Advantages of PM Machines for High-Speed Applications

A rotating electric machine is an energy converter; we call it a motor when electrical energy is converted to mechanical energy, and an alternator when the conversion is reversed.  In either case, the electric machine may be the same device even though its function is reversed.  Electric machines can be much smaller and have better efficiency when they can operate at a higher speed than the 3,600 rpm limit imposed by 60 Hz power systems.  Solid state inverters remove the speed limit.

The power rating of a given machine increases directly with speed until structural, thermal or dynamic stability limits are reached.  A good high-speed design is where these limits are approached in harmony.  Heat extraction becomes a challenging problem at high speed because the power rating and losses of a machine increase with speed. This puts more heat in the same size package. 

Commercial and military applications of high-speed turbines for electric power generation have existed for many years.  Originally, speed reduction gear boxes connected the high-speed turbines to 60 Hz alternators or in some cases 400 Hz alternators. The advent of solid-state inverters with their unique ability to change the frequency has made it possible to eliminate gear boxes and to greatly reduce the size, complexity and weight of electrical machines by trading speed for torque. In addition, the concurrent developments of gas bearings, foil bearings, magnetic bearings and high energy magnets have also expanded the range of applications where high-speed machines may be used.

Inverters are not inexpensive, but their cost must be weighed against the virtues of eliminating a gear box and related issues about service life, lubrication, noise and weight.  Another profound advantage offered by inverters is they make it possible to use more robust rotors that don’t need insulated windings, slip rings, and sliding contacts (brushes).  Inverters usually can accommodate frequencies up to 3,000 Hz.  Since the eddy current component of iron loss in electric machines increases with the square of frequency, there is generally not a great incentive to exceed 3,000 Hz so far as machine efficiency and cooling is concerned.   

Some attractive benefits that high-speed alternators or motors can offer include:

  • Portability, small size and light weight
  • Low maintenance
  • Reliability in a wide range of environments
  • Good efficiency
  • Quiet operation
  • Freedom from lubricants and other contaminants

 

There are different machine topologies for high-speed applications, and each has its own benefits and drawbacks.  The permanent magnet machine type is considered the most superior in terms of performance due to its unique characteristics, including robust construction that is well suited for high-speed operation, and zero excitation power requirements that result in unity power factor operation.

Permanent Magnet (PM) Machines

When efficiency and weight are primary concerns, a machine with a permanent magnet rotor is clearly superior in most applications. This is because of the following:

  • Zero excitation power is required.
  • The machine can run at unity power factor (reactive stator current is not required for excitation).  Efficiency of 95% or even higher can be achieved.
  • The rotor is smooth, and the air gap is relatively large. This reduces windage loss, tooth ripple loss and provides a passage for cooling air.
  • The rotor has high resistivity and very low permeability. This inhibits losses that might otherwise be induced by stator flux ripples due to stator teeth and stator current. Permeability of rotor magnets is almost the same as air!
  • Inverter size and loss is favored by unity power factor.

Other considerations that are also taken into account when selecting this type of machine for high-speed applications include:

  • The magnet material is costly, but this is offset by benefits due to high efficiency, smaller inverter, easier cooling, smaller size of other parts and lower bearing loads.
  • Spring rate of the attraction force between the rotor and stator is minimal because the rotor has very low permeance; the flux changes only slightly when rotor moves from center. This is an important advantage in soft bearing systems in high-speed machines where foil bearings or resilient bearings mounts are employed.
  • The rotor structure is stiff, stable, and durable when it is encased in an Inconel or stainless steel hoop to retain magnets.
  • The rotor is always excited.  If a sustained fault occurs, the prime mover must shutdown to avoid a high temperature hazard.  The typical short circuit current is 3 per unit.
  • Magnets are not suitable for hot environments. Certain magnet materials have higher temperature capability than others, but the practical limit is about 200oC. Magnetization is irreversibly reduced when temperature approaches the magnet’s Curie temperature.  The safe temperature for a magnet depends on its physical properties; the less expensive materials have a lower Curie temperature.

The motors require a synchronous start-up procedure; induction starting can overheat and demagnetize the rotor.

Induction Machines

Induction machines are the workhorse of industry. They are used everywhere and have many good features. The simple, low cost squirrel cage rotor structure is particularly appealing. Excitation is provided by the stator current, which induces and reacts with current in conductive rotor bars. This type of machine has the following characteristics:

  • Since the stator current must include a reactive component to excite the machine, the stator and inverter bear the burden for this need. The magnitude of this exciting current is quite significant and is determined by the machine’s winding reactances, the airgap between the rotor and stator, and the permeance of the stator and rotor iron. Generally a power factor of 0.85, and an efficiency of .9 are reasonable expectations. An induction rotor has significant losses in its rotor iron and rotor cage.
  • A short air gap is required to obtain a reasonable power factor. Since the rotor has high permeability, conditions exist to promote stray loss due to stator and rotor slots.  Rotor laminations mitigate the loss.
  • The rotor must slip with respect to rotating exciting flux. This produces a current in cage bars at slip frequency and the flux linking the rotor iron moves at slip frequency.  If the rotor slip speed is 1% of rated speed, the rotor loss will be 1% of shaft power.
  • The inverter must provide about 18% more Volt Amperes than required for a Unity PF machine.
  • The spring rate of the attraction force between the rotor and stator is high because the rotor has very high permeance and the air gap is short; large flux change occurs when the rotor moves from center. This might be a problem in soft bearing systems in high-speed machines where foil bearings or resilient bearings mounts are employed.
  • The rotor structure is a stack of ferromagnetic laminations held together by cage bars.
  • Excitation can be varied to reduce losses at partial load; even turned off.  The machine cannot produce a sustained short circuit current.  Alternators may not self-excite with the load circuit connected.
  • The rotor surface speed and allowable rotor temperature depend on the properties of the materials and construction used.

Synchronous Reluctance Machines

Synchronous reluctance machines have a very stiff, high strength rotor that can operate at surface speeds up to 1,100 feet/second. The rotor can also operate at fairly high temperatures without detriment – possibly 600-700 ⁰F. The rotor is constructed of layers of ferromagnetic steel separated by equal layers of non-magnetic material to form salient poles of low reluctance in the direct axis, but high reluctance in the cross axis.  Both materials are brazed together and have very high strength. The rotor is a smooth bimetallic cylinder. This type of machine has the following characteristics:

  • The stator current must include a reactive component to excite the machine. The magnitude of this exciting current is very significant and is determined by the machine’s winding reactances, the airgap between the rotor and stator, and the permeance of the stator and rotor iron. Power factor of 0.7 and efficiency of .92 to .95 are probably realistic.  The rotor surface losses are significant but the reluctance rotor can tolerate high temperature better than most other types.
  • The rotor has salient slots. Also, a short air gap is required to obtain a reasonable power factor.  The rotor has high permeability and is not laminated, which are conditions that promote stray losses.
  • The rotor is synchronous with the rotating exciting flux. There is no current induced in the rotor other than stray eddy currents.
  • Inverter must provide about 43% more Volt Amperes than required for a Unity PF machine.
  • The spring rate of the attraction force between the rotor and stator is high because the rotor has very high permeance and the air gap is short; large flux change when rotor moves from center. This might be a problem in soft bearing systems in high-speed machines where foil bearings or resilient bearings mounts are employed.

Excitation can be varied to reduce losses at partial load; even turned off.  The machine cannot produce a sustained short circuit current.  Alternators may not self-excite with load circuit connected.