ABB Motor-VSD Sizing

ABB Ltd Application Advice

Selecting a VSD when using an Over sized motor

Author F.T.Griffith

Applications Group

Date 05/07/2011

Introduction

In the past switchgear, contactors etc, were rated to match the current (power) of the associated motor.

This ensured that there was always sufficient thermal margin in the starter components to cater for any variations in actual running duty. The fact that the actual running duty may have been less than the rating of the motor was of no real consequence since the cost of matching switchgear components to the reduced rating did not yield any real cost benefit.

It is natural to apply this same rating philosophy to Variable Speed Drive (VSD) controlled motors. However, this can result in accepting a cost premium which is not really a requirement , especially if the reduced running duty is not expected to change.

VSDs are rated according to CURRENT not Power, so, where a fully rated motor operates at reduced duty, then advantage can be taken of using a lower rated VSD (based on operating current rather than motor RATED current).

This note shows by example how the selection of a lower rated VSD can be of benefit.

Method

The example chosen is a screw compressor rated at 450kW @ 1500 RPM which has operating Duty points of 350kW, 200kW & 100kW respectively. For all of the cases the 450kW 4 pole motor is retained.

Using the DRIVESIZE programme first the motor was selected for a Constant torque load, corresponding to 350kW @ 1500RPM, using this motor data individual inverters were selected for Duty points of 350kW,200kW & 100kW respectively.

The results are shown in the table (next page) below.

The 100kW duty point has two selections for an inverter, the first is one being run at its’ absolute maximum which, although the smallest inverter size, would not be recommended.

All of the inverter selections are smaller than the inverter size that would have been chosen based upon the motor rating.

The inverter selected for a motor rated Duty point would have been a 610-3 (610kVA rated : 500kW) unit. In comparison the selected inverters are as listed below ;

1) 350kW Duty : 440-3 (440kVArated, 355kW)

2) 200kW Duty : 260-3 (260kVArated, 200kW)

3) 100kW Duty : 260-3 (260kVArated, 200kW) or 205-3 (205kVArated, 160kW)



Author F.T.Griffith

Applications Group

Date 05/07/2011

Project Technical Data SheetFile name: Undersize motor

Case: 1

Revision: A

Item Drive name Speed range P Motor nominal values

No. min base max [nbase] Un fwp Pn Poles In cos eff. Tmax

[rpm] [rpm] [rpm] [kW] Type designation [V] [Hz] [kW] [A] phi [%] /Tn

1.1.1 350kW Duty 300 1500 1500 350 M3BP 355 MLB 4 400 50 450 4 780 0.86 97 2.9

1.1.2 100kW Duty Smallest Inverter 300 1500 1500 100 M3BP 355 MLB 4 400 50 450 4 780 0.86 97 2.9

1.1.3 200kW Duty 300 1500 1500 200 M3BP 355 MLB 4 400 50 450 4 780 0.86 97 2.9

1.1.4 100kW Duty Larger Inverter 300 1500 1500 100 M3BP 355 MLB 4 400 50 450 4 780 0.86 97 2.9

Motor calculation result Frequency converter Inu calculation

Temp.rise/ Tcont Tmax Icont Imax In Icont ImaxInsul. margin margin [A] [A] Type designation [A] margin marginB/F 4% 119% 665 665 ACS800-02-0440-3 679 4% 53%

B/F 263% 668% 321 321 ACS800-01-0205-3 285 -10% 9%B/F 81% 284% 431 431 ACS800-02-0260-3 440 3% 36%

B/F 263% 668% 321 321 ACS800-02-0260-3 440 39% 83%

Conclusion

From the short description above it can be seen that lower rated inverters can be chosen to operate with a larger rated motor , than that of the selected Duty point,

As such this shows that where a lower Duty point power is required and using a higher rated motor, say, for mechanical reasons, it is not necessary to select a fully rated inverter for a Duty related to motor rating.

F.T.Griffith Applications Group

Note : Technical charts and data from the DRIVESIZE programme shown in the Appendix.



