Selection Criteria for Ac Drives and Motors - Spm

7/28/2019 Selection Criteria for Ac Drives and Motors - Spm

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AC MOTOR AND DRIVE SELECTION

AC Motor Selection

kWm = mechanical kW requirement (shaft kW) as per M&K List (kW)

N1 = base speed as per motor list. Upto this speed, the motor will operate in the “constant torque” zone. (rpm)N2 = maximum speed as per motor list. In this region, the motor operates in “constant power” zone. (rpm)

P = Number of poles (nos)

N = rated RPM of the motor (rpm)

Tm = mechanical torque required at base speed (Nm)

Tm = 9550 X kWm / N1

Amb = ambient tmperature in deg C

K AMB = ambient deration factor as follows

< 30 deg C = 1.07

30 – 40 deg C = 1.00

45 deg C = 0.96

50 deg C = 0.92

55 deg C = 0.87

60 deg C = 0.82

All above factors are applicable for altitude upto 1000 metres

C-Tq = ratio of speed deration (minimum speed/ rated speed)

= minimum speed at which the motor is required to deliver the rated torque continuously

When a self cooled motor is run at low speed, the cooling fan which is mounted on the motor shaft also

rotates at a lower speed, thereby causing a reduction in cooling capacity of the motor. The motor, under

this condition cannot be loaded upto its rated torque value. Thus a corresponding deration is to beconsidered.

KCT = deration factors due to speed below rated speed

1 : 2 (C-Tq = 2) = 1.0

1 : 5 (C-Tq = 5) = 0.9

1 : 10 (C-Tq = 10) = 0.7 *

* for 8 Pole motors running at speed > 1024 rpm, this deration factor is 0.61

Note : For force cooled motors, the KCT value is 0.95, irrespective of the speed range and number of poles.

Note : For motors driving loads with square law characteristics (fan, pump etc), the KCT value is 1.0,

irrespective of the speed range.

T1 = Torque required to be delivered by the motor so that the deration factors can be taken care of.

T1 = Tm / KCT / K AMB

Note : Incase the temperature rise is to be limited to Class B limits, Tm is to be further divided by 0.9.

TFR = Frame Torque of the motor selected.

The motor is to be selected so that the TFR > T1 ⇐ CHK1

CLIENT : WUXI PACIFIC SALES REF : A-104870001

MILL BUILDER : FPE, INDIA S. O : 30166954

SKIN PASS MILL – SM01 ITEM No. : 10 QTY. : 1 X

D ATE: 28.01.03 DESCRIPTION :

PREP.ATD AC MOTOR AND DRIVE SELECTION

CKD. WORK :Siemens Ltd SH.

ISSUE REMARKS D ATE N AME HEAD OFFICE DETAILS : DESIGN CALCULATION 1ORIGINAL / REPLACEMENT FOR / DRG. NO. 5REPLACED BY ; FPE-SM-01-E-GN-15-0-120-A SH.



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authority. Offenders will be liable for damages. All rights

including rights created by patent grant or registration



AC Motor Selection (cont’)

Irated = rated current as per the selected motor frame rating

EFFrated = efficiency as per the selected motor frame rating

PFrated = power factor as per the selected motor frame rating

TBW = Breakdown torque or pullout torque of the motor

TBW/TFR Ratio of the Breakdown torque : rated torque is selected as per the selected motor frame

TBW = TBW/TFR X TFR

When the motor is run at frequencies above 50 Hz (i.e. rated speed N), the voltage is maintained constant

and only the frequency is increased. This results in reduced V/f ratio and hence weakens the magnetic flux

in the motor.

The breakdown torque of the motor is proportional to the (V/f)2 . Hence at higher frequencies, theBreakdown torque reduces rapidly. Hence the magin between the Breakdown torque and the operating

torque keeps reducing. For satisfactory and stable operation, the Breakdown torque should be atleast

1.5 times the operating torque of the motor.

Tm2 = mechanical torque required at maximum speed (Nm)

T2 = Torque to be delivered by the motor at peak speed so that the deration factors can be taken care of.

