Many small DC motors with brushes and permanent magnets for field magnets are used for devices like windshield wipers, power windows, and power seats in automobiles. A DC motor with small brushes is simple in structure and low in cost and has a large starting torque. But when it rotates, a spark is discharged at the point of contact between brushes and commutators. This causes noise and wears the brushes. A ring varistor can be attached to a commutator and used as a surge protection device to absorb and reduce noise, and protect the contacts.

TDK's ring varistor VAR-18 series use a strontium titanate semiconductive ceramic material whose electrical characteristics and physical performance are greatly improved compared to the conventional levels. This lineup covers a wide variety of shapes and dimensions as well as conventional sizes to meet various requirements. This flexibility and ease of use are great advantages for this lineup.

What is a Varistor?

The term "Varistor" is coined from "variable resistor", meaning a component whose resistance value varies depending on the voltage. A high resistance value is maintained and almost no electric current passes until the voltage reaches a certain value (varistor voltage). When the voltage exceeds the value, the resistance value suddenly drops and a large electric current is allowed to pass. This property allows it to be used as a surge protection device to prevent circuit malfunction and destruction of an IC due to a surge of abnormally high voltage. For electronic devices, many disc type and multilayer chip type varistors are used. In DC motors with brushes, used for automobile components and the like, thin ring-shaped varistors are widely used for noise reduction and contact protection.

Role of Ring Varistors in Micro Motors

A ring varistor is attached to the commutator of a DC motor with brushes. The commutator is a device that switches current direction according to the rotation of the rotor. At the point of contact between the commutator and a brush, high surge voltage generated at the moment of switching current direction causes a spark, which leads to noise and wear of the brush. The ring varistor absorbs the surge voltage and prevents the spark (Figure 1).

Easy and Effective Motor Noise Solutions Using Ring Varistors

The following are principles and characteristics of varistors and details of noise solutions for automotive DC motors offered by TDK's ring varistors.

Special Semiconductor Ceramics Which Do Not Obey Ohm's Law

Figure 2: Voltage-current characteristics of varistors

A pure resistive element obeys Ohm's law, V = RI (V: voltage, R: resistance, I: current). The current increases proportionately with the applied voltage and is represented as a straight line in a graph. However, some resistive elements do not obey Ohm's law and their resistance varies depending on the applied voltage. A varistor uses this characteristic. It absorbs irregularly generated high voltage such as surge voltage and ESDs (electrostatic discharges) and discharges them to the ground.

The voltage-current characteristic of a varistor is approximated by the formula I = KV^α. K is a constant and α is a voltage nonlinear coefficient (α coefficient). The coefficient α is 1 for a pure resistive element, but it is greater than 1 for a varistor. The larger the value is, the more the nonlinear voltage-current characteristic deviates from Ohm's law (Figure 2). Semiconductive ceramics such as ZnO (zinc oxide) and SrTiO₃ (strontium titanate) are used as materials of varistors.

Suppresses a Wide Range of Noise to Prevent Negative Impact on Automotive Electronic Devices

With recent improvements in safety and convenience, the number of small DC motors used in some automobiles has increased to over 100. An automotive DC motor may have a negative impact on an electronic device. For example, operation of the windshield wipers of an automobile may cause noise that can be heard through the radio. This is caused by a spark discharging at the point of contact between the commutator of a DC motor and a brush. The noise widely ranges from low frequency to high frequency. As a noise countermeasure in automotive DC motors, ring varistors, as well as components such as capacitors, are widely used, especially in small DC motors (Figure 3).

Figure 3: Electrically-driven devices with small DC motors for which ring varistors may be used

Features of TDK's Ring Varistors

With the VAR-18 series, TDK offers various ring varistors using a SrTiO₃ (strontium titanate) semiconductor ceramic material. The main features and electrical and physical characteristics of TDK's ring varistors are explained below.

●Combined surge absorber and bypass capacitor functions

SrTiO₃ is a material which has a high dielectric constant across a wide temperature range. Using a material with a high dielectric constant in a varistor produces a varistor with a large capacitance and a large α coefficient. For high-level voltage, such as a surge, it works as a varistor, and for low-level voltage, it works as a capacitor and suppresses the noise component by sending it to the ground. In other words, it has both the characteristics of a surge absorber and a bypass capacitor.

●Copper electrodes and a ceramic element with excellent heat resistance and flexure strength

TDK's ring varistors employ copper electrodes and a ceramic element with excellent heat resistance and flexure strength. As a result, there is no need to worry about electrode erosion or thermal cracks - even at the higher soldering temperatures that are used in lead-free soldering. The ceramic element has excellent flexure strength and is suitable for automated motor assembly.

