What does it mean when we “control the speed”?  According to Fig.1. working point is defined by intersection between motor torque and load characteristics M m = M t  Control the speed means that we want to change motor torque characteristics M m in M m ‘ in order to get the intersection between the load characteristics just with DESIRED SPEED n! Motor torque caharacteristics M m is changing by changing synchronous motor speed n s in n s ‘ - changing stator voltage FREQUENCY, see Fig.1. Fig.1. Motor torque M m and load characteristics M t

 Speed of the motor is: The change of the pair poles is possible only in discrete way, i.e. 1, 2, 3,.. pair poles. That kind of speed control was used in the past time, when.power electronics components were expansive. For some discrete pair poles values, and constant frequency, the synchronous speed is: Keeping frequency constant, f= 50 Hz, and changing the pole pairs, we get for: p=1, synchronous speed is 3000 rpm, p=2, synchronous speed is 1500 rpm, p=3, synchronous speed is 1000 rpm. Example: IM for washing machine. Motor has two windings, one with 2p=2, (for high speed) and another with 2p=12, for low speed (14). Switching windings from 2 poles to 12 poles configuration by mechanical switch, we switch speed from 3000 rpm to 500 rpm! Electromechanical Systems – how to change the speed of IM (1)

IM speed control by stator voltage Define positive and negative aspects of IM voltage control ? If the voltage supply changing under constant frequency, machine torque is changing according to M em ~f(U 2 ). For any other voltage motor has new characteristics. Because load characteristics usually doesn’t change, then working point will be defined by motor’s characteristics. We have two working points at n t and n f ; load is M t, voltage is U n and 0,8U n Fig. 2. Voltage control characteristics M=f(U), a), Motor’s characteristics with nominal stator voltage Un and 0,8U n ; load is M t, b). a) b)

IM speed control by stator voltage and frequency Rotor speed is directly proportional to frequency, Changing the frequency, magnetic field (induction) in motor changes as well. As a consequence, developed torque is changes. (4) If the stator voltage frequency f 1 decrease and stator voltage keep constant, magnetic induction B as well as magnetic field Φ will increase. Increasing induction B is not acceptable because of magnetic saturation. If the frequency f 1 increase, magnetic field Φ decrease (5), as well as motor torque (not desired!). Because of that, speed is controlled by (7): (7) (3) (4) (5) (6)

It is clear why we want to control the speed of induction motor by U/f=const.(3). Main reason is that changing the voltage and frequency in the same way, magnetic field keeps constant !!! Frequently, for such control we use the term SCALAR CONTROL !! Fig. 3. Motor torque characteristics (f 1 -f 3 ) with scalar control, a), motor torque characteristics (f 1 -f 5 ) with load torque characteristics, b). IM speed control by stator voltage and frequency a) b)

U/f=const. control for different type of loads  For one case study, choose one frequency (one motor characteristics) and compare the slip for constant and centrifugal load. Comment!! Fig. 4. Motor torque characteristics (in the region of small slip) with different frequencies - with constant load, a) and centrifugal load b). a) b)

f 1 is base frequency for which is motor is built. Decreasing the voltage U 1 and base frequency f 1 we are getting new characteristics, U 21 / f 21 = const.,ie. voltage and frequency are decreased proportionaly. This reagion is called Constant torque region. Increasing frequency and voltage according U/f = const. is not alowed, because voltage transcend nominal value! Increasing only frequency (without increasing voltage) magnetic fields (6) and motor torque (4) decreasing. This is the region of field weakening, or Constant power region. Scalar control (U/f=const.) for extended speed region Region of constant maximal Torque and constant maximal Power

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