VSD Motor Protection -- via direct temperature sensing

Intro:

The main requirements for the protection of AC converters and AC induction motors has been covered in considerable detail in: Protection of AC converters and motors.

This -- covers some of the detail of the direct temperature sensing methods of protecting electric motors.

Microtherm (thermostat)

A thermostat is a temperature dependent device that uses a bi-metallic strip to change the position of a pair of contacts at the preset rated response temperature. When the temperature exceeds a preset level, the contacts are used to switch an external control device, such as a relay or contactor. To avoid 'hunting', some sort of hysteresis is usually built into the device to ensure that the set and reset take place at different temperatures. Microtherms, which are commonly used with electric motors, are miniature precision thermostats, sufficiently small for direct insertion into the windings of a motor or transformer to allow a close thermal association with the winding. The contacts, typically rated at 2.5 amp at 240 volt, are capable of switching a contactor or relay directly. Several microtherms are usually fitted into a motor, each designed to operate at a temperature related to the design temperature of the area in the motor where it’s placed. Typical strategic locations are the windings, air ventilation path and bearings. The manufacturers of DC motors tend to prefer Microtherms, while the manufacturers of AC motors tend to prefer thermistors, which are described below.

Microtherms are usually used in groups of two, with one group having a rated reference temperature of 5 C or 10 deg. C lower than the other to provide a temperature pre-warning alarm. The second group is used to trip the motor to prevent damage to the winding insulation. On a motor of significant rating and thermal inertia, the pre-warning alarm would give an operator several minutes to clear the process machine or rectify the overload condition before the overload trip signal occurs.

In DC motors, two groups of microtherms are generally used. The mounting position of the first group is usually at the hottest point of the hottest interpole, usually the one carrying the armature current. This location provides protection for armature current overload. The second group is usually located in the shunt field, providing protection for both the shunt coil temperature and the general temperature within the motor.

In a modern shunt wound DC motor, the working temperature of the shunt winding is very similar to that of the armature. Any loss or restriction of the cooling air, which is difficult to monitor other than by direct measurement, will result in a fast rise in the temperature of the field winding and will be detected by the microtherm.

Thermistor sensors and thermistor protection relays

A thermistor is a small non-linear resistance sensor, which can be embedded within the insulation of a motor winding, to provide a close thermal association with the winding. It’s made from a metal oxide or semiconductor material. The relationship between resistance and temperature is non-linear and the resistance varies strongly with small temperature changes around the set point.

By correct positioning, thermistors can be located close to the thermally critical areas, or hot-spots, of the winding, where they closely track the copper temperature with a certain time lag, depending on the size of the thermistors and how well they are installed in the winding.

Thermistors are most easily inserted into the non-rotating parts of motors, such as the stator winding in an AC motor or the interpole and field windings of a DC motor.

++++Characteristic curve of a PTC thermistor sensor to IEC TC2 RRT = Rated response temperature IEC specified temperature/resistance limits are clearly marked.

The main advantages of thermistors are:

• Their small size allows them to be installed in direct contact with the stator winding.

• Their low thermal inertia gives rapid and accurate response to winding temperature changes.

• They measure temperature directly irrespective of how these temperatures are initiated.

• They can be used to detect overload conditions in motors driven by frequency converters.

The temperature coefficient can be positive (PTC - positive temperature coefficient), where the resistance increases with temperature, or negative (NTC - negative temperature coefficient), where the resistance decreases with temperature. The type most commonly used in industry is the PTC thermistor, whose typical resistance characteristic is shown in the curve below.

The resistance at normal temperatures is relatively low and remains nearly constant up to the rated response temperature (RRT). As the RRT is approached and exceeded, the gradient of the resistance increases sharply, giving the PTC thermistor a high sensitivity to small changes of temperature. At the set point, a temperature rise of a few degrees results in a large increase in resistance. The resistance is monitored by a thermistor protection relay (TPR) and, when the sharp change in resistance is detected by the thermistor protection relay (TPR), it operates a contact to initiate an alarm or to trip the protected device.

