Switched reluctance motors used to be a favorite topic among Ph.D. candidates doing thesis work in electric motors. The reason was pragmatic and had little to do with the promise of SR technology: It was relatively easy to cobble together an SR motor that would work in some fashion.

It was far more difficult, though, to devise an SR motor that would work efficiently enough to challenge existing motor technology on its own turf. But that recently happened with the advent of fast-switching power electronics. Now SR drives can be significantly more efficient than comparable induction motors and drives in several applications.
Inherently simple, SR motors were developed in the 1800s when switching devices were primitive. With the rising emphasis on energy efficiency, SR motors are taking more prominent roles in appliances, industrial uses, and in commercial and vehicular applications.

An SR motor is simple and rugged. The rotor consists of stacked steel laminations with a series of teeth. The rotor requires no windings, rare earth materials or magnets of any kind. The teeth are magnetically permeable, and the areas surrounding them are weakly permeable by virtue of slots cut into them.

Unlike induction motors, there are no rotor bars and consequently no torque-producing current flow in the rotor. The absence of any form of conductor on the SR rotor means that overall rotor losses are considerably lower than in conventional motors, which utilize conductors on the rotors.

Lower rotor losses are especially relevant during start-up where, in the SR motor, the rotor losses are no greater than when the motor operates at its rated condition. This permits a virtually unlimited capability for prolonged operation in the stall condition and for repeated starting under full-load. Such performance is often not possible with conventional drives because of the large electrical losses they experience on their rotors and the subsequent rotors heating under such conditions.

Torque produced by the SR motor is controlled by adjusting the magnitude of current in the stator electromagnets. Speed is then controlled by modulating the torque (via winding current), in the same way that speed is controlled via armature current in a traditional brush dc motor and drive. Torque production in an SR motor is proportional to the amount of current put into the windings. Torque production is unaffected by motor speed. This is unlike ac motors where, in the field-weakening region, rotor current increasingly lags behind the rotating field as motor rpm rises.

The SR motor’s torque density can easily exceed that of a typical induction motor. This factor frequently lets equipment manufacturers eliminate gear boxes or greatly decrease the number of reductions necessary to handle specific applications.

SR motors also offer performance advantages in motion control. An SR motor can produce 100% torque at stall indefinitely. The reason is that the rotor produces no heat at stall. Rotor bearings stay cool as well. Only the stator coils get warm, and they can be cooled via fins on the stator housing, or by other conventional means.

The SR stator windings are much simpler than those required for induction motors or permanent-magnet ac motors. Each slot in the stator contains windings for only one phase. A winding that emerges from the stator slot needs only to loop back around one slot, rather than around multiple slots as on induction motors. This minimizes the volume of end windings and significantly reduces the risk of a phase-to-phase insulation failure.

This winding construction also minimizes the energy lost on coil overhangs at the slot ends, because magnetic fields generated at the end of the slot do not contribute to output power. A smaller end-winding area also minimizes the length of the motor and the amount of heat to be dissipated. Consequently, an SR motor can be one or two frame sizes smaller than an equivalent induction motor.

The overall losses within the Switched Reluctance motor are concentrated within the stator, where they are relatively easily dissipated: In the case of a standard totally–enclosed machine, heat conducts away through to the relatively cool exterior of the motor frame. The minimal rotor losses mean that the rotating parts of the machine, including bearings and lubricant, run relatively cool, often promoting long bearing and lubricant life.

Inside an SR drive

The energy efficiency of SR motors and drives meets or exceeds that of the best ac machines and drives operating at their “sweet spot.” The energy efficiency of ac induction motors drops dramatically when the motor operates at less than 50% load, or when used in the field-weakening range at higher speeds. In contrast, complete SR systems (including all losses – motor and inverter) will typically maintain efficiency well over 90% across a wide range of load conditions.

There is no fundamental high-speed limit for SR motors. Nidec Motor Corp. has run some units at 70 krpm and is evaluating operation at 100 krpm for certain small machines. High speeds are constrained only by the bearing system and the yield strength of the rotor steel. Moreover, SR motors generate no back EMF, so there is no need to expend energy for field weakening at high speeds, as is the case with permanent-magnet drives.

