For the wide variety of DC motor applications, there are those where a straight shunt motor is preferred and others which seem to require the greater starting torque of a series field. Why are there different field designs and are they interchangeable? What about the nameplates marked “stab shunt” or “str shunt?” The purpose of this article is to clear up lingering confusion about the types of fields as well as the benefits of each.

Para la amplia variedad de aplicaciones de motores de CC, existen aquellas en las que se prefiere un motor shunt directo (straight shunt) y otras que parece que necesitan el mayor torque de arranque de un campo serie. ¿Por qué existen diferentes diseños de campo y son intercambiables? ¿Qué sucede con las placas de datos marcadas como “stab shunt” o “str shunt?” El propósito de este artículo es aclarar la confusión permanente sobre los tipos de campos, como también los beneficios de cada uno.

Occasionally a customer wants a spare DC machine, and you find a replacement that is almost – but not quite – identical. Often, either the original or the spare is compound wound. The customer then asks: “What do the series fields do?” and “Can we just isolate the series leads?”  There are a couple of considerations, but the answer is that “it depends.”

When rewinding field coils, there are a couple of common problems that make life difficult for the service center. One problem occurs when the newly rewound set of shunt fields is returned to the service center, roasted again. What causes this, and whether we can improve our winding procedure, has been the subject of much discussion. We all realize that something caused that first failure. If the subsequent failure looks a lot like it - the shunt fields were charred in both cases then it seems logical that the underlying cause could be the same. Sometimes it is. Let's look at why the coil is burnt. Well sure, it got too hot. But why? Is it because the shunts are energized when the machine is otherwise shut down? If so, the fields can be automatically de-energized after "x" minutes of machine inactivity. Installing a timed relay in the controls will avoid future problems. It may be that the customer is able to trace the problem to a new operator, who needs more training.

Trickle heating is another very practical option to heat AC stator windings or DC field windings while they are not energized.

Covers all necessary steps for layer-winding DC field coils, how to layer-wind a shunt field coil, how to reinsulate an interpole field coil, and how to install large field coils in the frame.

Illustrates procedures for manufacturing random-wound interpole and main field coils for DC machines. Covers everything from removing old coils and taking data to installing and connecting new coils, including how to construct winding forms and jigs, how to shape coils to conform with the curvature of the field frame and how to insulate field coils. Both “wet” and “dry” winding techniques are illustrated.