How Protection and EMI Suppression Components Improve Safety and Noise Suppression in Permanent Magnet DC Motors

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Introduction

Permanent magnet (PM) DC motors are used as actuators and are critical electronic control system elements in many automotive and transportation applications. These applications include power windows, front and rear wipers, seat adjustments, sunroofs, automatic door openers, and more. PM DC motors are an ideal solution for many applications due to their relatively low cost and abundant supply, as well as the availability of simple control electronics when more precise field control is not required.

PM motors typically consist of a stator with permanent magnets and a rotor. From a safety perspective, manufacturers add protection to the motor in case the rotor locks up or an overload condition occurs during operation. This article provides an overview of the working principle of PM DC motors and a comparison of the two most common forms of thermal protection for DC motors: bimetallic and polymeric PTC protection devices.

Additionally, dual function capacitor-varistor filter solutions are presented that provide protection against load dump and jump start transient overvoltages in automotive applications, and these components also help suppress electromagnetic interference (EMI) caused by the spinning rotors and brushes of DC motors in automotive and industrial applications.

Figure 1: Typical PM DC motor application (Courtesy of Bourns)

Operating characteristics of PM DC motor

Figure 2: Structure of a PM DC motor *Provided by Bourns

In addition to the stator and rotor containing permanent magnets, the PM DC motor's construction includes a commutator with coils electrically connected to an electronic circuit. The rotor rotates when current flows through its winding coils, generating a magnetic field that is attracted to the permanent magnets, whose magnetic field opposes that of the windings. The timing of the current in each winding is designed so that its direction is always opposite that of the successive stator magnets. In this way, the rotor continues to rotate by being continually attracted to the magnets in the stator.

Figure 3 shows the conditions under which PM DC motors are most inefficient. The efficiency loss occurs at no load and zero work or at maximum current and maximum torque. However, under certain conditions of current and very low efficiency, the magnetic field density around the windings experiences very large current fluctuations. These fluctuations (Faraday's law of electromagnetic induction) induce large eddy currents in the windings, which eventually cause overheating unless the motor is disconnected from the power supply. The same phenomenon also occurs with transformer windings that are too close to the core gap and is known as fringing. Under such conditions, the windings are known to melt and the rotor stops due to overload (mechanical blockage). Motor manufacturers define the time that their motor designs can safely withstand a motor jam or blocked rotor condition.

Figure 3: PM DC motor efficiency *Courtesy of Bourns

Protection Requirements

Improved materials and design techniques have resulted in thinner, more efficient and cost-effective motors. Nevertheless, motor designers must consider two key factors when considering motor protection:

1. Trip time:
Each DC motor has its own characteristic curve. The trip time and trip current of the protection device must be tailored to the characteristics of each motor. Tests are performed in the motor with locked rotor to ensure that the trip time is coordinated with the protection device. The desired result is minimal variation in trip time.

2. Cycle life:
A typical cycle life consists of short motor revolutions, short stops, and then periods of rest. A motor may go through hundreds or even thousands of cycles, and the pass criterion is that the protection device never trips. Figure 4 shows an example of how to check that a protection device does not trip nuisance.

Figure 4: Cycle life of PM DC motor *Courtesy of Bourns

Motor manufacturers must also pay attention to two important motor conditions:

In a locked rotor condition, the high locked rotor current through the coil can cause it to burn out.
This is mostly caused by a mechanically locked motor.
Burnout can also occur under overload conditions. This happens if the coil temperature rises, if a heavy load condition occurs, or if the supply voltage is too low.

In these conditions the motor needs sensors for overcurrent and thermal protection of the motor coils.

The motor protection drives are directly coupled to the rotor as shown in the photograph in Figure 5. Each drive has space for mounting a protection device which can be plugged in when the motor drive is assembled. An example of a typical connection, generally made from brass, is also shown in Figure 5.