Drive electronics for SR motors resemble those for conventional variable-frequency drives to some degree. Ordinary six-switch inverters for VFDs and SR motor drives both contain identical numbers of power switches (usually IGBTs) and freewheeling diodes.

However, the SR drive has an efficiency advantage when compared with VFDs that necessarily use relatively high PWM (carrier) frequencies to approximate a sinusoidal motor current. Switching losses can be appreciable in VFDs, causing the inverters to run hotter than a drive powering a typical eight-pole SR motor. SR switching takes place at eight times the physical rotation speed of the motor. Thus, for a 3,600 rpm motor, switching frequency per phase is 480 Hz, about ten times slower than for the equivalent inverter. Therefore, the switching losses are ten times lower as well, allowing power loss in an SR inverter to be reduced by as much as half that of an inverter for an ac motor.

The SR power converter contains a ‘characterisation’ map which, for any given motor design, incorporates the optimum control parameters for operating across the motor’s whole torque and speed rating. This ensures that for any load point, the system delivers maximum efficiency. Designers know this efficiency performance at the outset. There is no the uncertainty about how one manufacturer’s drive will perform with another’s motor.

The characterisation map also lets the performance envelope of the drive be adjusted such that the drive can never operate outside a user-defined envelope of speed and torque set. Designers set these limits to be within those of the end equipment.

14. UCC3817 demo board used to produce RCI data.

Design a PFC Resonant Coupled Inductor That Doesn’t Distort Power Factor

Here’s a step-by-step design of a resonant-coupled inductor employed in a PFC EMI filter that doesn’t impact power factor.

Harry Dellamano

Jul 01, 2015

Over the past 40 years many of my designs included Power Factor Correction (PFC) ranging from 50 W to over 5.0 kW and operating from 30 kHz to 150 kHz switching frequency, utilizing CCM, CrCM and DCM modes and 50 Hz to 850Hz input power.

Recently, I was designing a PFC unit for 800 Hz operation and had difficulty designing the input filter that eliminates EMI, but does not distort the power factor caused by the large currents in the X capacitors. My challenge was that I could not divert the switching current from the input filter and circulate it in another loop. Then I remembered Lloyd Dixon’s seminars for Unitrode that covered coupled inductors and “current steering”. That is what I needed but did not understand the theory at that time. So I went back and searched the many papers on the subject, finding lots of higher math and transformer equivalent circuits. There is a coupled inductor where one winding has only AC current and the other with DC current. Why were they all using transformer circuits to describe this coupled inductor? In transformers, current goes in the primary dot and out the secondary dot. This design has the current being shared (split into ac and dc) by the primary and secondary.

So back to the bench, declaring that if two inductors share the same core the inductance is either mutual or leakage. I measured the total inductance of each inductor and their mutual inductance, and Figure 5 (see all figures below):

Ldc = Lm + LdL (1)

Lac = Lm + LaL

Where:

Ldc = Total inductance of dc winding

Lm = Mutual inductance between L_AC and L_DCwindings

Lac = Total inductance of ac winding

LdL = Leakage inductance in the L_DC winding

LaL = Leakage inductance in the ac winding

Using SPICE simulation, I realized the simplicity of this inductor configuration. If this is true, then why is it not in general use, why can’t I buy these Resonant Coupled Inductors (RCIs) off the shelf? The PFC Inductors I can buy are single windings of high power dissipation due to high resistance at the switching frequency (Rac losses). The designer did not have a good understanding of skin effect nor proximity losses. I believe that the previous papers have made this design very difficult to understand and scared off the engineers and magnetics manufacturers. This is not just about PFC inductors but input and output filters. Any place we have an ac and dc current component in an inductor winding, we should look at this topology to reduce the filter components.

To meet this design problem I came up with a solution and patented it. The RCI patent is currently pending (US 62/170,844) and the search results are very promising. I would like to hand off this design to the right company. Why did I patent this configuration when so many papers have been written on it? Yes, there is lots of prior art but the theory given is very confusing. Maybe I can educate the engineering community and reap some small benefits.

This design has been simulated in SPICE and bench tested on a T.I. Eval board. The Eval’s improvement was evident and all readings and waveform match the SPICE simulation. There is no magic, just solid engineering.

