Power Electronics

Next-Generation Shunt Resistors Meet Critical Circuit Protection Needs for Motors Controls

To control system operation, motors require a current measurement, which can be achieved using shunt resistors that provide a simple and cost-effective solution.

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Motors are used in countless consumer electronics applications, and technology improvements that have led to more cost-effective designs are expanding the way motors are used in a broad range of additional applications. Motor types range from specific-purpose variable frequency motors that can be deployed in compact form factors, all the way up to massive motors that are used in HVAC or other large-scale applications. Regardless of size, an important requirement is the ability to measure the current of the motor. There are many methods of interpreting the reported current value in a motor, and each can benefit from the accuracy and reliability provided by a shunt resistor.

It is critical to the operation of a motor to have an accurate current measurement in order to detect a short circuit or an overcurrent condition. Current measurement information is also required by the motor's controller. Because the monitoring of circuits can quickly become quite complex, a shunt resistor can be a valuable part of the design of a motor and provides valuable circuit protection. A shunt resistor is a simple, accurate, cost effective component that introduces little inductance to the system while providing crucial information for long-term performance.

The following overview on motors includes their inverters and electronic speed controllers. It will also address shunt resistors (sometimes referred to as current sense resistors) and their role in providing current measurement to the motor controller. Accurate current measurement and a broad scope of resistance values with very high thermal stability make next-generation shunt resistors an optimal circuit protection solution for motor inverter designs.

MOTORS OVERVIEW

For the purpose of simplification, we will divide motors into three types. The three motor types vary according to:

  • Stator, the motor's stationary part
  • Stator current
  • Rotor, the motor's rotating part

Induction motors operate with ac in both the stator and rotor. Synchronous ac motors use dc in one portion and ac in the other, and are often referred to as stepping motors because they can operate at a range of speeds. They are generally larger and are sometimes designed with permanent magnet rotors. The third type of motor is called a dc motor and operates with dc applied to both the stator and rotor.

There are two varieties of dc motors. The first type uses a mechanical system that rotates with brushes and a commutator system. The second type is a brushless dc motor identified by permanent magnets in the rotor, which operates with a switched dc fed to the stator. Brushless dc motors are gaining popularity over brush dc motors because maturing technology continues to drive down the price of these systems. Brushless dc motors do not require contact with brushes for rotation, have no sparks, and are able to perform with greater reliability and efficiency over a longer lifespan (thanks to the absence of electrical and friction losses associated with the contacting parts). Another added benefit of brushless over brush dc motors is its ability to be cooled by conduction so that no airflow inside the motor is necessary.

Motors can be optimized for ac or dc operation. The main difference between a brushless dc motor and a fixed-magnet ac motor is the way in which they are controlled. AC motors are connected to ac power to create the rotating magnetic field within the stator that makes the ac rotor turn. The controller requires dc power, and thus inversion is required to generate a supply for the controller, typically in the form of an inverter. For systems operating on a battery, a brushless dc configuration offers an advantage of the controller using the same dc battery and requiring no conversion. A separate power supply may be included for noise reduction and stability in the motor. A three-phase ac motor is considered brushless and is usually the first choice due to its cost advantage. Brushless dc motors are similar to synchronous ac motors, because they typically do not operate at fixed speed. A distinct advantage of brushless dc is their size: they tend to be much smaller than the synchronous ac variety.

MOTOR AND CONTROLLER

Motors have a preset relationship between voltage and frequency that remains constant over the entire operating range, regardless of load. There are situations that can cause this relationship to change due to low speeds, light loads, excessive slip, or during start-up. Various methods exist to adjust the voltage of a motor in such situations, and two are briefly introduced. Voltage boost adds a fixed amount of voltage and is especially good at low speed and low voltage conditions. However, it is important to note that the circuit can become saturated with this type of adjustment when driving a small load. IR compensation can be added to boost voltage, so that it is proportional to the amount of current in a motor. In this manner, no voltage will be applied that would be high enough to cause saturation, and precise current feedback to a controller is critical.

