IET
Decrease font size
Increase font size
Topic Title: Reduce Motor Starting Current
Topic Summary:
Created On: 01 October 2012 12:06 PM
Status: Read Only
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 01 October 2012 12:06 PM
User is offline View Users Profile Print this message



thamee97

Posts: 14
Joined: 13 February 2003

Greetings All,

One of our industrial customers need to reduce their starting current on the motors currently direct-on-line. The motor supplier recommended that VFDs can not be used with the blowers installed at site as they are not designed to accept frequency variation and up to 10 second starting is needed to avoid any issue related to critical speed for "over-hang" type compressors. Is there any other way of industrial practice in starting to bring down the current for such type of motors?

Regards
 01 October 2012 01:42 PM
User is offline View Users Profile Print this message


Avatar for aroscoe.
aroscoe

Posts: 91
Joined: 18 October 2002

Depending on the present wiring of your motors and the accessibility of terminals, you might have the option of starting them in star and then changing to delta, but this will only help if they are presently wired in delta and you can access the terminals to add the required switching to make the changeover.

-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 01 October 2012 08:12 PM
User is offline View Users Profile Print this message



dlane

Posts: 690
Joined: 28 September 2007

Hi,

To reduce the starting current of a motor you need to reduce the starting voltage and the most popular way of doing that, as already mentioned above, is to utilise a star-delta starting system if the motor is suitable. The cost of the control panel and some more wiring will probably be the most economical option anyway.

If star-delta cannot be used then the alternative to reducing the voltage is to use an auto-transformer starter, a reactor starter or a resistance starter. These 3 methods would probably be able to utilise the existing motor. The modern equivalent to theses starters is the soft-starter which is similar in operation to a VSD but only varies the voltage through a controllable ramp rate. If the motor has a frequent start rate though you may struggle to get a device rated for the number of starts required.

If none of those options are applicable then you start to look at Low Inrush Current motors. ABB are one manufacturer you could look at, but this is probably going to be the most expensive option for you. They work exactly the same as a DOL motor but are specifically wound to reduce the starting current.

Kind regards

Donald Lane
 01 October 2012 08:26 PM
User is offline View Users Profile Print this message


Avatar for Dave69.
Dave69

Posts: 441
Joined: 16 July 2011

lets start with the basic questions
what size are the motors?
how many motor terminals are there in the motor?
how many starts per hour?
 01 October 2012 10:33 PM
User is offline View Users Profile Print this message



jcm256

Posts: 1840
Joined: 01 April 2006

The letter code below should help in determine what your motor/s starting current is.
Yes, pity none are wound rotor motor, opens a whole lot of other possibilities.
LOCKED ROTOR CURRENT
Steady state current taken from the line with the rotor at standstill (at rated voltage and frequency). This is the current seen when starting the motor and load.

INRUSH CURRENT
When started with full line voltage, AC motors draw line currents substantially greater than their full load running current rating. The actual magnitude of this current is called "inrush current." It is a function of the motor horsepower and design characteristics. It is also called Locked Rotor Current.
A letter used to indicate the "Design Code Rating" on the nameplate is referred to as the "Code Letter." The Code letter of the motor is an indication of the locked rotor KVA per horsepower. It is a function of the motor's design. Code letter ratings indicate the starting current motor will draw. Code letters below F indicates a low starting current; beyond F indicates a high starting current.
The motor's Code Letter is helpful in determining the maximum rating of the motor's electrical circuit protection. A replacement motor should have the same rating as its predecessor.


