Induction Motor Control Theory (3) - Finish

7. Frame Classification

Induction motors come in two major frame types, these being Totally Enclosed Forced air Cooled (TEFC), and Drip proof.
The TEFC motor is totally enclosed in either an aluminium or cast iron frame with cooling fins running longitudinally on the frame. A fan is fitted externally with a cover to blow air along the fins and provide the cooling. These motors are often installed outside in the elements with no additional protection and so are typically designed to IP55 or better.

Drip proof motors use internal cooling with the 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 that the environment is clean and dry to prevent insulation degradation from dust, dirt and moisture. Drip proof motors are typically IP22 or IP23.

8. Temperature Classification

There are two main temperature classifications applied to induction motors. These being Class B and Class F.The temperature class refers to the maximum allowable temperature rise of the motor windings at a specified maximum coolant temperature. Class B motors are rated to operate with a maximum coolant temperature of 40 degrees C and a maximum winding temperature rise of 80 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. 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. Note: Some Class F motors are designed for a maximum coolant temperature of 60 degrees C, and so there is no derating necessary up to this temperature.Operating a motor beyond its maximum, will not cause an immediate failure, rather a decrease in the life expectancy of that motor. A common rule of thumb applied to insulation degradation, is that for every ten degree C rise in temperature, the expected life span is halved. Note: the power dissipated in the windings is the copper loss which is proportional to the square of the current, so an increase of 10% in the current drawn, will give an increase of 21% in the copper loss, and therefore an increase of 21% in the temperature rise which is 16.8 degrees C for a Class B motor, and 21 degrees C for a Class F motor. This approximates to the life being reduced to a quarter of that expected if the coolant is at 40 degrees C. Likewise operating the motor in an environment of 50 degrees C at rated load will elevate the insulation temperature by 10 degrees C and halve the life expectancy of the motor.

9. Power factor correction

Power factor correction is achieved by the addition of capacitors across the supply to neutralise the inductive component of the current. The power factor correction may be applied either as automatic bank correction at the main plant switchboard, or as static correction installed and controlled at each starter in such a fashion that it is only in circuit when the motor is on line. Automatic bank correction consists of a number of banks of power factor correction capacitors, each controlled by a contactor which in turn is controlled by a power factor controller. The power factor controller monitors the supply coming into the switchboard and adds sufficient capacitance to neutralise the inductive current. These controllers are usually set to adjust the power factor to 0.9 - 0.95 lagging. (inductive)Static correction is controlled by a contactor when the motor is started and when the motor is stopped. In the case of a Direct On Line starter, the capacitors are often controlled by the main DOL contactor which is also controlling the motor. With static correction, it is important that the motor is under corrected rather than over corrected. This is because the capacitance and the inductance of the motor form a resonant circuit. While the motor is connected to the supply, there is no problem. Once the motor is disconnected from the supply, it begins to decelerate. As it decelerates, it generates voltage at the frequency at which it is rotating. If the capacitive reactance equals the inductive reactance, i.e. unity power factor, we have resonance. If the motor is critically corrected (pf = 1) or over corrected, then as the motor slows, the voltage it is generating will pass through the resonant frequency set up between the motor and the capacitors. If this happens, major problems can occur. There will be very high voltages developed across the motor terminals and capacitors causing insulation damage, high resonant currents can flow, and transient torque's generated can cause mechanical equipment failure. The correct method for sizing static correction capacitors, is to determine the magnetising current of the motor being corrected, and connect sufficient capacitance to give 80% current neutralisation. Charts and formula based on motor size alone can be totally erroneous and should be avoided if possible. There are some power authorities who specify a fixed amount of KVAR per kilowatt, independent of the size or speed. This is a dangerous practice.

10. Single phase motors

In order for a motor to develop a rotating torque in one direction, it is important that the magnetic field rotates in one direction only. In the case of the three phase motor, there is no problem and the field follows the phase sequence. If voltage is applied to a single winding, there are still multiples of two poles which alternate between North and South at the supply frequency, but there is no set rotation for the vectors. This field can be correctly considered to be two vectors rotating in opposite directions. To establish a direction of rotation for the vector, a second phase must be added. The second phase is applied to a second winding and is derived from the first phase by using the phase shift of a capacitor in a capacitor start motor, or inductance and resistance in an induction start motor. (sometimes known as a split phase motor.) Small motors use techniques such as a shaded pole to set the direction of rotation of the motor.

11. Slip Ring Motors

Slip ring motors or wound rotor motors are a variation on the standard cage induction motors. The slip ring motor has a set of windings on the rotor which are not short circuited, but are terminated to a set of slip rings for connection to external resistors and contactors. The slip ring motor enables the starting characteristics of the motor to be totally controlled and modified to suit the load. A particular high 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 gradually moved towards synchronous speed. This results in a very high starting torque from zero speed to full speed at a 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. The down side of the slip ring motor is that the sliprings and brush assemblies need regular maintenance which is a cost not applicable to the standard cage motor. If the rotor windings are shorted and a start is attempted, i.e the motor is converted to a standard induction motor, it will exhibit an extremely high locked rotor current, typically as high as 1400% and a very low locked rotor torque, perhaps as low as 60%. In most applications, this is not an option. Another use of the slipring motor is as a means of speed control. By modifying the speed torque curve, by altering the rotor resistors, the speed at which the motor will drive a particular load can be altered. This has been used in winching type applications, but does result in a lot of heat generated in the rotor resistors and consequential drop in overall efficiency.

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