Syafruddin Hasan

Motorinputin

stator

Statorcopperandironlosses

Rotorinput

Rotorcopperlosse

MechPower

developedor

GrossTorque

Windageand

Frictionlosses

RotoroutputorBHP

Power Flow DiagramPower Flow Diagram

Input powerPsup

Stator Copper loss3 Ista

2 Rsta

Rotor Copper loss3 Irot

2 Rrot

Stator Iron loss3 Vsta

2 / Rc

Ventilation andfriction losses

Output powerPout

Air gappower Pag

Developed powerPdv = 3 Irot

2 Rrot (1-s)/s

Air gap

Figure Motor energy balance flow diagram.

Induction MotorsInduction MotorsFigure shows the energy balance in a

motor. The supply power is:

The power transferred through the air gap by the magnetic coupling is the input power (Psup) minus the stator copper loss and the magnetizing (stator iron) loss.

The electrically developed power (Pdv) is the difference between the air gap power (Pag) and rotor copper loss.

*stasupsup IVS 3Re)Re(Psup

Induction MotorsInduction Motors

The electrically developed power can be computed from the power dissipated in the second term of rotor resistance:

The subtraction of the mechanical ventilation and friction losses (Pmloss) from the developed power gives the mechanical output power

s

sR trot

13P _

2

dv rot_tI

mlossdvout PPP

Power and Torque in an IMPower and Torque in an IM

From Fig.7-12Input current : I1 = VΦ / Zeq

Pcu Stator: Pscl = 3 I12R1

Core Losses : Pcore = 3 E12 Gc

Air-gap power : PAG = Pin – Pscl – Pcore = 3I22R2/s

Pcu Rotor: PRcl = 3 IR2RR = 3 I22R2

Note

PAG : Pconv : PRCL = 1 : (1-s) : S

Example 7-3

Induction MotorsInduction MotorsThe motor efficiency:

Motor torque (shaft load torque):

sup

out

P

P

m

outP

T

Equivalent CircuitEquivalent Circuit

Stator Rotor

Xrot_m = rot Lrot Rrot

Irot

Vrot = s Vrot_s

Rsta Irot_t

VstaVsup

IstaXmRc

Xsta = sy Lsta

Figure Single-phase equivalent circuit of a three-phase induction motor.

Induction MotorsInduction Motors

Stator Rotor

Xsta Xrot Rrot/s

IrotVrot_s

Rsta Irot_t

VstaVsup

IstaXmRc

Figure Modified equivalent circuit of a three-phase induction

motor.

The rotor impedance is transferred to the stator side. This eliminates the transformer

Induction MotorsInduction Motors

Figure Simplified equivalent circuit of a three-phase induction motor.

Stator Rotor

Xsta Xrot_t Rrot_t/s

Irot_tVsta

Rsta

Vsup

IstaXmRc

Air gap

Induction MotorsInduction MotorsThe last modification of the equivalent

circuit is the separation of the rotor resistance into two parts:

The obtained resistance represents the outgoing mechanical power

trottrot

trot Rs

sR

s

R__

_ 1

trotR

s

s_

1

Blocked-Rotor testBlocked-Rotor test (cont…)(cont…)

Vblocked_ln

Vblocked

3 Vblocked_ln 21.939 V

Pblocked_A

Pblocked

3 Pblocked_A 160 W

Re

Pblocked_A

Iblocked2

Rrot_t Re Rsta

The stator resistance was measured directly

IM Design ClassesIM Design Classes

NEMA & IEC :Class A: normal Tst, normal Ist, normal S

Class B: normal Tst, low Ist, low S

Class C: high Tst, low Ist, low S

Class D: high Tst, low Ist, high S

Class E: low Tst, normal Ist, low S

Class F: low Tst, low Ist, normal S

Starting Induction MotorsStarting Induction Motors

Problems : - High Starting current - Low starting torque

Determining of Istart

Read the rated voltage, hp and code letter from name plate.

Starting apparent power :Sstart = (rated hp)(code letter factor)

Istart = Sstart / √3 VT

Example 7-7

Q U I Z Q U I Z 04-10-200504-10-2005

1. Why the circuit equivalent of an induction motor can be approached by equivalent circuit of a transformer and draw it

2. The output of a 3 phase induction motor is 9 kW. Rotor copper losses is 0.5 kW.

Motor runs at 5 % of slip. Stator loss is 0.75 kWa). Calculate the mechanical power that converter by this motorb). Determine the input powerc). Calculate the efficiency

Speed Control of IMSpeed Control of IM ns = 120 f / p

by changing the electrical frequency (f)

by changing the number of poles (p)(1) The methode of consequent poles(2) Multiple stator windings

by changing the line voltage:n proportional to V2

by changing the rotor resistance Solid-state IM drives

Motor ProtectionMotor ProtectionOver Current ProtectionOver Load Protection

IM RatingsIM Ratings1. Output power2. Voltage3. Current4. Power Factor5. Speed6. Nominal efficiency7. NEMA design Class8. Starting Code9. Service Factor (SF)