Thermistor protection relays are required to trip reliably when the sensor resistance rises above about 3 k-ohm. They will also respond to an open circuit, either in the cable or the thermistor sensor, thus providing fail-safe protection. Modern TPRs are also designed to detect a thermistor sensor short circuit, when sensor resistance falls below about 50 ohm. The specified operating levels are:

• Thermistor over-temperature protection according to IEC

- Response level = 3300 ? ± 100 ?

- Reset level = 1650 ? ± 100 ?

• Thermistor short-circuit protection according to IEC

- Response level = 15 ?

In AC variable speed drives, PTC thermistors are commonly used to protect the AC squirrel cage motor fed from inverters. Many modern AC converters have a thermistor protection unit built into the converter, avoiding the requirement for a separate thermistor protection relay. In DC motors, PTC thermistor sensors are increasingly used instead of microtherms, which are described in the section above.

The rated response temperatures (RRT), which are commonly selected for the various classes of insulation on electric motors, are summarized.

Insulation class; Rated temp; Alarm temp; Trip temp; Class B Class F Class H

++++ Typical temperature level settings used on rotating electrical machines Due to the relatively slow transfer of heat to the sensors through the insulation medium, PTC thermistors don’t provide sufficiently fast protection for short circuits in motors or transformers. Also, since they are usually located in the stator windings, they don’t provide adequate protection for rotor critical motors or for high inertia starting or stalled rotor conditions. In these cases, to achieve complete protection, it’s recommended that PTC thermistors should be used in combination with electronic motor protection relays, which monitor the primary current drawn by the motor.

The application of PTC thermistors as temperature sensors is only effective when:

• The rated response temperature (RRT) of the thermistor is correctly selected for the class of insulation used on the winding.

• The thermistors are correctly located close to the thermally critical areas.

• There is a low thermal resistance between the winding and the PTC thermistor. This depends on the electrical insulation between the winding and the thermistor. Since thermistors need to be isolated from high voltages, it’s more difficult to achieve a low heat transfer resistance in HV motors, which have greater insulation thickness.

Several thermistor sensors may be connected in series in a single sensor circuit, provided that the total resistance at ambient temperatures does not exceed 1.5 k?. In practice, and as recommended by IEC, up to six thermistor sensors can be connected in series.

For a 3-phase AC motor, two thermistor sensors are usually provided in each of the 3 windings and connected in two series groups of three. One group can be used for alarm and the other group for tripping of the motor. The alarm group is usually selected with a lower rated response temperature (RRT); typically 5o C or 10 deg. C lower than the tripping group. If the operator takes no action, the tripping group is used to trip the motor directly to prevent damage to the winding insulation. In many cases, users choose both groups to have the same RRT. In this case, only one group of thermistors is used (one in each phase) and these are then used for tripping the motor. This provides for one spare thermistor in each phase.

The physical location of the thermistor sensors in an AC motor depends on the construction of the motor, whether it has a cylindrical rotor or salient pole rotor, and several other design and manufacturing variables. In some cases, the optimum location may have to be determined from test experience.

Thermistor protection relays (TPR) are designed for mounting inside a control cubicle or motor control center, usually on standard terminal rails. The ++++ shows a typical connection of two thermistor protection relays, and their associated groups of thermistor sensors.

For alarm and trip control of a 3-phase AC induction motor. The performance of thermistor protection relays can be affected by external electrical interference, where voltages can be induced into the sensor cable. Consequently, cables between the thermistor protection relay and the PTC thermistor sensors should be selected and installed with a view to minimizing the effects of induced noise. Cables should be kept as short as possible and should avoid running close to noisy or high voltage cables over long distances.

++++ Typical connection of thermistor protection relays

During testing, care should be taken not to megger across the thermistors as this can damage them. The correct procedure is to connect all the thermistor leads together and to apply the test voltage between them and ground or the phases.

