Cyndi Nyberg
Former EASA Technical Support Specialist 

There are a number of different types of wind­ing protection devices used with motors. However, they all basically do the same thing; they sense a change from the normal operating temperature and either sound an alarm or take the motor off line when the specified temperature limit has been met or exceeded. 

Temperature protection is not limited to just large motors. A smaller motor that is critical to op­eration would be a good candidate for winding temperature protection if a failure would cause significant costs due to downtime. 

Areas of protection 
Winding protection will sense a change in the total winding temperature that could potentially be harmful to the motor. We know that as tem­perature increases, the insulation life decreases. For every 10ºC increase in total winding temperature, the thermal insulation life will be cut in half. Winding pro­tection is designed to protect against a number of condi­tions that can cause the winding to heat up rapidly. 

When a mechanical fail­ure causes the rotor to lock, the temperature will increase rapidly, first in the rotor and then to the stator. 

Unexpected loads can oc­cur during starting and stats, thermocouples, thermistors, and resistance tem­perature detectors (RTDs). Any of these devices that are imbed­ded in the motor winding will give an accurate assessment of the total winding temperature, and any changes to the normal operating level. 

Temperature ratings 
When winding protection is replaced or added to an existing design, there are a couple of simple rules to follow to ensure that the right critical tempera­tures are selected. In some 
running, so the winding protection can detect the high temperatures that will develop due to in­creased current. Winding protection also will protect the motor from a high ambient temperature.

For that reason, some motors might trip out more during hot periods in the summer. 

Voltage unbalance can have a negative effect on the motor’s performance, and one effect is on the total winding temperature. A good rule of thumb is that the winding temperature (in ºC) increases by two times the square of the voltage unbalance. For example, if the voltage unbalance is 2%, then the temperature will go up around 8 degrees, and that could make the temperature high enough to start breaking down the insu­lation. So winding protection can prevent permanent damage. 

A high or low voltage supply can increase the current that a motor draws, which will in turn increase the winding temperature. In the case of ventilation failure, maybe a fan blade has broken off, or an air vent has become blocked because of something in the environment. In that case, the temperature can increase very quickly. 

Any of the previous conditions can cause per­manent damage to a motor. There are a number of devices used to protect the windings from damage due to high temperatures. These include thermerature are selected. In some cases, like with a winding RTD, you may need to check with the user to see which type is used, to make sure the right one is put back into the motor. However, you can easily measure the resistance before burnout if the device has not been de­stroyed. 

First, you will need to know the class of the winding insulation. Class F insulation is good for up to 155°C; Class H is good for up to 180°C. Some older designs use Class B insulation, but most service centers use Class F or H when re­winding stators. The alarm and shutdown points of the device are based on the maximum winding temperature the insulation can handle. However, many motors are nameplated, for example, “Class F insulation with Class B rise.” This means that even though the insulation is good for 155°C, the expected temperature rise under full load is 80°C, assuming a 40°C ambient temperature. In this case, the temperature protection will be based on the to­tal temperature rise of 120°C, rather than the maximum of 155°C. 

Based on the insulation class and/or the ex­pected temperature rise of the winding, you can next determine the alarm and shutdown points. For Class F insulation, 145-150°C would be the typical rating of the protection device, unless the name­plate indicates a lower temperature rise. If an alarm point is used, it is typically set 10°C lower than the shutdown point. Caution: One thing to keep in mind is the actual load the motor sees. If, for ex­ample, a motor is running very lightly loaded, winding protection may not sense what may actu­ally be happening inside the motor. 

For example, if a bearing fails (and there are no bearing RTDs), it can cause significant damage to the rest of the motor before the winding temperature protection kicks in, if the winding temperature is far below the maximum point allowed. If you know that the motor is running at a much lower than rated load, the winding temperature will be lower to begin with, so the protection device may need to be rated for a lower critical temperature. 

Thermostats 
Winding thermostats are simple temperature on-off switches normally installed in the end turns of the motor winding (U.L. specifies they be inserted in the opposite fan end.). They can activate a warn­ing device, or shut down the motor when the winding temperature exceeds the temperature rating of the switch. 

Thermostats have contacts that are either nor­mally closed that open at the specified temperature, or normally open that close at the specified tempera­tures. The thermostat is automatically reset after the temperature drops below the critical point. No exter­nal controls are necessary for use with thermostats. 

There are normally three thermostats provided, one for each phase, at a specified temperature rat­ing. U.L. explosion-proof motor requirements specify thermostat temperature ratings, and they may be lower than typically specified for a standard mo­tor to ensure that the frame temperature of the motor does not exceed the maximum temperature of the hazardous material in the environment. 

Thermocouples 
Thermocouples are basically two metal conduc­tors that generate a voltage between them. The voltage produced is a function of the winding tem­perature where the thermocouple is placed.

They can be put into the slot with the winding, but there are not many buried in the end turns. 

A separate control is required to convert the volt­age to a temperature reading that in turn operates the alarm and/or shutdown. There are a number of types of thermocouples; the most common types are cop-per-constantan, chromel-constantan and iron-constantan. The materials will have a different relationship between the voltage produced and the temperature it indicates. Typically, two thermo­couples per phase are installed in the windings. 

Resistance temperature detectors 
Resistance temperature detectors (RTDs) have a definite relationship between resistance and tem­perature. One type of RTD is the thermistor. It is made of a semi-conductive material, and as the tem­perature rises, so does the resistance. As the set cutoff temperature is reached the resistance rises quickly and dramatically, causing an alarm or shut­down. Thermistors are normally put on the end turns of the winding. 

Standard RTDs have a purely linear relationship between resistance and temperature. They are placed in the slots in form wound stators and in the end turns in random wound stators. There will typically be two RTDs per phase. For RTDs installed in the slots, they typically will be put into the center of the slot length, equally spaced around the winding, since that will be the hottest part of the stator. By putting them there, you can ensure that the temperature is as accurate as possible. 

There are several types of RTDs and the differ­ence between them has to do with the resistance characteristics of the material from which they’re made. As the temperature increases, so does the re­sistance. (Bearing temperature detectors operate on the same principles to protect the bearings.) 

The most common RTDs used in motor wind­ings are 10 ohm (copper), 100 ohm (platinum), or 120 ohm (nickel). Each type of RTD has a specific resistance vs. temperature characteristic, so it is im­portant to know the type for replacement since the controller is matched to the RTD material. Tip:  If you are installing winding and/or bearing RTDs, you will want to use the same type for both so they can use the same controller.