1. AC Machines

2. Contents Electric Motors AC Motors Synchronous AC Motors
Asynchronous AC Motors
Losses in Motors
Power Calculations for Motors

3. Electric Motors
If an electric current flows through a conductor in a magnetic field, a magnetic force effects the conductor. A simple electric motor can be formed if this conductor has a point to rotate around. Faraday electric motor and Barlow Wheel are the first experimental representations of the electric motor.

4. Electric Motors Faraday Electric Motor
There is a free rotating wire which is inserted in a glass full of mercury (or salt water) in Faraday Electric Motor. The glass full f mercury has a permanent magnet on center.
If a current flow through the wire, it starts to rotate.
This motion is the representation of the magnetic field produced because of the current flows on a wire.

5. Electric Motors
An electric current passes through the hub of the wheel to a mercury contact on the rim; this is contained in a small trough through which the rim passes. Due to health and safety considerations brine (salt water) is sometimes used today in place of mercury. The interaction of the current with the magnetic field of a U-magnet causes the wheel to rotate. The presence of serrations on the wheel is unnecessary and the apparatus will work with a round metal disk, usually made of copper.

6. What is an Electric Motor?
An electric motor is a machine that converts electrical energy to mechanical energy.
Used is compressors, pumps, air condition fans, electric vehicles, robot mechanisms, cranes, etc.
More than the two thirds of the load in industry are the load of electric motors.

7. Electric Motors
It is the ‘Lorentz Force’ that effects the charge of ‘q’ which has the velocity of ‘V’ in magnetic field ‘B’. The directions of this force, the current and the magnetic field can be seen in the figure.
|FL|=q V B Sin α

8. Electric Motors
The rotating part of an electric motor is called as rotor whereas the fixed part as stator. If the rotor is consist of windings, brushes are used to transfer the current. The brush is a carbon part which has a contact with the terminals of the coils on the axle of the rotor. In some DC motors, permanent magnets are used in rotors and these types are called as Brushless DC Motors (BLDCM). The problem for these types is to sense the position of the rotor. Information about the position of the rotor is needed to be sent to the driver of the motor.

9. Electric Motors
In some AC motors, Aluminum bars are used as the rotor of the motor. These type of motors are called as Squirrel Cage type electric motors. When the current is changing periodically, the rotor follows the current.

10. Electric Motors

11. Alternate Current Motors (ACM) Induction (Asynchronous
Electric Motors
Electric Motors
Alternate Current Motors (ACM)
Direct Current Motors
(DCM)
Synchronous
Induction (Asynchronous
Three Phase
Mono Phase
Self-Exited
Externally Excited
Series
Schunt
Compound

12. Alternatif Akım Motorları
Alternate Current Motors (ACM)
Synchronous ACM
Induction (Asynchronous) ACM
Squirrel Cage ACM
Slip Ring ACM

13. Alternate Current Motors
The ACM’s are simplier in structure and more economic than DCM’s.
An ACM generates more power comparing with a DC motor that has the same weight. Maintenance of ACM’s is easier. However, their speed control is harder. They can be connected to the AC source directly. If accuracy in velocity or position control is needed, DCM’s are used. But, ACM’s are used more than DCM’s in industry.

14. ACM’s Basic Definitions
Free Running Current (I0): It is the current consumed from the grid with nominal voltage and frequency, but without any load on motor.
Maximum Starting Current (Ik): It is the maximum current on nominal voltage and frequency when starting a motor.
Starting Torque (MA): It is the torque generated by the motor during starting under nominal voltage and frequency.
Nominal Moment (MN): It is the toque generated by the motor under nominal power and speed.
Stall Torque (Mk): It is the maximum torque generated by the motor with nominal voltage and frequency.
Pull-up Torque (Ms): It is the minimum torque delivered by the motor with nominal voltage and frequency, between zero velocity and the velocity with the stall torque.
1 kgm = 9,81 Nm ~ 10 Nm,
MN = 9550 x Nominal Power [kW] /Nominal Velocity of Rotor [RPM]

15. ACM’s Basic Definitions MA Torque [Nm] Revolution [RPM] MS MK MN

16. Synchronous Machines
“ Synchronous Machine is a machine that runs at a constant speed which is proportional to frequency and number of poles. It can be run as a generator or a motor. However, because of the constant running speed these machines are generally used as generators. They are the most common machines used in power plants. They can be manufactured to generate electricity up to 2000 [MVA]. Cost effectivity due to unit power generated, higher efficiency in greater power generation, less maintenance and control processes made them to be manufactured in greater powers.
(*see references)