Some practical recommendations for the type of cables that should be used are as follows:

Distances = 20 m Standard parallel cable is acceptable Distances = 20 m, = 100 m Twisted pair cable is necessary Distances = 100 m Screened twisted pair (STP) cable is necessary High level of interference Screened twisted pair (STP) cable is necessary

The screen should be grounded at one end only

For cable distances to the sensors of greater than 200 meters, the cross-sectional area of the conductors should also be considered. The following are recommended:

Conductor Cross-section Maximum Length Type of Cable

0.5mm_

200m Screened twisted pair (screen grounded at one end only)

0.75mm_

300m Screened twisted pair (screen grounded at one end only) 1.0mm_

400m Screened twisted pair (screen grounded at one end only) 1.5mm_

600m Screened twisted pair (screen grounded at one end only) 2.5mm_

1000m Screened twisted pair (screen grounded at one end only)

++++ Recommended cable size to thermistor sensors

Thermocouple Thermocouples consist of two lengths of dissimilar metals joined at one end to form a junction. At the other open end, a small voltage is produced which is dependent on the temperature at the junction. This is known as the peltier effect. As the temperature changes, the developed thermionic voltage changes to give an indication of the temperature.

++++ Connections between a thermocouple sensor and its controller There are several national standards, which specify the performance characteristics of thermocouples, such as voltage/temperature, error limits and color-codes for connecting wires. The most commonly used standards are listed below. These standards are generally interchangeable in terms of their relationship between voltage and temperature.

• ANSI M96.1 - American National Standards Institute (also known as the NBS standard)

This is one of the most widely used standards for instrumentation. The ANSI color-code always uses a red negative leg with a different color for the positive leg indicating the type and temperature range of the thermocouple.

The overall sheath color is brown.

• BS 1843 - British Standard This standard uses a blue negative leg color-code, with a different color sheath and positive leg indicating the type and temperature range of the thermocouple.

• DIN 43714 - German Standard This standard uses a red positive leg color-code, with a different color sheath and negative leg indicating the type and temperature range of the thermocouple.

• JIS C1610 - Japanese Standard This standard uses a red positive leg and white negative leg color-code, with a different color sheath indicating the type and temperature range of the thermocouple.

• NF C42-323 - Normes Françaises (French Standard)

This standard uses a yellow positive leg color-code, with a different color sheath and negative leg indicating the type and temperature range of the thermocouple.

• IEC 584 - International Electrotechnical Commission (IEC) This is a new standard that will start to gain acceptance in the future.

Hopefully, it will overcome much of the confusion that currently exists with thermocouple color-codes.

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Type Temp range Metals

+ve -ve NBS colors BS colors J-Type

0ºC to 750ºC Iron Constantan White/Red Brown Sheath Yellow/Blue Black Sheath K-Type

-200ºC to 1250ºC Copper Nickel Yellow/Red Brown Sheath Brown/Blue Red Sheath N-Type

-270ºC to 1300ºC Nicrosil Nisil Orange/Red Brown Sheath Orange/Blue Orange Sheath E-Type

-200ºC to 900ºC Chromel Constantan Purple/Red Brown Sheath Brown/Blue Brown Sheath T-Type

-200ºC to 350ºC Copper Constantan Blue/Red Brown Sheath White/Blue Blue Sheath

===

++++ Details of the most common base-metal thermocouples, with the ANSI-NBS and BS 1843 color-codes

As temperature sensors, thermocouples have the following main advantages:

• Robust: very suitable for the industrial environment

• Good accuracy: typically 0.5% per 10 C

• Low cost: consist of a junction of two dissimilar metals

• Self powered: thermal energy is converted into electrical energy

• Wide temp range: types are available for most temperature ranges

The materials used for thermocouple junctions are either base metals or noble metals.

