Control of electric motors.

08/30/2024

Practice 2:
Continuity Testing and Interlocking System.

Universidad Autónoma de Querétaro.

San Juan del Río.

Career:
Electromechanical engineering.

Subject:
Control of electric motors.

Student:
Ismael Pacheco Villeda - 308477.

Professor
Dr. Juan Pablo Amézquita Sánchez.

August - 30 - 2024.

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I. OBJECTIVE.
Create, simulate and implement the force and control diagrams for a three-phase electrical system, as well
as make the necessary adjustments to be able to create an enclave in the system that keeps it active.

II. INTRODUCTION.
A three-phase AC motor interlocking system is a crucial safety and control mechanism designed to manage
the operation of motors in industrial environments. It ensures that motors do not start or continue running
under unsafe or undesirable conditions, thereby preventing equipment damage, accidents, or operational
failures. The interlocking system coordinates the sequence of motor operations and prevents conflicting
actions, such as starting the motor during an overload or a fault condition.

Key components of this system include contactors, thermal relays, and push buttons. Contactors are used to
switch the motor on and off by controlling the power flow to the motor, while thermal relays protect the
motor by detecting overheating or excessive current and disconnecting the circuit if necessary. Push
buttons—typically used for start and stop commands—allow manual control of the motor, ensuring that it
can only be operated under safe conditions.

The system works by integrating these elements to prevent actions such as the simultaneous operation of
conflicting motors, starting a motor under an overload condition, or bypassing safety mechanisms. In case
of an emergency, the interlocking system, through emergency stop buttons, can shut down the motor
immediately. This arrangement provides comprehensive protection for the motor, ensuring operational
safety and reliability in industrial settings.

III. MATERIALS, INSTRUMENTS, TOOLS AND RESOURCES.

A contactor is a special type of relay used to make or break an electrical circuit. They are most commonly
used with electric motors and in lighting applications. Using a contactor provides a level of isolation, away
from the high electrical currents associated with those applications, protecting workers and equipment [1].

Fig. 1. Contactor.
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The thermal relay is a device designed to protect motors and electrical installations from overloads and
excessive heating, which can cause damage to the equipment and also cut off and damage the supply. It is a
way to keep your motor or electrical installation in good condition, prolonging its life and providing peace
of mind to your home [2].

The operation of the electrical thermal relay is based on a bimetallic plate (usually iron, nickel and brass are
used). This plate identifies possible internal fluctuations in electricity and temperature, as it reacts by
expanding [2].

When the plate expands, the electric current is automatically cut off. Until the plate cools down and
recovers its position, the supply is not enabled again [2].

Fig. 2. Thermal relay.

Materials:
● Banana-Alligator clips.
● Banana-Banana clips.
● 1 contactor.
● 1 Thermal relay.
● 1 Push Button NO.
● 1 Push Button NC.
● 1 multimeter.
● PC.

Software:
● SolidWorks Electrical.
● CADe_SIMU.

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IV. METHODOLOGY.
Simulation:
● Design the control system to generate the interlocking system.
● Recreate the force diagram.
● Verify the operation of the system.

Physical assembly:
● Check the continuity of the cables to be used.
● Check the continuity of the components and their connections.
● Recreate the control system with the real elements.
● Check the operation of the system.

V. EXPERIMENTATION.
Simulation:

The simulation section is divided into two parts: the CADe_SIMU part and the SolidWorks Electrical part.
The first part shows the control diagram for the interlocking system and the force diagram in operation.
In SolidWorks Electrical, the force diagram is shown to adapt the elements and connections to the
specifications of the standard used (IEC).

CADe_SIMU:

Fig. 3. Control diagram for the interlocking system. Fig. 4. Force diagram for the interlocking system.
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Figure 3 shows the control diagram for the interlocking system. Where F represents the thermal magnetic
switch that protects the power line. The NC push button (Stp) represents the STOP button in the system,
while the NO push button (Str) represents the START button. The normally open contact (k) represents the
normally open contact of the contactor, and the coil (k) represents the contactor coil.

In Figure 4 you can see the force diagram controlled by the control diagram in Figure 3, where F is the
thermal magnetic switch, k is the contactor, rel the thermal magnetic relay and M the test motor in the
simulation.

SolidWorks Electrical:

Fig. 5. Force diagram in SolidWorks Electrical.

