Methods of Motor Winding Temperature Protection

Winding protection is providing protection against excessive temperature. Excessive winding temperature can permanently damage the winding, greatly reducing winding life and can cause complete winding insulation breakdown and failure. Down time in many motor applications are prohibitive and a good monitoring system is essential to minimize costs which can be incurred. A complete protection system for a winding is designed to remove the motor from the line when any of the following conditions occur:

1. Locked Rotor - a mechanical failure of some type which locks the rotor and prevents it from turning when power is applied to the motor.

2. Starting Overload - an excessively high overload due to increased friction or inertia on the drive system can cause excessive temperature on the winding and permanent damage.

3. Running Overload - an abnormal condition which overloads the motor when it is running causing it to draw higher current then designed for. This will permanently damage the motor winding.

4. Abnormally High Temperatures - an environmental condition where the motor is exposed to abnormally high temperatures can cause the total winding temperature to reach a point where permanent damage can occur.

5. Voltage Unbalance - since the winding temperatures increases by a percentage equal to 2 times the square of the voltage unbalance, this can easily cause excessively high winding temperatures resulting in permanent winding damage.

6. High or Low Voltage - depending on winding design, high or low voltage can cause excessive currents and permanent winding damage.

7. Ventilation Failure - blocked air openings, broken fans, or anything which may disrupt the normal ventilation system on a motor to open when the motor is running will cause abnormally high winding temperatures resulting in failure.

There are three basic methods of protecting a motor from winding failure.

1. Current Sensitive Devices - these include circuit breakers, fuses, motor starter heaters of all types and instantaneous current relays. These are usually always external to the motor and are used in accordance with "National Electrical Code" which sets installation of motor branch circuits.

2. Combination Current & Temperature Sensitive Devices - these consist of a thermal disc and heater in on e unit are line break devices. These devices must be properly sized for each rating and application. They are often used in high volume U/L listed applications such as air conditioning.

3. Temperature Sensitive Devices - since temperature is the limiting factor in protecting a motor, a direct sensing of motor winding temperature is the logical approach. There are a variety of winding temperature devices and systems all being designed to open the pilot circuit to the motor starter. These winding temperature protection systems can be classified according to their mode of operation. The basic detector systems in use today are:

A. Temperature Switches

B. Resistance Temperature Detectors (RTDs)

C. Thermistors

D. Thermasentry

E. Thermocouples