Improvements in Devices for Starting Electric Induction Motors.
Year: 1899

page_text: Date of Application, 7th June, 1899-Accepted, 5th May, 1900
COMPLETE SPECIFICATION.
Improvements in Devices for Starting Electric Induction Motors.
We, THE BRITISH THOMSON HOUSTON COMPANY, LIMITED, of 83, Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature of this
invention (which has been communicated to us from abroad by The Union Elek-
tricitäts-Gesellschaft and Arnoldo P.. Zani, .of Berlin, Germany, Electrical En-
gineers, and in what manner the same is to be performed to be particularly
described and ascertained in and by the following statement:
-
An electric induction motor requires a low resistance in the induced circuits
to operate efficiently. If in starting such motors the inducing circuits are directly
connected to full supply voltage they will take a very large current and the torque
will be small, owing to the reaction of the secondary induced currents and their
displacement out of phase with the field interlinked with. both circuits.
The different methods of starting induction motors, to have moderate starting
currents with the highest possible torque, can be divided into two categories.
The first method acts on the primary or inducing circuits, either by change of con-
nections, or by a decrease of the voltage in starting. The second, acts on the
secondary or induced circuits, either by changes in the connections of the winding
or by the introduction of the resistances in series with the induced circuits;
such resistances being cut out or short circuited as the motor increases in speed.
lf the induced circuit is the rotor, the resistance can be either inside the rotor
or outside of it, and connected to the terminals of the different phases of the
rotating part by means of slip rings. The above variation of connections in the
secondary circuits are made either automatically by the rotation of the rotor
itself, or by hand. It is evident that in all cases, a change is needed in the
electrical connection of a circuit when actually carrying current; thus the use
of sliding or surface contacts is necessary, and the possibility of sparking is al-
ways present. Aside from the well known objectionable features of sliding con-
tacts both electrically and mechanically, the danger of sparking is more objection-
able, considering that such motors are often used in places where, from the con-
ditions of the surrounding atmosphere, there is danger of explosion.
With the present invention all the electrical connections of the induced circuits
remain unchanged, the magnetic condition only of a part of the induced circuit
being changed; the moving part being of solid or laminated pieces of magnetic
material.
Fig. 1 shows the second.ry or induced winding of an induction motor which,
for simplicity, has been represented with a threephase Y-connected winding,
A', A, and A³ being the three phases. The terminals of the three phases are
connected directly or by means of slip rings to the three starting resistances R,
thus closing the circuit.
The spools D, of high self induction (obtained by the use of a magnetic circuit
of very low reluctance) and of very low electrical resistance compared with the
resistance of the induced winding, are severally connected in parallel with the
resistances R, in the manner indicated.

page_text: Fig. 2 shows a second method of connection practically equivalent to that of
Fig. 1.
In starting, owing to the high impedance of the coil 1), very little current
flows through it, the greater part going through the resistances R. The motor
will thus start as a motor with resistance in the induced circuit, taking a small
current in the inducing circuit and giving a high starting torque. The intro-
duction of the spools D modifies but slightly the intensity and phase relation, of
the induced or secondary currents.
Instead of cutting out or short circuiting the resistances R after the motor is
well started, as has been done until now, the self induction of coil D is decreased,
and the reluctance of its magnetic circuit increased in some suitable way, as for
instance by withdrawing the iron from the spools in part or entirely thus leaving
the resistances R shunted by circuits of very low electrical resistance and practic-
ally negligible self induction; and hence obtaining, for every practical purpose,
a short circuited induced winding.
If the magnetic material employed in D is not laminated eddy currents will
be generated therein by the inductive action of the secondary currents themselves
induced by the primary currents of full frequency. The impedance D will thus
be decreased and large currents will flow through the circuit, but, as these cur-
rents are in phase with the currents in R and with the electro-motive force im-
pressed in the secondary or induced circuit of the motor they will be active in
producing torque. This fact permits a reduction in the value of the resistance
in the resistance R.
Having now particularly described and ascertained the nature of our said in-
vention, (as communicated to us from abroad) and in what manner the same is
to be performed, we declare that what we claim is:—
1
In electric induction motors the use of auxiliary circuits in series and closing
the main secondary or induced circuits, so electromagnetically proportioned that
the induced secondary current in virtue of the relative higher periodicity of the
induced current during starting is limited to the suitable value of intensity and
phase relation for obtaining the desired torque in starting, and means for changing
the magnetic reluctance of these auxiliary circuits as the motor increases in
speed to permit the main secondary circuits being short circuited with but a
slight increase in their resistance and reactance in the running condition of the
motor.
Dated Is On O1 June 16.
THE BRITISH THOMSON-HOUSTON CO. LTD.
W. MARTINDALE,
I. THEO. MERZ,
} Directors.
ALFRED CLEMENTS,
Assistant Secretary.