![INDUCTION MOTORS
STARTING
• EXAMPLE: what is starting current of a 15 hp, 208 V,
code letter F, 3 phase induction motor?
• Maximum kVA / hp is 5.6 max. starting kVA of this
motor is Sstart=15 x 5.6 = 84 kVA
the starting current is thus:
IL=Sstart / [√3 VT] = 84 / [√3 x 208] = 233 A
• Starting current may be reduced by a starting circuit:
a- inductor banks
b- resistor banks
c-reduce motor’s terminal voltage by autotransformer](https://image.slidesharecdn.com/speedcontrolofim-240205122202-f91de2a6/85/Starting-and-speed-control-of-induction-motor-ppt-3-320.jpg)
Starting and speed control of induction motor.ppt
- 1. INDUCTION MOTORS STARTING • An induction motor has the ability to start directly, however direct starting of an induction motor is not advised due to high starting currents, may cause dip in power system voltage; that across-the-line starting not acceptable • for wound rotor, by inserting extra resistance can be reduced; this increase starting torque, but also reduces starting current • For cage type, starting current vary widely depending primarily on motor’s rated power & on effective rotor resistance at starting conditions
- 2. INDUCTION MOTORS STARTING • To determine starting current, need to calculate the starting power required by the induction motor. • A Code Letter designated to each induction motor, which can be seen in figure 7-34, may represent this. (The starting code may be obtained from the motor nameplate) In example: for code letter A; factor of kVA/hp is between 0-3.15 (not include lower bound of next higher class) 3 start L T S I V
- 3. INDUCTION MOTORS STARTING • EXAMPLE: what is starting current of a 15 hp, 208 V, code letter F, 3 phase induction motor? • Maximum kVA / hp is 5.6 max. starting kVA of this motor is Sstart=15 x 5.6 = 84 kVA the starting current is thus: IL=Sstart / [√3 VT] = 84 / [√3 x 208] = 233 A • Starting current may be reduced by a starting circuit: a- inductor banks b- resistor banks c-reduce motor’s terminal voltage by autotransformer
- 4. INDUCTION MOTORS STARTING • Autotransformer starter: • During starting 1 & 3 closed, when motor is nearly up to speed; those contacts opened & 2 closed • Note: as starting current reduced proportional to decrease in voltage, starting torque decreased as square of applied voltage, therefore just a certain reduction possible if motor is to start with a shaft load attached
- 5. INDUCTION MOTORS STARTING • A typical full-voltage (across-the-line) motor magnetic starter circuit
- 6. INDUCTION MOTORS STARTING • Start button pressed, rely coil M energized, & N.O. contacts M1,M2,M3 close • Therefore power supplied to motor & motor starts • Contacts M4 also close which short out starting switch, allowing operator to release it (start button) without removing power from M relay • When stop button pressed, M relay de- energized, & M contacts open, stopping motor
- 7. INDUCTION MOTORS STARTING • A magnetic motor starter circuit has several built-in protective features: 1- short-circuit protection 2- overload protection 3- under-voltage protection • Short-circuit protection provided by fuses F1,F2,F3 • If sudden sh. cct. Develops within motor causes a current (many times greater than rated current) flow; these fuses blow disconnecting motor from supply (however, sh. cct. by a high resistance or excessive motor loads will not be cleared by fuses)
- 8. INDUCTION MOTORS STARTING • Overload protection for motor is provided “OL” relays which consists of 2 parts: an over load heater, and overload contacts • when an induction motor overloaded, it is eventually damaged by excessive heating caused by high currents • However this damage takes time & motor will not be hurt by brief periods of high current (such as starting current) • Undervoltage protection is also provided by controller If voltage applied to motor falls too much, voltage applied to M relay also fall, & relay will de-energize The M contacts open, removing power from motor terminals
- 9. INDUCTION MOTORS STARTING • 3 step resistive starter • Similar to previous, except that there are additional components present to control Removal of starting resistors • Relays 1TD, 2TD, & 3 TD are time-delay relay
- 10. INDUCTION MOTORS STARTING • Start button is pushed in this circuit, M relay energizes and power is applied to motor as before • Since 1TD, 2TD, & 3TD contacts are all open the full starting resistor in series with motor, reducing the starting current • When M contacts close, notice that 1 TD relay is energized, however there is a finite delay before 1TD contacts close, cutting out part of starting resistance & simultaneously energizing 2TD relay • After another delay, 2TD contacts close, cutting out second part of resistor & energizing 3TD relay • Finally 3TD contacts close, & entire starting resistor is out of circuit
- 11. INDUCTION MOTOR SPEED CONTROL • Induction motors are not good machines for applications requiring considerable speed control. • The normal operating range of a typical induction motor is confined to less than 5% slip, and the speed variation is more or less proportional to the load • Since PRCL = s PAG , if slip is made higher, rotor copper losses will be high as well • There are basically 2 general methods to control induction motor’s speed: - Varying synchronous speed - Varying slip
- 12. INDUCTION MOTOR SPEED CONTROL • nsync= 120 fe / p • so the only ways to change nsync is (1) changing electrical frequency (2) changing number of poles • slip control can be accomplished, either by varying rotor resistance, or terminal voltage of motor • Speed Control by Pole Changing • Two major approaches: 1- method of consequent poles 2- multiple stator windings
- 13. INDUCTION MOTOR SPEED CONTROL 1- method of consequent poles relies on the fact that number of poles in stator windings can easily changed by a factor of 2:1, with simple changes in coil connections - a 2-pole stator winding for pole changing. Very small rotor pitch • In next figure for windings of phase “a” of a 2 pole stator, method is illustrated
