Validation of Electrical Noise of a DC Motor through Controlled Varistor Cracking: An Experimental Study

The varistor is an electronic component that protects the DC motor’s circuitry from electrical noise or transients that can cause damage. It works as a voltage-dependent resistor that can change its resistance according to the applied voltage. Once the voltage surpasses a specific threshold, the varistor conducts and directs the excess voltage away from the motor’s circuitry. In small DC motor manufacturing, ring varistors are vital for reducing electrical noise, minimizing spark-induced damage to the commutator and brush, and extending the motor’s lifespan. Additionally, they prevent damage to electronic parts in the customer’s mechanism set. The objective of this study is to investigate the impact of varistor cracks or chips that may occur during the soldering process of varistors to the commutator. To confirm the occurrence of cracks or chips, intentional damage will be inflicted on the varistors. The study aims to determine how the presence of cracked or chipped varistors affects the electrical noise produced by a DC motor during its operation. The resulting spark was observed through an oscilloscope, and it was found that the effect could be substantial, up to 5 to 10 times the rated voltage supplied to the motor. In the next phase of this study, further tests will be conducted on motors without varistors to provide a comparison.


Introduction
A varistor is a type of electronic component used to protect electronic devices, including DC motors, from transient overvoltages.It is a voltage-dependent resistor that can provide a low impedance path to ground in response to high voltage spikes, thereby protecting the sensitive components of the motor from damage.When a varistor is connected in parallel to a DC motor, it can limit the voltage across the motor by shunting excess voltage away from it, which helps prevent electrical noise, voltage surges, and voltage spikes from damaging the motor.This can help to prolong the lifespan of the motor and improve its overall performance.Cracking a varistor that is connected to a DC motor can cause the varistor to fail or become damaged, which can result in a loss of protection against transient overvoltages.As a result, the DC motor may become more susceptible to damage from voltage surges, electrical noise, and other transient voltage threats.Cracking the varistor can cause a partial or complete breakdown of the material within the varistor, which can result in a permanent decrease in its ability to suppress transient overvoltages.If the varistor is completely destroyed, it may cease to function altogether and offer no protection to the DC motor.In this experiment, the authors were purposely cracking the varistor and confirming the effects of voltage surge on DC motors.
Varistors can also be categorized based on their physical differential, such as axial lead varistors, radial lead varistors, surface mount varistors, and strap varistors etc.In this experiment, the varistor used in DC motor is a type called Ring Varistor [1] as shown in Figure 1.

Figure 1. Ring varistor
DC motor used in this experiment is a small size motor normally used in automotive applications dimensioned of diameter 12mm x length 20mm [2] shown in Figure 2. A small DC motor is a type of electric motor that is relatively compact in size and designed to produce rotational motion through the use of direct current (DC) electricity.These motors are commonly used in a wide range of applications, including in consumer electronics, toys, appliances, and automotive systems.Small DC motors typically have a simple design and consist of a rotor, a stator, a commutator, and brushes.The rotor is the rotating component of the motor, while the stator is the stationary component that surrounds the rotor.The commutator is a component that allows the motor to change the direction of the current flowing through the coils in the rotor, while the brushes make contact with the commutator and conduct the electricity that powers the motor.Small DC motors are available in various sizes, power ratings, and configurations, and can be designed to meet specific performance requirements.They are valued for their efficiency, reliability, and ease of control, and are used in a wide range of industrial and consumer applications.

Figure 2. M1N10 motor
This paper provides a detailed description of the experimental procedure used to confirm electrical noise in DC motors, which involved dismantling the motor, cracking the varistor, and reassembling the motor.Section 3 outlines the methodology used for this purpose, while the results of the experiments are presented in Section 4. The paper concludes with a discussion and conclusion in Section 5. Additionally, the paper summarizes previous studies related to DC motors and electrical noise in the following section.