Figure 5 shows the force diagram adapted to the symbols and colors dictated by the standard provided by
the IEC, with the color for neutral being blue, and for the different lines being brown, black and grey.

In addition to this, the elements present in the system are also shown, such as the thermal magnetic switch
Qt, the contactor K1 and the thermal relay RT1, in addition to the motor M.

It is important to mention that these elements are designed for a three-phase system, however, the control
system would be the same for a 2-phase or 1-phase system, simply the elements used must be selected
under these criteria, in addition to the current and voltage present in the system.
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Physical assembly:

To verify the correct operation of the system, the interlocking system was first recreated with a 5V relay
and two pushbuttons. I simulated the contactor and its coil:

Fig. 6. Recreation of the interlocking system with 5 V relay.

Figure 5 shows the recreation of the interlocking system, where the relay represents the contactor and its
coil, in addition to adding two pushbuttons, one for the STOP button and another for the START button.

Once this system has been created and its operation confirmed, the connections for the real system are
made, following the diagram shown in the software simulation parts:

Fig. 7. Assembly of the real system.

Figure 7 shows the assembly of the real system, where two reference lights were added to know in which
state the circuit is operating, as the diagram shows, only the use of a contactor was necessary.
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VI. RESULTS.
Simulation:

Fig. 8. Control and force diagram in simulation.

Fig. 9. Control and force diagram in simulation, Str pressed.

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Fig. 10. Control and force diagram in simulation, Stp pressed.

Fig. 11. Control and force diagram in simulation, rel pressed.

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Physical assembly:

Fig. 12. Interlocking system with 5V relay.

Figure 12 shows the verification of the operation of the interlocking system using a 5V relay, which kept
one of its contacts closed when the START button was pressed, this served to confirm that the circuit was
set up correctly.

Fig. 13. Circuit not operating.

Fig. 14. Circuit with START button pressed.

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Fig. 15. Circuit with STOP button pressed.

VII. DISCUSSION.
In order for the practice to be carried out correctly, it was important to define the force diagram that
describes the system, especially the protection elements for the motor and the power line. This was because
this diagram was the basis for establishing the control diagram, which has some differences with the force
diagram.

In the control diagram, it was necessary to correctly select the elements to be used, for example, the NC
and NO buttons, since if they were placed the other way around, the system would remain off even when
the START button was pressed. The same goes for the auxiliary contact of the contactor, since if an NC
contact was selected, the system would never stop operating, which is a problem.

For the simulation, it was important to define and name each element of the power and control system with
a specific letter or word, since within the software they are related to each other in order to perform the
correct operation of the elements, in addition to the fact that if they were not related to these labels, the
circuit does not operate properly.

For the real system, the continuity of all the wires, contacts and connections of the different elements to be
used was checked, in order to ensure that the connections were useful. For example, some banana-banana
cables had continuity faults and were discarded, and some contactor terminals were not properly fastened
and the wires did not make good contact, so they were tightened to avoid open circuit faults.

Finally, in the real system, connections that were exposed to the air had to be protected with insulating tape
to prevent them from touching unwanted elements, generating short circuits or accidents when energizing
the system, this guaranteed the safety of the system and of the students who operated the elements.

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VIII. DESIGN CONSEQUENCES.

The circuit generated for this practice only allows the control of turning on and off a three-phase motor in a
single direction of rotation, since it does not have elements that provide a change in the direction of rotation
of the motor. In addition to this, the motor is protected only by the thermal relay in its three phases, since
the thermal magnetic switch is designed to protect the electrical line and not the motor.

Another point of the created system is that the design allows the START button to be pressed more than
once without affecting the operation of the circuit, which prevents failures in it. The same occurs if the
STOP button is pressed more than once or even at the same time as the START button, the system is not
negatively affected and can continue operating normally.

In addition to this, it is important to consider that the contactor to be used must have at least one NO
auxiliary contact to be able to build the proposed circuit, since without this contact, the circuit is impossible
to build as shown in this document.

IX. CONCLUSIONS.

An interlock system for a three-phase motor with a contactor and its normally open auxiliary contact is an
effective and safe solution to control the start and stop of the motor, ensuring that it remains on without the
need to hold down a push button. The interlock is based on the activation of the contactor by means of a
start push button, which, when closed, supplies power to the contactor coil. This contactor, when activated,
also closes its normally open auxiliary contact, keeping the coil supplied even when the start push button is
released, ensuring that the motor continues to run.

