Page 278 - Industrial Power Engineering and Applications Handbook

Philosophy of quality systems and testing of electrical machines 11/257
Methods (2) and (3) are suitable for machines having Power Factor = -_______ 1
slip of not more than 5%.

Stroboscopic method
On me end of the motor $haft a single black radial line where
is painted upon a white background. The slip is easily W, = the higher of the tu0 reading\ and
measured by counting the apparent backward rotations W? = the Iowcr of the two reading\.
of the black line over ;I given period of time.
If W2 gives a negative reading it should be considered as
Slip-coil method a minus quantity. From the above fomiula. graphs can
be plotted for Power Factor versus ( W2/\V',). W2/WI being
A suitable slip-coil. hating approximately 700 turn5 of the ratio of lower wattmeter reading to the higher
1 mm diameter insulated wire. is pasjed axially over wattmeter reading.
the motor and its two ends are connected to a centre- If W, is negative, the ratio of ( W2/\Vl) should be
iero galvanometei-. When the motor is running, the considered as 21 ininus quantity. The falling curves 5hould
gal\anometer pointer will oscillate. The number of be designated 'for ( W2/WI)' and the ri5ing cur\c$ 'for
oscillations should be counted in one direction only, (+W21WI)'. Similarly, ordinates on the left-hand side
that is to the left or to the right, for a period of. say. should be designated for (-W2/Wl) and on the right-hand
20 seconds. side for (+W2/Wi ). See Figure 11 3.
The following formula will determine the percentage
dip: ,Vote If two \~;iltic\ of the Power FactoI dctci-mined h> the \vat[ to
\olt-ampere ratio md two-m attiiietci- tnethod\ do riot tall) for J
three-phaw niotor. the tc\t may he repeated to climinate the error.
Howevci-. \\ her-e the load i\ fluctuating. ii Poser Factoi- dctcrmined
where by a two-\battiiieter method \\ill he hisher h~ti that determined by
the matt to volt-ampere ratio method. In thir case. the higher value
S = slip (%) \hould be taken a\ the cort-ect reading. The ilill'ercnce I\ due to the
11 = number of oscillations incluhioii of a pul\attng coinlwnent 01' cui-rent in volt-;impcrcs.
t = time in seconds taken for II oscillations. and whlch i\ ;I function of load rathct- than ol the riiotor it\clf. The
,j'= supply frequency in Hz. Power Factor determined h-on1 the ratio of atLnieter I-cadi ng i\ nut
atfected by the presence ol ii pirlsating curt-ciit.
Magnetic needle method
Power Factor meter method
In this inethod ;I magnetic needle suspended on a sharp
point (so that it can rotate freely) is placed on the body In this method. a Power Factor meter is directly connected
of the inotor in the horizontal plane. The needle will in the circuit and ii direct reading i.; obtained at any
oscillate and the number of oscillations should be counted loading.
for ;I period of, say. 20 seconds. The percentage slip is
then calculated by the formula given above. 11.4.4 Overspeed test

11.4.3 Power Factor measurement All niotors are designed to withstand 1.2 times the
maximum rated speed. The test is simple and may be
The Power Factor indy be mea\ured by one ot the carried out by running the motor for 2 minutes at the
lollowing three method\ highei- specd. After the test. the motor must have no
deformation or any fault that may prevent it from operating
Watt to volt-ampere ratio normal 1 y.
Two-wattmeter
Power Factor meter
11.4.5 Test for speed-torque and speed-current
curves
Watt to volt-ampere ratio method
The speed-torque characteristic is the relationship between
The Power Factor is obtained by the ratio of the algebraic
sum of wattmeter readings to volt-ampere readings. For the torque and the speed. in the range from ~ero to
synchronous >peed. This relationship. when expressed
a three-phase system; as a curve. will include breakdown torque (pull-out torque).
Watts pull-up torque and starting torque. The speed-current
Power Factor = characteristic is the relationship of the current to the
\'3 x line volts x line amperes
speed.
Two- wattmeter method
Methods
On a three-phase motor where the load is pulsating the
Power Factor may be checked by the following formula. Speed-torque and speed-current te\t\ may be carried
obtained from independent wattmeter readings: out by the following method\