Motor overload safety device

A rotor of a motor has a bearing mounted thereon, the bearing having a tab which is urged in one direction by the fluid friction of the lubricant in the bearing, the tab being urged against a spring, the tab actuating a micro-switch. The force exerted by the tab on the spring is a function of the rotor speed. Upon reduction of rotor speed the spring urges the tab rearwardly so that the micro-switch open circuits in turn disconnecting power from the motor which comes to a standstill.

This invention relates to an overload safety device which is responsive to 
reduction of angular velocity of a rotor in a motor which has a rotor 
journalled for rotation in bearings in a stator, and power means to induce 
said rotation. 
BACKGROUND OF THE INVENTION 
Overload devices are commonly used both in electrical and internal 
combustion motors, in electrical motors the overload devices being slow 
heating current sensors which deflect bi-metallic strips, or alternatively 
fuses or circuit breakers which change state upon sharp increase of 
current. Overload devices for internal combustion engines are more 
complex, and usually sense the fuel intake of the engines. 
In both cases, overload conditions can exist which will result in a 
reduction of angular velocity of the rotor (which term includes 
crankshaft) and under some circumstances damage can be effected to the 
engine or motor without the overload devices functioning. Thus for example 
in the case of a vehicle engine, it is necessary for the fuel flow to be 
sufficient to supply short term high power requirements, and it is 
necessary for the overload condition to be sensed above that relatively 
large flow. In the case of an electric motor, a thermal overload device 
will not function to fully protect a motor under conditions of extreme 
cold, for example if the motor is operating under frosty or other very 
cold conditions. 
In most instances however, both internal combustion motors and electric 
motors have overload conditions associated with loss of angular velocity, 
and the main object of this invention is to provide a safety device which 
is responsive to the reduction of angular velocity. 
BRIEF SUMMARY OF THE INVENTION 
Briefly in this invention, the rotor of a motor has a bearing mounted 
thereon, the bearing having an operator which is driven into engagement 
with a switch by the rotational force of the rotor imparted to the 
operator by the friction of the bearing. The operator movement however is 
resisted by the resilient return means, and the resilient return means is 
selected or adjusted such that upon loss of angular velocity of the rotor, 
the operator moves with respect to the switch to such a degree that the 
switch ceases to be actuated. The switch is arranged to control 
disconnection of the power means, for example to operate a solenoid to 
close the flow of disel fuel or to operate a relay to disconnect power to 
an electric motor. Thus under overload conditions a motor merely comes to 
a standstill condition. 
With this invention, the device which is used can be extremely simple in 
construction, mechanical, can be mounted where it is readily accessible 
for maintenance purposes, and the device can be made substantially 
independent of temperature or extreme condition requirements. Its use 
provides additional safety, in that an operator is unlikely to be 
subjected to safety hazard when the motor reaches its standstill state. 
More specifically, in this invention a safety device responsive to 
reduction of angular velocity of a rotor in a motor which has a rotor 
journalled for rotation in bearings in the stator, and power means to 
induce said rotation, comprises bearing means freely rotatable on and 
carried by the rotor, said bearings means having an operator thereon, 
switch means arranged to be actuated by the operator by rotational force 
of the rotor imparted to the operator by friction of the bearing means, 
and resilient return means engaged by the operator and operable to release 
the operator from the switch actuation upon reduction of the bearing 
friction occasioned by reduction of rotor angular velocity, the switch 
controlling disconnection of the power means upon release from switch 
actuation. 
Conveniently the bearing means can be a ball or roller race of a sealed 
type containing a grease of so-called "constant viscosity" type, wherein 
the viscosity of the grease varies by only a limited amount with variation 
of temperature. The operator can conveniently be a cam, but is preferred 
to be a tab or finger carried on the outer race of the bearing. The 
resilient return means can merely be the return spring of a micro-switch. 
The micro-switch can be directly connected to the motor in the case of a 
small electric motor, or can be arranged to operate a relay or solenoid in 
the case of a larger electric motor, or diesel engine or petrol engine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It will be clear to those skilled in the art that the invention is equally 
as well applied to an internal combustion engine, but the embodiments 
described herein are restricted to electric motors. 
In the first embodiment of FIGS. 1, 2 and 3, an electric motor 10 has a 
motor housing 11 containing bearings (not shown) in which are journalled a 
rotor (also not shown). However the rotor carries on its end a boss 12 
which is centrally bored, and retained on the end of the rotor with a 
locking bolt 13. The boss 12 has fitted to it the inner race of a sealed 
ball race 14, while the outer race of the sealed ball race 14 carries a 
sleeve 15 having an operator 16 depending from it, the operator 16 being a 
depending tab. 
