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SLIP METER.

A Thesis
Submitted to the Faculty
ot_
MICHIGAN AGRICULTURAL COLLEGE

by

A

I 1/;
. L.“

e” 53
Benjamin P. Carr 3. D. Shaffmaeter
«fin» . fL,
Candidates for the Degree
of
BACHELOR OF SCIENCE.

June, 1924.

?&o.

THESIS

i; 04,) . an,

r ’18

TABIE OF CONTEII TS

Introduction.
Available Met hode .

Difficulties Encountered in Available Methods.

Complete Discussion of Apparatus Designed
and Built in the Shape.

Adjustments.

Precautions.

Suggestions for Further Work.

Result of Test.

BibliOgraphy

.. " :5. ‘ 4“".

Page

61

and-30:

10
11

IN TRODUCTI ON .

It is frequently desirable to obtain the slip of an
induction motor to a greater degree of accuracy than can be '
obtained by the comparison of two speed indicators. The slip
of an induction motor is the differeme between load speed
and synchronous meed as a decimal of synchronous speed.

When Syn. r.p.m. . 120:
P

It is necessary to obtain the slip accurately in order to
show the true performance or an induction motor.

The following discussion will show why two speed
indicators are very inaccurate. If the speed of a motor is
taken with an ordinary speed counter, an error of one-half
of one percent is not unusual. With a machine operating at
900 r.p.m. this would mean an error of 4.5 r.p.m. EIpressed
in terms of clip, this error would become 10% and usually
the error would be greater. Tachometers, especially the
hand type, if constructed for any great range, are difficult
to read and usually unreliable for precision work. There-
fore, it may be seen that some special method is required to
obtain the slip accurately.

AVAILABLE METRO DS .

After eating a study of references and devising
many methods for masuring slip, the one presented in this
paper was chosen.

Methods Reviewed:

‘ 1. Various etroboscopic methods.

8. Devices imploying a vibrating reed.

3. An escapement wheel which ccuntsthe
cycles slip. I

4.. Wound rotor induction motor with voltmeter
attached to field windings the generated voltage
indicating the slip. .

6. Cone and a disc with an are light to
indicate synchronous speed. .

6. Two similar shunt D. C. mohinee one on e
synchronous motor the other on the motor in question,
with their voltages bucking each other, any differ-
ence in voltages indicating slip.

7. Two series D. C. nachines arranged as in ,
(6), but the voltage of the one indicating the
eyn.r.p.m. and the voltage of the other indicating

the motor r.p .m.

DIFFICULEIES ENCOUNTERRD 1H AVAILABEE HETHODS.

Any method which depends on the human element to
count the slip or any which.mnst be read over a period of
time will have inacuracy of time, or if the slip is too great
it cannot be counted accurately. Therefore, the following
methods are eliminated l, s. a, and 5. inc. 4 is not accurate
because the wave form of the impressed voltage would effect
the voltmeter readings. Where relative high speeds are used,
the differences of which indicate a relatively small slip
the results are inherently inaccurate because a small error
on a large number when shifted to the small one causes a
much larger error. Hence 6 and 7 are eliminated. Compact
devices have been placed on the market by means of which.the
slip can be measured. One device cmsiste of a rotating
commutator having the sane number of segments as the motor
has poles. This commutator is pressed against the end of
the motor shaft and thus rotates at the same speed. An
ammeter is connected in series with the commutator and a
resistor to one phase of the power supply. As the rotor lags
behind the revolving field the pointer of the ammeter oscill-
ates at a frequency prOportional to the slip. This again
depends upon time and a person to count the oscillations.
Another slip meter censists of a conical cylinder which

rotates with the shaft of the motor in question and at the

same speed. On an axis parallel to the conical cylinder and
pressing against it is a wheel of the same diameter or slight-
ly larger than the small and of the cylinder. This wheel
carries a rotating commutator as described above. The con-
neotions are as above described.. In Operation the speed of
the wheel is increased by moving it towards the large end of
the cylinder until the ammeter ceases to oscillate. The
distance that it has been moved is graduated directly in

terms of slip. This last method seems to be the most likely
and is the one adOpted with slight variations.

t

COHPIETE IIISOIBSION OF APPARATUS DESIGNED AND
BUILT IN THE SHOPS.