Code KVA/HP
A 0-3.14
B 3.15-3.54
C 3.55-3.99
D 4.0-4.49
E 4.5-4.99
F 5.0-5.59
G 5.6-6.29
H 6.3-7.09
J 7.1-7.99
K 8.0-8.99
A 0-3.14
B 3.15-3.54
C 3.55-3.99
D 4.0-4.49
E 4.5-4.99
F 5.0-5.59
G 5.6-6.29
H 6.3-7.09
J 7.1-7.99
K 8.0-8.99
L 9.0-9.99
M 10.0-11.19
N 11.2-12.49
P 12.5-13.99
R 14.0-15.99
S 16.0-17.99
T 18.0-19.99
U 20.0-22.39
V 22.4 & UP

Edited: 01 October 2012 at 10:40 PM by jcm256
 02 October 2012 04:20 AM
User is offline View Users Profile Print this message



Jaymack

Posts: 4586
Joined: 07 April 2004

Originally posted by: thamee97
One of our industrial customers need to reduce their starting current on the motors currently direct-on-line. The motor supplier recommended that VFDs can not be used with the blowers installed at site as they are not designed to accept frequency variation and up to 10 second starting is needed to avoid any issue related to critical speed for "over-hang" type compressors. Is there any other way of industrial practice in starting to bring down the current for such type of motors?

I wonder why there is a need to reduce the starting current ........ tariff/M.D. or capacity/voltage drop? It doesn't bode well for any future additional loads, time to increase the supply capacity perhaps. You could look at a mechanical solution such as initial throttling of the air, or a hydraulic coupling for the load takeup .......... after the motor is started off load. Expensive methods, but maybe borne of necessity.

Regards
 03 October 2012 07:40 AM
User is offline View Users Profile Print this message



thamee97

Posts: 14
Joined: 13 February 2003

Thanks for all the inputs.

Please find below three -Phase - induction motor (with Squirrel Cage Rotor) Operating and installation Data :

Rated:
Power 1170kW Torque class KL
Voltage 6000V Connection Y
Frequency 50Hz Class of rating S1
Current 130A Absolute alitude <1000m ab.s.l
Speed 2973 r/min Coolant temperature (Water) 35 °C
Torque 3758Nm Therm class (design/uti)155 (F) /130(B)
Power Factor 0.9
Torque of driven machine: M-n2 up to 59% Mrequired
Driven Machine: Turbocompressor
Required Power 1056kW
Moment of Inertia 16.5kgm2
Starting Time 4s (at U/Un=1) and 6s(at U/Un=.8)
Perm no. of starts 3 cold, 2 warm (1000/year)
Recommended start: (0-->3000 rpm =full motor speed ) in max 10 seconds, 4 to 6 seconds in certain particular cases, up to 10s to avoid any issue related to the critical speed for "over -hang " compressors.

Reducing motor stating is required as it is causing unnecessary trip of upstream power utility feeder and limiting the capacity of upstream power transformer, which is also serving several other customer loads.

Your earliest feedback is appreciated.

Regards,
 03 October 2012 11:18 AM
User is offline View Users Profile Print this message



dlane

Posts: 690
Joined: 28 September 2007

Okay, lets forget about the star-delta starter then....

At that kind of voltage and power my initial thoughts would be a reactor or auto-transformer starter, my first option would be to speak to someone at ABB. Laurence Scott may be able to help but they are mostly motor manufacturers, they would certainly be able to help with a low inrush current motor which I would look at as a price comparison.

I have seen VSDs at that voltage but you may be able to get a soft starter unit, again I would probably look at ABB for some guidance.

Whatever you do, its going to be a big dent in the pocket for this kind of equipment and probably also quite long delivery times and installation.

I would be curious as to how the situation has arisen. Are you the new load that has been added and have created the issue or have you always required this kind of load for many years and then the situation has arisen by the DNO allowing other loads to be connected to the local transformer? If it is the latter, I think I would be arranging a meeting or 2 with the DNO.

Kind regards

Donald Lane
 03 October 2012 07:39 PM
User is offline View Users Profile Print this message



stableford

Posts: 64
Joined: 04 April 2006

Looking at the data supplied its a HV unit, so any solution will have a high cost and long lead time. You dont see megawatt+ sized motors every day.

I had wondered whether a soft start(VFD could also fill this need) doing PWM modification of the line voltage only(no frequency adjustment as many VFDs do) with current feedback, and time control, coupled with a bypass contactor might work. This might avoid the harmonics issue, and reduce the starting surge.

Possibly someone with an oil and gas distribution background might be able to offer an insite, as the application sounds very much like a gas compressor station.
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.