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Permanent vs. Field Winding

  1. Mar 15, 2009 #1
    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?
    Last edited: Mar 15, 2009
  2. jcsd
  3. Mar 15, 2009 #2
    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.
  4. Mar 15, 2009 #3
    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.
    Last edited: Mar 15, 2009
  5. Mar 16, 2009 #4
    :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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