Page 67 - Industrial Power Engineering and Applications Handbook

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Inference through an auto-transformer, even with a tapping as high as
On an ON-LOAD start, the starting time increases 80% although the motor possesses some accelerating torque
disproportionately, depending upon the type of switching. This at each Point during Pickup (cuwe of Figure 2.18).
load therefore cannot be accelerated within a safe stall time
Relevant Standards

IEC Title IS BS
60034- I2/ 1980 Rotating electrical machines 8789/1996 BS EN 60034-
starting performance of single-speed three-phase cage 121996
induction motors for voltages up to and including 690 V
60079-7/1990 Electrical apparatus for explosive gas atmosphere. 638111991 BS 5501-611977
Increased safety protection e
Related US Standards ANSVNEMA and IEEE
NEMA/MG-1/1993 Motor and generators ratings, construction, testing and performance
NEMA/MG-2/1989 Safety Standards (enclosures) for construction and guide for
selection, installation and use of rotating machines
Notes
1 In the tables of relevant Standards in this book while the latest editions of the standards are provided, it is possible that revised editions
have become available. With the advances of technology and/or its application, the updating of standards is a continuous process by
different standards organizations. It is therefore advisable that for more authentic references, readers should consult the relevant
organizations for the latest version of a standard.
2 Some of the BS or IS standards mentioned against IEC may not be identical.
3 The year noted against each standard may also refer to the year of its last amendment and not necessarily the year of publication.

List of formulae used also, H = W . 6. 8 (2.8)
W = weight of heated portion in kgm
Effect of starting current on torque 6 = specific heat of the material of windings, in
wattdkgm. "C.
I: xR,
T, - 8 =temperature rise in "C
S
Sharing of heat

&- = (&)' (for the same rotor resistance R,) H, = rotor heat in W.S.
Tct2 H, = stator heat in W.S.
(2.3)
Heating during a no-load start
or - T,, 1 = (F)' x 2 (for different rotor resistances)
Tstz st2 (2.10)
(2.4)
Heating in a motor during a stalled condition:
Time of start-up
for the stator
GD? . N,
ts = (2.5) HSte . tstc = 8 . C (2.1 1)
375 . T,
t, = time of start in seconds Hste = heat generated during stalled condition per second
GD; = GD; + GD; in watts
GDi = motor and tste = time for which the stalling condition prevails in
seconds
GD; = load weight moment of C = heat capacity of the motor
inertia referred to the motor speed
or tstc = seconds (2.12)
JA X 0.00552
J,, = current density during start in A/cm2
GD: = weight moment of inertia of load at a speed NL
Further reading
Motor heating during start-up
Machinery Hand Book, Industrial Press Inc., 200 Madison Avenue,
H = I$ .R.t, (W.S.) (2.7) New York, USA.