## Power Conversion

Although the small-scale tubular machine has better generating characteristics than the VHM in terms of power factor, some consideration must be given to the effect of inductance at larger scale tubular machines. For an accurate design of air cored tubular machine, FEA of actual topologies is recommended. For the purpose of demonstration, however, the inductance approximation given in Section 5.1.7.3 is used to examine the power characteristics of the generators proposed in the preceding section. It was demonstrated in Chapter 5, Section 5.4.2.1, that the tubular machine could be modelled as a pure emf source with a series reactance like that of Figure 8.9, which allows the simple phasor diagram, also shown in Figure 8.9, to be used to analyse the voltage.

Figure 8.9: Phasor diagram of tubular machine with resistive load The electrical machines were designed for a rated current and maximum speed, the value of a purely resistive load that ensures the rated current flows at the maximum speed can thus be chosen using geometric relationships derived from Figure 8.9.

Figure 8.9: Phasor diagram of tubular machine with resistive load The electrical machines were designed for a rated current and maximum speed, the value of a purely resistive load that ensures the rated current flows at the maximum speed can thus be chosen using geometric relationships derived from Figure 8.9.

Table 8-2: Generator characteristics of tubular machines

Model |
A |
B |
C |
D |
E |

Internal |
0.2 |
1.1 |
0.37 |
1.6 |
0.042 |

resistance (Ohm) | |||||

Load (Ohm) |
2.18 |
7.13 |
4.75 |
16.6 |
0.455 |

Inductance (mH) |
14.5 |
232 |
28.9 |
464 |
6.9 |

Power/coil (kW) |
3.36 |
11.0 |
7.32 |
25.6 |
11.2 |

Power factor |
0.98 |
0.90 |
0.98 |
0.92 |
0.95 |

Table 8-2 shows the resistor values necessary for individual coils and the power dissipated within them at the rated current. As the airgap for all the machines is equal to 5 mm, it is always less than 5% the value of the radius, a close approximation to the coils being surface mounted. It is likely that the value of inductance, based on a method which ignores the airgap and treats the coils as point current sources, is reasonably accurate. The resulting power factor is consistently above 0.9. These values are used in Table 8-3, which gives the peak phase power as the product of coil power and number of poles, the average machine power as 1.5 times this value, and the total configuration power as the sum of all the generators.

Table 8-2 shows the resistor values necessary for individual coils and the power dissipated within them at the rated current. As the airgap for all the machines is equal to 5 mm, it is always less than 5% the value of the radius, a close approximation to the coils being surface mounted. It is likely that the value of inductance, based on a method which ignores the airgap and treats the coils as point current sources, is reasonably accurate. The resulting power factor is consistently above 0.9. These values are used in Table 8-3, which gives the peak phase power as the product of coil power and number of poles, the average machine power as 1.5 times this value, and the total configuration power as the sum of all the generators.

A |
B |
C |
D |
E | |

Peak phase |
167 |
275 |
366 |
641 |
370 |

(kW) | |||||

Total |
250 |
412 |
549 |
962 |
555 |

machine(kW) | |||||

Total (MW) |
3.8 |
3.3 |
3.8 |
3.8 |
3.9 |

he product o he product o velocity and force, is

The power removed from the AWS, equal to th 3.96 MW for all the machines. Table 8-3 shows the power delivered to a simple load resistor and is a function of the internal resistance and coil inductance. The efficiency of alternative configurations may hence be investigated and varies from 83 to 98 % for alternative configurations.

Consideration of the machine masses imply that machine A and E are the most favourable. Machine B has the largest mass and delivers the smallest power to the load. The large magnet width in this machine requires a large coil pitch, adversely affecting the inductance and internal resistance of the machine.

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