Literature studies
This section aims to summarize previous papers related to varistors and DC motors, highlighting the importance of utilizing varistors to reduce electrical noise and achieve optimal DC motor performance.
In his paper, Y.T. Cho [3] conducted a study on small DC motors that are widely used for their low cost and compact structure.Despite their simple design, the sources of motor noise and vibration can be complex.The study aimed to identify these sources and visualize them across a wide range of frequencies.Near field sound pressure measurements were used to reconstruct the particle velocity of the motor.Additionally, the study measured the particle velocity of a stationary motor due to an impact hammer and motor noise at different rotational speeds.By utilizing these three methods, the study successfully identified the sources of motor noise with accuracy.However, the study did not propose any methods for reducing motor noise, with the common approach being to attach a varistor inside the motor circuits.P.M. James, et.al. [4] in their research on using Broadband KuTEM Omni-Cell for Small DC Motors for a Low-cost Filter solution applied two different motor filter designs which are used to limit the amount of unwanted noise that passes through the motor.The first design consists of 7 components, including 2 inductors, 2 X-capacitors, 2 ferrite beads, and a 0.47 uF Cap-Varistor, all of which work together to bypass the noise to ground and clamp it to 14 Volts.The ferrite beads provide high impedance at the frequencies of the unwanted noise, while the Cap-Varistor absorbs the noise and dissipates it as heat.The second design is a 5-component network that uses 2 ferrite beads and a 0.47 uF Cap-Varistor to clamp the noise to 14 volts, and 2 Y-capacitors to bypass the remaining noise to ground.Overall, both filter designs are used to minimize electrical noise and ensure optimal performance of the motor.
In F. Diouf's study on Wideband Impedance Measurements and Modeling of DC Motors for EMI Predictions [5], suppression filters were used to prevent arcing during commutation in a specific motor.The suppression filters were resonant and composed of a cell containing a capacitor, varistor, and inductance.The varistor acted as a resistor, and the impedance behavior of the filter changed with frequency.At very low frequencies, the resistor dominated the impedance behavior, while at higher frequencies, the impedance of the cell became capacitive.At frequencies ranging from 10-350 kHz, the impedance of the capacitor dominated, and the filter behaved capacitive.At higher frequencies, the inductance's impedance dominated, resulting in the filter's impedance becoming inductive.The filters were placed between the brush and voltage supply input and played a vital role in preventing arcing during commutation.I. Oganezova et al. [6] concluded in their paper that the proposed EMC model for low-voltage DC motors utilizes a time-varying circuit topology that considers all major motor components, such as brushes, coils, commutator segments, iron core, and chassis.The simulation setups are suitable for analyzing the frequency response of a single-speed wiper motor.By predicting the noise generated by the DC motor, the EMC model enables the analysis of prototype designs for conductive and radiation disturbance at various stages of motor design, including the use of embedded filters.Simulation analysis can aid in the identification of the best filter elements for noise reduction and optimal motor performance.The researchers used three different models in their simulation: one without a filter, and two with filters.The first filter used in the simulation was a three-component design called the LC filter, which included two 3.4 uH inductors to restrict the amount of noise passing through, and one 0.2 uF capacitor to redirect the noise to the ground and motor case.The second filter used was called the LV filter, which replaced the capacitor with a TNR5V330K metal oxide varistor.Regrettably, based on the authors' understanding, no research paper offers a comprehensive experimental analysis of the influence of varistor cracks on electrical noise in small DC motors.

Instead of electrical noise reduction, the varistor also being effectively use in Electromagnetic
Interference filtering as reported in 2013 by Q. Hou et al [7].They conducted a study on electromagnetic interference (EMI) in window lifter motors, which are considered a potential source of EMI in automobiles.The study involved an analysis of the principles of window lifter motors and the mechanisms by which EMI is generated.Following this, the motor's EMI characteristics were tested and analyzed, and the authors proposed EMI suppression methods.The study compared two filter methods -one involved adding a varistor and inductance filter to the original X2Y filter.The designed filter was found to be more effective in suppressing EMI, with conducted emission and radiated emission levels found to be lower than the second level limits of CISPR 25-2007+.

