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Electric motors - machines that transform electricity into mechanical energy- are ubiquitous in the engineering world. They are the cornerstone of engineering feats such as elevators, pumps, and even electric vehicles, thanks to the ability to leverage the electromagnetic induction effect. These so-called induction motors use AC current and electromagnetism to generate rotational motion and come in many configurations. A special type of AC induction motor, known as wound rotor motors, will be the main focus of this article. While only used in special scenarios, these motors have a distinct advantage over other popular choices (squirrel cage, synchronous motors, etc.) due to their unique characteristics. The anatomy and operation of these motors will be explored, as well as the specific characteristics that make them so vital to applications where other, more popular induction motors cannot be implemented.
Wound rotor motors are a specialized type of AC motor and work in much the same way that other induction motors function. They consist of two main components, the outside stator, and the inside rotor, which are separated by a small air gap. The stator is generally the same across all induction motors, and consist of metal laminations that hold windings of copper or aluminum wire in place. There are three separate coils in the stator, which are fed by a three-phase AC current, which simply means they are each powered by a separate AC current. This is not always the case, with some motors being single-phase motors, but wound rotor motors are generally always three-phase. Nevertheless, these three phases generate a magnetic field that shifts with the alternating currents. This creates a rotating magnetic field (RMF), which acts on the rotor. In wound rotor motors, the rotor is “wound” with wire similar to the stator, with their terminal ends connected to 3 slip rings on the output shaft. These slip rings are attached to brushes and variable power resistor banks, where operators can change the speed of the motor by varying the resistance through the rotor coils. These slip rings allow for speed and torque control and are the defining feature of wound rotor motors (it is also why these motors are often referred to as “slip-ring” motors).
We recommend reading our article all about induction motors to grasp the basic laws common to all induction machines, but this article will briefly explain the science behind wound rotor motor operation.
These motors are classified as asynchronous motors, where a discrepancy (known as “slip”) exists between the stator RMF speed (synchronous speed) and output speed (rated speed). When generating the necessary current, voltage, and magnetic force in the rotor windings, the motor will always experience slip between the rotating field and the rotor. Feel free to visit our article on the types of AC motors to learn more.
Wound rotor motors differ in how their rotor interacts with their stator. The rotor windings are connected to a secondary circuit containing slip rings, brushes, and external resistors, and are powered by a separate three-phase AC current. Upon start-up, the external resistance imparted on this secondary circuit causes the rotor current to reduce the strength of the stator RMF (it runs more “in phase” with the stator RMF). This means that the speed of rotation can be controlled by altering the resistance as the motor reaches 100% speed, allowing operators to choose the starting torque and running characteristics. This results in a smooth start-up, high initial torque, low initial current, and the ability to adjust rotational speed, which cannot be achieved with simpler designs such as the squirrel cage motors (more information on this design can be found in our article on squirrel cage motors).
The specifications for a wound rotor motor involves understanding the specifications for all induction motors, which can be reviewed in our article all about induction motors. This article will highlight the important concepts for wound rotor motors that must be understood before purchasing one of these but know it is not all-inclusive.
The stator RMF rotates at full speed when a three-phase induction motor starts, while the rotor is initially at rest. The rotor experiences induced current when the RMF of the stator passes through it, and the only limiting factor to this current is the resistance of the rotor windings (current = voltage/resistance). This results in a greater current in the rotor, which increases the current demand in the stator, and therefore causes an “inrush” of starting current into the motor. This current may be two to seven times higher than the nameplate current rating and can cause serious issues in high voltage scenarios. As the motor reaches its rated speed, the rotor generates a “back EMF” into the stator, which lowers the stator current back down to rated levels. The inrush current is what is minimized in wound rotor motors by increasing the resistance in the rotor windings (I=V/R, where R is increased), and why they have such smooth starting characteristics.
The most important specification for wound rotor motors is how they operate once powered, and this is visualized through torque-speed graphs. Induction motors can greatly exceed both their rated torque and current when not at 100% speed; torque speed curves display this transient behavior, and Figure 1 displays a general torque speed curve for induction motors with significant points designated.
The starting torque is the torque generated from the initial current inrush, which is always higher than the rated torque. The pullout torque is the maximum torque reached before steady-state, and the rated torque is what is provided once the motor is at 100% speed. This associated speed is not exactly equal to the synchronous speed of the RMF, and this slip is visualized in Figure 1.
Motors using the popular squirrel cage designs only have so much control over their torque-speed curves (learn more in our article on squirrel cage motors). The squirrel cage rotor bars are shorted; this results in an inability to change the resistance of the rotor, which means the only way to affect the speed of rotation is by changing the voltage (I=V/R, where R is constant). This can cause problems in large motors, where the necessary input current can get dangerously high. Wound rotor motors solve this issue by altering the rotor resistance using the secondary circuit attached to the variable power resistance bank and slip rings. By increasing the resistance in the rotor via the slip rings, the pullout torque can be achieved at much lower speeds, allowing higher initial torque and lower starting current. When reaching synchronous speed, the rotor resistance can also be shorted, making the wound rotor motor behave as if it was a squirrel cage motor. Figure 2 shows the effect of increasing rotor resistance on output torque.
Through this graph, it is clear that the wound rotor motor addresses current, torque, and speed control much better than other designs. By varying the resistance, these motors will need less initial inrush current to compensate, have a stronger starting torque, and can maximize their starting torque by also making it the pullout torque (example curve R2 in Figure 2). This approach results in a speed-controllable-motor with high starting torque and low starting current, with the ability to alter these characteristics to the operator’s liking.
Wound rotor motors can handle what other asynchronous motors cannot, namely speed, current, and torque control. The ability to increase rotor resistance when starting the motor allows for heavy loads to be smoothly accelerated to rated speed. When the inrush current must be minimized, or there is an inrush current limitation lower than what squirrel cage motors/synchronous motors can handle, consider using a wound-rotor motor.
There are disadvantages of wound rotor motors, and they are a consequence of their complex designs. The secondary circuit introduces more opportunities for error, and the slip ring brushes can be a safety concern if not regularly checked (worn brushes can spark and increase the risk of fire). These motors are also expensive to maintain, which adds to their already costly price tag. Their complexity also lowers the overall motor efficiency, and a squirrel cage motor should be chosen if efficiency is a primary concern or design constraint.
Though expensive and less efficient, the wound rotor motor and its adjustable torque-speed characteristics are great for driving large ball mills, large presses, variable speed pumps, cranes, hoists, and other high inertial loads. They also are great for any application that desires smooth startup and the ability to change speeds. They cover the bases that other induction motors cannot, and are invaluable to designers who need absolute control over speed and torque output.
This article presented an understanding of what wound rotor motors are, how they work, and what their primary characteristics are that establish when they should be specified over standard induction motors. 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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