●The temperature characteristic of the varistor voltage is positive

The temperature characteristic of a TDK's ring varistor is positive, which means that its resistance value increases as the temperature rises. This eliminates the risk of decreased varistor voltage at high temperatures and large currents flowing through the varistor as a result. There is no need to compromise noise reduction levels when setting the room temperature varistor voltage (E₁₀ value) higher, which gives an advantage in design. Furthermore, it eliminates the problem of noise level being amplified at low temperatures and having a negative impact on the life of the motor.

●Excellent noise absorption and suppression across a variety of frequency bands achieved by optimal capacitance and α coefficient

For a small low-power DC motor, a ring varistor only measure is effective enough for suppressing high frequency radiation noise (Figure 4).

Figure 4: Noise suppression with a ring varistor

Mounted Easily and Securely in Small Spaces without Using PCBs

The combination of a ring varistor and an electronic component such as a capacitor realizes a more effective DC motor noise solution. Generally, as a noise countermeasure for electronic devices, SMD components such as MLCCs (multilayer ceramic chip capacitors) are used. However, using a PCB for mounting SMD components for an automotive DC motor causes various problems including space restrictions, increased costs, and deterioration of noise reduction effects due to wires. Components with lead wires are recently drawing attention as a way to resolve these issues.

SDM components soldered on automobile PCBs, which are used in inhospitable environments, are frequently exposed to thermal and mechanical stresses and have risks of solder crack, etc. On the other hand, components with lead wires can be easily and securely mounted in small spaces by welding or swaging. This ensures high reliability and may solve the problems of space restriction and costs. Components with lead wires have the advantage that they can be used without a PCB.

Figure 5 shows an example of a ring varistor and MLCCs with dipped radial leads mounted in a small DC motor. Two MLCCs with dipped radial leads are welded into a small brush holder. MLCCs with dipped radial leads are resin-coated. Thus they can meet requirements for weather resistance, moisture resistance, and the like.

Figure 5: Motor noise solution through a combination of MLCCs with dipped radial leads and a ring varistor

Clears Requirements for CISPR 25 Class 5, an Extremely Stringent Noise Regulation

A DC motor emits two types of noise: conduction noise and radiation noise. As shown in the graphs in Figure 6, the combination of a ring varistor and MLCCs with dipped radial leads can greatly suppress both conduction noise and radiation noise, enabling compliance with CISPR 25 Class 5, an extremely stringent vehicle noise regulation.

In addition, motor units for systems such as a powertrain system and a driving and steering system are increasingly being placed inside engine compartments. Therefore, the need for electronic components that can withstand temperatures up to 150°C is increasing. TDK offers a lineup of various automotive grade products that can withstand temperatures up to 150°C, including MLCCs with dipped radial leads.

Figure 6: Noise suppression using a combination of MLCCs with dipped radial leads and a ring varistor

We Offer Both Ring Varistors and MLCCs with Dipped Radial Leads

Figure 7 shows application examples of ring varistors and MLCCs with dipped radial leads in automotive DC motors. With 30 to 40 mm as a threshold, ring varistors are used for smaller diameter motors and MLCCs with dipped radial leads are used for larger motors. As it is anticipated that noise regulations for automobiles will become more stringent in the future, it is widely expected that the combination of a ring varistor and MLCCs with dipped radial leads will be the motor noise solution. One of TDK's strengths, as an electronic component and device manufacturer, is that we produce both ring varistors and MLCCs with dipped radial leads. We can promptly provide optimal solutions satisfying customers' demands through our wide-ranging product lineup.

Figure 7: Application examples of ring varistors and MLCCs with dipped radial leads in automotive DC motors

TDK's VAR-18 series ring varistors use a strontium titanate semiconductive ceramic material whose electrical characteristics and physical performance are greatly improved compared to conventional levels. They are simple, easy, and effective protection devices attached to the commutators of DC motors with brushes, such as motors for automobile components, that can prevent noise generation and wear. For the improvement of safety and convenience, noise regulations for automobiles are anticipated to become more stringent in the future. The combination of a ring varistor and MLCCs with dipped radial leads enables compliance with CISPR 25 Class 5. Please take advantage of this excellent performance in your products.

Product lineup of VAR-18 Series, ring varistors for micro motors

Plane surface electrode type

Side surface electrode type

PERFORMANCE