Patent Features:

The RCI will divert >30db of switching current from the input filter.
The resultant magnetic package is about the same volume as a standard PFC inductor but in a lower profile that aids system packaging, thermal flow and noise shielding.
Due to the separation of ac and dc currents, a higher efficiency unit can be made in the same volume.
Manufacturing costs are similar to a PFC inductor.
Core magnetics with some needed cutting are readily available.
This device will operate in CCM, CrCM and DCM of operation.
Can be used in any topology, input or output, diverting switching current to ground.

Figure 1 is prior art for reference only. Figure 2 shows both a two-winding and three-winding PFC circuit to help explain the RCI operation, but there can be many implications as shown in Fig. 3.

In Fig. 4, L2 has three windings and in Fig. 5, L2 is a two winding device. Both two- and three-winding devices operate in the same manner as explained below:

Ldc and Lac, which when loosely coupled (0.5< K<0.90) each form a leakage inductance. (LdL + LaL) that reduce their original (K=1) inductance such that:

Ldc = Lm + LdL (2)

Where:

K = Magnetic coupling factor

And:

Lac = Lm + LaL (3)

Where:

Therefore:

Ldc > Lac for best results

Lm = K (Lac × Ldc)^0.5

K = Lm/(Lac × Ldc)^0.5

Cr₌Resonant capacitor

LdL impedes the flow of ac current to the external circuit, but has low resistance to Idc, current in the dc winding. LaL resonates with the resonant capacitor (Cr) to form a low-impedance series resonant circuit at the switching frequency and shunts the ac current to ground, away from the output and the I_DC current is blocked by Cr. The ac winding carries high ac current so its resistance at the switching frequency (Rac) must be constructed as low as possible, so Litz wire is sometimes used. The dc winding only requires a low dc resistance, so normal wire is used. The placement of core gaps is also important to Rac.

Ldc > Lac improves the ac rejection ratio at the operating frequency of Iac (current in ac winding). As the K factor is reduced, Lm gets smaller and the leakage inductances get larger. As Lm is reduced, the I_AC component increases, affecting the external circuit. A K of ~0.6 is a good tradeoff.

The ac winding is sized according to the needs of the converter, which is known by those in the art. It has typical values of 500µH to control ripple current in the winding and switching device.

A low coupling factor (K) is obtained by placement of the two windings on opposite sides of the core and gap. The gap should be placed away from the windings or the fringing flux will increase winding resistance. Prior art coupled inductors were composed of one winding containing both ac and dc currents. This caused the winding to be large and difficult to package and keep cool. This unit with its smaller dual windings or triple windings can be packaged in a lower profile package, allowing better thermal flow. Full complete winding rows offer the best results.

Resonant Capacitor Calculations

Determine this capacitor value to resonate with Lac, leakage inductance.

1. Using an LCR meter, measure L_AC, Ldc, and Lm. You can directly measure Lm with most LCR meters with four leads. Attach the drive leads across Ldc, the sense leads across Lac and read Lm directly. The sign displayed is the phase.

2. LaL= Lac – Lm

Use LaL to calculate Cr at the switching frequency:

Cr = 1/[(2pF)² × LaL] (4)

It is important to note that a custom device is not necessary to get good results. You could obtain a coupled inductor (K>0.95) that has suitable ac and dc windings with the inductance to maintain proper PFC operation. Then add a low-frequency inductor (Powder Iron to operate at 120Hz) to connect the dc winding to the bridge. Then add a high-frequency (Ferrite) core to the ac winding and terminate with the resonant capacitor. These three devices would simulate the RCI. Not a pretty package, but a good start in understanding this topology.

The word PFC (Power Factor Correction) is used throughout this document to denote one configuration that is widely known to the profession, but the RCI can be used in many different configurations as shown in Fig. 3 in both two and three winding packages. Additionally, any electronic device that draws more than 25 Watts off line should be power factor corrected. The RCI will scale from 25 watts to kW and it will operate from 50Hz to over 850 Hertz, filling the needs of commercial and military devices.

Figures 6 and 7 are SPICE simulations for the RCI. Figures 8 to 14 are data taken from a TI UCC3817 (250W) demo board rev 5 with the original TI inductor (L1) and a two-winding RCI.