The rapid development of high-energy PMs with a rather linear demagnetization curve led to widespread use of PM synchronous motors for variable speed drives. As electric machines are reversible by principle, the generator regime is available, and, for direct-driven wind generators in the hundreds of kilowatt or MW range, such solutions are being proposed.
Super-high-speed gas-turbine-driven PM synchronous generators in the 100 kW range at 60 to 80 krpm are also introduced. Finally, PM synchronous generators are being considered as starter generators for the cars of the near future.
There are two main types of rotors for PM synchronous generators:
• With rotor surface PMs (Figure 2.10) — nonsalient pole rotor (SPM)
• With interior PMs (Figure 2.11a through Figure 2.11c) — salient pole rotor (IPM)
The configuration in Figure 2.10 shows a PM rotor made with parallelepipedic PM pieces such that each pole is patched with quite a few of them, circumferentially and axially.
The PMs are held tight to the solid (or laminated) rotor iron core by special adhesives, and a highly resilient resin coating is added for mechanical rigidity. The stator contains a laminated core with uniform slots (in general) that house a three-phase winding with distributed (standard) coils or with concentrated (fractionary) coils.
The rotor is practically isotropic from the magnetic point of view. There is some minor difference between the d and the q axis magnetic permeances, because the PM recoil permeability (μrec = (1.04 – 1.07) μ0 at 20°C) increases somewhat with temperature for NeFeB and SmCo high-energy PMs.
So, the rotor may be considered as magnetically nonsalient (the magnetization inductances Ldm and Lqm are almost equal to each other).
To protect the PMs, mechanically, and to produce reluctance torque, the interior PM pole rotors were introduced. Two typical configurations are shown in Figure 2.11a through Figure 2.11c. Figure 2.11a shows a practical solution for two-pole interior PM (IPM) rotors. A practical 2p1 = 4,6,… IPM rotor as shown in Figure 2.11b has an inverse saliency: Ldm < Lqm, as is typical with IPM machines.
Finally, a high-saliency rotor (Ldm > Lqm), obtained with multiple flux barriers and PMs acting along axis q (rather than axis d), is presented in Figure 2.11c. It is a typical IPM machine but with large magnetic saliency.
In such a machine, the reluctance torque may be larger than the PM interactive torque. The PM field first saturates the rotor flux bridges and then overcompensates the stator-produced field in axis q.
This way, the stator flux along the q axis decreases with current in axis q. For flux weakening, the Id current component is reduced. A wide constant power (flux weakening) speed range of more than 5:1 was obtained this way. Starters/generators on cars are a typical application for this rotor.
As the PM’s role is limited, lower-grade (lower Br) PMs, at lower costs, may be used. It is also possible to use the variable reluctance rotor with high magnetic saliency (Figure 2.11a) without permanent magnets. With the reluctance generator, either power grid or stand-alone mode operation is feasible.
For stand-alone operation, capacitor self-excitation is needed. The performance is moderate, but the rotor cost is also moderate. Standby power sources would be a good application for reluctance synchronous generators with high saliency Ldm/Lqm > 4.
PM synchronous generators are characterized by high torque (power) density and high efficiency (excitation losses are zero). However, the costs of high-energy PMs are still up to $100 per kilogram.
Also, to control the output, full-power electronics are needed in the stator (Figure 2.12). A bidirectional power flow pulse-width modulator (PWM) converter, with adequate filtering and control, may run the PM machine either as a motor (for starting the gas turbine) or as a generator, with controlled output at variable speed.
The generator may work in the power-grid mode or in stand-alone mode. These flexibility features, together with fast power-active and power-reactive decoupled control at variable speed, may make such solutions a way of the future, at least in the tens and hundreds of kilowatts range.
Many other PM synchronous generator configurations were introduced, such as those with axial airgap. Among them, we will mention one that is typical in the sense that it uses the IPM reluctance rotor (Figure 2.11c), but it adds an electrical excitation. (Figure 2.13).
In addition to the reluctance and PM interaction torque, there will be an excitation interaction torque. The excitation current may be positive or negative to add or subtract from Id current component in the stator.
This way, at low speeds, the controlled positive field current will increase and control the output voltage, while at high speeds, a negative field current will suppress the electromagnetic torque, when needed, to keep the voltage constant.
For DC-controlled output only a diode rectifier is necessary, as the output voltage is regulated via DC current control in four quadrants. A low-power four-quadrant chopper is needed.
For wide speed range applications such a hybrid excitation rotor may be a competitive solution. The rotor is not very rugged mechanically, but it can easily handle peripheral speeds of up to 50 m/sec (10,000 rpm for 0.1 m diameter rotor).
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