Control of electric motors.

10/19/2024

Practice 5:
Reduced voltage starting with resistors on a three phase motor.

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.

October - 19 - 2024.

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I. OBJECTIVE.
Create and simulate the force and control diagrams for a three-phase motor at reduced voltage with the help
of calculated resistors, contactors and timer, in addition to physically implementing the control system, to
reduce the current consumed during motor start-up.

II. INTRODUCTION.
Reduced voltage starting in three-phase motors is a technique used to reduce the starting current, protecting
both the motor and the electrical network from high current peaks. One of the simplest ways to implement
this starting is by using resistors in series with the motor windings. By including these resistors, the voltage
applied to the motor at the time of starting is reduced, which in turn reduces the starting torque and current.

The system is complemented by the use of contactors and timers. Initially, a contactor connects the resistors
in series with the motor, thus limiting the voltage at start-up. As the motor accelerates and stabilizes its
speed, a timer acts to disconnect the resistors and connect the motor directly to the full power line through
another contactor. This stepwise process allows the motor to reach its rated speed gradually, reducing
mechanical and thermal impact.

Advantages:
● Reduction of the starting current, avoiding peaks that could damage the motor or the electrical
network.
● Simplicity of the system, easy to implement and maintain.
● Relatively low cost compared to other methods such as star-delta starting or variable frequency
drives.
● Less mechanical stress on the motor and the connected equipment, due to a smoother start.

Disadvantages:
● Energy losses in resistors during start-up.
● Limited efficiency, as it does not regulate current or torque optimally under all conditions.
● Increased heating in resistors, which may require additional dissipation.
● Less control over start-up time and torque compared to more advanced methods such as variable
frequency drives.

Fig. 1. Current-speed, Torque-speed graphics [1].

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III. MATERIALS, INSTRUMENTS, TOOLS AND RESOURCES.

Timer or time-controlled contactor: is an electrical device that allows the switching on or off of electrical
circuits based on a preset time [2]. This type of contactor integrates a timer that determines when a load,
such as a motor, lighting, or any electrical equipment, should be switched on or off without constant
manual intervention [2].

Operation:

The timer works by setting a time interval during which the contactor (an electrically controlled switch)
closes or opens its contacts [2]. Depending on the configuration, the timer can:

● Delay connection (ON delay): The contact closes after a predetermined time from the moment the
activation signal is applied [2].
● Delay disconnection (OFF delay): The contact opens after a predetermined time from the moment
the activation signal is removed [2].
● ON/OFF cycle: Automatically alternates between switching on and off a load in repeated cycles,
according to a time set for each state [2].

Fig. 2. Timer or time-controlled contactor.

Materials:
● Banana-Alligator clips.
● Banana-Banana clips.
● 2 contactors.
● 1 Thermal relay.
● 1 Push Button NO.
● 1 Push Button NC.
● 1 multimeter.
● PC.
● Oscilloscope.
● Ammeter clamp.
● Three-phase motor 220V, 60Hz

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.

Numeric:

● Obtain the motor information.

● Compute the starting current at full voltage.
● Define the limitation of the starting current.
● Compute the voltage per phase and at the motor terminals.
● Compute the voltage drop across the motor resistance.
● Compute the voltage drop across the motor reactance.
● Compute the resistor voltage.
● Compute the resistor impedance.
● Compute the power absorbed by the resistor.

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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 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. Fig. 4. Force diagram.

Figure 3 shows the control diagram required to create the reduced voltage starting system with
autotransformer in a three-phase motor, based on the force diagram in Figure 4, where F refers to the
thermal relay that protects the motor (M); KM1 is the contactor for the direct line, KM2 is the contactor
required for the resistors to operate when starting the motor, R are the resistors and Q represents the thermal
magnetic switch that protects the power line.

In the designed circuit, contactor KM1 starts open, while KM1 starts closed, allowing current to flow
through the resistors that connect to the motor, which reduces the voltage that it initially consumes, which
decreases the maximum current it reaches during the start-up transient.

When the timer reaches the time for which it was programmed, its contacts change, opening the NC contact
and closing the NO contact, this causes KM1 to close and KM2 to open, allowing current to flow now
directly from the 220V power line, which allows all the power of the motor to be used once the start-up
phase has been completed.
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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 gray.

In addition to this, the elements present in the system are also shown, such as the thermal magnetic switch
Q1, the contactors K1 and K2, the thermal relay is F1, in addition to the motor M.

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Physical assembly:

Fig. 6. control stage, connected.

In the figure above the boxes represent the following components:

● White: KM1.
● Pink: KM2.
● Yellow: Thermal relay.
● Blue: Timer.
● Green: Control buttons.

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Numeric:
To carry out the corresponding analyses, some values are taken from the motor data plate:

Fig. 7. Motor data plate.

