In this article, we will consider resistors in motor applications.
AC, DC, and specialty motors are used in a wide range of applications. Some motor applications are relatively simple, and the only thing to focus on is the start-up and shutdown of the motor. Here, cost, simplicity, and reliability are the main concerns, and motor control resistors are the common solution.
At another extreme speed, torque and position are critical, and the motor requires a high level of control. AC and DC drives are now common in speed control applications, but resistors are still used to facilitate motor braking.
Since typical motor armatures tend to have low resistance, inrush currents can be high at start-up. In AC and DC motors, surge resistors limit the inrush current to prevent damage to the motor windings. When the motor is close to full speed, these resistors are switched out of the circuit.
Impulse events can be intentional or temporary. Pulse-width modulation (PWM) is used for AC inverter and DC motor speed regulation. Transient events such as switching events or electromagnetic interference (EMI) can create electrical impulse events on the motor power line. Resistors are used as gate resistors in PWM applications, or as components in pulse protection circuits.
Today, electronic drive control (VSD and VFD) is common in motor speed and torque control applications, but traditional systems and relatively simple applications continue to use discrete components.
The current and voltage changes of the motor control the speed and torque. Current sense resistors are used for measurement and feedback elements.
Resistance values, physical dimensions, power ratings, tolerance temperature and voltage coefficient (TCR VCR), frequency performance, and resistance stability are all considerations when considering resistance in motor control applications.
In motor control applications, size and frequency performance are often not an issue. In many applications, tolerance and TCR VCR are secondary considerations. However, they are important in current sense resistors.
When selecting a surge resistor for a motor application, it is important to ensure that it can withstand both the maximum inrush current and the energy generated by that current. Energy is a function of inrush current, input voltage, and time.
The choice of pulse resistance depends on the pulse condition, peak, duration, and energy. Determining the main components of a pulse is a major challenge when selecting a resistor. Motor power lines can be affected by a variety of external events, resulting in many potential pulsing conditions.
Resistors in AC and DC motor control applications can be located in the main motor power line or in the low-voltage control circuit. Those in the power cord are always on, so it is essential to know the steady-state conditions of the motor. They must also be able to survive pulse or surge events.
Since current sense resistors are used in measurement applications, tolerance and stability are more important than power handling capabilities. The temperature coefficient and resistance value of the resistance are also important considerations.
As mentioned above, current and power rating are important factors in surge control applications. Therefore, the choice is often between thick film and wirewound power resistors.
For pulsed applications, wirewound resistors and thick film resistors are common choices, but ceramic resistors are also an option. The choice of control resistor depends on its position in the circuit. In some applications, power handling capability is critical. In other cases, tolerance and stability are a bigger issue.
There are many applications of EAK resistors in electric motors. The choice of resistor and resistor technology depends on the needs of the specific application.