Author F.T.Griffith

Applications Group

Date 05/07/2011

Motor Technical Data Sheet

Item No. 1.1.1 M3BP 355 MLB 4Specifications DOL Catalogue data

Name [undefined] Product code 3GBP 352 420 (FI)

No.of motors 1 Voltage [V] 400

Motor type Process performance Frequency [Hz] 50

FrameMaterial Not specified Power [kW] 450

Family Not specified Poles 4

Polenumber 4 Speed [rpm] 1490

Design Basic (Cenelec) Max mech.speed [rpm] 2000

Connection Not specified Current [A] 780

IP class IP55 Torque [Nm] 2884

IC class IC411 self ventilated Tmax/Tn 2.9

IM class IM1001, B3(foot) Power factor 0.86

Max. speed rule Standard Efficiency [%] 96.8

Temp. rise B (<80 K) Temperature rise class B

Tmax margin 43% Insulation class F

Motor load Calculations

Load type Constant torque Torque [Nm]

Overload Type Simple n min 2228 2313 4 %

n min [rpm] 300 n base 2228 2579 16 %

n base [rpm] 1500 Power [kW]

n max [rpm] 1500 n min 70 72.7 4 %

Pbase [kW] 350 n base 350 405 16 %

Olbase [%] 100 Overload [Nm]

Olmax [%] 100 n min 2228 5855 163 %

Temperature [°C] 40 n base 2228 4889 119 %

Altitude [m] 1000

Ol definition: No RMS

300-1500 rpm / 100 % 590s

300-1500 rpm / 100 % 10s

Appendix

Motor data



Author F.T.Griffith

Applications Group

Date 05/07/2011

Drive Technical Data Sheet

Item No. 1.1.1 ACS800-02-0440-3Specifications Catalogue data

Name [undefined]

Inverter amount 1 Voltage [V] 400

Type Auto selection Nominal power [kW] 355

Pulse 6-pulse Nominal current [A] 679

Imax [A] 1017

Ihd [A] 590

Pulse 6

Frame type R8

Icont max [A] 693

IP Class IP21

Inverter load Calculations

Load type Constant torque Limit

Overload type Simple cyclic Icont [A] 665 693 4 %

Icont [A] 665 590s Imax [A] 665 1017 53 %

Imax [A] 665 10s Pmax kW 362 474 31 %

cosfii Temperature 6 %

Selection method: IGBT temperature

300kW Duty Inverter



Author F.T.Griffith

Applications Group

Date 05/07/2011



Name [undefined]




Imax [A] 588

Ihd [A] 340

Pulse 6

Frame type R8

Icont max [A] 445

IP Class IP21



Overload type Simple cyclic Icont [A] 431 445 3 %

Icont [A] 431 590s Imax [A] 431 588 36 %

Imax [A] 431 10s Pmax kW 207 241 17 %



200kW & 100kW Duty Inverter



Author F.T.Griffith

Applications Group

Date 05/07/2011



Name [undefined]




Imax [A] 351

Ihd [A] 234

Pulse 6

Frame type R6

Icont max [A] 290

IP Class IP21



Overload type Simple cyclic Icont [A] 321 290 -10 %

Icont [A] 321 590s Imax [A] 321 351 9 %

Imax [A] 321 10s Pmax kW 103 212 105 %



100kW Duty Inverter



Author F.T.Griffith

Applications Group

Date 05/07/2011

0

500

1000

1500

2000

2500

3000

3500

0 200 400 600 800 1000 1200 1400 1600

Speed [rpm]

Torq

ue [N

m]

cont loadability 100kW 200 kW 350 kW

Fig 1 : Operating Torque for differing Duty power



Author F.T.Griffith

Applications Group

Date 05/07/2011

0

100

200

300

400

500

600

700

800

0 200 400 600 800 1000 1200 1400 1600

Speed [rpm]

Torq

ue [N

m]

440-3 260-3 205-3 260-3 350 kW 200 kW 100kW

Fig 2 : Operating Current for differing Duty power

Documents

ABB Motor-VSD Sizing