Permanent vs. Field Winding

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Discussion Overview

The discussion centers on comparing permanent magnets and field windings in brushless DC motors, specifically for a single geared vehicle drive system aimed at achieving the widest power band within a given package size. Participants explore the implications of using neodymium permanent magnets versus soft iron field windings, considering factors such as magnetic flux density and operational efficiency.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the magnetic flux density values (B_r for neodymium and B_R for soft iron) can be directly compared, highlighting the differences in effective pole cross-section due to copper requirements in soft iron.
  • Another participant argues against comparing the two, stating that neodymium iron boron permanent magnets provide significant magnetic fields without the need for amp-turns, and notes the typical configuration of PM motors with magnets in the rotor and coils in the stator.
  • A participant raises a question about the cost in field current associated with field windings, suggesting that they could potentially double low-end power, referencing their use in diesel electric locomotives.
  • Another participant reiterates the question about the cost in field current and discusses field excitation in AC machines, mentioning its application in synchronous condensers for power factor improvement.

Areas of Agreement / Disagreement

Participants express differing views on the comparison between permanent magnets and field windings, with no consensus reached on the advantages or disadvantages of each approach. The discussion remains unresolved regarding the implications of field current costs and the effectiveness of each method in achieving desired performance outcomes.

Contextual Notes

Participants acknowledge the complexity of comparing magnetic flux densities and the operational characteristics of different motor types, but do not resolve the implications of these factors on performance.

Phrak
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This is a question about brushless DC motors. (I should have put that in the title)

How do you compare the two for an single geared vehicle drive system to get the widest power band in the same package size?

I've been looking over the max flux density of soft iron vs. neodymium permanent magnets. The field strength of high grade neodymium is quoted at a B_r of 14K Gauss, and B_R of soft iron at about 18K Gauss. But for the soft iron, the required copper reduces the effective pole crosssection.

(Can B_r and B_R be directly compared?)

Variable field winding strength can nearly double the operating speed range by decreasing the field stength at higher RPM, or so I've read. Where do I start?
 
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Don't compare the two. Neodymium iron boron permanent magnets give you large fields for free. No amp-turns required. All large PM motors have the PM in the rotor, and the coils in the stator, and a Hall-effect sensor to determine the armature position. The stator polarity and current (and motor torque) is controlled by solid state switches. Usually the motor's max torque is at zero RPM.
 
What's the cost in field current?

Field windings could nearly double the low-end power. This may be the strategy used on diesel electric locomotives. Don't know.
 
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Phrak said:
What's the cost in field current?

Field windings could nearly double the low-end power. This may be the strategy used on diesel electric locomotives. Don't know.

:smile: Hi, field excitation is used when u require to vary the magnetic field strength in applications such as 'synchronous condensers' for PF improvement. Varible excitation is used in large alternators to adjust MVars sourced by the alternators to the power system.

But these are all mostly as applied to AC machines.
 

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