Selection of AC Electric Motors
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For the selection of a motor for a particular application, the following points should be considered:-

1) Load torque and speed required:-
· Variable torque with adjustable speed
· Constant torque with variable speed
· Constant horsepower with variable speed

2) Starting torque
3) Cost
4) Efficiency
5) Type of enclosure
· Totally enclosed type forced air cooled (TEFC)
· Drip proof type
6) Temperature classification
· Class B motors
· Class F motors

1) LOAD TORQUE AND SPEED REQUIRED:
The slip ring induction motor enables the “STARTING CHARACTERISTICS” of the motor to be totally controlled and modified to suit the load. The torque speed curves with low and high “ROTOR RESISTANCE”, with a “CONSTANT TORQUE LOAD” and at different reduced stator voltages is shown in Fig.31. a particular high rotor resistance can result in the pull out torque occurring at almost zero speed providing a very high locked rotor torque at a low locked rotor current. As the motor accelerates, the value of the resistance can be reduced altering the start torque curve in a manner such that the maximum torque is moved towards synchronous speed. This results in a very high starting torque from zero speed to full speed at relatively low starting current. This type of starting is ideal for very high inertia loads allowing the machine to get to full speed in the minimum time with minimum current draw.Transformer

With stator voltage reduction and high and low rotor resistance the curves of Fig. 56 show a wide variation. The break down torque varies inversely as the square of the supply voltage; hence the curves are widely separated for a voltage reduction of 70%.
In case of high rotor resistance motors the drop in speed is considerably more than in case of low rotor resistance motors.

Thus it must be considered while selecting a slip ring induction motor for applications like ‘winching’, suitable speed-torque curves can be obtained by means of variation of rotor resistance and stator voltage.

· CONSTANT TORQUE APPLICATIONS:
For constant torque applications, the machine’s torque requirement is independent of its speed. The torque speed characteristics of the motor should be such that torque remains constant at any speed and is not affected even when the direction of rotation is reversed. Such a characteristics is exhibited by cranes during hoisting, winches, piston pumps acting against constant pressure, conveyors handling constant weight.

· VARIABLE TORQUE APPLICATIONS:
In applications like centrifugal pumps, the motor should be suitable for adjustable speed with variable torque. The torque characteristics of a variable torque load is such that the load falls off rapidly as the motor speed is reduced. The load torque is proportional to the square of the speed.

· CONSTANT HORSE POWER APPLICATIONS:
In this type of applications the product of load torque and speed is constant, for example a typical load of this kind is a winder drive for take-up spool of wire, paper etc. Besides, some kind of lathes, boring machines, milling machines etc, show such a non-linear characteristics. The voltage is “clamped” at a constant level while the frequency is adjusted further to achieve the desired maximum speed. The motor becomes “voltage starved” above the “clamping point” and torque decreases as speed increases, resulting in “CONSTANT HORSE POWER”. Output. Hence this type of drive provides constant horsepower, variable torque operation.

2.STARTING TORQUE
High starting torque with low starting current requires a high rotor resistance. For applications requiring high starting torque with variable speed control, rotor resistance control is used, where external resistance in rotor circuits can be varied statically and steplessly by a high frequency thyristors chopper circuit.

3.COST
The cost is also an important factor for selecting a drive system. By the use of slip recovery system in a slip ring induction motor results in a substantial energy system, which was being wasted as heat dissipation. Hence the sub-synchronous converter cascade system of speed control gives smooth and stepless regulation of speed and also gives substantial energy savings, as the slip power is fed back to the system.

4.EFFICIENCY
While selecting a motor for any industrial applications, it is important to select “Energy efficient motors”. If the losses of the motor are reduced, the reduction of power consumption can be extremely valuable. The losses consist of:-

1.NO LOAD LOSSES: Windage, friction, and core losses
2.LOAD LOSSES: Stator copper losses, rotor copper losses and stray load losses. (for copper windings). For reducing the No-load losses amorphous core laminations are used which can reduce the core losses by more than 70%.

Further there is a correlation between the power factor of the motor, and the motor efficiency, such that the power factor will begin to fall when the efficiency of the motor falls. Hence the efficiency of the motor can be improved by reducing the iron loss in the motor.

5. TYPE OF ENCLOUSER:
Induction motors come in two major frames types:-
Totally enclosed forced air cooled (TEFC)
Drip Proof

The TEFC motor is totally enclosed in either aluminum or cast iron frame with cooling fins running longitudinally on the frame. A fan is fitted externally with cover to blow air along the fins and provide the cooling. This type of motors are often installed outside in the elements with no additional protection.

Drip Proof motors use internal cooling with cooling air drawn through the windings. They are normally vented at both ends with an internal fan. This can lead to more efficient cooling, but requires a clean and dry environment. Selection has to be made out of these two types of enclosure.

