shorted coil turns. Shorted turns in a coil act like a closed secondary on a transformer. The voltage drop variation with shorted coils is often quite dramatic, typically much greater than the acceptance judgment value. That makes the AC test the preferred drop test method. Typically 120 (or
240) volts AC is applied to the shunt field circuit and the voltage drop across each coil is measured. A variation from average of greater than 10% is an indicator that a coil is shorted. However, variations in the magnetic circuit can lead to unequal drop test voltages even though the coils are sound.
If a coil tests shorted by the AC drop test method, it should be removed from the pole piece, if possible, and comparison tested to another removed coil that has tested satisfactory and is also separate from its pole piece. The reason for this is that iron differences, such as a split in the frame or even a terminal box opening, can affect the AC drop test. Some other examples that illustrate iron differences are:
- Coils placed flat on a concrete floor may be affected by the proximity of reinforcing rod in the floor.
- Coils placed on a steel frame table may be affected by the proximity of the steel framework.
- Field coils placed side-by-side for testing may indicate a difference in the coils at each end, since they do not have the same amount of iron on both sides.
- Sometimes, a shorted coil will cause a lower test value for the coils immediately adjacent to it.
AC voltage drop testing of interpoles, series or compensating windings can be done using an AC welder if the service center test panel can not supply the low voltage necessary to limit the current to below rated amperes. Typically, the voltage needed is less than 120 volts. The voltage drop test with lower voltage uses the same acceptance criteria as higher voltage; however, the lower coil voltage values must be measured more precisely.
Because many of these high current coils have relatively few turns, a coil-to-coil resistance comparison may also reveal shorted turns. The resistance to be measured is very low, typically requiring a digital low resistance ohmmeter (DLRO) capable of reading into the milli- or micro-ohm range. A variation in excess of 5% from average for any one coil is an indicator of either a short or a high-resistance connection. A coil resistance that is too low indicates a short, and if too high, a poor connection (e.g., a “cold solder joint”).
A variation of the AC drop test is the impedance test. AC voltage is applied to a single coil, and the exact voltage and current recorded. The exact same voltage is applied to each of the other coils, and the currents are compared. Impedance is the ratio of voltage to current, and by keeping the voltage constant, any change in current directly reflects a change in impedance. Being an AC drop test, the tolerance for acceptable impedance variation is 10%. As with the AC drop test, the proximity of other iron will affect the results. Table 1 summarizes the tolerance values for both AC and DC drop tests.
Interpole polarity
Interpole polarity should be such that the interpoles oppose the magnetic flux of the armature. If the interpole polarity is incorrect, severe sparking at the brushes typically results. In some cases, reversed interpole polarity can lead to flashover.
If the brushholder leads are accidentally swapped, the polarity of the armature relative to the interpoles will be reversed. Depending on the design, the brushes may not arc until the motor is loaded. The arcing tends to be proportional to the load, and arcing at full load can be severe enough to cause a flashover. This is one of the most common repair problems experienced unless the technician follows the procedure to confirm interpole polarity.
Low voltage AC, typically 30-60 volts, can be applied to the armature and interpole circuit to verify correct interpole polarity. The voltage is applied on two brushholders of opposite polarity, and the output voltage is measured on the A1 and A2 leads in the terminal box. The output voltage will 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. If the voltages are the same, either the interpoles are disconnected, or an equal number of them are opposing each other. Therefore, check for incorrect polarity. Motors with compensating (pole-face) windings will typically develop a very low output voltage. Figure 1 illustrates the interpole polarity test circuit.
Compound (series-to-shunt) field polarity
The series and shunt field coil polarities should almost always be the same as each other; that is, cumulative compound. Because of their instability, there are almost no real-world applications for differential compound motors. Differential compounding is the condition where the series field coil polarity opposes that of the associated shunt field coils.
In almost all cases, if a differentially connected motor is operated with load, sparking at the brushes will occur due to increased armature current. Output power or torque is a function of field strength multiplied by armature current.
The formula that relates this is: Torque = k F I; where k is a constant, F is field flux (strength) and I is armature current. The armature current must increase to compensate for the weakened field with a differential connection.
To verify series-to-shunt field polarity, connect an analog DC voltmeter (no more than 3 volts scale) to the series field leads, with the positive meter lead on S1. Using a 1224 volt DC supply, flash (briefly contact and immediately disconnect) the shunt field, applying positive voltage to the F1 lead, and observe the deflection on the voltmeter.
If the meter indication is upscale (positive), the fields are cumulative compound, which is almost always the correct relationship.
If the meter deflects downscale (negative) the fields are differentially compounded and probably incorrect.
In Part II next month, we will learn about these DC motor tests: brushholder spacing, setting neutral and two-way run testing.
Brushholder spacing
Unequal brush spacing can lead to sparking at the brushes. To measure brush spacing, measure the circumference of the commutator and divide that distance by the number of brush posts. Use a machinist’s scale and mark the calculated spacing intervals on a paper strip (adding machine paper works well for this). Wrap the marked paper around the commutator and align the first mark with one brush, then check the position of the remaining marks relative to the other brushes.