● Engine power (P): 0.5 Hp ≅ 0.37285 kW.

● Supply voltage (SV): 220 V.
● Rated current (IN): 1.6 A.
● Phases: 3.
● Frequency (F): 60 Hz.
● Speed: 3450 min-1.
● Increase in the number of times the nominal current (NT): 7.42 times.
● Starting current limitation (IL): 70%
● Power factor (FP): 0.88

The starting current at full voltage for the motor in question is given by:

𝐼𝐴 = 𝑁 𝑇 · 𝐼𝑁 (1)

𝐼𝐴 = 7. 42 · 1. 6 𝐴

𝐼𝐴 = 11. 872 𝐴

The start-up current limitation will be 70%:

𝐼𝐿 = 𝐼𝐴 · 70% (2)

𝐼𝐿 = 11. 872 𝐴 · 70%

𝐼𝐿 = 8. 3104 𝐴

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The voltage per phase is obtained:

𝑆𝑉
𝐸 = (3)
3

220 𝑉
𝐸 =
3

𝐸 = 127. 0170 𝑉

Voltage at motor terminals:

𝐸𝑀 = ( )(
70%
𝑁𝑇 )
𝐸𝐿 (4)

𝐸𝑀 = ( 70%
7.42 )(127. 0170 𝑉)
𝐸𝑀 = 11. 9827 𝑉
And also we have:

−1
θ = 𝑐𝑜𝑠 (𝐹𝑃) (9)
−1
θ = 𝑐𝑜𝑠 (0. 88)

θ = 28. 3576°

The voltage drop across the motor resistance is calculated from the triangle relationship of the vector
diagram as follows:

𝐸𝑅𝑀 = 𝐸𝑀 · 𝑐𝑜𝑠(θ) (10)

𝐸𝑅𝑀 = 11. 9827 𝑉 · 𝑐𝑜𝑠(28. 3576)
𝐸𝑅𝑀 = 10. 5448 𝑉
In a similar way that, for the voltage drop across the motor resistance, the voltage drop across the motor
reactance is calculated as follows:

𝐸𝑋𝑀 = 𝐸𝑀 · 𝑠𝑖𝑛(θ) (13)

𝐸𝑋𝑀 = 11. 9827 𝑉 · 𝑠𝑖𝑛(28. 3576)

𝐸𝑋𝑀 = 5. 6914 𝑉
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The resistor voltage is calculated from the trigonometric relationship in the vector diagram:

𝐸𝑅 = (𝐸𝐿)2 − (𝐸𝑋𝑀)2 − 𝐸𝑅𝑀 (14)

2 2
𝐸𝑅 = (127. 017 𝑉) − (5. 6914 𝑉) − 10. 5448 𝑉

𝐸𝑅 = 115. 9057 𝑉

To know the impedance of the resistor, from Ohm's law:

𝐸𝑅
𝑅= (15)
𝐼𝐴

115.9057 𝑉
𝑅=
11.872 𝐴

𝑅 = 9. 7629 Ω

The power absorbed by the resistance will be:

( )2 · 𝑅
𝑃 = 𝐼𝐴 (16)

2
𝑃 = (11. 872 𝐴) · 9. 7629 Ω

𝑃 = 1. 376 𝑘𝑊

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VI. RESULTS.
Simulation:

Fig. 8. Motor initially powered by the resistors lines by pressing Str.

Fig. 9. Motor supplied with full voltage after the timer countdown is complete.
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Fig. 10. Thermal relay protection test.

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Physical assembly:

Fig. 11. Timer during the test.

Fig. 12. KM1 closed and KM1 opened after the timer reached the programmed time.

Numeric:

P = 1.3760 kW

R = 9.7629 Ω

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VII. DISCUSSION.
During the development of the practice, the power part was omitted for the actual assembly, since in the
motor control laboratory, the required resistors needed maintenance to operate correctly, so this stage could
not be added. However, the connections of the control diagram were made, managing to create the start
with the resistors, starting with KM2 closed and then unlocking it and closing KM1.

During the development of the practice, some complications arose that delayed the completion of the tests a
little, one of them was that some banana-banana cables with which the connections were made presented
continuity failures, so they had to be tested one by one to ensure that those used worked correctly. Another
important point was that when the contactor coils were activated, the mechanical knocking caused some
cables to come out of their connection terminals, this due to defects in the cable terminals, so each
connection had to be secured more tightly.

After all these complications, the necessary corrections were made to the circuit elements such as the
cables, in addition to isolating the exposed connections to avoid possible short circuits. With this done, the
diagram shown in Fig. 3, physically implemented (Fig. 6) operated as shown in Fig. 8 and Fig. 9.

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VIII. DESIGN CONSEQUENCES.