The thermal relay, integrated into the circuit, provides protection to the motor against overloads. This
device monitors the current passing through the motor phases and, in the event of detecting a prolonged
overcurrent, disconnects the contactor coil, interrupting the power supply to the motor. In this way,
overheating and possible damage to the motor are avoided, guaranteeing its safety and prolonging its useful
life. This interlocking system, in conjunction with the thermal relay, offers safe and effective control in
industrial applications where continuous motor operation is required with protection against possible
electrical faults.

The interlocking system for a three-phase motor with contactor and thermal relay has several advantages,
but also has certain limitations:

Current consumption in the contactor coil: The system requires energy to keep the contactor activated,
since the coil must be constantly energized during motor operation. This consumption is small compared to
that of the motor, but it can become significant if the system is in operation for long periods or if many
contactors are used in an installation. In addition, in the event of power supply failures, the motor would
stop abruptly.

Motor rotation reversal: The basic interlocking system does not contemplate the reversal of motor rotation.
If it is desired to change the direction of rotation of the three-phase motor, an additional circuit with two
contactors and a cross-interlocking system that reverses two of the motor phases is required. In this case,
simply including a normally open auxiliary contact is not sufficient to change the direction of rotation.
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X. NORMATIVE REFERENCES.

The IEC 60446 standard governs the color codes for electrical cables in three-phase systems, widely
applied in industrial and electrical installations to ensure safety and clarity. In a three-phase, four-wire
system, the standard defines the following wire colors:

● Phase 1: Brown.
● Phase 2: Black.
● Phase 3: Grey.
● Neutral: Light Blue.
● Protective Earth (Ground): Green/Yellow.

These color codes are adopted in various countries, including the European Union, the United Kingdom,
Argentina, China, and others​.

Importance of Cable Colors:

Correct identification of phase, neutral, and ground wires is critical for electrical safety and efficiency,
especially in three-phase motors, where an incorrect connection could lead to significant equipment
damage or injury.

Motor Protection Standards: IEC 60947-4-1:

The IEC 60947-4-1 standard outlines the requirements for protective devices like contactors and motor
starters in three-phase motors. These devices provide essential functions, including:

● Overload Protection: Prevents motors from running beyond their capacity, safeguarding against
overheating.
● Short-Circuit Protection: Shuts down the motor in the event of a short circuit, protecting both the
motor and the electrical installation.
● Phase Failure Protection: Detects when one or more phases are missing or unbalanced, preventing
potential motor damage.

Motor starters often include thermal or electronic overload relays that monitor the motor's status and
disconnect the power in hazardous conditions.

Interlocking Systems:

Interlocking systems are also critical in motor protection, ensuring that certain operations cannot occur
simultaneously, thus preventing dangerous malfunctions. For example, interlocks in a motor's star-delta
starter ensure that both starting methods do not activate at the same time, protecting the motor from
damage.

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General Electrical Protection: IEC 60364:

The IEC 60364 standard covers low-voltage electrical installations, focusing on the safety of both
equipment and users. It includes guidelines for:

● Residual Current Devices (RCDs): Protect against earth leakage currents, which can cause electrical
shocks and fires.
● Circuit Breakers and Fuses: Protect electrical systems from overcurrent, which could cause
equipment damage or fire.
● Earthing Systems: Ensure that stray currents are safely directed to the ground, reducing the risk of
electric shock.

REFERENCES.

[1] c3controls. (2024). Los conceptos básicos de un Contactor y diferentes tipos de Contactores. c3controls
line of IEC Contactors are ideal for motor, actuator, solenoid, and other power switching applications.
https://www.c3controls.com/es/documento-tecnico/los-conceptos-basicos-de-un-contactor-y-diferentes-tipo
s-de-contactores/

[2] ENDESA (2023). Protege tu instalación eléctrica con el relé térmico.

https://www.endesa.com/es/blog/blog-de-endesa/luz/proteger-instalacion-electrica-rele-termico

[3] International Electrotechnical Commission (2024). IEC 60446:2007. Basic and safety principles for
man-machine interface, marking and identification - Identification of conductors by colours or
alphanumerics. https://webstore.iec.ch/en/publication/16226

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