Variable speed drives and soft starters 2

[majuba]

Hi

Can somebody please explain me the difference between VSDs and soft starters?

Will a softstarter cause a motor to trip while the motor is running?

How can I check if a soft starter is faulty?

 

[Marke]

Hello majuba

Soft starters and speed drives (VSD) are completely different systems.

A soft starter is just a solid state reduced voltage starter. It will reduce the start current and start torque alowing the motor and machine to start more gently that with Full voltage starter. The soft starter has a number of advantages over traditional reduced voltage starters, but the main advantage is that there are no steps in the output voltage, current and torque. The soft starter is not open transition, infact there is no transition so there are no transients. Being electronic, the start curves can be customised to better start the load than with electromechanical starters. The soft starter will always cause the motor to operate at line frequency and therefore design speed except during start and stop when the motor is essentially in a semi stalled state. The voltage applied to the motor is controlled by SCRs in series with one or more phases of the supply to the motor.

A Variable speed drive is designed to operate the motor at speeds other than design speed. This is achieved by changing the voltage and frequency of the voltage applied to the motor. The incoming supply is rectified and converted to DC and then is chopped up and converted back to AC. The frequency of the output voltage is set to the speed that the motor is required to operate. The voltage applied to the motor is controlled in a manner to limit the flux in the stator. With a VSD, the motor is never operated under high slip conditions, to start the machine, the frequency of the VSD is started from 0 Hz and ramped up to the final speed.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[GGOSS]

Hello majuba,

As Marke has done an excelent job of explaing the difference between soft starters and VSD's, there's no need for any additional comment there.

Below are my comments to your other questions.

Q> Will a softstarter cause a motor to trip while the motor is running?

A> Most soft starters offer some form of motor and/or system protection. If the soft starter detects an abnormal condition it may trip to protect the motor/system from potential damage.

Q> How can I check if a soft starter is faulty?

A> The shorty answer is, please refer to the troubleshooting section of the installation manual. Unfortunately to cover all fault possibilities here is simply not possible.

If you do not have a copy of the installation manual, please advise soft starter manufacturer and part number details and I'll guide you further.

Regards,
GGOSS

 

[jbartos]

Suggestion: Visit

http://www.thomasregister.com

and type Controls: Motor, Soft Start under Product or Service, which will return 67 companies to approach to for more detail product literature.
The autotransformer also provides a smooth soft start, and it is preferred for applications where the electronic soft starter output is not acceptable because of waveform distortion, harmonic, EMI interference, ground fault currents, etc.
Again, visit the above site and type Drives: Motor under Product or Service, which will return 169 companies to inquire about suitable literature.
Again, the output of the VFD should be checked for its waveform, EMI, ground fault currents, etc.
The soft starter may trip during motor run if its input parameters are out of its designed range, and if its output parameters are out of its designed range.
The soft starter troubleshooting instructions should be followed. Contact the soft starter manufacturer tech support for details.

 

[CHIPIE]

Majuba,

Further to the above comments, I wish to add that although auto-transformer starting is one of the smoothest methods of electro-mechanical starting it still has transitions. The number of transitions can vary. These transitions are not as significant in magnitude to a DOL transision but will still create transitent (as with any step change).

A soft starter on the other hand will provide a smooth acceleration from zero speed to full speed.

The comments made by jbartos above regarding soft starters and the effect of harmonics, EMI and ground faults seem to relate to VSDs and not soft starters.

If your soft starter trip is occuring whilst running (not during soft start/stop) it is most likely that it is the motor protection that is causing the trip to proctect the motor (as indicated by GGOSS). This could be a factor of how your motor protection is setup or it could be actually telling you that there is a problem developing with the motor or load. The manufacturers user manual should be able to direct you to the fault based on the fault code or message received.

Regards,
Chipie

 

[jbartos]

Comment: If one examines technical data of a soft starter carefully, one may notice filters on the output side. Visit

http://www.wegelectric.com/Brochures/ssw03-04/ssw03-04.pdf

for an optional EMC output filter

 

[Marke]

jbartos
Soft starters do cause harmonic currents to flow during start only. These harmonic are low order odd harmonics and do not cause problems with the motor or supply. Soft starters do not cause additional harmonic voltages or currents to flow once up to speed except for those using an energy saving algorithm. You do not need to add harmonic filtering to soft starters. Occasionally, there may be a problem with radio frequency interference during start in very weak signal areas. I suspect that the filter you refer to is for suppression of RFI during start.
The soft starter provides line voltage and frequency to the motor once up to speed in the same way that a contactor does.
Best regards

Mark Empson

http://www.lmphotonics.com

 

[GGOSS]

Hello Jbartos,

I have reviewed the document referred to via your web link, and have further discussed its contents with engineering team at WEG. There are a few points worth noting.

1. The WEG SSW-04 soft starter product is manufactured and tested to the applicable standard IEC 60947-4-2. As certification to this standard is not possible without compliance to all relevant low voltage EMC directives, the product is also EMC compliant and CE marked.

2. The additional filter suggested in the brochure is NOT for harmonic mitigation purposes. It is in fact an additional EMC filter to further reduce conducted emmissions.

3. The engineer I spoke to at WEG suggested this additional filter was intended for use in (Class B environments (domestic, hospital etc) and NOT required for industrial application as noted in the brochure. He further commented that the contents of the brochure appeared to be in-correct and that he would confirm accordingly.

4. The harmonics produced by soft starters are low order odd harmonics that do not cause problems with the motor or supply. As they are also transitional (present during soft starting and soft stopping only) soft starters do not contraviene any standards relating to harmonic genneration. You should note these comments apply only to soft starters that do NOT offer features such as energy saving, phase rebalancing and controlled running features eg slow speed operation etc.

Regards,
GGOSS