Electronic speed and slowing down control with intrinsic safety

An electronic system is disclosed for controlling both the ascendant and the descendant speed in vertical machines such as cranes, elevators, etc., in which it is required to lift different loads in a regulated and self-controlled manner, laying aside the traditional mechanical and/or pneumatic control systems. Such control is achieved by virtue of the voltages included in the secondary of the motors of the wound rotor type used in such machines to lift or to bring down different loads, which may vary within a wide range from practically the vacuum to an overload, being possible by such control to select and to maintain both the ascendant and the descendant speed of the load.

BACKGROUND OF THE INVENTION 
The present invention relates to an electronic speed control for a motor 
and more particularly, to such a control for a motor which raises and 
lowers loads. 
The prior art as exemplified by U.S. Pat. No. 3,810,253 discloses an 
electronic circuit for controlling the speed of a three phase motor of the 
wound rotor type, which circuit includes a plurality of electronic 
elements each adapted to regulate the amount of alternating current passed 
therethrough and induced to an associated phase of the rotor. The 
electronic elements provide a controlled short circuiting for all three 
phases of the rotor due to the setup of unidirectional current paths 
related to the biasing of the current. Each electronic element has a gate 
electrode connected to a master control circuit to be enabled thereby and 
thus pass a controlled current in the appropriate amount and at the 
precise time with a substantial suppresion of the Joulean effect. The 
control circuit is integral within a motor housing and rotates with a 
shaft supporting the rotor. Slip rings may be utilized to provide 
electrical connections between the gate electrodes and the master control 
circuit which may be also exterior of the motor housing. 
In order to decrease the time required to raise and lower loads with 
cranes, devices formed by pulleys, hoists, elevators, etc., the industry 
of vertical transportation has developed a multiplicity of braking 
systems, which have used different types of signals controlled by 
micro-switches, photo-cells, tachometers, comparative circuits, etc., with 
the view to achieve an effective braking deceleration on the descendant 
load, such systems constituting, as it might be the case, complex and 
therefore expensive installations. 
All the systems used up to date seek as a main purpose a high efficiency as 
well as a security during its operation, governing the descendant speed 
through all the course of the load. Thus, the prior art requires the 
addition of further equipments and circuits which are complex in their 
structures and designs, to control the descent of the lifted loads, so 
excessively increasing the total price of the equipment. 
SUMMARY OF THE INVENTION 
This invention consists of a circuit to control the speed of three phase 
motors of the wound rotor type, such circuit being modified to control the 
speed of those motors when they are used to bring down a load, using 
thereto the induced voltages in the rotor or secondary of said motor, when 
it rotates in a reverse sense by action of the potential energy contained 
in the previously lifted load. The modification to the control circuit to 
push the load up, is that of the inclusion of a further resistance which 
makes that the discharge signal to the gates of the thyristors forming the 
motor speed control circuit be triggered at a higher induced voltage. In 
this manner, such modified speed control may choose and maintain a 
descendant speed of the load at a predeterminated rate independent from 
the potential energy contained in the load to be brought down. 
OBJECTS OF THE INVENTION 
It is therefore an object of the invention to provide a circuit to control 
in an effective manner a load descent, without modifying in a substantial 
form the circuit to control the speed of a three phase motor of the wound 
rotor type. 
It is another object of the invention to provide a braking circuit to be 
used both, to control in a regulating manner, the ascendant and the 
descendant speed of a load, without adding or substantially modifying the 
ascendant control circuit. 
It is still a further object of the invention to provide a circuit with 
intrinsic security to control the descendant speed of a load.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The circuit for speed control 10 illustrated in blocks in FIG. 4 of the 
accompanying drawing is one of the type shown in U.S. Pat. No. 3,810,153. 
With reference to FIG. 4, the control 10 jointly with R.sub.1 and with the 
master control circuit 12, form the circuit to control the speed of the 
rotor 14 of the three phase motor of the wound rotor type 16, to effect a 
controlled ascension or for any other application in which a precise 
control on the operation speed of said motor, and therefore its developed 
operation is required. 
The above mentioned circuit, in a summary description, is constituted by a 
plurality of electronic elements or thyristors fixedly mounted with 
respect to the motor shaft and connected by their respective cathodes and 
anodes to pairs of the three phases of the rotor of the motor, so as to 
make a number of unidirectional current flow paths, when actuated, between 
all three phases of the rotor. Each electronic element has a gate 
connected to the master control circuit through slip rings. The flow of 
current is controlled by actuating the gates of the electronic elements to 
cause the short circuit of the current flow in the three phases of the 
rotor, all such as described in the above mentioned U.S. Pat. No. 
3,810,253. 
The present invention constitutes an improvement to the circuit of U.S. 
Pat. No. 3,810,253. 
