The depth of an armature slot is limited by several factors, including the tooth density, eddy losses in the armature conductors, available core depths, and commutation. For reasonable frequencies (up to 50 Hz on medium and large dc machines), slots about 2 in deep can ordinarily be used.

Acceptable slot pitches range from 0.75 to 1.5 in. Small machines have shallower slots and a lower range of slot pitches. For medium and large machines, a reasonable tooth density usually results if the ratio of slot width to slot pitch is about 0.4.

Eddy losses in the conductors can be large compared with their load I 2R losses. Sometimes these must be reduced by making each armature conductor from several strands of insulated copper wire.

The number of strands and their size depend on the frequency and the total depth of the conductor. An approximate formula for reasonable eddy losses is

No. of strands = (0.168) (f ^0.83)(de^ 0.4)

where f is the frequency in hertz, (r/min # poles)/120, and dc is the total depth of a conductor.

The insulation space required depends on the type used. Typical conductor strands have about 0.018 in of glass strands and varnish total. Mica wrappers, binding tapes, and varnish and slot finish allowance (0.010 in) total about 0.085 in on the coil width.

If the space for the wedge and its retainer is included, the two coils depth wise total about 0.315 in.

Approximate Slot Design

Width [see text preceding and 0.563 in
following Eq. (8-17)] 0.4 # 1.407
Depth 2.0 in
Approx. total cond. depth 0.875 in
Frequency 42.8 Hz
No. of strands/conductor [Eq. (8-17)] 3
Slot width, in Depth, in
Size 0.563 in 2.000 in
Insulation 0.139 (0.085 # 0.054) 0.423 (0.315 # 0.108)
Bare copper 0.424 in 1.577 in
Strand size 0.141 in 0.263 in
Use 3 (0.144 0.289) in strands/conductor
Use available slot 0.570 in 2.250 in

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