Maximum Start Current With Soft Starters

[marke]

Hi

 

Most of the soft starters out on the market that have a current limit, have a maximum current limit setting that seems to vary from as low as 400% current, to around 550% current.

 

Some soft starters are designed for a maximum start current of 400% and that is a thermal limit. You could oversize the starters to increase the thermal limit, but there is a problem with the current limit setting.

 

With modern "Hi Efficiency" motors, the locked rotor current has trended upwards from an average of around 600 - 650% to an average of 700 - 750%.

This means that to develope a high torque start as required for loaded conveyors, mono pumps, loaded drying drums and high inertia loads, the start current is required to be well above 400% rated current.

 

With the standard motors that were used 20 years ago, it was very rare for a start current to be required to be above 500%. Today, to achieve the same start torque, the current may need to be closer to 600%.

This raises the issue, "what are the manufacturers doing to enable soft starters to be used in these applications?" If you considerably oversize the starter in order to remain within reasonable thermal limits, you loose the protection features of the soft starter and you need to add external protection devices.

With some starters, you can turn OFF the current limit and operate in an open loop voltage ramp mode. This may be OK for a loaded conveyor, but would not be any good for a high inertia load.

 

Are there any starters out ther that cater for the 580% start current required in starting the loaded conveyor referred to in another post?

 

Best regards,

[GGOSS]

Although the preference is to keep all communications here of a technical ntaure, there are some commercial implications for those manfacturing and/or selling soft starters.

 

As we all know, it has been the aim of most soft start manufacturers to produce products that are more commercially competitive against electro-mechanical RVS alternatives in order to make it easier for customers to migrate to soft start technology.

 

However as the typical values for motor LRC have risen considerably, any cost reductions achieved through improved manufacturing techniques has been off-set through the need to now offer a larger soft starter (for a given application) that may have been necessary say 5 years ago.

 

As an example, with a standard efficiency motor, an un-loaded screw compressor could easilly be started with a current limitation of 3.5 x FLC over an approximate 8 to 10 second period. With a high efficiency motor we find that the very same compressor now requires 4.2 x FLC for 12 to 15 seconds. The longer 'run up time' also implies that we do not have the same level of acceleration torque and therefore if we wanted to start within the 8 to 10 second period, and even higher level of starting current would be required.

 

There's lots of other information that can be added here and I may attend to that in subsequent posts.

 

Regards,

GGOSS

[niallnz]

Hello Marke

 

Interesting point. I checked the specs on the ABB PST range of Softstarters, and its current limit can only goto 500%. However, it does have a "Kck Start" function that will apply a percentage of voltage to the motor for a set period of time, before reverting to a current limit start up. The voltage applied to the motor is setable from 50% to 100%, and can be applied for between 0.1 and 1.5 seconds. With is function you can apply the required 580% current but only for a maximum of 1.5 seconds, and hope that the conveyor starts moving enough for the standard current limit start to take over and continue to accelerate the machine.

 

Cheers

Niallnz

[marke]

I see this as a very real problem in some installations and propose that in a non commercial manner, we identify starters that can be used for higher start currents and how that can be done.

I would suggest that posters respect the impartiality of this forum and avoid emotional or non factual posts. If it appears to get out of hand, I will delete inappropriate posts.

 

What are we interested in?

• Maximum Current limit setting
• Maximum starter rating with and without oversizing
• Integrity of internal motor protection when higher start currents used.
• Can the current limit be defeated?
• Can the trip curve be altered for high inertia high torque starts. i.e. class 10 - class 30, or motor start time constant etc.

Any other ideas?

 

Please post about starters with a separate reply per starter. If there is enough quality information, I may make a sub thread and sort the postings by starter type.

 

Best regards,

[marke]

AuCom IMS2

• Standard rating : 300% current.
• Starter can be rerated up to 550% current by changing models.
• Current limit can be set up to 550% current.
• Current limit can not be defeated.
• Motor protection fully operational and correct up to 550% current.
• Starter could be rated above 550% byoversizing, but the motor protection would be compromised.
• Trip curve can be tuned to a match the motor by setting the motor start time constant.