Spacing should be as close as possible, but should not exceed more than 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.
As noted last month, there are many tests that can be performed on a DC motor to verify the integrity of windings, correct coil polarities and proper running performance. We will continue with more simple tests that can help assure a motor operates properly when the customer applies it.
Setting neutral
When an armature coil is rotated through magnetic fields, 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 ideal commutation, the brush should short the armature leads at the lowest possible induced voltage. If the brushes are not set in the neutral position, sparking at the brushes will occur. The further from neutral the brushholders are set, the greater the severity of sparking.
When a motor is disassembled it is a good practice to mark the brushholder yoke to identify the neutral setting as received. This can be done by using a metal marking paint to draw a line between the yoke and frame as illustrated in Photo 1.
Tip: In addition to marking the brushholder yoke to the frame, mark the end bracket to the frame prior to disassembly. The reason for this marking is that if there is clearance between the end bracket bolts and the end bracket, the bracket position could change. Another good practice is to indelibly mark the brushholder leads and their connection point.
Photo 1: A good practice is to mark the brushholder-to-end bracket position prior to disassembly, and to check it again after assembly.
There are several ways to verify or set the neutral. The most common of these methods are the DC inductive kick, AC induced voltage and two-way run test. The AC induced voltage test is more accurate than the DC inductive method and is therefore the preferred motor-stationary neutral setting method.
The AC induced voltage method is essentially a continuous inductive kick.
When AC is applied to the field leads, voltage is induced into the armature, with the magnitude depending on how far “off neutral” the brushes are. At the neutral position, no voltage will be induced into the armature coil. For designs with a connecting lead from the brushholder to auxiliary fields, the lead must be temporarily disconnected from the brushholder before proceeding.
For compound-wound motors (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 a compound-wound motor, the series and shunt act as a transformer. Voltage applied to the shunt induces voltage into the series which, if connected to the interpoles, will affect the voltmeter reading.
Begin the test by connecting the field leads to a 120 - 240 volt AC supply, then connect an AC voltmeter across two adjacent brush posts and read the induced voltage. A digital voltmeter is usually more suitable than an analog meter for this test because the final target voltage magnitude will be less than 10 millivolts (0.01 volts).
Next, shift the brushholder yoke assembly until the lowest voltage reading possible is obtained. The voltage across the adjacent brush posts should be less than 0.01 volts AC. If the voltage reading exceeds 0.01 volts AC, check for other items that could affect neutral such as uneven brush spacing, brushes that are not fully seated or a spacing problem with the shunt fields or interpoles.
Brush spacing around the circumference of the commutator should be equal (within 3/64” or 1.2 mm). If the brushes are only partially seated there may be unequal distances between brushes. Further, as the brushes “wear-in”, the neutral setting will shift and cause sparking. A spacing problem with field poles or interpoles could arise if the bolt holes in a field, interpole, or the frame are oversize. The result could be inexact indexing of the fields and interpoles. The 3/64” (1.2 mm) spacing tolerance also applies to field poles and interpoles.
After the neutral has been set by any of the above methods, note the location of the as-found brushholder yoke neutral position. If the new setting varies from the original, investigate the cause.
A commutator that has been turned will have a smaller outside diameter. The height of the brushboxes above the commutator may have been adjusted and that could explain a change in neutral setting if the boxes are not perpendicular to the commutator.
If the armature has been rewound and the lead placement in the commutator has changed, that would also explain a change in neutral setting. Another cause would be that the bolts attaching the end bracket to the frame might have enough clearance that the end bracket can rotate slightly and thus not be in the exact location as prior to disassembly. Marking the end bracket to frame position can help avoid this situation.
Two-way run test
This test is the rotating (dynamic) equivalent of the stationary (static) neutral tests. The motor is operated in one direction, readings taken, then stopped and rotated in the opposite direction of rotation and another set of readings are taken. Comparison of the results of the two test run sequences indicates whether or not the neutral setting is acceptable. This method is useful for motors with no wound fields, such as permanent magnet (PM) motors.
The initial direction of rotation can be either clockwise (CW) or counterclockwise (CCW) facing the commutator end of the motor. We will use clockwise rotation initially in our explanation of the test. Begin by running the motor at no load in the CW direction, applying rated field current and gradually increasing armature voltage to its rated value. Record the exact field current and voltage, and armature current and voltage. Using a digital tachometer, measure and record the exact operating speed (to within 1 rpm). Maintain rated field current and reduce the armature voltage to zero.
Measure, record exact rpm
Reverse the armature circuit leads and run the motor in the CCW direction of rotation. At exactly the same armature voltage as the CW test run, and with rated field current (not necessarily the same field volts), measure and record the exact rpm. It is critically important that field current (not voltage) be the same for both directions of rotation. The speed in both directions should be within 1% of being equal. Also record the exact field current and voltage, and armature current and voltage for future reference.
In the case of the PM motor, an alternative is to drive the shaft at the same speed, in both directions, and compare the output voltage of the armature A1 and A2 leads. The output voltages for both directions should be within 1% of being equal.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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