Reduced voltage starting with resistors presents a series of consequences and particular characteristics that
affect both the behavior of the motor and the electrical system. Below, I will detail some of the most
important ones:

Characteristics:

● Reduction of the starting current: During starting, the resistors limit the current that flows to the
motor. This helps to reduce the initial impact on the electrical network, avoiding current peaks that
could cause voltage drops or protection trips.

● Reduction of the applied voltage: By inserting resistors in series with the motor, the voltage that
reaches the motor windings is reduced. This makes the motor start more smoothly, reducing
mechanical stress.

● Progressive increase in speed: As the motor accelerates, the resistors are removed from the circuit,
allowing the motor to gradually receive the nominal voltage and reach its normal operating speed.

● Simple control: Using resistors for starting is a simple and relatively inexpensive technique,
making it suitable for applications where basic motor control is required without the complexity of
other methods such as starting with variable frequency drives.

Consequences:

● Loss of energy in the resistors: A disadvantage is that part of the energy is dissipated as heat in the
resistors during starting. This implies a loss of energy efficiency, since that energy is not used to
move the motor, but is lost.

● Greater mechanical wear than in other methods: Although starting is smoother than direct
starting, there may be greater stress on the system compared to other methods such as starting with
variable frequency drives or autotransformers, especially if the resistors are not well calculated.

● System heating: The resistors must be designed to dissipate the heat generated during starting,
which may require additional ventilation elements or cooling systems to avoid overheating.

● Limited use in large motors: For large motors or in applications where speed control during
starting is crucial, resistor starting may not be the best option, as the number of resistors required
may be considerable, and energy losses would be high.

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IX. CONCLUSIONS.

The force and control diagrams for a three-phase motor at reduced voltage were created and simulated with
the help of calculated resistors, contactors and a timer, in addition to physically implementing the control
system, to reduce the current consumed during motor start-up. There were some complications mainly with
the quality of the banana-banana cables used for the connections, which delayed the tests for the real
control system a little; however, these complications were successfully resolved without any major
problems.

In conclusion, reduced voltage starting with resistors in three-phase motors is a simple and effective
technique to reduce current peaks during start-up, protecting both the motor and the electrical network. This
method is characterized by the inclusion of resistors in series with the motor windings, which temporarily
limits the current and torque, facilitating a smoother and more controlled start-up.

Among its main advantages, its low cost and simplicity of implementation stand out, making it an ideal
option for applications where a soft start is required without resorting to more complex systems. In
addition, it significantly reduces mechanical stress on the motor and connected equipment, prolonging their
lifespan.

However, this method has disadvantages such as energy losses in the resistors and lower efficiency
compared to more advanced alternatives such as variable frequency drives. Also, heating of the resistors
can be a limiting factor in certain applications, and control over torque is limited during the initial start-up
phase.

In summary, resistor starting is suitable for systems that do not require precise control of the starting torque,
but that demand a low-cost method to protect the network and motor from overloads at start-up.

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X. NORMATIVE REFERENCES.
IEC 60947 covers resistor-based reduced voltage starting for motors, focusing on control and protection
devices such as switches, contactors and starters. Below is a breakdown of the key features that this
standard regulates for resistor-based starting:

● Utilization Categories: IEC 60947-4 defines several utilization categories depending on the type of
load and its behavior when connected. For motors with resistor-based reduced voltage starting,
categories AC-1 (resistive loads) and AC-4 (squirrel-cage motor starting and braking) are mainly
used. These categories define the devices' ability to withstand currents during starting and
operations under load.

● Overcurrent Protection: The use of resistors implies that the protection device must be able to
withstand the high initial starting current without tripping prematurely. The standard establishes
how the device must be selected to avoid overcurrent faults during starting.

● Switching Capacity: Switching devices must be able to handle both the starting current and
switching operations when the motor is running. Switching characteristics fit into the categories
mentioned and must meet the values ​defined for each type of load.

● Resistors and Current Limiting: Reducing the voltage during starting using resistors helps limit
the starting current and reduces the mechanical stress on the motor. The regulations specify how
these resistors must be sized and placed to meet starting requirements without compromising the
safety of the equipment.

● Safety Considerations: Emphasis is placed on safety measures when implementing this type of
starting, ensuring that protective devices, such as overload relays, are correctly configured to
prevent damage to both the motor and the electrical system.

This approach ensures that reduced voltage starting with resistors is effective and safe, protecting both the
equipment and the electrical infrastructure, complying with international standards.

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REFERENCES.

[1] Chapman, S. J. (2005). Electric machinery and power system fundamentals (1st ed.). McGraw-Hill.

[2] Petruzella, F. D. (2011). Electric Motors and Control Systems. McGraw-Hill.

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

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