[marke]

Danfoss MCD3000

• Standard rating : 300% current.
• Starter can be rerated up to 550% current by changing models.
• Current limit can be set up to 550% curent.
• Current limit can not be defeated.
• Motor protection fully operational and correct up to 550% current.
• Starter could be rated above 550% by oversizing, but the motor protection would be compromised.
• Trip curve can be tuned to a match the motor by setting the motor start time constant.

[marke]

Emotron MSF

• Standard rating : 500% current.
• Current limit can be set up to 500% curent.
• Current limit can be defeated.
• Torque ramp start could be used for high torque start.
• Motor protection fully operational and correct over 500% current if unit over sized and torque ramp used.
• Starter could be rated above 500% by oversizing. Motor protection would not be compromised unless current limit set to 500% and the motor current setting increased.
• Trip curve can be tuned to a match the motor by setting the motor start time constant.

[niallnz]

ABB PST

• Standard rating IEC60947-4-2 AC53a
• Current limit 500%
• Current limit cannot be directly defeated, but has "Kick Start" Function
• Motor protection fully operational to and correct to 800%
• Motor protection curve selectable class 10A, 10, 20 & 30, separate protection class can be selected for starting and running

[jraef]

Motortronics XLD Series can be adjusted up to 600% and are rated 500% for 60 seconds. They have also been tested at 600% for 30 seconds, but not beyond, because the thinking was that this covered a "Mill Duty" class 30 OL motor application. Their new VMX Series can also be adjusted to 600% CL but is rated for only 10 seconds at that level (for use with Class 10 OL protection) and 350% for 30 seconds.

 

 

Not that they pay me to care anymore......

[marke]

Schneider Altistart 48

• Standard duty 400% 12 seconds or 300% 23 seconds
• Severe duty applications 400% 25 seconds
• Standard duty protection Class 10
• Severe duty protection Class 20
• Max Current limit setting 500%

[GGOSS]

Rockwell Automation / Allen Bradley SMC 3

 

Rated to IEC 60947-4-2 AC53b:3.5-15:3585 Note: varies between models, also important to view published starts/hour curves as more starts are possible than those implied by the IEC rating.

Max Current Limit Level 450%

Timed Voltage Ramp (TVR) and Kick-Start also available.

Inbuilt electronic motor overload protection with adjustable trip class (OFF, 10, 15 & 20)

 

Rockwell Automation / Allen Bradley SMC Delta

 

Rated to IEC 60947-4-2 AC53b:3.5-15:3585 Note: varies between models, also important to view published starts/hour curves as more starts are possible than those implied by the IEC rating.

Max Current Limit Level 350%

Inbuilt electronic motor overload protection with adjustable trip class (OFF, 10, 15 & 20)

 

General Note: The motor must operate within the FLC setting range at all times. Therefore oversizing SMC 3 & SMC Delta products to compensate for higher starting currents may lead to nuisance tripping, particularly if the motor runs below the minimum FLC setting during periods of unloaded operation.

 

Rockwell Automation / Allen Bradley SMC Flex

 

Rated to IEC 60947-4-2 AC53b:3-50:1750 Note: suspect there may be other published data for max starts/hour as per SMC 3 & Delta products but have not sighted them.

Max Current Limit Level 600%

Timed Voltage Ramp (TVR) and Kick-Start also available.

Inbuilt electronic motor overload protection with adjustable trip class (OFF, 10, 15 20 & 30)

 

Regards,

GGOSS

 

 

[mariomaggi]

Dear all,

I've not yet seen on the market a software for soft starters that take into consideration current harmonics during starting.

I'm convinced that with a better software it is possible to get better results, using the same power devices.

 

Regards

Mario

[jOmega]

 

I've not yet seen on the market a software for soft starters that take into consideration current harmonics during starting.

I'm convinced that with a better software it is possible to get better results, using the same power devices.

 

Regards

Mario

 

 

Ciao Mario,

 

Perhaps you could provide additional details and specifically address what your concerns are .... and how you think that 'current harmonics' affect the performance of SCR type SoftStarters.