Test Methodology
The experiment of this overvoltage from a faulty varistor DC motor begin by disassembling the armature assembly from the DC motor and removing the bracket assembly from the frame assembly.Before proceeding further, it is better to identify the three main components of a DC motor.
1. Frame assembly: It is the outer casing or housing of the motor, which encloses all the other components.It provides mechanical support and protection to the motor's internal parts.The frame assembly typically consists of a metal or plastic frame, bearings, and other support components.2. Armature assembly: It is the rotating component of the motor, which contains the coils of wire that interact with the magnetic field to produce torque.The armature assembly typically consists of a varistor connected to a commutator, windings, laminations, and a shaft.It rotates inside the frame assembly and is connected to the motor shaft.3. Bracket assembly: It is the part of the motor that holds the armature assembly in place and allows it to rotate freely.The bracket assembly typically consists of a metal bracket, or bearings, a pair of brushes and terminals.It is mounted inside the frame assembly and supports the armature assembly.
Create a single line crack or a small chip in the varistor, then connect a magnet wire from one of the commutator risers to the shaft and solder it in place.The connection shown in Figure 3 works in the following way: first, when a brush is connected to a rotating commutator segment on one side of the segment, there is a significant voltage difference that can cause sparks.However, the over-voltage is absorbed by the varistor, reducing the spark.Secondly, by short-circuiting to the shaft, the voltage difference is transferred to the shaft and then to the frame assembly, where it is collected by the probe and displayed on the oscilloscope.

Figure 3. Short-circuit connection
After completing these steps, reassemble all of the parts, including the frame assembly, armature assembly, and bracket assembly, following the procedures below.
Step 1. Connect the motor to the power supply as shown in Figure 4.Then, attach one probe of the oscilloscope to the motor's body and the other probe to the motor's terminal.
Step 2. Adjust the DC power supply voltage to match the motor's rated voltage and turn it on to start the motor.Step 4. Disassemble the motor and remove the armature assembly.Make another crack on the varistor to observe any increase in the spark peak on the waveform.Keep in mind that this will cause the varistor to lose its ability to absorb over-voltage or sparks.
Step 5. Finally, compare the results with those obtained from a working varistor.This experiment is useful for validating new varistors introduced in the motor's design or production process.It can also be used to establish a limit for over-voltage inspections in the production line to prevent faulty varistor motors from being sent to customers.

Result
Table 1 presents the recorded peak waveform results from measuring electrical noise at a motor rated voltage of DC 2.0 V.The measurements were taken under different conditions, namely no crack on the varistor, one crack, two cracks, and three cracks.The data in the table represents the average data obtained from ten repetitions of each initial condition.The intensity of the spark will be greater with a higher supply voltage to the motor, reaching up to 5 to 10 times the voltage supplied in the case of a completely faulty varistor.This can be verified by testing a motor that does not contain a varistor.
An example for single crack condition waveform recorded from the oscilloscope is depicted in Figure 6.
Figure 6.One varistor crack waveform

Discussion and Conclusion
The functionality of a varistor can be confirmed not only by examining the motor waveform but also by its E10 characteristic, which relates to its energy absorption capability.The E10 value indicates the maximum energy that a varistor can absorb without failing or sustaining damage, and it is typically expressed in joules (J).When selecting a varistor for a particular application, the E10 value is an important parameter to consider because the higher it is, the more energy the varistor can absorb, and the better it can protect against transient voltage spikes or surges.It's important to note that while the E10 value is a crucial factor to consider, it's not the only one.The overall performance of a varistor in a specific application is influenced by various factors, including the voltage rating, current handling capacity, response time, and other considerations.
The reason for the fault of crack or chipping in a varistor may be due to the soldering process during the attachment of the varistor to the commutator riser.The causes of this fault may include excessive heat application from the soldering iron to the varistor, mechanical stress from the soldering jig to the varistor, sudden temperature changes during the soldering process, and poor soldering technique.To avoid the occurrence of varistor cracking or chipping during the soldering process, it's essential to use appropriate soldering techniques and carefully monitor the temperature and duration of the heat exposure.Additionally, it's a good practice to inspect the varistors before and after the soldering process to detect any signs of damage.
To conclude, it is important to have a fool-proof process that eliminates the possibility of cracking or chipping.Additionally, implementing a 100% inspection process before packaging and shipping the product to the customer is advisable.The inspection could involve monitoring the motor waveform while it runs or automatically detecting over-voltage from the waveform.The E10 characteristic cannot be confirmed after the soldering process to the commutator, so it is recommended to conduct a single part incoming sampling inspection or to use it for the purpose of evaluating new parts.