T2 = 9550 X kWm / N2 / K AMB

FWK = Field weakening ratio = Maximum speed / rated speed of the motor

TBW2 = Breakdown torque or pullout torque of the motor at peak speed (maximum frequency)

= TBW / (FWK)2

T2A = Maximum permissible torque at peak speed

= TBW2 / 1.5

T2A should be greater than T2 ⇐ CHK 2

Incase of Payoff Reel and Tension Reel, special tailor-made motors are selected to provide improvedDynamics. For these motors, the above calculation is not required. The motor data sheets areAttached.





PREP.ATD AC MOTOR AND DRIVE SELECTIONCKD. WORK :

Siemens Ltd SH.

ISSUE REMARKS D ATE N AME HEAD OFFICE DETAILS : DESIGN CALCULATION 2

ORIGINAL / REPLACEMENT FOR / DRG. NO. 5REPLACED BY ; FPE-SM-01-E-GN-15-0-120-A SH.









Inverter Selection

The following data are picked up from the motor calculation for drive selection

Irated = rated current as per the selected motor frame rating (amps)

PFrated = power factor as per the selected motor frame rating

TFR = Frame Torque of the motor selected. (Nm)

Tm = mechanical torque required at base speed (Nm)

PFrated = cos φ

The motor will continue to draw its rated magnetising current, irrespective of the actual load. The load

component of the current will vary and will be proportional to the mechanical torque required.

Imagn = Magnetisation current of the motor (amps)

Imagn = Irated X √(1- cos2 φ)

Iload rated = Rated load component of the current (amps)

= Irated X cos φIload actual = Actual load component

= Tm / TFR X Iload rated

Iactual = Actual current drawn by the motor to deliver Tm

= √(I2load actual + I2magn)

O/L Overload due to acceleration

During acceleration, the motor will draw larger current and deliver larger torque due to the moment of inertia

of the load. 150% is considered as a reasonable value for acceleration

During acceleration, the torque to be delivered by the motor increases

Tmmax = Maximum torque demand of the load

= Tm X O/L

Iload max = Maximum load component = Tmmax / TFR X Iload ratednote 1)

Imotmax = Maximum current drawn by the motor to deliver Tmmax

= √(I2load max + I

2magn)

Note 1 Due to overload in torque, only the load component of the motor increases. The magnetising current remainsconstant.






Siemens Ltd SH.

ISSUE REMARKS D ATE N AME HEAD OFFICE DETAILS : DESIGN CALCULATION 3












Inverter Selection (cont’)

Iinv = continuous current to be delivered by the inverter

= Iactual / KTA

KTA = ambient deration factor for Inverters

= 1.0 for ambient upto 40 deg C

Iinvmax = maximum current to be delivered by the inverter

= Imotmax / KTA

The inverter should be so selected so that IG > Iinv and Imax > Iinvmax

Rectifier Selection

While calculating the rectifier rating, we have to consider that some of the motors will be in regenerationmode while the others will be in motoring mode. This there will be a load balancing at the dc bus level.

To calculate this, the actual current drawn by each inverter from the dc bus is to be calculated

Idc-act = actual current drawn by each inverter from the dc bus

= Iactual / IUN X rated dc link current

After the Idc-act is calculated for each inverter, a regenerative factor is applied on this depending on the

application. The regeneative factor can be – 0.6 to –0.7 for the high inertia regenerative drives. However,

in the present case, we have considered –0.5 as the regenerative factor for all the regenerative drives.

RF = Regenerative factor

= 1.0 for motoring drives; -0.5 for regenerative drives

Idctotal = n Σ Idc-act X RF where n = number of inverters

The calculation is done for two cases – i.e with Bridle and without Bridle. Incase the mill is run with bridles, themill stand motor takes about 20% of the load (refer tension diagram).

The Rectifier is to be selected so that the base load dc link current of the rectifier is greater than I dctotal

Incase there are multiple rectifiers connected in parallel, the resultant base dc link current is as follows

= 0.9 X n Σ Ibase dc link where n = number of rectifiers in parallel






Siemens Ltd SH.

ISSUE REMARKS D ATE N AME HEAD OFFICE DETAILS : : DESIGN CALCULATION 5


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Selection Criteria for Ac Drives and Motors - Spm