 

Please be mindful that SCR devices are only gated switches; i.e., they are told when to turn-on but are dependent upon the commutation of the ac supply voltage to turn them off.

 

Thank you.

jΩ

 

[mariomaggi]

Dear jOmega,

this is only an idea in my mind, I've not seen any practical study or web site about this specific argument.

Shortly, during starting you can see that the 3 currents at the oscilloscope have many harmonics. Some of these current harmonics generates momentarily some braking torques (i.e., 5th and 11th and 17th harmonics).

 

Looking to motor side, during starting you will have in a 3-phase motor a complex system of currents at different frequencies. If you are able to avoid certain conduction angles you will avoid to work in some critical situation, where the acceleration torque is lowered by counter-rotating fields generating braking torque during acceleration.

 

At the end, using the same semiconductors and the same level of true Irms, you will have a better starting torque and less heating.

 

Regards

Mario

[jOmega]

this is only an idea in my mind, I've not seen any practical study or web site about this specific argument.

Shortly, during starting you can see that the 3 currents at the oscilloscope have many harmonics. Some of these current harmonics generates momentarily some braking torques (i.e., 5th and 11th and 17th harmonics).

 

Looking to motor side, during starting you will have in a 3-phase motor a complex system of currents at different frequencies. If you are able to avoid certain conduction angles you will avoid to work in some critical situation, where the acceleration torque is lowered by counter-rotating fields generating braking torque during acceleration.

 

At the end, using the same semiconductors and the same level of true Irms, you will have a better starting torque and less heating.

 

Ciao Mario,

 

Thank you for sharing your thoughts. It is most opportune that you give attention to your ideas at this time.

 

The July/August 2006 issue of IEEE TRANSACTIONS ON INDUSTRY APPLICATION Volume 42, Number 4; has an article entitled, "Soft-Started Induction Motor Modelling and Heating Issues for Different Starting Profiles Using a Flux Linkage ABC Frame of Reference".

 

The article is authored by Mark G. Solverson, MSc., Behrooz Mirafzal, PhD., and Nabeel A.O. Demerdash, PhD..

 

From the article Abstract— :

 

" .... Investigation of semi-optimum soft-starting voltage profiles was implemented using a flux linkage ABC frame of reference model of a soft-started three-phase induction motor. A state-space model of the soft-starter thyristor switching sequence for the motor and load was developed and implemented in a time-domain simulation to examine winding heating and shaft stress issues for different starting profiles. Simulation resulfts of line starts and soft starts were compared with measured data through which validation of the model was established. "

 

In the article, "different induction machine soft-start profiles are shown, and comparisons of starting times, torque profiles, and heating losses are made."

 

The empirical data was obtained using a fan load on the motor.

 

 

From the article, I offer this for your consideration ....

 

 

" A Fourier transform spectral analysis was performed on the measured and simulated starting current waveforms to provide added validation to the simulation. The spectrum ..... shows good agreement for the 1st, 5th, 7th, 11th, and 13th harmonics. " [/b]

 

"Although the spectrum ..... illustrates the added harmonics that are imposed on the system, it is a relatively short transient event and is not a major concern for most applications. "

 

From examination of the spectral analysis graphic, the 5th harmonic was ≈ 20% of the fundamental; the 7th harmonic was ≈ 6.25% of the fundamental; 11th harmonic was 1.5% of the fundamental, and the 13th harmonic was ≈ 2% of the fundamental ...

 

Truly not a significant contributor to the Heating Index determined in the study.

-----

 

From my previous post, Mario, .......... because SCRs (thrystors) can only be gated on... and require commutaiton of the the ac source to turn them off... manipulation of the firing angle alone, in a schema to to reduce harmonic currents, is impractical if not impossible.. A further concern would be the introduction of a dc component into the system by changing voltage wafeform symmetry in order to control harmonic currents. This, in and of itself, would further exacerbate the harmonic currents and introduce even harmonics into the spectrum; which would contribute to increased heating as well as torque pulsations.

