Rotating Machines temperature measurement methods; Motors temperature

In every machine it becomes always very important to measure temperature of machines as we know what is ambient temperature on which machine runs and running temperature of machine, machine breakdowns can be reduced. If we know that machine temperature is rising then we can easily understand from that there is some abnormality with the machine. So it becomes prime importance to know and monitor the temperature of machines irrespective of their design. In this article we will study about various methods of temperature measurement.

Method of measurement of Temperature rise in Machines:-

In machines there is very difficult to measure temperature as there is complex heat flow through materials having different conductivities and heat transfer co-efficient.

There are following methods for temperature measurement in machines:-

1.    Thermometer method

2.    Resistance method

3.    Temperature detector

1.    Thermometer Method:-

Thermometer is usually applied at surface of machine. Therefore this method provides temperature of surface at one point of machine only. This includes non-embedded thermocouples and resistance thermometers provided they are applied to the points accessible to the usual bulb thermometers.  When bulb thermometers are employed in places where there is any varying or moving magnetic field, alcohol thermometers should be used in preference to mercury thermometers as the latter are unreliable under these conditions.

2.  Resistance Method:-

In this method temperature winding is determined by increase in resistance of winding. This method involves measurement of resistance of winding, both cold and hot, and estimating the average temperature rise by use of resistance temperature co-efficient.

The temperature rise T= T2-Ta

Ratio of resistance is :-

R2 = T2+k

R1    T1+k

Here T1 is the temperature (°C) of the winding (cold) at the moment of the initial resistance measurement;

T2 is the temperature (°C) of the winding at the end of the thermal test;

Ta is the temperature (°C) of the coolant at the end of the thermal test;

R1 is the resistance of the winding at temperature T1 (cold);

R2 is the resistance of the winding at the end of the thermal test;

k is the reciprocal of the temperature coefficient of resistance at 0 °C of the conductor material.

For copper k = 235

For aluminium k = 225 unless specified otherwise.

For particle purposes the following formula is used;-

T= T2-Ta= R2-R1 (k+T1) + T1- Ta

                   R1

In case of A.C. Machines resistance measurements may be made without interruption of the test by method of Superposition which consists of applying a small DC measuring current superposed upon the load current.

3.  Temperature detector method:-

In this method temperature detectors are embedded into machines during construction of machines. These detectors are may be Thermometers or thermocouples. There are approximately 6 detectors built into the machine  and distributed around circumference and placed in positions along the length of the core at which highest temperatures likely to occur.

In this method detectors are protected from contact with cooling medium when the machine has two sides per slot, the detectors are located between the insulated coil sides within the slot.

Effect of Environment on Induction Motor

Effect of Environmental Factors on Rating of Electrical Machines

Every electrical machine is designed to deliver its rated performance under standard environmental conditions defined by manufacturers and international standards (such as IEC, NEMA, IS). However, in real-world applications, factors like temperature, altitude, and humidity may vary significantly. These deviations directly impact the cooling, efficiency, insulation life, and overall rating of the machine.

Below, we discuss how different environmental parameters affect the performance and rating of induction motors and other electrical machines.

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1. Effect of Temperature

The ability of a machine to handle load primarily depends on how efficiently it can dissipate heat generated during operation.

• Low Ambient Temperature:
  Machines perform better in colder climates because the temperature difference between the winding and ambient air increases, improving cooling efficiency.
  👉 Example: A 100 kW motor of intermittent rating may safely deliver 200 kW continuously at the North Pole (ambient –80 °C) since the natural cooling prevents overheating.
• High Ambient Temperature:
  When operating in hot environments, the cooling effect decreases, and the winding temperature rises quickly, which reduces the motor’s permissible load. Manufacturers generally derate machines above 40 °C ambient temperature.

✅ Key Insight: For every 10 °C rise in temperature beyond the permissible limit, the insulation life reduces by nearly 50% (Arrhenius law of thermal aging).

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2. Effect of Altitude

Cooling of electrical machines depends on air density. At higher altitudes, the air becomes thinner, reducing its ability to absorb and transfer heat.

• Up to 1000 m above sea level → effect is negligible.
• Beyond 1000 m, derating is necessary as per standards.

Altitude Derating Table

Height Above Sea Level (m)	Rating (%) at ≤1000 RPM	Rating (%) at >1000 RPM
0 – 1000	100%	100%
2000	95%	92%
3000	90%	85%
4000	85%	72%

👉 Example: A 200 kW motor at 3000 m altitude, running above 1000 RPM, can only deliver 170 kW safely.

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3. Effect of Humidity

High humidity adversely impacts both insulation and metallic parts of machines.

• Insulation Deterioration: Moisture absorption reduces dielectric strength, increasing the chances of short circuits and insulation breakdown.
• Corrosion: Metallic components like laminations, stator frame, and bearings are prone to rust, reducing reliability and lifespan.
• Condensation Risk: Sudden changes in temperature cause condensation inside enclosures, which is especially dangerous in tropical and coastal regions.

✅ Best Practices:

• Use tropicalized insulation systems in humid climates.
• Provide space heaters inside large motors when idle.
• Ensure proper ventilation and dehumidification in motor rooms.

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🔑 Conclusion

The rating of electrical machines is not fixed universally but varies with environmental conditions.

• Low temperatures → allow higher loading.
• High temperatures → reduce machine rating.
• High altitudes → lower cooling, hence derating is mandatory.
• High humidity → reduces insulation life and accelerates corrosion.

👉 Engineers must always consider site conditions while selecting, installing, or operating machines to ensure long service life and safe operation.

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Transformer Off-Load Tap changer

There are two types of Tap changers in Transformers depending upon the rating of transformer.

1. Off-load Tap changer
2. On-Load Tap changer

If a tap-changer is designed to operate with the transformer out of circuit the it is called off-load tap changer. In that Transformer line should be disconnected completely before changing the tap.

If a Tap changer is designed to operate with the transformer in circuit then it is called as On-Load tap changer i.e. tap can be changed even when transformer will remains in line.

Read about about why and where tap changer required in Transformer:-

Read here

Let’s discuss here about off –load or No load Tap changer:-

No Load Tap Changer:-

This type of Tap changer is used for seasonal voltage variations as in Summers load on HT increases leads to drop in HT voltage so O/P can be increased by using Transformer Tap changer and in Winters when load get reduced HT voltage restores to normal levels then tap changer position can be done according to value of HT Voltage.

A tap changer is shown below:-

There are six studs marked as 1-6. The winding is tapped at six points equal to no. of studs. The tapping leads are connected to six corresponding marked stationary studs arranged in circle. The face plate carrying six studs can be mounted anywhere on Transformer. The Rotatable arm R can be rotated by means of Hand-wheel from outside transformer tank.