Doubly-Fed Electric Machines

Doubly-Fed Electric Machines are very sensitive to the synchronous relationship between speed and excitation frequency and as a result, are susceptible to instability without introducing extraordinary control means.

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Like any synchronous machine, losing synchronism will result in alternating torque pulsation and other related consequences. The Wound-Rotor Doubly-Fed Electric Machine, the Brushless Wound-Rotor Doubly-Fed Electric Machine, and the so-called Brushless Doubly-Fed Electric Machines are the only examples of doubly-fed electric machines.

Like any synchronous electric machine, Doubly-fed electric machines require electronic control for practical operation and should be considered an electric machine system or more appropriately, an adjustable-speed drive.

The electronic controller conditions bi-directional (i.e., four quadrant), speed synchronized, and multiphase electrical power to at least one of the winding sets (generally, the rotor winding set). Using four quadrant control, which must be continuously stable throughout the speed range, a wound-rotor doubly-fed electric machine with two Poles (i.e., one pole-pair) has a constant torque speed range of 7200 rpm when operating at 60 Hz.

The electronic controller is smaller, less expensive, more efficient, and more compact than electronic controllers of singly-fed electric machine because in the simplest configuration, only the power of the rotating (or moving) active winding set is controlled, which is less than half the total power output of the electric machine.

All electric machines are categorized as either Singly-Fed with one winding set that actively participates in the energy conversion process or Doubly-Fed. Although sometimes described as doubly-fed, the wound-rotor induction machine (slip-energy recovery) and the field-excited synchronous machine are singly-fed machines because only one winding set actively participates in the energy conversion process.

Features of doubly fed machines
Uniquely, doubly-fed electric machines can operate at constant torque to twice synchronous speed for a given frequency of excitation with each active winding set rated at half the total power of the machine (i.e., contiguous operation between sub-synchronous through super-synchronous speed range).

The sum of the power ratings of the multiphase winding sets determine the total electro-mechanical conversion power rating of the machine.

Two multiphase winding sets with similar pole-pairs and equal power rating are placed on the rotor and stator bodies, respectively. The wound-rotor doubly-fed electric machine is the only electric machine with two independent active winding sets, the rotor and stator winding sets, occupying the same core volume as other electric machines. Since the rotor winding set actively participates in the energy conversion process with the stator winding set, utilization of the magnetic core real estate is optimized.

A multiphase slip ring assembly (i.e., sliding electrical contacts) is traditionally used to transfer power to the rotating (moving) winding set and to allow independent control of the rotor winding set. The slip ring assembly requires maintenance and compromises system reliability, cost and efficiency. Attempts to avoid the slip ring assembly are constantly being researched with limited success (see brushless wound-rotor doubly-fed electric machines).

Electric Motors