Introduction to Induction Motors //
All types of industries are invariably required to install different types of electric motors as the prime mover for driving process equipment in their respective production line up. The continuous process of technical development has resulted in the availability of highly diversified types of electric motors.
Hence, the utmost care should be taken in the selection of the most appropriate type of motor considering a number of technical factors for each application, so that the motor will provide the desired and optimum performance.
The characteristics of motors vary widely with the nature of their application and the type of duty they are expected to perform. For example, applications like constant speed, constant torque, variable speed, continuous/intermittent duty, steep/sudden starts, frequent start/stops, etc. should be taken into consideration carefully when deciding for the type of a motor for that specific application.
Moreover, the motors are quite often required to perform under abnormal conditions during their total service life.
In view of above, incorrect motor selection always lands the industrial buyer in all sorts of problems, including premature motor failure causing severe production curtailments.
As mentioned above, a number of other factors and design features like weather conditions, stringent system conditions, abnormal surroundings, hazardous area, duty cycle, motor efficiency, etc. should be considered when deciding the rating and subsequently drawing out the technical specifications of the motor.
Abnormal conditions and effects //
The usual abnormal conditions encountered by motors are given below.
1. Abnormal System Conditions
- Unbalance in 3-phase
- Single phasing
- Voltage surges
- Low frequency
- High frequency
2. Abnormal Operating conditions
- Locked rotor or stalled rotor
- Reswitching/frequent start-stops
- Momentary interruption/bus transfer
- Improper cable sizing
3. Environmental conditions
- High/low ambient temperature
- High altitude
- High humidity
- Corrosive atmosphere
- Hazardous atmosphere/surroundings
- Exposure to steam/salt-laden air/oil vapour
4. Mechanical problems
- Seized bearings
- Incorrect alignment/foundation levelling
- Incorrect fixing of coupling
- High vibration mounting
- External shock due to load
5. Condition at location
- Poor ventilation
- Dirt accumulation
- Exposure to direct sunlight
The above mentioned abnormalities may prevail for short or long duration’s or may be transient in nature, and the major impact is overheating of the motor along with one or several other effects such as change in the motor performance characteristics like drawl of more power and consequent deterioration in motor efficiency, etc.
Increase in mechanical stresses leading to:
- Shearing of shafts
- Damage to winding overhang
- Bearing failures
- Insulation failures
Increase in stator and rotor winding temperature leading to:
- Premature failure of stator or rotor insulation (For wound rotor motor)
- Increased fire hazard
- Breakage of rotor bar and/or end ring (For squirrel cage motors)
All motors encounter few or several of these abnormalities during the course of their service lives. Consideration of listed abnormal conditions at design stage greatly helps to minimise the effects of abnormal conditions and maintain a consistent performance.
Design Considerations //
Following are the most important design factors to be considered when selecting a motor for any of the diversified industrial applications.
Output in kW/HP
There are two principle limitations for selecting the motor output:
- Mechanical limitation
The breakdown torque, which is the maximum torque that the motor can produce when operating without stalling. This is a critical design factor in motor applications, particularly for the motors subjected to occasional extreme load conditions.
Another critical factor is the locked-rotor torque, which is the maximum torque that the motor can produce during startup from steady-state condition, a critical design feature for conveyor drives.
- Electrical limitation due to insulation provided on the motor windings
The electrical load on the motor can be imposed until the winding insulation is able to withstand the prescribed temperature rise over an ambient for that particular class of insulation. Life of the motor greatly depends on the temperature rise of the windings.
Anticipated life-span of the motor can be achieved provided it is operated at its rated output without overloading and the prescribed preventive maintenance practices are religiously followed.
Speed of the Motor //
Most of the motors are directly coupled with the driven equipment where in the speed of the motor and the driven equipment will be same.
In order to meet the speed of the driven equipment, the devices like gearbox, chains or belts are introduced between motor and driven equipment. In this case, it may be necessary to provide the rotor shaft suitable for its attachment with the speed decreasing or increasing device and hence the specification should include such a specific requirement.
In case a variable speed drive is to be used for the speed variation, the motor should be compatible for this specific application. The standard motor may not provide desired performance when operated via variable speed drive.
Power Supply Voltage and Frequency Variations //
Variations in the power supply parameters, i.e. voltage and frequency significantly affect overall performance of the motor. As provided in IS:325-1996, the permissible voltage variation is ±5 to ±10%, permissible frequency is 50Hz ± 3%, and permissible combined variation is ±6 %.
The effect of undervoltage is more serious than that of overvoltage.
The higher torque, resulting from overvoltage, can handle a little overload without undue heating of the winding, but only for a short duration. Continuous operation with undervoltage condition increases the current at the rate of about 20% for every 5% reduction in the supply voltage, increasing the rated copper loss.
This results in heating and prolonged temperature rise, and finally the burning of winding. During a motor start-up, the torque reduces by 10% for each 5% reduction in the supply voltage, causing more starting current and consequently more rapid heating of the winding.
The motor offers reduced efficiency at either overvoltage or undervoltage. Power factor drops sharply with higher voltage and improves with lower voltage. Even when the motor is lightly loaded, over-voltage causes a rise in current and temperature thus reducing the life of the motor. The variation in frequency by +5 % decreases the torque by about 10% and vice-versa at – 5% frequency, the torque increases by about 10%.
It is, therefore, of utmost importance to consider the combined effect of variation in voltage and frequency when purchasing the motor.
Unbalance in the supply voltage results in a current unbalance of 6 to 10 times the percentage voltage unbalance. This in turn results in generation of negative sequence currents in the rotor causing its overheating and premature failure.
It is therefore vital to specify the permissible limits of variations in the power supply parameters for the motor in accordance with the requirement of the driven equipment. However, the permissible limits should never be more than provided in the applicable Indian Standard IS:325-1996 (Reaffirmed in 2002).
To be continued…look out for Part Two coming next week!
Source Reference: Electrical Engineering Portal (EEP)