- 14. INDUCTION MOTOR SPEED CONTROL • A view of one phase of a pole changing winding • In fig(a) , current flow in phase a, causes magnetic field leave stator in upper phase group (N) & enters stator in lower phase group (S), producing 2 stator magnetic poles
- 15. INDUCTION MOTOR SPEED CONTROL • Now, if direction of current flow in lower phase group reversed, magnetic field leave stator in both upper phase group, & lower phase group, each will be a North pole while flux in machine must return to stator between two phase groups, producing a pair of consequent south magnetic poles (twice as many as before) • Rotor in such a motor is of cage design, and a cage rotor always has as many poles as there are in stator • when motor reconnected from 2 pole to 4 pole , resulting maximum torque is the same (for :constant-torque connection) half its previous value (for: square-law-torque connection used for fans, etc.), depending on how the stator windings are rearranged • Next figure, shows possible stator connections & their effect on torque-speed
- 16. INDUCTION MOTOR SPEED CONTROL • Possible connections of stator coils in a pole-changing motor, together with resulting torque-speed characteristics: (a) constant-torque connection : power capabilities remain constant in both high & low speed connections (b) constant hp connection: power capabilities of motor remain approximately constant in both high- speed & low-speed connections (c) Fan torque connection: torque capabilities of motor change with speed in same manner as fan-type loads Shown in next figure
- 17. INDUCTION MOTOR SPEED CONTROL Figure of possible connections of stator coils in a pole changing motor (a) constant-torque Connection: torque capabilities of motor remain approximately constant in both high-speed & low-speed connection (b) Constant-hp connection: power capabilities of motor remain approximately constant in … (c) Fan torque connection:
- 18. INDUCTION MOTOR SPEED CONTROL • Major Disadvantage of consequent-pole method of changing speed: speeds must be in ratio of 2:1 • traditional method to overcome the limitation: employ multiple stator windings with different numbers of poles & to energize only set at a time Example: a motor may wound with 4 pole & a set of 6 pole stator windings, then its sync. Speed on a 60 Hz system could be switched from 1800 to 1200 r/min simply by supplying power to other set of windings • however multiple stator windings increase expense of motor & used only it is absolutely necessary • Combining method of consequent poles with multiple stator windings a 4 –speed motor can be developed Example: with separate 4 & 6 pole windings, it is possible to produce a 60 Hz motor capable of running at 600, 900, 1200, and 1800 r/min
- 19. INDUCTION MOTOR SPEED CONTROL • Speed Control by Changing Line Frequency • Changing the electrical frequency will change the synchronous speed of the machine • Changing the electrical frequency would also require an adjustment to the terminal voltage in order to maintain the same amount of flux level in the machine core. If not the machine will experience (a) Core saturation (non linearity effects) (b) Excessive magnetization current
- 20. INDUCTION MOTOR SPEED CONTROL • Varying frequency with or without adjustment to the terminal voltage may give 2 different effects : (a) Vary frequency, stator voltage adjusted – generally vary speed and maintain operating torque (b) Vary Frequency, stator voltage maintained – able to achieve higher speeds but a reduction of torque as speed is increased • There may also be instances where both characteristics are needed in the motor operation; hence it may be combined to give both effects • With the arrival of solid-state devices/power electronics, line frequency change is easy to achieved and it is more flexible for a variety of machines and application • Can be employed for control of speed over a range from a little as 5% of base speed up to about twice base speed
- 21. INDUCTION MOTOR SPEED CONTROL • Running below base speed, the terminal voltage should be reduced linearly with decreasing stator frequency • This process called derating, failing to do that cause saturation and excessive magnetization current (if fe decreased by 10% & voltage remain constant flux increase by 10% and cause increase in magnetization current) • When voltage applied varied linearly with frequency below base speed, flux remain approximately constant, & maximum torque remain fairly high, therefore maximum power rating of motor must be decreased linearly with frequency to protect stator cct. From overheating • Power supplied to : √3 VLIL cosθ should be decreased if terminal voltage decreased • Figures (7-42 )
- 22. INDUCTION MOTOR SPEED CONTROL • Variable-frequency speed control (a) family of torque-speed characteristic curves for speed below base speed (assuming line voltage derated linearly with frequency (b) Family of torque-speed characteristic curves for speeds above base speed, assuming line voltage held constant
- 23. INDUCTION MOTOR SPEED CONTROL • Speed control by changing Line Voltage • Torque developed by induction motor is proportional to square of applied voltage • Varying the terminal voltage will vary the operating speed but with also a variation of operating torque • In terms of the range of speed variations, it is not significant hence this method is only suitable for small motors only
- 24. INDUCTION MOTOR SPEED CONTROL • Variable-line-voltage speed control
- 25. INDUCTION MOTOR SPEED CONTROL • Speed control by changing rotor resistance • In wound rotor, it is possible to change the torque-speed curve by inserting extra resistances into rotor cct. • However, inserting extra resistances into rotor cct. seriously reduces efficiency • Such a method of speed control normally used for short periods, to avoid low efficiency