 

Kind regards,

jΩ

[mariomaggi]

Dear jOmega,

thank you very much for your info.

Being a small self-employed, I've not access to many high-level information.

 

Kindest regards

Mario

[robwill77]

Hi

 

Most of the soft starters out on the market that have a current limit, have a maximum current limit setting that seems to vary from as low as 400% current, to around 550% current.

 

Some soft starters are designed for a maximum start current of 400% and that is a thermal limit. You could oversize the starters to increase the thermal limit, but there is a problem with the current limit setting.

 

With modern "Hi Efficiency" motors, the locked rotor current has trended upwards from an average of around 600 - 650% to an average of 700 - 750%.

This means that to develope a high torque start as required for loaded conveyors, mono pumps, loaded drying drums and high inertia loads, the start current is required to be well above 400% rated current.

 

With the standard motors that were used 20 years ago, it was very rare for a start current to be required to be above 500%. Today, to achieve the same start torque, the current may need to be closer to 600%.

This raises the issue, "what are the manufacturers doing to enable soft starters to be used in these applications?" If you considerably oversize the starter in order to remain within reasonable thermal limits, you loose the protection features of the soft starter and you need to add external protection devices.

With some starters, you can turn OFF the current limit and operate in an open loop voltage ramp mode. This may be OK for a loaded conveyor, but would not be any good for a high inertia load.

 

Are there any starters out ther that cater for the 580% start current required in starting the loaded conveyor referred to in another post?

 

Best regards,

 

[jraef]

jOmega,

Thanks for that information, I'm going to look for that paper. In the mean time, did they offer an assessment as to whether soft starting increases motor heating compared to other forms of starting? I have heard arguments on both sides and I have taken the position that heating is equal, just spread out over a longer period of time. Some Allen Bradley engineers wrote a very good white paper on this a few years back which forms the basis of my conclusions, but unfortunately A-B has chosen to discontinue providing access to that paper. I'd be curious if this study addressed that issue, even indirectly.

[GGOSS]

Hello Jraef & jOmega,

 

My undertanding is that the I2t remains the same in almost all cases. The exceptions seem to result from commissioning error and occur when the soft starter's current limit settings are too low thereby causing the motor to labour excessively throughout the starting period.

 

Technically speaking, a good soft starter with features like excessive start time protection and/or motor thermal modelling will still protect the motor IF of-course they have been left at factory default (genrally over-protecting) or increased beyond that correctly.

 

It is also fair to say that similar over-heating will occur with electromechanical reduced voltage starters that have been incorrectly applied or set up.

 

Hope that helps in some small way.

 

Regards,

GGOSS

[jOmega]

jOmega,

...........................In the mean time, did they offer an assessment as to whether soft starting increases motor heating compared to other forms of starting? ................. X snip X

 

JRaef,

 

I've read, and re-read the article and am unable to find any statements pertinent to a comparison of thermal stress consequent to DOL vs Soft-Start.

 

Here is a quote from their paper that is relevant to this discussion:

 

"If the motor and load cannot withstand large torques during starting, e.g., with a belt-connected load in which slippage can occur when the motor torque exceeds a certain value, then larger values for Γ (sic) (Greek Alphabet Letter Gamma used to represent the notch angle in the sine wave) ... will result in lower values of peak torque during the starting transient. The operator should be aware that the consequence of this leads to a longer starting time and an increase of the stator heating index."

(A bit more text than necessary but didn't want to obscure the context.)

 

Here then is the equation for determining the Stator Heating Index .......

 

 

 

The ABC relates to the ABC frame reference of Flux Linkage.

 

(see SimPower Systems ~ Asynchronous Machine for discussion on ABC Frame Reference)

 

One of the authors of the paper, Mark G. Solverson, M.Sc. , wrote his Masters Theisis on "Soft Started Induction Motor Modeling and Heating issues for Different Starting Profiles Using a Flux Linkage ABC-frame of Reference". I am attempting to obtain a copy of same and will report additional insight gained if successful.

 

Kind regards,

jΩ