As shown in FIG. 1, a current flow is applied to the gate G of a thyristor 
to attain the conduction between its elements cathode (k) and anode (a) as 
well as the thyristor polarity shown in the elemental circuit represented 
thereon. 
In FIG. 2 there is constituted the relationship existing between the 
voltage E.sub.AK applied between the cathode K and the anode A of the 
thyristor and its gate current flow Ig, in which it is shown that such 
parameters are inversely proportional with each other and that each 
voltage rate existing between thyristor cathode and anode, has in strict 
sense a current flow rate in its gate to conduct to such thyristor. 
Superimposed on FIG. 3 there is the representation of how a controlled 
diode works when it receives a voltage E.sub.R proceeding from the 
secondary of a motor of the wound rotor type in connection with the gate 
current flow Ig applied to said controlled diode, as well as said voltage 
E.sub.R in connection with the speed V developed by the rotor of such 
motor, from which it is deduced that the rotor speed is also inversely 
proportional to the induced voltage in the secondary of the motor. 
As disclosed in U.S. Pat. No. 3,810,253, the speed control of a motor of 
the above mentioned type is attained by considering the inverse 
relationship existing among the parameters of the induced voltage E.sub.R 
in the rotor and its voltage V as well as such induced voltage E.sub.R and 
the triggering signal Ig applied to the gates of the thyristors of the 
control circuit, which constitutes the speed control illustrated in FIG. 4 
by block 10. This situation is illustrated in FIG. 3, in the diagram 
corresponding to axis A-O and O-B. The above mentioned situation controls 
the motor speed as it goes forward and where by such circuit it is 
possible to choose and maintain the motor speed within the range of an 
equal value from zero to substantially the rotative speed of the 
electromagnetic field of the stator, which would be equal if the 
phenomenon so-called "sliding" cosisting in an out-of-phase relationship 
for the magnetic fields of the rotor and of the stator, would not exist. 
While U.S. Pat. No. 3,810,253 obtained a precise control of the "forward" 
speeds of the three phase motors of wound rotor, an operation that 
logically may be applied to lift a load, the problem is now how to control 
the descent of such load by using the same circuit, which problem is 
solved by the present invention, as is readily apparent from the following 
description FIGS. 3 and 4 of the accompanying drawings. 
In FIG. 3 it is shown that in the vertical axis of the drawing therein 
illustrated, the value of the rotor speed 14 may adopt negative speeds, 
that is "Backward" speeds. This situation has been illustrated in the 
diagram of said figure and corresponds to the moment in which the load 
comes down, thereby inducing in the secondary of the motor a greater 
voltage by virtue of the increase in the out-of-phase relationship between 
the magnetic fields of the stator and of the rotor. Under such a 
circumstance, it can still control the trigger of the thyristors and make 
them work by applying to their gates a control signal, which obviously 
shall be less than the one required to operate the rotor in a forward 
manner. 
As it is now apparent, the Forward control of the motor is attained by 
regulating the R.sub.1 value. As precisely depending upon the flow current 
value supplied to the gates of the thyristors of control 10, a broad 
margin is obtained in the speed value, which goes from zero up to 
substantially the rotational speed of the electromagnetic field of the 
stator. To effect the controlling of the thyristors trigger when there is 
a greater tension between its cathodes and its anodes, it will only be 
required to decrease the gate flow current Ig value, which may be attained 
by the addition in the control circuit of the resistance R.sub.2, thereby 
achieving a precise control of the backwards speed of the motor, 
independent of the value of the load to be brought down. 
In the diagram of FIG. 3 it is shown that in the descent of the load the 
induced voltage in the rotor is directly proportional to the rotor 
backward speed since while the stator magnetic fields rotate in one 
forward direction and the rotor rotates backward by action of the weight 
of the load, the out-of-phase magnetic fields of the stator and of the 
rotor increase. This induces a greater voltage in the rotor, which being 
still energized to go forward, rotates in a reverse sense by action of the 
load, thus being enough to increase or to decrease the value of R.sub.2 to 
bring down the load at a pre-determined speed. The resistance R.sub.2 is 
introduced as a control element in the load descent, by only opening the 
small switch B.sub.1 ; as it is shown in FIG. 4, it conducts only flow 
current values in the range of the order or micro-amperes. When the small 
switch B.sub.1 is closed, the resistance R.sub.2 is suppressed and then 
the circuit controls and establishes the forward speed of the motor. 
From the above description it is evident that according to the present 
invention, the circuit to control the motor operation achieves a precise 
control in its application to control the descent of a load in all those 
equipments such as cranes, devices formed by pulleys for raising heavy 
bodies, elevators, etc., which use in their operation a three phase motor 
of wound rotor. 
Inasmuch as the present invention is subject to many modifications, 
variations and changes in details, it is intended that all matter 
contained in the foregoing description or shown on the accompanying 
drawing shall be interpreted as illustrative and not in a limiting sense.