In this post, we consider resistors in electric motor applications.
AC, DC, and special purpose motors are used in a vast array of applications. Some motor applications are relatively simple and the only concern is the motor start-up and shutdown. Here cost, simplicity and reliability are the primary issues and motor control resistors are common solutions.
At the other extreme speed, torque and/or position are critical, and the motor requires a high level of control. AC and DC drives are now common in speed control applications, although resistors remain to facilitate motor braking (discussed elsewhere on this blog).
Motor Start-Up
As the resistance of a typical motor armature tends to be low, inrush currents can be high at start-up. In AC and DC motors inrush resistors limit the inrush current to prevent damage to the motor windings. These resistors are switched out of circuit as the motor approaches full speed.
Pulse Protection In Motor Applications
Pulse events can be intentional or transient. Pulse Width Modulation (PWM) is used in AC drives and for DC motor speed regulation. Transient events such as switching events or Electromagnetic interference (EMI) can generate electrical pulse events on motor supply lines. Resistors are employed as gate resistors in PWM applications, or as elements in pulse protection circuits.
Resistors in Electric Motor Control Applications
Today, electronic drive controls (VSD and VFD) are common in motor speed and torque control applications, but legacy systems and relatively simple applications continue to use discrete components.
Varying current and voltage to the motor controls speed and torque. Current sense resistors are used in measurement and feedback elements.
Resistor Selection Criteria
When considering resistors in electric motor control applications, resistor value, physical dimensions, power rating, tolerance temperature and voltage coefficient (TCR/VCR), frequency performance and resistor stability are all issues to consider.
In motor control applications dimensions, and frequency performance is generally not an issue. In many applications, tolerance and TCR/VCR are minor considerations. They are important, however, in current sense resistors.
When choosing an inrush resistor for electric motor applications, it is important to ensure it can both withstand the maximum inrush current and the energy generated by that current. Energy is a function of the inrush current, the input voltage and time.
Pulse resistor selection depends on the pulse condition, its peak, duration and energy. Determining the main elements of the pulse is the major challenge when choosing the resistor. The electric motor supply lines could be affected by several external events, resulting in many potential pulse conditions.
Resistors in AC and DC motor control applications can be in the main motor supply lines or low voltage control circuits. Those in supply lines are always on, so the steady state conditions of the motor must be known. They must also be able to survive pulse or surge events.
As current sense resistors are used in measurement applications tolerance and stability are more of an issue than power handling capability. Temperature coefficient of resistance and the resistor value are also important considerations.
Choosing A Resistor Technology
As mentioned above current and power rating are the important factors in inrush control applications. The choice, therefore, tends to be between thick film vs wirewound power resistors.
For pulse applications, wirewound and thick film resistors are a common choice, but ceramic resistors are also an option. The choice of a control resistor depends on its position in the circuit. In some applications power handling capability is critical. In others, tolerance and stability are more of an issue.
There are many use cases for resistors in electric motors. The selection of the resistor and the resistor technology depends on the demands of the specific application.
In most applications a standard resistor device will match the specification, in others, only a custom resistor will suffice. This is more of an issue in legacy electric motor systems where the original resistor may be hard to source or obsolete.