Standard Chorus motors can achieve up to five times the start-up torque of conventional AC induction motors of the same sized frame while remaining within Class B heating limits.

Motor technology has seen refinements recently in both permanent magnet DC motors and variable frequency drives based on pulse width modulation control. But a radically different approach, the Chorus drive system, offers advantages over both in systems that exploit its special characteristics.

While using all the same materials and components which have existed for decades, Chorus’ primary advance is in understanding how to use all the electrical forces which are in play within an electrical machine.

Specifically, Chorus harnesses “stray” electrical waves which have plagued electric motors for as long as they have existed. These waves are called harmonics. Harmonics in motors create electrical fields which do not rotate in sync with the main electrical field. As a result, harmonics in conventional motors actually prevent motors from operating at very high power levels because the motor reaches the point of diminishing returns once it reaches saturation.

An analogy might be that if people are talking in a crowded room quietly, they can easily be heard. But if, as people talk louder, the rest of the room keeps raising the volume, then it becomes difficult to be heard. At some point, the noise so saturates the room that even if that person yells, he cannot effectively communicate. Yet if somehow everyone can be made to talk quietly, multiple conversations can easily be had once again.

What Chorus does is bring all the conversations in the room together. So instead of individuals trying to out-shout each other in different conversations, all of the waveforms work in concert — they speak in a “chorus” that is easy to hear because, effectively, everyone in the room is saying the same thing at the same time.

The Chorus motor technology is an AC inverter-based induction motor of high-phase order in which harmonic content of the drive waveforms are enlisted to act in concert with the main wave pattern of the motor.

AC inverter-based induction motors have advantages over DC motors. They are typically smaller and lighter. They are more reliable, and they need less maintenance. An immediate benefit of applying the Chorus technology to an AC motor is a large increase in peak torque.

Peak torque is needed for short periods when a motor undergoes a sudden extra load. A typical example might be starting an electric vehicle or elevator, where the power needed to get the machine moving is much greater, for a short period, than the power needed to keep it moving once it is already going.

A major limit on the efficiency of AC motors in the past has been losses due to harmonic waveforms generated by the motor in addition to the main waveform, or “fundamental.” In a standard two pole three phase motor, some of the harmonic waveforms rotate in the opposite direction to the fundamental, and act as a drag on it. Others rotate at different speeds to the fundamental, again, reducing efficiency. The ideal would be to have all the harmonic waveforms rotating at the same speed and in the same direction, adding their energy to the energy of the fundamental.

In practice, most research efforts to date have concentrated on damping down the effects of harmonics or trying to eliminate them altogether. That may prevent them from working against the fundamental, but their energy is still being wasted and must count as an efficiency loss.

The research team at Chorus Motors (CHOMF) has been working for the past decade on motor designs which, far from eliminating harmonics, revels in them — organizing the phases of the motor so that the harmonics are co-opted to rotate with the fundamental.

Because this energy is no longer contributing to the heating of the motor, but to the rotation of the drive shaft, the motor can be driven harder, drawing more power at saturation without over-heating.

Results show that for short periods of time, standard Chorus motors can achieve up to five times the start-up torque of conventional AC induction motors of the same sized frame while remaining within Class B heating limits.

Moreover, continuous torque has been achieved at least 30 percent higher than standard VFD motors with a similar frame size, enclosure, and fan. Alternatively, driven at the same rate as a conventional motor, a Chorus motor should last longer, as the harmonic waveforms no longer produce heat which degrades the motor and shortens its lifespan.

Fig. 1. This torque-speed curve for a 20 HP Chorus Meshcon machine illustrates two important concepts. First, with an inverter sized for continuous high-speed application, low-speed torque is more than five times greater than 3-phase or brushless machine performance (which are comparable to Chorus Meshcon machine operating at fundamental frequency). Second, by transitioning between operation at harmonic and fundamental frequencies, the Chorus Meshcon machine can realize this increase in low-speed torque without sacrificing high-speed performance.

Chorus differs from conventional motors physically in two important respects.

Firstly, it has many phases, normally at least 12, where conventional motors have three.

Secondly, each pole phase group of a Chorus motor occupies a single slot in the stator winding. Full span windings are used with unity chording and distribution factors. Conventional motors, which use partial and distributed chording to try reduce harmonic waveforms, by necessity use more turns and thinner wire, thus increasing the resistance of the windings. Chorus, because it does not need to eliminate harmonic waveforms, uses fewer turns of wire for a given size of motor, thus decreasing the apparent resistance of the windings and increasing the efficiency of the motor.

Furthermore, the wire windings on the stator create spatial harmonics arising from the angle of the windings in relation to one another. Again, if the number and angle of the windings corresponds with the number of phases in a high-phase order motor, spatial harmonics up to the number of phases will generate rotating fields of the same synchronous speed as the fundamental field.

Once all the poles of the harmonic are represented upon the stator, (by using the Chorus winding) the cyclic quality of the harmonic in the drive waveform is represented upon the stator. In this manner, the Chorus approach allows each harmonic, up to the number of phases in the motor, to synchronize speed and direction with the fundamental.

The second innovation introduced by Chorus is the Chorus Meshcon control system. This uses a harmonic mesh effect to dynamically change the V/Hz ratio needed to drive the motor, thus allowing the full capabilities of the inverter to be used over a wider speed range, decreasing the cost of the inverter substantially when low speed starting torque is required.

If the V/Hz ratio is increased, the required current for a given output is reduced but the required voltage is increased, and vice versa. Normally you would have to do this by changing the number of turns of wire connected in series in the motor windings. But with Meshcon, instead of changing the number of series turns, a software controller changes the harmonic current being fed to the motor. At low speeds, an extremely high V/Hz ratio can be developed, and the motor then uses the full output voltage capability of the inverter at reduced current, to provide higher torque at low speed.

Meshcon can then change the harmonic fed to the motor to reduce the V/Hz ratio, permitting the motor to operate at full speed, with a lower torque. Thus, with Chorus Meshcon, a single drive can optimize performance for both start-up and continuous running — something standard AC Motors do not do.

Building Chorus motors is not difficult, and leading manufacturers, including power electronics specialists Semikron, are working with Chorus to provide reliable, certified product.

The annual maintenance costs of large D.C. motors, with brushes and commutators which continually wear out, may well make Chorus an attractive proposition, while AC motors will receive a considerable boost in power and efficiency with a Chorus rewind.

As a corollary, Chorus does not need to be engineered to eliminate or mitigate harmonics. Thus, it can use rugged inexpensive SCR controllers instead of more sophisticated but fragile IGBTs.

Chorus motor torque and efficiency figures are comparable to, and in many cases exceed, DC motors of similar power. DC motors have brushes and commutators which wear out frequently, and overheating problems with locked rotors. Chorus motors avoid such problems because they are AC induction motors. In consequence, Chorus may replace many DC motor applications.

The complexity of a Chorus motor, with its many phases, means that for applications requiring less than about 1HP, Chorus is unlikely to be competitive. But for heavy duty traction applications, or where there is a need to stop and start frequently under load, Chorus motors are ideal.

One application of particular interest might be commercial or industrial washing machines. In a washing machine, high torque is required at low speeds, switching to high speed spin cycles. This is a market where specialist motors have been developed, with 2-18 poles, the number of poles being varied to provide different speeds of rotation. At the same time, there’s an increasing demand for faster spinning speeds to remove more water from the load.

Chorus Meshcon is capable not only of changing the operating mode of the motor to switch from low-speed washing cycles to high-speed spin cycles without loss of efficiency, but can also respond automatically to varying loads and demands. In other words, it can adjust the performance of the motor to match the conditions it meets.