Improved Method of Regulating the Speed of Series Electric Motors.
Year: 1897

page_text: Date of Application, 18th Mar., 1897-Accepted, 29th May, 1897
COMPLETE SPECIFICATION.
Improved Method of Regulating the Speed of Series Electric
Motors.
I, HORACE FIELD PARSHALL, of 83, Cannon Street, in the City of London,
Electrical Engineer, do hereby declare the nature of this invention, and in what
manner the same is to be performed, to be particularly described and ascertained
in and by the following statement :-
My invention relates to the regulation of the speed of series-motors particularly in
electric traction and has for its object to secure the known advantages of a
series-motor in starting or accelerating or retarding the speed of a load such as a
tram-car, that is, to make the effort per ampere a maximum but at the same time to
gain the advantages of a shunt-motor so that the maximum speed of the motor
may be automatically restricted and limited as required and the motor cannot race
when unloaded.
It is well known that the current intake of a series-motor is approximately
inversely proportional to its speed when the fields are not saturated. Thus at
slow speeds the current intake is comparatively great and at high speeds comparatively
small.
The difference of potential on the terminals of the series coil is directly proportional
to the amount of current flowing into the motor and consequently inversely
proportional to the speed of the motor. If therefore a storage battery or equivalent
electrical arrangement be connected in shunt to this series coil owing to the difference
of potential at the terminals of the series coil with heavy loads current will be flowing
into the storage battery or equivalent, and with light loads owing to the diminished
potential at the terminals of the series coil due to the current from the line, current
will flow from the storage battery through the series coil. Thus the tendency of such
an arrangement is to maintain constant current in the series coil of an electric motor
and therefore to maintain a practically constant speed.
According to my invention therefore I place a storage battery or other equivalent
acting electro-motive-force in shunt either to the whole or to a part of the series coil
of an electric motor and in order to maintain the advantages of a series-motor 1 may
use also a resistance (variable or not) in series with the said battery and so applied
that the greatest possible proportion of the current flows through the series coil at
heavy loads, and the discharge through the series coil is restricted by some
pre-determined amount at light load, owing to the greater electro-motive-force
required to charge the secondary battery than that given out in its discharge, and
this also tends to make the action of the motor in starting that of a series-motor. In
electric traction practice where only a prearranged maximum speed has to be controlled,
preferably batteries of high internal resistance are used so that the difference in
voltage as between charging and discharging becomes considerable, in which case
for a given maximum speed the amount of current shunted from the fields in starting
becomes a minimum. This action is obviously augmented by the use of a resistance
in series with the storage battery. When the speed is to be maintained constant
over any considerable range of load batteries of minimum internal resistance and with
no external resistance will be used.
In further description of my invention I will refer to the accompanying
drawings in which the figures diagrammatically represent three ways of carrying
out my invention.

page_text: In all the figures A is the armature of a series-wound motor and M the field
magnet. Current passes through line L from the trolley or other source of electrical
energy T through the main series coil C to earth at E. B is a storage battery, R a
resistance, and. Sa variable switch. In Fig. 1 the battery Bis, in shunt to
the whole of the main series field, the switch S acting on the resistance R for varying
:same
In Fig. 2 the battery is in shunt to a part only of the main series field.
In. Eig. 3 the, resistance. R is omitteil and the battery. is placed. in shunt, to
part of the main series field; the variable switch S acting upon some of the coils
"thereof.
Since the electro-motive-force in the field C varies with the current and speed
of the motor, at slow speeds the battery B is charged and at high speeds, that is
when the prearranged maximum is exceeded, the battery supplies current to retard the
motor and so maintain a constant ratio of speed and this is variable as required by
means of the switch S. I would state that I am well aware that a primary battery of
high resistance might. accomplish, in some degree the, same, result as, the storage
battery above referred to, therefore I do not limit myself, to the use of a storage
battery.
Having now particularly described and ascertained the nature of my...said: inven-
tion, and in what manner the same is to be performed, I declare that: what I
claim is;
The hereindescribed, method of, regulating or limiting the speed of electric motors
which consists, in the use of a storage battery or equivalent acting,electro-motive-force,
in, shunt, to the whole, or to: a part of the series.coil, of, an electric, motor, and
either with or without a resistance (variable or not) in series therewith, substantially
2.set forth
Dated this 18th day of March 1896.
For the Applicant,.
J. C. CHAPMAN,
70 Chancery Lane, London, W.C., Chartered Patent Agent.