Chuck Yung
EASA Senior Technical Support Specialist

More than 400 people have at­tended EASA’s Fundamentals of DC Operation and Repair Tips seminar since it was introduced in 2003. Even the most experienced and well-trained DC technicians will appreciate picking up some more testing tips. 

The early DC motors (see Figure 1) did not have interpoles, requiring an operator to shift the brush-rigging position each time the load changed. We know that interpoles were added to eliminate this inconvenience. The in­terpoles provide an equal-but-opposite force to counterbalance the armature flux. If you have ever seen a motor with the brushholder leads reversed, you recognize that even though the brushes may arc when a load is applied, the mo­tor might exhibit no symptoms when running unloaded. With low armature current (no load), the interpoles really are not necessary.

The extremely low resistance of the inter­pole circuit limits our testing options. Follow­ing Ohm’s Law, apply­ing 10 volts across an interpole circuit of 0.001 ohms would draw 10,000 amps. So a DC drop test is limited to very low voltages.

Bypassing interpoles
If you suspect an interpole problem, bypass the interpoles. Use leads con­nected to brushholders of each polarity and test run the motor unloaded. If the performance problem disappears, you know the problem is in the interpoles. There may be a shorted interpole or a polarity/connection problem. But now you can focus on the interpoles as the problem (see Figure 2). This method certainly beats dismantling the motor to verify whether or not the polarities are correct.

Compound wound machines can be connected cumulatively or differ­entially (see the “Assembly and Test” section of Fundamentals of DC Opera­tion and Repair Tips manual). Virtually all compound wound DC motors are connected cumulatively, meaning that the shunt and series fields on each pole have the same polarity. When our polarity testing indicates otherwise, there is a tendency to assume that the customer is connecting the motor differentially for a reason. The truth is that the polarity test just revealed that the leads are marked incorrectly.Either the customer is connecting the leads incorrectly or he is about to do so – unless we make sure they under­stand that we found and corrected a lead marking problem.

Drop test
One of the routine tests we use to detect shorted fields is the drop test. When using the AC drop test, apply AC voltage to the shunt fields and measure the voltage drop across each field. If the shunt fields test okay, we then perform a drop test on the series fields, right? Wrong!! When energizing the shunt or series with AC voltage, we have a transformer. The induced voltage will approximate the turn ratio between the shunt and series fields. 

Consider a typical 4-pole DC motor with a 1,000 turn shunt field and 2-turn series field on a common pole. Apply­ing 120 volts AC to the shunt fields, we expect to measure a 30 volt drop across each shunt field. Using the turn ratio (in this case, 1,000:2), we could measure 0.06 volts AC induced in the series coil – without having to apply voltage directly to the series fields. Even more importantly, that transformer ratio works in both directions. 

If we apply 10 volts AC to the series fields, expecting a voltage drop of 2.5 volts across each series field, we have induced 1,250 volts in the shunt field. Four shunt fields, connected in series, would result in 5,000 volts across the field leads. That’s potentially deadly.

But there is good news: the reason the AC drop test works so well is that a shorted turn acts like a closed sec­ondary of a transformer – so a shorted series field will cause the same behav­ior. If the AC voltage drop test of the shunt fields is good, the series fields are not shorted either. 

There is no reason to perform a second drop test on the series fields. By accurately measuring the voltage drop across the shunt and series field, we can closely estimate the turns/coil of the shunt field coil. That might be useful information if the shunt fields require rewinding in the future. Most compound wound motors have only a few turns on the series.

However, there is another uncom­mon problem for which to test. While rare, sometimes a short occurs between the shunt and series field circuits. Use an ohmmeter or test light to make certain there is no continuity between the shunt fields and series.

Testing field coils 
The options for electrically testing field coils include:
•    Surge testing
•    Measuring resistance
•    Drop test

A surge test may not detect shorted turns if the pulse dissipates too quickly. Comparing resistance relies only on Ohm’s Law and is not always reliable. That leaves the drop test.
Drop test:  AC or DC

The drop test can be performed us­ing AC or DC voltage. Both methods have their advantages and drawbacks. In both methods, voltage applied to a number of identical coils connected in series should be equally distributed across those coils.

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 problem with a DC drop test is that it relies on Ohm’s Law to detect shorted turns. With a tolerance of +/- 5%, a coil with 1,000 turns could have a short between two turns less than 50 turns apart and still be within tolerance.

The AC drop test is easily done using household current. When a coil has shorted turns, they act like a closed secondary on a transformer. The volt­age unbalance can be quite dramatic. That makes the AC test the preferred drop test.

Iron differences
When there are significant differ­ences in the iron distribution around the coils, the results of an AC drop test may be misleading. In those cases, a DC drop test is useful to determine whether the AC results are due to shorted turns or interference from the iron.

A short piece of extension cord with alligator clips on the end opposite the male plug makes this test convenient in the service center or on the job site.

Iron differences can affect the AC drop test or a surge test. For a synchro­nous rotor, which often has lightening holes in the web, or poles which may be retained by tapered wedges rather than bolted, an AC voltage drop test may give false indications of shorted coils.

Other items that affect the voltage drop test include:

• 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.
• A frame with one pole removed will often give erroneous test re­sults for the coils on either side of the missing coil.
• Sometimes, a shorted coil will cause a lower test value for the coils immediately adjacent to it.

Interpretation of drop test

Shorted turns will cause a DE­CREASE in the voltage drop across the shorted coil. If the voltage drop across a coil is HIGHER than across the remaining coils, look for a differ­ence in the iron (large holes in the web or structure, wedged pole not seated). A shorted coil, with an AC drop test, usually results in a voltage drop similar to that shown in Figure 3.
When in doubt, remove and com­pare the poles with the poles spread out on a wood surface.

Categories: DC motors, Testing, Motor testing

Tags: Motor testing

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