Practical tips on how not to burnout an electric motor

Motor heating

There are many reasons why an electric motor could start heating up. For example, when another starting duty is used then the informed one on the motor nameplate may result in motor overheating and consequent motor damage.

Due to high starting currents on electric induction motors, the time required to accelerate high inertia loads will result in a sudden motor temperature rise. If the interval between successive starts is very short, motor windings can experience some overheating that will cause some damage or reduce their lifetime.

The temperature of a motor winding is affected by heat coming from various sources. These sources can be internal to the motor resulting from its operation, or they can be external to the motor resulting from its environment. Temperature is also affected by the ability of the motor to dissipate this heat.

Let’s discuss the most important topics related to induction motor heating:

1. 1. Winding heating up
      1. Losses
      2. Heat dissipation
      3. Outer surface temperature of the motor
   2. Motor lifetime
   3. Insulation classes
   4. Winding temperature rise measurement
   5. Electric motor applications leading to overheating
      1. General
      2. Variations of motor loading
      3. Repetitive starts and stops
      4. Load inertia
      5. Voltage and frequency fluctuations
      6. Operation with adjustable frequency drives
      7. Inadequate altitude
      8. Bad ventilation

1. Winding Heating Up

1.1 Losses

The effective or useful power output supplied by the motor at the shaft end is lower than the power input absorbed by the motor from the power supply, i. e., the motor efficiency is always below 100%.

This heat removal must be ensured for all types of motors.

In the automobile engine, for example, the heat generated by internal losses has to be removed from the engine block by water flow through radiator or by fan, in the case of air-cooled engines.

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1.2 Heat dissipation

The heat generated by internal losses is dissipated to the ambient air through the external surface of the frame. In totally enclosed motors this dissipation is usually aided by a shaft mounted fan.

Good heat dissipation depends on:

1. Efficiency of the ventilating system
2. Total heat dissipation area of the frame
3. Temperature difference between the external surface of the frame and the ambient air (t_ext − t_a)

Recommendations

Action #1 – A well designed ventilation system, as well as having an efficient fan capable of driving a large volume of air, must direct this air over the entire circumference of the frame to achieve the required heat exchange.

A large volume of air is absolutely useless if it is allowed to spread out without dissipating the heat from the motor.

Action #2 – The dissipation area must be as large as possible. However, a motor with a very large frame require a very large cooling area and consequently will become too expensive, too heavy, and requires too much space for installation.

To obtain the largest possible area while at the same time keeping the size and weight to a minimum (an economic requirement), cooling fans are cast around the frame.

Action #3 – An efficient cooling system is one that is capable of dissipating the largest possible amount of heat through the smallest dissipation area.

Therefore, it is necessary that the internal drop in temperature, shown in figure 7.1, is minimized. This means that a good heat transfer must take place from the inside to the outer surface of the motor.

Internal drop in temperature depends on different factors which are indicated in Figure 1 where the temperatures of certain important areas are shown and explained as follows:

Where:

A – The winding hottest spot is in the centre of the slots where heat is generated as a result of losses in the conductors.

AB – The drop in temperature is due to the heat transfer from the hottest spot to the outer wires. As the air is a very poor conductor of heat it is very important prevent voids inside the slots, i.e. the windings must be compact and perfectly impregnated with varnish.

B – The drop in temperature through the slot insulation and through the contact of the insulation material with the conductors and by contact with the core laminations.

BC – Drop in temperature by the transmission through the stator lamination material.

C – Drop in temperature by contact between the stator core and the frame. Heat transmission depends on the perfect contact between the parts, good alignment of the laminations, and accuracy in the machining of the frame.

Uneven surfaces leave empty spaces, resulting in poor contact and consequently bad heat transmission.

CD – Drop in temperature by the transmission through the frame thickness.

Due to modern design, use of first class material, improved manufacturing processes, and continuous quality control, electric motors MUST ensure excellent heat transfer properties from the motor inside to the outside thus eliminating “hot spots” in the windings.

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1.3 Outer surface temperature of the motor

Figure below shows the recommended places where the outer surface temperature of an electric motor should be checked with calibrated temperature measuring instruments:

Important! Measure also the ambient temperature (at a max. distance of 1 m from the motor).

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2. Motor Lifetime

As you already know, useful lifetime of the motor depends almost exclusively on the life of the winding insulation.

The lifetime of a motor is affected by many factors, such as moisture, vibration, corrosive environments and others. Among all these factors, the most important is the working temperature of the employed insulation materials.

You should know that an increase from 8 to 10 degrees above the rated temperature class of the insulation system can reduce the motor lifetime by half.

When speaking about decreasing the useful lifetime of the motor, we are not talking about high temperatures where the insulation system burns and the winding is suddenly destroyed. For the insulation lifetime this means a gradual ageing of the insulation material which becomes dry, losing its insulation properties until it cannot withstand the applied voltage.

This results in a breakdown of the insulation system and a consequent short-circuit of the windings.

This temperature limit is well below the “burning” temperature of the insulation system and depends on the type of used insulation material.

This temperature limit refers to the hottest spot in the insulation system, but not necessarily to the whole winding. One weak point in the inner part of the windings will be enough to destroy the insulation system.

It is recommended to use temperature sensors as additional protection devices for the electric motor. These protection devices will ensure a longer lifetime and more process reliability.

The alarm and/or shutdown setting should be performed according to the motor temperature class.

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3. Insulation Classes

Insulation class definition

As previously mentioned the temperature limit depends on the type of used material used. In order to comply with the  standards the insulation material and insulation systems (each one formed by a combination of several materials) are grouped in INSULATION CLASSES.