As shown best in FIG. 2, the depending tab 16 engages a resilient return 
spring 17, and the resilient return force is adjusted by means of an 
adjusting screw 18. This adjusting screw 18 is threadably engaged in a 
bracket 19 which is fixed to the motor 11 housing by means of screws 20. 
The bracket 19 is also provided with an upstanding stop 22 which limits 
clockwise rotation of the sleeve 15, and carries a switch 23 which in this 
embodiment is a micro-switch, the tab 16 engaging the leaf arm 24 of 
switch 23. 
As shown in FIG. 3, the micro-switch 23 is in the active line 26 of a power 
supply, and its normally open contacts disconnect the windings of motor 10 
from the active line, the neutral line 27 being connected to the motor 10 
through the main power switch as in the ordinary way. When power is 
applied, the "fault lamp" 28 is illuminated (via line 26, n.c. contacts of 
switch 23, lamp 28 and line 27) until such time as the push button 29 is 
depressed to start the motor running. When the motor has reached speed, 
the micro-switch 23 is actuated by the operator 16, and thereupon the push 
button 29 can be released, the "fault lamp" 28 becomes open-circuited by 
the opening of the normally closed contacts of the micro-switch 23, and 
the "safe run" lamp 30 becomes illuminated (via line 26, closed contacts 
of switch 23, lamp 30 and line 27). 
Since use is made of a ball race 14, under normal conditions the friction 
of that race will vary only slightly with increased speed. However the 
ball race 14 is, as said, packed with a grease having relatively constant 
viscosity characteristics, and thus the friction increases rapidly with 
speed due to the fluid friction of the grease being displaced by the 
elements of the ball race. As the fluid friction increases with speed, the 
operator 16 is retained in engagement with the leaf arm 24, but increase 
of load on the motor 10 will cause a reduction of speed, to which the 
rotor current will be approximately proportional over the useful range. 
This can be quite accurately adjusted in this invention, and on overload 
conditions being reached, the speed reduces to a point where the friction 
in the ball race 14 is so small that the operator tab is urged by the 
spring of switch 23 and also the spring 17 away from the micro-switch by a 
sufficient amount that the switch ceases to be actuated, thereby returning 
to the conditions shown in FIG. 3 until such time as the press button 29 
is again depressed. At the same time, the "safe run lamp" 30 extinguishes 
and "fault lamp" 28 illuminates. 
The invention can of course be extended to use on reversing motors, and in 
FIG. 4 there is shown an electrical circuit wherein use is made of two 
micro-switches respectively designated 35 and 36, each similarly 
associated with respective push buttons 37 and 38, and each 
micro-switch/push button assembly actuating a respective electromagnet 39 
or 40 of a reversing relay 41. As in the first embodiment, the "fault 
lamps" are designated 28 and lie across the respective push buttons 37 and 
38, and the "safe run" lamps 30 become illuminated upon energising of the 
respective relay electromagnet 39 or 40. In other respects the wiring of 
the motor 43 is in accordance with known art. 
The third embodiment of FIG. 5 shows the electric motor 10 having a stator 
44 and a rotor 45 carried on a shaft 46 journalled in bearings 47 and 48 
in respective end plates 49 and 50. The sealed ball race 14 is carried on 
shaft 46, and its outer race carries operator 16 on sleeve 15 as in the 
first embodiment, operator 16 cooperating with micro-switch 23. However, 
micro-switch 23 is itself carried by end plate 50, and is partly or wholly 
contained within the housing of motor 10. This affords some protection to 
the safety device. In other respects the arrangement is similar to that of 
FIG. 1. 
The invention of course is not limited to the use of a ball race 14 or any 
other type of race (for example a roller race), but use can also be made 
of a sleeve-type bearing having a high degree of response to fluid 
friction. If desired, use may be made of impeller plates or other types of 
plates within the bearing 14 to make it even more responsive to fluid 
friction. Furthermore, the ring 15 can have associated with it an "over 
centre" spring which causes it to snap into engagement with the 
micro-switch 23 or away from engagement with that switch, thereby 
providing a snap action for the mechanism. It may be noted that for a ball 
race 14 having an outside diameter of about 50 mm, the load imparted by 
the operator 16 can be between 15 and 50 grams, whereas the load required 
to actuate the micro-switch is only about 1/2 gram. It will thus be seen 
that a wide safety range exists, and the device is found to function even 
when the bearing loses some of its grease. However, this is most unlikely 
to occur owing to the very light load imparted to the bearing. 
Various modifications in structure and/or function may be made by one 
skilled in the art to the disclosed embodiments without departing from the 
scope of the invention as defined by the claims.