Figure 5: Typical rotor connection *Courtesy of Bourns

DC motor protection testing inside the motor is always required. Bourns has a UL recognized DC motor test lab that can perform the following common motor protection tests:
・Locked rotor
- Thermal protection (winding temperature measurement)
・Trip time (TTT)
・Reset time
Cycle life
Trip durability

Load dump transients in automotive electronics occur when the vehicle battery being charged by the alternator is suddenly disconnected (Figure 6). As a result, the load connected to the alternator experiences an overvoltage surge (VP) of up to 120V that can take 400ms (tf) to decay (Figure 7). These transients must be suppressed, and are typically clamped to 40V in 12V systems and 60V in 24V systems.

Figure 6: Load dump occurrence *Provided by Bourns — Figure 6: Load dump occurrence

Figure 7: Typical alternator output voltage after battery disconnection (Provided by Bourns) — Figure 7: Typical alternator output voltage after battery disconnection

DC motors are a common source of EMI (electromagnetic interference) that can cause performance degradation in other electronic systems and even corrupt data. In the worst case, EMI can cause complete failure of the electronic system. DC motors can generate both conducted and radiated emissions (Figure 8). It is therefore recommended to suppress this unwanted EMI noise below required values to ensure electromagnetic compatibility.

Figure 8: Noise emission test setup (Courtesy of Bourns)

Overview of protection technology

bimetal

Bimetals for small automotive DC motors are compact devices, typically 3-5mm thick, with a variety of terminal options to facilitate insertion into the DC motor. Bimetallic protection devices are typically designed to operate in ambient temperatures up to 80°C and, like polymer PTCs, are intentionally designed with slow reaction times to avoid false tripping.

polymer PTC Unlike conventional devices, many bimetallic devices on the market today are cooled because they do not carry leakage current after tripping. The construction of bimetallic devices physically breaks the contacts. This construction allows bimetallic protection devices to be used with polymeric devices. PTC The following disadvantages may occur compared to thermistors:
If the rotor is still locked when the bimetal device cools and resets, the windings will heat up
・If a bimetal device is unstable, multiple inrush currents occur in the windings, causing the average temperature of the windings to rise (see Figure 9).
If the bimetal device is unstable, the equipment will produce higher emissions and increased noise.
*Polymer PTC thermistors are designed to maintain a high impedance state
- Equipment fails prematurely due to contact welding from continuous cycling

Figure 10: Comparison of a bimetallic PTC, which flickers open and close the protective device, causing the rotor temperature to rise above the desired level, and a Multifuse® polymer PTC, which stabilizes the rotor temperature at a low level. (Courtesy of Bourns)

Bourns offers a variety of Thermal Cut-Off (TCO) mini-breakers utilizing bimetal technology. Currently, these devices are used primarily for Li-Ion battery protection, but there is growing interest for their use in DC motor applications.

Bourns TCO mini-breakers differ from the designs above and do not suffer from the drawbacks listed. Bourns TCO devices are constructed with a separate bimetal trigger mechanism and current carrying arm, improving long term reliability and impedance values. Bourns TCO devices also incorporate an internal ceramic PTC that holds the arm mechanism open when tripped. This allows the circuit to cool significantly, preventing the flickering effect of opening and closing.

Polymer PTC Thermistor

Polymeric Positive Temperature Coefficient (PPTC) thermistors are resistive components with a non-linear temperature resistance curve. The temperature can be increased by controlling the ambient temperature in a chamber or by self-heating that occurs when current flows through it (power = current squared x resistance). The resistance of the thermistor depends on the physical dimensions of the component and the resistivity of the material.

Figure 11: Resistivity of a PPTC thermistor — Figure 11: Resistivity of PPTC thermistor *Provided by Bourns

The resistivity of the material is controlled by the amount of conductive particles (usually carbon black), so the resistance is inversely proportional to the terminal dimensions and proportional to the thickness of the device.

For DC motors PPTC The devices are typically made of polyvinylidene fluoride (PVDF) High temperature polymer formulations are used to withstand ambient temperatures up to 125℃ One advantage of high temperature polymers is that they are compatible with low melting point polymers (typically with a maximum rated operating temperature of 85℃) has improved resistance drift over temperature.

In recent years, automotive DC Motor sizes are becoming smaller and lighter to meet fuel efficiency targets. PPTC The protection devices have a small footprint compared to bimetallic protection devices, making them an ideal solution for the limited space inside motor brush cards.

