Today, electric motors are more closely scrutinized as a result of broad spectrum application use, which is strictly regulated. Electric motors consume more than half of all electric energy produced. For this reason, more attention is being focused on how electric motors are designed to deliver energy efficiency and possible cost savings. Attention to power handling, power dissipation, and temperature specifications are important considerations for designers when selecting the right components for more efficient motor system applications.

Design efficiencies can be achieved by increasing the motor power output and through better use of system design layout space, and advancements in performance and component miniaturization are contributing to enhanced integrated solutions. This article will provide an overview of motor technology and discuss specific components such as current sense resistors for motor drive design. It will also introduce a full motor system with suggested components for power, circuit protection, and control panel adjustment to implement new electric motor systems that can help reduce energy, decrease costs and improve the overall efficiency.

Motor system overview
A motor system contains several coordinating units. The basic operation of a motor converts electrical energy into mechanical energy and operates at a specific voltage. The motor receives electrical power from a motor drive, which is controlled by a motor/motion controller. The controller receives feedback from the motor and motor drive. The motor drive is the interface between the motor and its controller, producing power conversion, amplification, and sequencing of waveform signals. In order to drive, the motor drive must match the control signal (power levels) and signal type (analog or digital) of the motor. The drive produces power conversion, amplification, and sequencing of waveform signals. The motor controller receives supply voltages and provides signals to the motor drive. The controller can have bidirectional interfaces with communication ports and user controls. Figure 1 examines the main units of a motor system and indicates the technology used in each. 

Figure 1. Motor system block diagram.

Current sense resistors to improve system efficiency
The fundamental function of the motor drive is to serve as the interface between the motor and controller, and it is essential for the drive to accurately translate the input of the controller. The drive must also provide feedback to the controller, and current sensing is the method most commonly used for these tasks. Current sensing provides a voltage signal that corresponds to the current flowing at a particular point in the circuit. Current information is provided from the drive to the controller to detect any overcurrent or short circuit conditions that may arise. For circuits under 5.5 kW, a current sense resistor is typically sufficient for providing the current measurement. For medium power motor applications, power losses to discrete shunt resistors limit their efficiency.

Insulated gate bipolar transistor (IGBT) modules are often used as an alternative to discrete resistors in motors. For current sensing, these IGBT modules can have integrated shunt resistors which provide an accurate measurement and are cost-effective. Current sense resistors are used at the power supply to motor driver interface and in the feedback loop in the drive section. Position feedback devices such as potentiometers and encoders can be used from the motor to the control unit for mechanical feedback of the position. Snubber resistors, also known as clamp resistors, and gate resistors are used at the motor driver interface. These types of resistors are part of RC filters and are used to remove any noise that might trigger the shutdown of the drive.

Increase system reliability with circuit protection
Components focused on protection and manual adjustment are used for the interfaces of the external portions of the motor system. For example, circuit protection components such as transient voltage suppression (TVS) diodes, Polymer Positive Temperature Coefficient (PPTC) resettable fuses, and metal oxide varistors (MOVs) provide port protection for communication interfaces that include Ethernet, RS-232, RS-485, CAN and others. Magnetic/power choke devices are used at the power supply, and in motor control and DC/DC conversion. Trimmers can also be used in motor control. The user interface or Human Machine Interface (HMI) to the motor control unit may include potentiometers and other panel control components to provide manual adjustment for speed and torque. The feedback from the load regarding the position of the motor may use position sensors such as single-turn or multiturn precision potentiometers, encoders or non-contacting Hall Effect sensors.

Select the right current sense resistor
The main selection criteria for current sense resistors includes tight tolerances on the initial resistance value, low inductance because of high changes in current over time, low resistance value to minimize power losses, and a low Temperature Coefficient of Resistance (TCR) for accuracy. TCR is an important factor in motors because all materials experience a unique change in resistivity with temperature changes. A high temperature rating is critical for reliability, and a high peak power rating is equally important to ensure the resistor can handle high current pulses for short durations.

Specific resistor characteristics to consider include rated to 3 Watts of continuous applied power and a high power density for circuit board space reduction. Resistors with a TCR specified as ±75 ppm/°C with the resistance being measured between an ambient temperature of 25 °C and either +125°C or -55°C are used for example. Other options include open air shunts, used in IGBT inverters that use current sense resistors. Snubber Capacitor Modules (SCMs) consisting of diodes and capacitors can be paired with external resistors to limit the IGBT switching overshoot, as shown in Figure 2. For the external resistors, thick film SMD resistors have good surge capability and are available with power ratings up to 2 W while thick film non-inductive resistors are available up to 35 W. 

Figure 2. IGBT circuit

Space and Power-Efficient Motor Redesign
The example below illustrates a redesign of a 3-phase inverter/motor drive, replacing the original through-hole current sense resistors with surface mount current sense resistors. A major concern for most redesigns is the power handling capability of the resistors. Prior to the availability of more compact components, two SMD resistors in parallel would have been suggested to replace each through-hole resistor. Today, space is constrained and, therefore, a more minimal part count is often required in many new applications. Component advancements make other options available. For example, a single current sense resistor can be used instead of two SMD resistors for higher power handling properties due to the Ni/Sn plating on the terminations. This plating method provides a more efficient transfer of heat from the resistor to the board. A redesign is now possible without increasing the number of components, and high reliability is maintained because the resistors are not susceptible to overheating. Figure 3 illustrates this solution. 

Figure 3. Motor redesign using next-gen current sense resistor

Options abound for new motor designs
Selecting next-generation components such as current sense resistors for a new motor design gives designers more options and provides a better use of system design layout. Current sense resistors are compact, have high power capability and contribute to the overall efficiency of motor drives. They also help to reduce costs and the energy output of motors. There is also a wide range of port protection solutions available for motor system communication interfaces such as Ethernet, RS-232, RS-485 and CAN. Integrating circuit protection components such as transient voltage suppression (TVS) diodes, resettable fuses, and metal oxide varistors (MOVs) helps keep motor systems running reliably.

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