17. Synchronous Machines
Stators of Synchronous Machines are manufactured using laminated cores which have slots to place the coils on them.
Synchronous Machines are divided into two groups according to the structure of the rotor that has exiting coil on it.
If the airgap between the stator and rotor is constant every where, then it is a round rotor (turbo) machine. Unless, it is a salient pole synchronous machine.
22 [MW], 13.8 [kV], 3,600 [RPM] *

18. Synchronous Machines
Round rotor synchronous generators are manufactured in small number poles and high synchronous revolution per minute. They are used in high velocity steam turbines. The length of the rotor is long and radius of the rotor is small in this type of turbines.
The salient pole synchronous machines are generally have more poles and are designed for lower synchronous rotational velocity.
Length of the rotors are short and the radius of the rotors are long. Salient pole synchronous machines are used in hydro elecric power plants and for compensating the power factor of the grid.
ns : velocity of the synchronous machine
f : frequency of the source
p : number of poles
ns = 120 f / p

19. Asynchronous Machine Mono Phase Induction Machine
Has only one stator coil.
Uses only one phase.
Rotor of an asynchronous machine can be a squirrel cage.
Needs a unit to start to motor.
Are used in applications needs 3 ~ 4 HP (Fans, washing machines, household devices… etc.)

20. Asynchronous Machines
Three Phase Induction Machine
Magnetic field is generated by three phases
Rotor can be either squirrel cage or composed of coils
Can be started easily
Has great power capacities
There are applications from 1/3 HP to hundreds of HPs: Pumps, compressors, conveyor drums, grinding machines and etc.
More than 70 % of the motors in industry are three phase induction machines.

21. Three Phase Asynchronous Machine
Industrial loads or high power loads are needed to be connected to three phase grid whose phases follow each other in 120 degrees instead of mono phase grid. Result of this usage is smaller currents.

22. Three Phase Asynchronous Machine

23. Three Phase Asynchronous Machine
Rotating Magnetic Field:
In a three phase AC motor, a rotating field might be achived using the coils which are located geometrically around stator (see Figure below).
Terminals for a three phase asynchronous machine:
Phase R  input terminal: U, output terminal X
Phase S  input terminal: V, output terminal Y
Phase T  input terminal: W, output terminal Z t1

24. Three Phase Asynchronous Machine
Rotating Magnetic Field in a Three Phase Machine
i,u t1 t2 t3 t4 t6 t5 R S T
1200 1 2 3 t1

25. Three Phase Asynchronous Machine
Rotating Magnetic Field in a Three Phase Machine t1 t2 t3 t4 t6 t5
Time Interval IR IS IT t1 + - t2 t3 t4 t5 t6 t1

26. Three Phase Asynchronous Machine
WYE Connection t1

27. Three Phase Asynchronous Machine
Delta Connection t1

28. Three Phase Asynchronous Machine
Three Phase Asynchronous Motor
If the frequency of the flowing current is f ,the number of rotation (or synchronous number of rotation or nember of rotation of rotating field) is n. Equation of the number of rotation of magnetic field is given below in unit of RPM.
f [Hz]: Frequency of the source
p [ ]: Number of pole pairs
Ns = 60 f / p
Three phase asynchronous machines do not form sparks. Their number of rotation do not change so much with changing loads. Thus they are said to be constant speed motors. Thus, they are called as constant speed machines. Their efficiencies are high. If the three phase grid is not present then the monophase motors are used. t1

29. Three Phase Asynchronous Machine
Speed and Slip
In an asynchronous motor, the speed of the magnetic field generated by the stator coils and the rotation speed of the rotor is not the same. The value of the rotational speed of the rotor is always smaller than the speed of the stator’s magnetic field. The reason of the word ‘asynchronous ’ is this. The difference of these speed is called as the slip. If ‘s’ is negative (rotor’s speed is greater) then the electric machine is running as a generator.
s = [(Ns – Nr)/ Ns ] x 100
The slip s is defined as 'the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm or in percent or ratio of synchronous speed'.
s [ %]: Slip
Ns [RPM]: Speed of the magnetic field.
Nr [RPM]: Rotational speed of the rotor t1

30. Asynchronous Machine
Efficiency – Speed – Torque Curves t1

31. Asynchronous Machine
Slip Ring Type

32. Losses in Electric Motors
Notation
Losses of mechanical frictions
Pks
Iron loss(hysteresis and eddy current losses)
PkFe
Ohmic power loss of armature
Pka

33. Losses in Electric Motors
Losses in Asynchronous Motors Pe
Losses
Notation
Friction and air flow losses
Pfw
Iron loss
Pfe
Loss of conductor
(stator - copper) PS
(rotor - alluminium) PR
Additinal load loss
PXL 2 3 1 4 1 Pm 5
Pfw
Pfe PS PR
PXL