Many integrated controllers are available for ac and brushless dc motors. Key components of controllers are inverters and electronic speed control (ESC) circuits. An inverter is a set of switches, typically digital, that can turn on and off with varying frequency to produce an electrical signal which is sent to the motor. The speed, direction, and starting current of a motor are dictated by the inverter's signal. Inverters require dc power to perform various control functions within the inverter stage of the circuit before the desired waveform is transmitted to the motor. A rectifier from the main ac power typically provides this fixed dc power. DC-DC inversion can be performed, or a transistor-based circuit can be employed. Variable speed brushless dc motors employ an ESC circuit to interpret the control information and set the switching frequency of the driver accordingly. The end result is the ability to control the speed, direction, and braking of the motor.

SENSE MECHANISMS

During the output phase of inverters, current sensors are required to measure the current. Inductive systems typically measure a magnetic field produced by current flowing through a conductor. Therefore, it is far more convenient to deal with measurements in the dc portion of a circuit, such as between the rectifier and inverter portions of an inverter circuit. A current sense resistor monitors the current in the circuit, translating the amount of current into a voltage, which is proportional to the current draw of the motor that is being measured or monitored. A control circuit interprets the feedback from a resistor simply as a voltage drop. It then calculates things such as the speed of the motor, since engine speed and voltage are directly proportional and engine torque and current are directly proportional. A current sense resistor at the gate is useful in finding short circuit conditions in the dc portion of a circuit. A calibrated resistor will sense the current that runs through it, interpret it as a voltage drop, and feed it to a control circuit to detect and monitor the value. Automatically reducing gate voltage in response to current flow over a given limit can prevent a latch-up condition in a solid-state inverter. When a resistor is introduced in series with the load current, a protection circuit can monitor its voltage to detect a short circuit. The resistor must have a resistance value low enough so as not to disturb conduction in the circuit or dissipate spurious power. It is important not to introduce resistance in the dc loop since the added inductance can severely reduce the performance of a system.

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Sense resistors are useful for providing information to systems with many types of motors. Fig. 1 illustrates a system with a motor driven by a separate dc drive, which also powers the controller. Main ac power is rectified to provide power for the inverter, or driver, which provides the motor with its input waveform. This configuration is well suited for brush dc motors.

Fig. 2 illustrates a versatile system for brushless dc and ac motors. The sense resistor is placed between the inverter and dc ground. The controller is connected to the resistor to measure and monitor its voltage drop, and a connection from the motor provides the additional signals the controller may require. The controller then interfaces with the three-phase inverter or driver, which drives the motor.

Fig. 3 presents a strictly dc motor system utilizing a dc battery and is similar to a three-phase inverter.

NEXT-GENERATION INVERTER SOLUTIONS

The low system cost and exact current measurement provided by next-generation shunt resistors make them an excellent candidate for placement in the dc circuit of an inverter. Some resistors offer resistance values ranging from 0.010 Ohms to 0.100 Ohms, which is consistent with the resistor values used in inverters. This low resistance allows large currents to flow through to the motor without dissipating much power, and also ensures that power is supplied to the motor rather than being wasted on the resistor. The resistor can be electrically isolated from the backplane and has a low inductance, allowing it to remain essentially invisible to the rest of the system.

Models that have high thermal stability and the ability to withstand short pulses are ideal for many inverter drives because they are able to absorb the pulse power of a misfired inverter switch, while continuously monitoring the voltage drop of the controller. Package designs may provide surfaces that are ideal for mounting to a heat sink, making it possible to dissipate higher amounts of power.

Size is an important issue in the design of electronics, so there are shunt resistors available in packages measuring 20% shorter per side than the JEDEC standard D2PAK. A reduction in size, however, does not reduce the continual power rating of the part. The smaller part provides 25 W rather than the JEDEC standard D2PAK rating of 20 W. In next-generation solutions, the size of the components will likely continue to shrink as power ratings increase.

Motor technologies are pervasive and are used in a wide range of applications. Current supplied to a motor must be consistent regardless of the conditions of the input power supply. Whether simple or complex, the motor control system will require a measure of current in the system to accomplish this.

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