The insulation classes used for electrical machines and their respective temperature limits is accordance with IEC 60034-1 are as follows:

• Class A (105 ºC)
• Class E (120 ºC)
• Class B (130 ºC)
• Class F (155 ºC)
• Class H (180 ºC)

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4. Winding Temperature Rise Measurement

It would be rather difficult to measure the temperature of the winding with thermometers or thermocouples since the temperature differs from one spot to another and it is impossible to know if the measurement point is near the hottest spot.

The temperature rise measurement by the resistance method, for cooper conductors, is calculated according to the following formula:

where:

• Δt – temperature rise;
• t₁ – winding temperature prior to testing, which should be practically equal to the cooling medium, measured by thermometer;
• t₂ – winding temperature at the conclusion of the test;
• t_a – temperature of the cooling medium at the conclusion of the test;
• R₁ – winding resistance prior to testing;
• R₂ – winding resistance at the end of the test.

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5. Electric Motor Application

5.1 General

The hottest spot temperature in the winding should be maintained below the maximum allowed temperature for the insulation class. The total temperature is the sum of the ambient temperature, plus temperature rise (∆t), plus the difference existing between the average winding temperature and the hottest spot.

Motor standards specify the maximum temperature rise ∆t, so the temperature of the hottest spot remains within the allowable limit based on the following considerations:

1. Ambient temperature should not exceed 40 ºC, as per standard. Above this value, working conditions are considered as special operating conditions.
2. The difference between the average temperature of the winding and the hottest spot does not vary very much from motor to motor and its value specifed by standard, is 5 ºC for Classes A and E, 10 ºC for Class B and F and 15 ºC for Class H.

The figures and the allowable temperature composition for the hottest spot are shown on Table 1 below:

Table 1 – Temperature composition as function of the insulation class

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5.2 Variations of Motor Loading

A motor operating at or above rated load will generate more heat and have a higher temperature rise than motor operating at fess than nameplate horsepower.

See Table 2 for typical operating data for an application requiring 1150 hp continuously, installing a 1000 hp, 1.15 service factor motor might initially cost 11% less than installing a 1250 hp, 1.0 service factor machine.

Table 2 – Temperature rise and efficiency 1250HP vs. 1000HP motor variations

** Temperature rise by resistance

However, the larger motor would have 3.9 times the expected insulation life and 0.8% (95.0 – 94.2) greater efficiency, which likely would result in lower life cycle cost.

Note that for operation at 1000 hp continuously, the larger motor would have about 2.8 times the life expectancy (10.7 divided by 3.8) and 0.6% greater efficiency (95.2 less 94.6).

It is typical of most induction motor designs that the ‘A load efficiency is greater than the full load efficiency. Conversely, the efficiency at 1.15 service factor generally is lower than at rated load.

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5.3 Repetitive Starts and Stops

When a motor starts under load, it typically draws six to seven times normal current while accelerating the load. This results in high short-term copper losses and heat build-up.

Repetitive starts and stops in a short interval of time will always have a deleterious effect on motor winding life – specifics will depend on the frequency of starts and stops, the nature of the load.

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5.4 Load Inertia

NEMA specifies standard inertia values for each motor rating. Starting loads with greater inertia will cause additional heat build up during acceleration, which can affect the insulation life.

Such applications should be checked with the motor manufacturer to insure proper design for the specific application.

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5.5 Voltage and Frequency Fluctuations

Fluctuations in system voltage or frequency can cause additional heat build-up and lead to premature winding failure.

NEMA specifies that motors be suitable for the following variations:

1. ± 10% voltage at rated frequency
2. ± 5% frequency at rated voltage
3. maximum 10% (absolute values) combined with 5% limit on frequency.

Motors with lower flux densities will be more effected by increased current flow in under voltage conditions.

Over-frequency can result in overloads on motors driving centrifugal machines; whereas under frequency can result in damage due to inefficient cooling on motors driving constant torque loads.

Similarly, unbalances of greater than 1% in phase voltages will cause negative sequence currents which may result in rotor overheating along with increases in motor winding temperature, noise and vibration levels.

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5.6 Operation with Adjustable Frequency Drives

Operation with an adjustable speed drive often causes harmonics in a motor which can result in heat build-up and localized hot spots. Harmonics from a “dirty” power system, even when the motor itself is not being used with a drive, can have the same effect.

For this reason, motors used on adjustable speed drives generally do not have a service factor greater than 1.0.

Such motors are generally specified: “90° rise by RTD at rated load (1.0 SF) on a 60 hz utility sine wave to be suitable for Class F rise when used on an inverter.”

Motors specially designed for use with drives can be made to compensate for this by use of blowers, oversized frames and/or special materials.

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5.7 Inadequate Altitude

Motors operating at altitudes above 3300 feet will be subject to higher temperature rises than those operating at sea level because the ambient air is less dense, and therefore will dissipate less heat.

It is recommended that the following derating factors to the nameplate horsepower be used when the motor is operated at higher altitudes:

1. 3% between 3300 and 5000 feet
2. 6% between 5000 and 6600 feet
3. 10% between 6600 and 8300 feet
4. 14% between 8300 and 9900 feet

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5.8 Bad Ventilation

Motors which operate under unclean or very confined conditions which inhibit proper ventilation of the motor will be subjected to overheating and shortened thermal life expectancy.

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References //

1. Specification of Electric Motors by WEG
2. Motor life: Effects of loading, service factor and temperature rise on insulation life by Bruce Campbell and Jose Galleno