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Electric motors have brought us nearly every modern comfort by allowing us to turn electrical power in physical motion. These machines have helped us create such wonders as vehicles, computers, air conditioning, just to name a few, and it is all thanks to the variety of electric motors available in industry. The DC motor – an electric motor that uses a DC power source such as a battery – is one of our oldest yet most widely used designs, and this article will take a look at one specific DC motor, the shunt DC motor. At first glance, it can be difficult to see this motor’s unique properties, but this article aims to help highlight those differences and show why engineers may choose this design over other models. By exploring the setup, operation, and specifications of shunt DC motors, this article will hopefully help designers make more informed choices when implementing the right machine for their application.
The shunt DC motor is simply a specific kind of brushed DC motor, so it will be useful to explain the basic principles common to all of these designs first (a similar explanation can be found in our article all about series wound DC motors). Figure 1 gives a simplified idea as to how DC motors work and is shown below:
There are two main parts to all DC motors: the stator – the outside housing containing the stator field – and the rotor – the rotating component connected to the DC power supply. The stator field can either be made of actual permanent magnets or a winding of wire (or a “field winding”, shown in Figure 1), which both produce a constant magnetic field through the rotor assembly. The rotor consists of the armature, armature windings, output shaft, commutators, and brushes. The armature winding is a coil of wire that snakes through the armature, or through the laminations of metal that guide the armature windings around the output shaft. These armature windings terminate at the commutator rings, which are mechanically separated from the DC power source (in other words, they “hover” over the output shaft, waiting to be pushed by the brushes). When the operator starts the motor, the brushes clasp down on the commutator rings and complete the circuit in Figure 1, causing current to flow through the brushes, commutator rings, and armature windings. When this happens, an electromagnetic field is produced in the armature, which opposes the permanent stator field. Since the rotor is free to rotate, the interaction between these two fields causes rotation on the output shaft, and ultimately useful speed/torque.
Now that we have shown the common principles to all DC motors, let’s take a look at the specific arrangement found in the shunt DC motor, shown below in Figure 2:
The field winding, which produces the constant magnetic field in the stator, is wired in parallel, or in shunt, with the armature windings in shunt DC motors. The same power supply is therefore used to power both the armature and the field windings, and the total current is split into two “parallel” paths.
The field winding in shunt DC motors is made of many windings of thin wire, to both increase the magnetic field’s strength and limit the current through the coil. By doing so, the current is reduced through the field coil and thus increases in the armature (remember, the current is shared when in parallel). The larger current in the armature produces a phenomenon known as back EMF – an electromotive force produced by the armature’s magnetic field rotating through the stator field – and the back EMF serves to reduce the current through the armature winding.
As the motor increases in speed, this back EMF increases (as it is proportional to speed), and similarly decreases if the armature rotation slows due to increased loading on the shaft. This gives shunt DC motors the unique ability to self-regulate their speed, especially when a larger load is imparted on the shaft, and is why they are colloquially known as constant-speed motors. So, in summary, shunt motors have low starting torque but constant speed; this is inverse to series DC motors, where their starting torque is high but there is virtually no speed regulation (review our article all about series wound DC motors for more information). They are also reversible by simply changing the polarity of either the armature coil or the field coil.
It is helpful to know what values to look for when choosing a shunt DC motor. This article will briefly go over some common specifications to look out for, but know that there is much more information on most spec sheets that what is provided here.
Due to the armature and field windings being wired in parallel, there are two separate voltages across each component (not across the whole circuit though; remember, they share the same power source). As a result, most spec sheets provide two rated voltages for a shunt DC motor, one for each coil, oftentimes with ranges. For example, a shunt motor could have a field voltage of 220 V with a max up to 500V and an armature voltage of 440 V with a max up to 600 V. Note that these values depend on frame size and construction of the motor. Also note that a DC motor should never be used with a power source less than its rated voltage, as it reduces the performance and can overheat.
Since these motors are considered constant speed, there is usually a base speed provided on the spec sheet, along with an associated power (in HP or kW). These values show what the motor can move and how fast it can move it, though shunt DC motors can regulate their speed even with a changing load (within safe tolerances).
There are standard frame sizes set by NEMA to give buyer ease of replacement between motor sellers, but usually, the motor’s dimensions will always be given if it is not standardized. The frame size will give the specifier an idea of how the motor will fit into any given application and provides a rough idea as to how powerful the motor will be (though size can be misleading with electric motors, so use caution).
Since a shunt DC motor uses brushes to connect the power source to the spinning armature, they will naturally degrade over time. Most DC motors give a brush life (in hours) so that operators can record how long the brushes have been in use and when to replace them. It is vital to maintain these motors by replacing the brushes when necessary, or they may become damaged or non-effective.
Unlike series DC motors, shunt DC motors excel in constant speed applications due to their feedback design. They can keep a precise RPM and torque, even under varying load conditions, making them useful for woodworking equipment, grinders, or any other rotating power tool where a user will be pushing against rotation. Note that these motors have a low starting torque, so these motors cannot be connected to a heavy load upon startup and must wait to be used at rated speed. They also slightly dip in speed when heavily loaded, as no electric motor runs at ideal conditions and all experiences losses.
These motors are very easy to install, with the ability to work with speed controllers. They are most often used in the aforementioned power tool applications, as well as car windshield wipers, car windows, computer fans, and more. While not as initially powerful as their series-wound cousin, shunt DC motors do not falter when providing their mechanical output, providing users with consistency over raw output power.
This article presented an understanding of what shunt DC motors are and how they work. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.
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