Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Starting Power Factor for Soft Starter (electronic type)

Starting Power Factor for Soft Starter (electronic type)

Starting Power Factor for Soft Starter (electronic type)

hi everyone,

I am hoping someone can advise me a typical starting power factor for an electronic soft starter.
The sort starter in question is SIEMENS 3RW5235-6AC04 rated at 75kW

The datesheet is attached and copied in this link below

Any comments would be highly appreciated


RE: Starting Power Factor for Soft Starter (electronic type)

Did you mean the power factor of the motor when started with a soft starter?
Why do you want to know?
Normally we concern ourselves with power factor to avoid power factor penalties and to lower KVA demand.
It is also a concern in sizing transformers generators and conductors.
PF Penalties are based either directly or indirectly on KVARHrs. (Kilo Volt Amp Hours.)
Motor starting times are typically measured in single digit seconds.
With a ratio of 1/3600 between KVAR Hours and KVAR seconds the effect of motor starting on overall Power Factor is negligible and generally not considered.
KVA demand measuring meters typically take 15 minutes to completely respond to a change in KVA.
In 3 minutes the demand will have registered 63% of the final demand.
In 36 seconds, the demand will have registered 40% of the final demand.
In 7.2 seconds the demand will have registered 25% of the final demand.
If motor starting is staggered, the effect of starting one motor in a large plant may be negligible.
The total motor running current must be considered in regards to transformer and conductor sizing to avoid overheating.
Motor starting time is so short that the heating effects on conductors and transformers may generally be ignored.
Peak current is a concern in regards to instantaneous trip settings on protective devices but Soft Starters reduce the starting current and so PF is of little concern.

BUT, that said;
Consider a motor start at 600% of running current and torque of 250% of rated torque.
Torque is related to current squared so we may estimate the real current at root 2.5 = 1.58 rounded to 160%
The reciprocal of 600% apparent current over 160% real current = 27%
Disclaimer. This result is a WAG (Wild Assed Guess)
I leave it to you to speculate on how a Soft Starter will skew this guesstimate of PF percentage.

Ohm's law
Not just a good idea;
It's the LAW!

RE: Starting Power Factor for Soft Starter (electronic type)

Also there are a few kinds of power factor to discuss when talking about electronics that can distort the waveform. Displacement power factor is associated with the angle between fundamental voltage and current (that's what most of us think of when we say power factor). Distortion power factor accounts for harmonics in the current. The overall power factor (real power over apparent power including effects of current harmonics) is something like the product of these two. I'm not positive I have the terminology right but that's the way I remember it. Link here.

(2B)+(2B)' ?

RE: Starting Power Factor for Soft Starter (electronic type)

The power factor starts very low or very inductive and then begins to improve as the motor approaches full speed. It could start -0.2 and ramp up to -0.9 then settle to the running power factor depending on load. This happens with a motor starter full-voltage too.

RE: Starting Power Factor for Soft Starter (electronic type)

As the others have said, Power Factor has little to do with the Soft Starter. Yes, there is some harmonic distortion while ramping that would represent a little more distortion power factor on top of the motor power factor, but it's not enough or happens long enough to be concerned about. But for the most part, the PF of the CIRCUIT will be determined by the motor, not the Soft Starter. The Soft Starter itself neither corrects nor (significantly) aggravates the PF.

Why don't you express what your real CONCERN is when asking about the PF?

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Starting Power Factor for Soft Starter (electronic type)

The power factor of an induction motor at 100% slip, (zero shaft speed) is typically in the order of 0.1 - 0.15. The actual value is dependent on the particular motor design.
As the motor accelerates and the slip reduces, the power factor increases reaching a maximum at the speed of maximum torque availability (almost full speed.)

Connecting a soft starter ahead of the motor will chop the waveform and lead to high harmonics and in addition to the poor displacement power factor, it will introduce a poor harmonic power factor component, however the displacement power factor will be the dominant power factor component at zero shaft speed.
I do not have any figures for the true power factor of a combined soft starter and induction motor under high slip conditions. I usually design around the displacement power factor only under these conditions.
Best regards,

Mark Empson
Advanced Motor Control Ltd

RE: Starting Power Factor for Soft Starter (electronic type)

thanks you guys

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close