6.TEMPERATURE CLASSIFICATION
There are two main temperature classifications applied to induction motors:-
a) Class ‘B’
b) Class ‘F’

Class “B” motors are rated to operate with a maximum coolant temperature of 40 degrees C. This leads to a maximum winding temperature of 120 degrees C,
Class “F” motors are typically rated to operate with a maximum coolant temperature of 40 degrees C and a maximum temperature rise of 100 degrees C resulting in a potential maximum winding temperature of 140 degrees C.

Hence a choice has to be made between the type ‘B’ or type ‘F’

Operating at rated load, but reduced cooling temperatures gives an improved safety margin and increased tolerance for operation under an overload condition. If the coolant temperature is elevated above 40 degrees C, then the motor must be derated to avoid premature failure.

Operating a motor beyond its maximum will not cause an immediate failure; rather decrease in life expectancy of the motor. A common rule is that for every ten degrees C rise in temperature, the expected life span is halved.

14.MOTOR OVERHAULING
Slip ring induction motors should be taken out of service and overhauled periodically, depending upon the severity of service. The following procedure should be followed:-

1.Cleaning of all parts including windings
2.Drying and backing of windings after revarnishing
3.Lubrication of all bearings
4.Checking of slip rings and brush mechanism
5.Checking of insulation resistance

1. CLEANING OF ALL PARTS INCLUDING WINDINGS
Motor windings should be cleaned with some solvent like carbon tetra-chloride, after blowing all dirt and dust. The work should be carried out in well-ventilated place as the fumes of carbon tetra-chloride are poisonous, and inhaling must be avoided. Excessive cleaning liquid should not be used, as it is injurious to insulation. All other parts of the motor should also be cleaned properly.

2. DRYING AND BACKING AFTER VARNISHING
The windings of the motors should be revarnished with a good quality of insulating varnish and then drying should be carried out by keeping in a oven through hot air circulation. It should be noted that the temperature of the windings should not exceed 80 degrees C. While the heating is carried out, insulation resistance test should be recorded every 4 hours. During the heating operation the insulation resistance drops and then gradually rises on constant heating. The heating operation should normally be carried out for four days.

3. LUBRICATION OF ALL BEARINGS
Proper lubrication of bearings is very essential and extends the life of bearings. While correct lubrication is desirable, excessive lubrication forces the lubricant into the machine and may damage the insulation. Under lubrication causes excessive bearing wear resulting into rotor rubbing on the stator or the shaft may wipe the bearing and freeze. Operating conditions of the bearing like proper alignment, proper belt or chain tension should also be attended at the time of lubrication of bearings. Loose belts cause excessive pounding of the bearings and tight belts cause wear by overloading. The amount of bearing wear can be determined by measuring the air gap between the stator and rotor. Measuring of air-gap between the rotor and the stator on both sides and the bottom is the accepted method. Large motors have one bearing insulated from the frame to prevent a current through shaft. This insulation has to be maintained by painting it with insulating varnish. A lead paint may short-circuit the insulation and cause pitting of shaft and bearings.

Sleeve bearings are provided with oil reservoir and settling chambers for collecting dirt. For cleaning, the old oil is drained and the space is cleaned with hot mineral oil and refilled with recommended grade of mineral oil. Ball bearing housing and bearings are cleaned using light heated mineral oil or approved solvent. Initially bearings are cleaned with solvent and then rinsed with light mineral oil to remove any solvent left. Then when the motor is running, grease should be added with hand operated grease gun until it begins to overflow through the relief hole and excess grease is driven out and the relief plug is placed when the motor has run for sufficient time to expel extra grease. While replacing a bearing it is pulled out with a bearing puller and a new one is driven with a block of hardwood.

4. CHECKING OF SLIP RINGS AND BRUSH MECHANISM
Slip rings should be checked for the colour, as the slip rings acquire a protective coating during operation. A good surface protective film is chocolate brown in colour. If this colour has turned jet black, then clean the slip ring surface by a wiper made of several layers of hard woven canvas at the end of a wooden stick, Brush mechanism should be checked for proper alignment and spring tension. Brush seating of brushes to the exact curvature of the slip rings is prerequisite to good performance. Of the brush, Improper seating may cause overheating and sparking which is destructive to slip rings, Before new brushes are fitted, the brush holders must be inspected and if necessary filed smooth. Pigtails or connecting leads to the brushes must be inspected for looseness and frayed wires.

5. CHECKING OF INSULATION RESISTANCE
Insulation resistance should be checked after the motor is overhauled. A megger of over 500 volts should be used for the measurement of insulation resistance. SEO Services