Because the resistance is expected to be very stable over the life of a PPTC device, designers often specify various temperature stress tests to simulate the useful life of the part. Typical specifications look like this:

At 90°C 500 Measure resistance before and after time
At 80°C 100 time, 80℃ from 40℃ Up to 10 Measure resistance in cycles

Resistance variation is expected to be within 12% (typical). As PM DC motor manufacturers continue to develop smaller, lighter, more efficient and lower cost motors, the trend is for protection devices to be similarly smaller, lighter and ultimately more cost effective.

Benefits of Bourns® PPTC protection in DC motors include:

・Economical: Bourns® PPTC The device offers a low-cost solution.
Allows designers to customize resistor characteristics, enabling fast trip times and a cost-effective process without the need for expensive tooling.
・Small size: Bourns® PPTC The device is a low profile solution that can accommodate space constraints within the drive.
・Low temperature sensitivity: Bourns® PPTC Advanced high temperature materials used in the thermistors allow them to operate at higher ambient temperatures.
moreover, Bourns® PPTC The construction of the device helps keep the windings cool during locked rotor testing.

Hybrid Dual Function Suppressor

Bourns offers hybrid dual-function capacitor-varistor suppressors consisting of a multilayer ceramic capacitor (MLCC) and a multilayer varistor (MLV). These hybrid components are an ideal solution for transient protection and EMI filtering. With both suppression functions integrated into a single radial leaded package, they eliminate the need for two separate parts and help significantly reduce the space required for mounting on a brush card (Figures 12 and 13).

Figure 12: A conventional DC motor brush card with capacitors and varistors as separate components. (Courtesy of Bourns) — Figure 12: A conventional DC motor brush card with capacitors and varistors as separate components
* Provided by Bourns

Figure 13: Advanced DC motor brush card using a single hybrid dual-function Bourns® OV Series component. (Courtesy of Bourns) — Figure 13: Using a single hybrid dual-function Bourns® OV Series component
Advanced DC Motor Brush Card *Provided by Bourns

Standard Music Video The series includes 14VDC from 125VDC Voltage ratings up to 10nF from 1000nF Capacitance ratings up to AEC-Q200 Testing and qualification is available upon request.
DC voltage range: 14V – 125V
・ Capacitance range: 10nF – 1000nF
・ Surge current capability (8/20µs): 150A
・ Temperature range:-40 ℃ ～+125 ℃

AEC-Q200 compliant O.V. The series includes 12V, 24V and 42V Compatible with the following power systems:
DC voltage range: 16V – 56V
・ Capacitance range: 470nF – 1500nF
・Surge current capability (8/20µs): 800A – 1200A
・ Load dump capacity (WLD): 6J – 12J
・ Temperature range:-40 ℃ ～+125 ℃
- AEC-Q200 compliant

Benefits of Bourns® DC Motor Protection Solutions

Low voltage DC motors are used in a variety of vehicle applications to operate seats, sunroofs, windows, mirrors, and door locks. To ensure the safe operation of DC motors, it is recommended to prevent overheating of the windings during locked rotor or overload conditions. Bourns® PPTC products offer an ideal protection solution that can be added to the drive section of a motor.

Customers can have their DC motors tested with Bourns® PPTC devices in Bourns'UL-certified laboratories. The ability to tailor precise data on protection device performance to customer requirements helps expedite the design cycle. Because DC motors are high-volume, low-cost components, Bourns® PPTC devices are well suited for these designs from both an economical and technical standpoint.

Additionally, Bourns PPTC solutions can be quickly and cost-effectively tailored to meet customer requirements. They also typically have a lower profile than bimetallic devices and maintain lower average winding temperatures during tripped, locked rotor or overload conditions. As DC motor technology advances, customers demand smaller, lighter and more cost-effective components. Bourns is committed to providing continuous technology improvements to meet these requirements.

Figure 14: Customized PCB-based suppressor (Courtesy of Bourns)

Based on the transient voltage protection and EMI suppression requirements of a design, Bourns can customize MV and OV series products as well as develop more complex customized filters that combine varistors and multiple capacitors onto a single compact PCB (Figure 14).

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