The Importance of Thermal Protection for Torque Motors

The Importance of Thermal Protection for Torque Motors

Brian Zlotorzycki

When talking about high-end machining or manufacturing applications that include direct-drive technology, one of the key advantages of utilizing this particular transmission method is its endurance. Because of the very nature of direct-drive motors they are able to operate at peak performance levels indefinitely — without any kind of wear or aging — as long as the motor isn’t pushed past its capacity. Unfortunately, because this isn’t a perfect world, unexpected things can happen which can cause the motor to overheat. Whether the heat source is due to a parameter being input incorrectly, or an unexpected external force causing more resistance than expected — it is important to have certain forms of thermal protection in place. Since torque motors are built in such a way that they cannot be repaired and yet maintain their efficiency, it is vital to prevent any overheating — thus precluding the need to purchase a new one.

There are currently a number of ways in which torque motors can be protected from overheating; they include having the controls monitor and maintain a certain amount of current, or using physical temperature sensors. Following are some of the methods possible to ensure optimum motor protection.

I²t values. Within controls, the algorithms that monitor the current being used are referred to as an I²t value. There are variations on how it is calculated depending on whose programs are used, but the principle is always the same. I²t is an algorithm that is a function of time and current, so it uses the amount input into the motor over time and sets a limit on how much can be applied before overheating. To give an example of this, we’ll take a look at the I²t programming in Etel’s AccurET control driver (Fig. 1).

Figure 1 A value of KF84 is set, which is dependent on the model of the motor. If the current reaches its limit of KF85 — an I2T error occurs.
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A value of KF84 is set, as determined by the model of the motor. As the amount of current used goes above and below this value, integration begins. If the current remains above the selected KF84 value for too long, and the integral reaches its limit of KF85, then an I²t error occurs. There is also an over-current limit at KF83 that sets a limit — regardless of any integration value.

Most drivers have some form of current- monitoring method to prevent overheating, and it is a way to use the motor’s continuous current value to prevent motor damage at the electronics level.

Temperature sensors. There are many types of motor temperature sensors on the market, but the focus here will be on some of the common ones; they are KTY, SNM, and S01 sensors.

KTY. KTY sensors are silicon temperature sensors that have a resistance value that changes based on temperature. The relationship between the temperature in the windings and between the resistance value the sensor outputs is linear (Fig. 2). Although KTY sensors are considered the most accurate of the three mentioned, one must compensate for a delayed response since the change in resistance is not instantaneous enough to detect sharp changes in temperature.

Figure 2 KTY sensors are silicon temperature sensors that have resistance values that change based on its temperature.
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SNM. An SNM sensor is typically used in low-voltage circuits and is meant to be connected to a controller working at an electronic voltage level typically around 5 VDC. Like a KTY sensor, it outputs a resistance value based on the temperature, but the change is a lot sharper (Fig. 3).

Figure 3 SNM sensors are typically used in low-voltage circuits.
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S01. An S01 sensor is a limit switch based on a bi-metallic mechanism. It is a digital signal that reads as either open or closed. If desired, it can directly cut off the power of some electronics in overheating scenarios, as it operates more as a “switch” rather than having a temperature/resistance relationship. It is able to operate in a medium- to highvoltage range of around 50 VDC.

Sensor configuration. There are two kinds of standard temperature configuration Etel utilizes for its torque motors (Fig. 4). The first is Configuration 8, which not only uses each type of sensor, but has all three phases monitored. Although the option for having all three sensor types is available, it is Configuration H that is recommended since there is a spare set of KTY sensors on each of the phases. This is important because KTY sensors — while the most accurate of the different types — are electrostatic- sensitive devices (ESDs), and there are many stages in the machine assembly process where the sensors can be damaged; having an extra set ensures that it wouldn’t be necessary to have the entire motor replaced on the offchance one of them is damaged.

Figure 4 There are two kinds of standard temperature configuration ETEL utilizes for its torque motors.
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Stall conditions. It is important to note that in each of the different configurations, all three phases are monitored (Fig. 5). Not all motors have this set-up because it is assumed that any overheating in one phase would be evenly replicated in the others. One scenario where this wouldn’t apply would be under stall conditions. In a machine tool duty cycle — where torque motors are commonly used — there may be an instance where a part is held in place for a certain amount of time and the current isn’t evenly distributed amongst all the phases; this is called a “stall condition.” If one phase is found to be above an acceptable level and it’s not the one that has a sensor on it, then a user runs the risk of a phase overheating without the driver even being aware of it. This is why monitoring all phases is important.