The slip meter consists of a conical cylinder which

m... the counter-shaft through a. movable disc. The size of

the disc is mch that at one end of the counter-shaft the disc
revolves at the same speed as the conical cylinder while at
the other end its speed is about 200 r.p.m. greater. Thus it
is seen that when the cylinder is connected to the motor under
test the disc may be so placed along the conical cylinder that
the speed of the counter-shaft will be the synchronous r.p.m.
of the motor under test. 0n the end of the counter-shaft is-
placed a commutator having four contacts. Placed opposite
this commutator and in contact with it is placed a double
contact brush which is driven by a 6 pole synchronous motor.
This ccnmutator, through the recessary brushes, closes the
circuit of a two volt storage battery and voltmeter. When
synchronous speed of the cmnter-shaft is obtained the volt-
meter indication remains constant. The position of the disc
indicates the per cent slip on a scale placed under it. As
made, the meter may be used only on 6 pole 60 cycle motors.
For a different number of poles, gears would be necessary to
6118-1180 1"0 equivalent 6 pole speed. An attachment for measur-
ing the period of hunting of a synchronous motor is described

under "Suggestions for Further Work."

The slip meter is shown and described in detail in
the accompaning diagrams and pictures.

It is desirable when measuring the slip to have
synchronous weed at a standard value, so a frequency meter

is a desirable adjunct fer the slip meter.

a.

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ADJUSTMEE TS .

The main adjustment is to keep the scale set so
that when the disc sets against the cylinder at the point of
the same diameter as the disc that the pointer will read
sere slip. As the disc~wears off it will be necessary to
change the poeiti on of the scale. This may be done by con-
necting the conical cylinder to a 6 pole synchronous motor
and setting'as described for testing motor. Then move scale
to zero reading on pointer. The tap or on the cylinder re-
maining constant the readings or divisions will remain the

same with reqiect to the zero point.

PRECAUTIONS.

Do not attempt to move the disc along the cylinder
when it is standing still as doing so will produce a flat
place on the disc which will cause a pound and also inter-

fere with accuracy.

SUGGESTIONS FOR FURTHER soar.

For measuring the period of hunting of a synchronous
motor a method has been devised and is offered as a suggestion.
It is as follows: On the shaft of the synchronous motor which
runs the contactor place a definite poled statorvexcited from
the A. 0. line. Inside of this place a rotor of the cane
number of poles, this is rotated by the counter-shaft.

By transformer action there will be a voltage in-
dueed in the rotor windings. The rotor and stator rotating
at the same speed in the same direction the poles will rennin

stationary. vith respect to each other, as in diagram.

 

 

A constant e.m.f. will be induced in the rotor-windings. and
with a voltmeter it is possible to read the e.m.f. induced.
As soon as the synchronous motor in quesii on begines to hunt
and the rotor deflects by an angle at the reluctance of the
flux path is increased and the induced e.m.f. changed. By

noting the oscillations of the voltmeter needle one can
determine the period of hunting.

Another mggestion is inst the cylinder be put in
a cradle and balanced at speeds between 1000 and 1800 r.p.m.
or obtain a fiber or composition cylinder which could be
turned down to suitable dimensions. This would be the most
desirable, as the fiber is lighter. It would also be
desirable to have a ball-bearing thrust to move the disc
along the cylinder.

10

RESULT OF TEST.

Slip as measured by Slip as measured
Stroboscopic Method. by Slip Meter.
R.P.n. R.P.M.
88 BO
19 21
12 11.5
8.2 8

The stroboscOpic method was used in checking the
accuracy of the slip meter. Sectors were painted on the end
of the pulley of the motor under test and an arclight, con-
nected to the same generator as the motor, was used to
illuminate the revolving sectors. By this method the clip
was easily found for low values.

The motor was loaded by means of a Pony Brake which
did not give a very constant load. For this reason the slip
varied some during the time of reading, and this accounts for

most of the discrepancy in the results.

I.

II.

III.

V.

VI.

VII.

VIII .

ll

BIBLIOGRAPHY.

Accurate method of Measuring motor Slip.
Electrical World «- Aug. 25, 191?.

Stroboscopic method of measuring Induction motor Slip.
Electric Journal ~ July, 1913.

A New Slip Indicator.
London Electrician - July 22, 1911.

Measuring the Slip and Speed of Induction Motors,
by Charles A. Perkins.
Electrical World - Oct. 31, 1908.

The Slip of An Induction motor, by B. F. Bartey.
Electrical World - April 22, 1905.

Measuring the Slip of Induction Motors.
Electrical World - June 10, 1905.

A Simple Device for Finding the Slip of An Inducti on Motor,
by Charles A. Perkins.
American Institute of Electrical Engineers - June, 22, '05.

Slip Meters.

E.M;F. Electrical Year Book, 1923 - page 816.
Measurement of Slip

E.H.F. Electrical Year Book, 1923 - page 573.

BIBLIOGRAPHY - (Continued)
IX. Slip Measurement
Electrical measurements in Practice, by Farmer - pp. 289-293

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