34. Losses in Electric Motors
Losses in Asynchronous Motors Pe
Friction and Air Flow
They are constant losses during motor run, independent from load and occur in bearings and cooling fan propellers. Pm
Iron Loss
Pfw
Pfe
Total affects of losses in cores of coils (hysteresis and eddy current losses). It can be neglected even the rotor composed of coils since the frequency of the induced voltage is low. It might be observed as heat in laminated cores when the motor is running. It is dependent to the material, thickness and dimensions of the laminated core, the frequency applied to the motor and the square of the voltage applied to the motor. It is constant if the frequency and the voltage that the motor is connected do not change. PS PR
PXL

35. Losses in Electric Motors
Losses in Asynchronous Motors Pe
Conductor Loss (Stator)
It is heat loss. The current flow through the stator coils creats this loss (I2RS ). Pm
Conductor Loss (Rotor)
It is heat loss. The current flow through the stator coils or cage bars creats this loss (I2RR ).
Pfw
Pfe PS
Additional Load Loss PR
PXL
It is the loss occurs in metal parts of the motor except the laminated cores in rotor and stator because of the leakage because of the load.
Losses
Friction and Air Flow Losses
% 0,5 ~ 1,5
Iron loss
% 1,5 ~ 2,5
Conductor loss (stator)
% 2,5 ~ 4,0
Conductor Loss (rotor)
Additional load losses
% 0,5 ~ 2,5

36. Power Calculations in Electric Motors
i,u t1 t2 t3 t4 t6 t5 R S T
Colours of wires (TS 6429)
Blue- Brown – Black – Gray – Yellow+Green

37. Power Calculations in Electric Motors
The nominal power of a DC motor might be expressed as the equation below. UDC [V] is the voltage applied to the motor, and IDC [A] is the current flow. Pinput [W] is the electrical power, Poutput [W] is the mechanical power or the nominal power, ωm [RPM] is the rotational speed of the axle of the motor, Tm [Nm] is the torque generated by the motor. Ploss [W] is the power loss, η [%] is the efficiency of the motor.
Pinput =UDC IDC
Poutput =ωm Tm
Ploss = Pinput − Poutput
η=( Poutput / Pinput )x100

38. Power Calculations in Electric Motors
In AC motors, because of the changing current characteristics, there is an important point that , there are three powers called as apparent, true and reactive. In AC motors, current is lagging voltage with angle . This divides the power into two vector parts.
[VAC] [Hz]
I [Amper] Lm Rm

39. Power Calculations in Electric Motors
Reactive (blind) Power
Q = U I sin
[VAR]
P = U I cos 
[Watt]
Apparent (imaginary) Power
S = I U
[VA]
ϕ = P / S
True (real) Power

40. Power Calculations in Electric Motors
Example: A mono phase asynchronous motor draws 12.3[A] from grid and its power factor is measured as What are the powers consumed?
Apparent Power= S = U I = 220 x 12,3 = 2706 [kVA]
Active Power = P = U I Cos  =220 x 12,3 x 0,94 = 2544 [kW]
Reactive Power = Q = U I sin  = 923 [kVAR]

41. Power Calculations in Electric Motors
Power Calculations in Three Phase Electric Motors
In a balanced three phase circuit:
P = √ 3 x U x I x cos 
Q = √ 3 x U x I x sin 
S = √ 3 x U x I
P : True Power [Watt] ;
Q : Reactive Power [VAR];
S : App. Power [VA]
U : 380 [V]  phase to phase voltage: 380 [V].
I : Current drawn from one phase: [A]

42. Power Calculations in Electric Motors
Power Calculations in Three Phase Electric Motors
The current drawn from the three phase grid by an asynchronous alternate current motor is 7 [A] and the power factor of the motor is measured as What are the powers consumed from the grid?
P : True Power [Watt] ; Q : Reactive Power [VAR]; S : App.Power [VA]
U : 380 [V]  Voltage btw. Phases: 380 [V]
I : Current drawn from one phase: [A]
Power in a balanced three phase circuit:
P = √ 3 x U x I x cos  = √ 3 x 380 x 7 x 0,85 = 3916 [W]
Q = √ 3 x U x I x sin  = √ 3 x 380 x 7 x 0,5268 = 2427 [VAR]
S = √ 3 x U x I = √ 3 x 380 x 7 = 4607 [VA]

43. Thanks for listening . References: http://ocw.mit.edu
Asenkron Elektrik Motorları, Ali Taner, 2011.