Base-metal thermocouples are most commonly used in industry because of their lower cost. Thermocouples made from noble metals are more expensive and are used for special applications, where corrosion may be a problem. There are also a number of very high temperature thermocouples, usually made from tungsten.

Conventional copper wire should not be used to connect thermocouples to the temperature controller. This would introduce additional junctions into the circuit and lead to substantial temperature sensing errors. Special thermocouple wire, using the same materials as the thermocouple junction, should be used to connect the junction to the controller. Thermocouple extension wires are usually also color-coded to match the thermocouple colors.

Thermocouple connections are also susceptible to external electrical interference and induced voltages, superimposed onto the junction voltage, will result in measurement errors. Extension wires should not be run along cable routes together with high voltage or high current power cables. Screened extension wires should be used in cases where there is a high level of noise. On industrial sites, it’s common practice to run thermocouple extension cables inside galvanized iron conduits, which provide physical protection and shielding against electrical noise.

Resistance temperature detector (RTD)

Name of Sensor Metal Resistance at 0ºC Cu-10 Pt-100 Ni-120 Copper Platinum Nickel 10 ? 100 ? 120

++++ The most common types of RTD sensors

Resistance temperature detectors (RTDs) monitor temperature by measuring the change of resistance of an accurately calibrated resistive sensor, usually made of copper, platinum or nickel. Tungsten is sometimes used for high temperature applications. RTD sensors can be of the wire wound type, which have a high stability over a period of time, or can be of the metal film types, which are lower cost with faster response but their characteristics can deteriorate over a period of time.

The type of RTD sensor most commonly used in electrical machines comprises a Pt-100 sensor element made of platinum, whose resistance is accurately calibrated to 100 ? at 0 deg. C. The sensor is usually insulated and mounted inside a cylindrical metal tube of dimensions typically 10 mm diameter and 200 mm length.

Since the RTD sensor is physically larger than other types of measuring sensors, it cannot easily be mounted in the windings or bearings of small electric motors.

Consequently, RTDs are only used on large machines, where they are installed within the stator slots during manufacture. A slightly different mechanical form is used for mounting in bearing housings. Thermistors or thermocouples are still the most commonly used temperature sensors for electric motors.

An RTD has a linear relationship between resistance and temperature, typically 0.4 ?/ 0 C for a Pt-100 sensor. A very sensitive measuring instrument, usually based on the Wheatstone bridge, is required to continuously measure the small changes in the resistance of the RTD. These instruments pass a small excitation current through the resistive sensor.

Although the excitation current can cause some problems with self-heating, this is seldom a problem because the currents are small, typically less than 1 mA, and RTDs have a high rate of heat dissipation along the connecting wires and to the measured medium.

Considering the small changes of resistance with temperature, the overall accuracy of the RTD resistance measurement is affected by the series loop resistance of the extension wire between the measuring instrument and the Pt-100 sensor. This is dependent on the cross-sectional area of the wires and the distance between the RTD sensor and the measuring instrument. This has led to the development of 3-wire RTDs, where a third identical extension wire is connected between the instrument and the sensor. The purpose of the third wire is to provide the measuring instrument with a means of measuring the wire loop resistance to the RTD sensor. To improve accuracy, this is subtracted from the total measured resistance. At the RTD sensor, the third wire is simply connected to one of the legs of the sensor.

++++ Connections for a 3-wire resistance temperature detector

In a similar way to thermistors and thermocouples, the RTD connections are also susceptible to external electrical interference and induced voltages, which can lead to measurement errors. Similar precautions need to be taken with the cable route selection and screening. RTDs have become very popular in industry because they provide low cost, high accuracy temperature measurement with a relatively fast thermal response.

Pt-100: Resistances for temperatures 0 deg. C to +299 C : DIN 43760

++++ Pt-100 sensor - variation of resistance with temperature over range 0 C to +299 C

Pt-100: Resistances for temperatures 0 C to -219 C : DIN 43760

++++ Pt-100 sensor - variation of resistance with temperature over range 0 C to -219 C