INFLUENCE OF VARIATIONS IN THE AIR GAP ON THE NO-LOAD CURRENT OF INDUCTION MOTORS

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP80-00809A000600310057-4
• Release Decision: RIPPUB
• Original Classification: C
• Document Page Count: 2
• Document Creation Date: December 22, 2016
• Document Release Date: August 12, 2011
• Sequence Number: 57
• Publication Date: May 10, 1950
• Content Type: REPORT

Sanitized Copy Approved for Release 2011/08/17: CIA-RDP80-00809A000600310057-4
C ONFIDENTIA
CENTRAL INTELLIGENCE AGENCY REPOR
INFORMATION FROM
FOREIGN DOCUMENTS OR RADIO BROADCASTS CD NO.
DATE OF
INFORMATION i osn
HOW
DATE DIST. 16 May 1950
PUBLISHED
Monthly periodical
WHERE
PUBLISHED
Moscow
NO. OF PAGES 2
DATE
PUBLISHED
Feb 1950
SUPPLEMENT TO
LANGUAGE
Russian
REPORT NO.
THIS DOCUMENT CONTAINS INFO RYATIOM AFFECTING TNl RATIONAL DEFENSE
OF THE UNITED STATES WITHIN THl MEANING OF E3PIOMAGE ACT EO
U. S. C.. SI AND SZ. AS AMENDED. ITS TRAXSMISSIOH ON THE REVELATION
OF ITS CONTENTS IN ANY MANNER TO AN UNAUTHORIZED PERSON IS PRO-
0111TED ET LAW. REPRODUCTION OF THIS FORM IS PROHIBITED
INFLUENCE CF VARIATIONS IN THE AIR GAP
ON THE NO-LOAD CURRENT OF INDUCTION MOTORS
Docent L. V. Litvak
Cand Tech Sci
STATE
ARMY
OONFIOENTIAL
Sanitized Copy Approved for Release 2011/08/17: CIA-RDP80-00809A000600310057-4
In induction motors the reluctance of the air gap amounts to 70-80 per-
cent of the total reluctance of its magnetic circuit. Hence, it may be assumed
that 70-80 percent of the no-load reactive power taken by an induction ;aotor
is due to the reluctance of the air gap.
Since, on an average, this reactive power constitutes 60 percent of the
total reactive power supplied to industrial enterprises, it may be concluded
that the air gaps of induction motors account for about half the total reac-
tive power balance. This is a very imposing figure. It indicates the neces-
sity of introducing into maintenance and operating procedures strict control
over the dimensions of the air gap, inasmuch as it predetermines the amount
of reactive power in the motor.
To obtain the best operating characteristics (power factor and efficiency)
modern induction motors of low or average power -- which constitute a majority'
of present-day electric motors -- are built with as small an air gap as possible
depending upon various mechanical factors: shaft vibration; permissible shaft
deflection; accuracy in surfacing the rotor and stator; and allowances for
wear and sag of the beax_ngs.' This "design" size of the air gap in various
types of motors with different powers and with rated rpm's of 3,000, 1;500,
1,000, and 750, varies within wide limits -- 0.25 to 1.75 millimeters.
Consequently, in regulating the air gaps in each concrete case, one must
be guided by the "design" size of the gap and not by "average" values which -
often exceed these "design" sizes.
 Sanitized Copy Approved for Release 2011/08/17: CIA-RDP80-00809A000600310057-4
,1O!FIOEHTIAE
It should be noted that, in practice, motors are often found in which
the air gaps are ccnsi3erabiy enlarged as a result of wear on the rotor
surface, and the "permissible' wear in these cases is largely based upon
the "averae" values for air gaps.
Analysis of carve:; showing the variation in no-load current as a func-
tion of air gap reveals that reactive no-load power increases quite rapidly
as the gea enlarges. Therefore, in view of the importance of this factor to
power economy, and the general lack of systematized published data on the
various motors in use, ar, extensive table of air-gap dimensions has been
compiled to serve as P. 3uide. [Note: nominal. air Kap, motor type, rpm, and
power range for over 100 induction motors, the majority for two or three
rated speeds, are available in original document in CI.A_7
At the Scientific and Technical Session of the Moscow Scientific and
Technical Society of Power Engineering on Maintainance of Electric Equip-'
meat of industrial Enterprises, held in Moscow in 1946, it was pointed out
that the advisability of repairing motors whose air gaps had increased by
more than 15 percent. was open to question. Undoubtedly, this statement is
too cautious. The operation of motors with sharply increased air gaps leads
to a correspondingly inadmissible deterioration in the technical and economic
characteristics of lectric power utilization, and should, therefore, be per-
mitted only in special cases.
CONFIDE aTm t
Sanitized Copy Approved for Release 2011/08/17: CIA-RDP80-00809A000600310057-4