# Siemens Soft Starter Theory



## nap (Dec 26, 2007)

"soft starter" is a generic term that includes VFD's, reduced voltage starters, and Delta wye starters. I presume you are speaking of a VFD but clarification would be a good idea.

Voltage on the T terminals; this was actually line voltage? Did you use a DMM? Make sure it is not phantom voltage (or whatever anybody wants to call it). Generally I do not see line voltage as phantom voltage but it would be possible in some circumstances.



> My theory is it sends the voltage at a low frequency but my meter assumes it is 60 Hz


Do you have a meter that reads freq? that would answer that question. The motor should not be recieving any voltage unless the VFD is programmed to in whatever situation you have at the moment.


> Do they alter the frequency until the motor stalls?


To do what? They do what they are programmed to do. Most do not utilize a feedback reading to ramp up or down but merely programmed timing.


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## nick (Feb 14, 2008)

*voltage on motor off*

iam not a expert on vfds , but here it gos ,vfd soft start are most common today and they work by switching scrs or triacs ,basic operation of these they change ac power 3 phase to 3 phase dc power , thur the dc 3 phase bridge rectifier and the phases are switched thur scrs electronically by a oscillating drive circuit programmed to switch a b c in rotation meaning a on /off of the gates of the scrs  which control the speed or freq function of the motor .meaning a is switched on then b switched on then c is switched on ect ect in rotation. and is controlled by a micro controller in that vfd , abc or cba cw ccw ,the freq of the motor is controlled by the drive circuit which is a pwm at what ever freq you may want zero thur 125 hz mostly .if your getting a false reading on your meter its ok its due to your meter its picking up the output of the electronics meaning the drive circuits and the dc bridge pwm whats your voltage is it 480 volt? if your at zero speed on your vfd the scr is still not off all the way it will pass voltage into your meter but so small in micro amps its not really there but seen on meter kind of like a dead battery that you see voltage but hook it up and it dies fast . heres what to do get your standard wiggy out try it ill bet it will not read any voltage , but you digital meter will? the vfd is never really fully off when your motor is or has stopped read your manual it will tell you that .to let it discharge for 5 minutes before service work is to be performed ,this is due to the dc power supply it has capacitors in it dc power supply has filter capacitors to make pure dc power they will hold a charge for a long time so even if your off meaning power switched off disconnected the drive is still on until it fully discharges the capacitors in that dc power supply does this sound ok ? i maybe wrong so dont listen to me ,take care and best to ya .


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## DennisR (Sep 6, 2008)

Thanks for the replies guys. To answer a few questions, I'm not sure what type of soft starter these units are. We often use vfd's in our trade for traction elevators which require precise motor control. Hydro's control the speed transitions through valves so I doubt, though i'm not sure, these starters are vfd's. I do know they do not have field adjustable accel/decel parameters like vfd's i've installed and adjusted on traction cars. My hunch is they are a high tech wye delta type starter, but I suppose they could be vfd's pre-programmed at a standard starting frequency for the given motor. 

Us elevator guys really just troubleshoot to the component, so if a soft starter went bad I can figure that out. I can't figure out why it's bad or what's wrong with it especially if i don't know how it works. I am not provided with literature on the starter as it is incorperated into the controller and pre-installed ready to field wire. I've asked controller manufacturer but they have no idea. I only use a regular fluke digital multi-meter and I show line voltage on the load side of the starter with the motor at rest. My meter is autosensing and shows AC voltage, but I've heard DMM's will sometimes read AC on rectified DC. Perhaps that is the answer, maybe there is no way for me to know with the tools i have.

Thanks again
Dennis


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## nick (Feb 14, 2008)

sorry we could not help ya out ,take care


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## micromind (Aug 11, 2007)

Electronic AC motor controls come in two types. VFDs and soft-starts. 


VFDs. These have the ability to control the frequency and voltage to a motor. The speed of an AC motor depends on frequency and number of poles built into the motor. The number of poles cannot be changed once the motor is manufactured, so the only other way to change the speed is to vary the frequency. The frequency of AC power from a utility cannot be changed. Therefore, the only way to change the frequency of an AC circuit is to 'manufacture' your own AC power. 

A VFD does of this by first changing the incoming AC power to DC. This is why changing two of the phases on the line side of a VFD does not change the rotation of the motor. Next, the VFD changes the DC power back into AC, but now the frequency can be easily controlled, as can voltage. This is one way a single phase power source can supply a 3 phase motor. The rate of change in frequency can also be controlled, so a VFD certainly can act as a soft-start.


Soft-starts. These are an electronic controller that cannot change frequency, but can 'ramp up' voltage or current. There are basically two modes of operation here, ramped voltage and constant current. 

In the ramped voltage mode (sometimes called ramped current), the voltage (and current) begin at near zero. Over the next few seconds, voltage is steadily increased until line voltage is reached. At this point, some sort of by-pass contactor is energized, (some are external, some are built into the soft-start), and the motor is now operating across the lines. The reason for the by-pass contactor is to reduce the amount of heat caused by running current through semi-conductors. 

In the constant current mode, a steady amount of current is applied to the motor. Usually more than the full-load current of the motor, or else it'd never get up to speed. After a certain amount of time, the by-pass contactor is energized. 

As you can see, they're two completely different things. Because of simpler operation, the soft-start is much less expensive than the VFD, and I'd bet nearly anything that your hydro elevators are soft-starts. 

Rob


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## nick (Feb 14, 2008)

yes micromind , good one i was wrong its a different puppy , i thought he was using a vfd same electronics to switch thyristors or scr s used to turn on the dc to the gates slowly ramp up then full on voltage and current . but i still think hes picking up the electronic rectifier thur the thyristors because there not fully off until fully discharged but i maybe wrong cant understand the factory does not know they built it whats that ?its interesting now ive got to find out why ? ive looked into soft starts guess nap had a good point also because there are many types, induction coil soft starts , resistive soft starts ,dc armature soft starts ,part winding soft starts ,scr soft starts , traic soft starts , auto transformer starting . wye delta contactors soft starts across the line starters ,then dc motors have a list a mile long ect ect , i always thought that a elevator motor was dc on our jobs when your in the penthouse the nameplate on the cable elevator motors say dc , the drives are dc and freq controlled ,guess theres alot to learn on this post .take care best to ya


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## DennisR (Sep 6, 2008)

Just about every traction (roped) elevator used to use DC motor/MG sets. Modern traction cars are now going in with VFD's and AC motors. (We appreciate the elimination of messy DC generators!) Hydro's are 99.9% AC motors but you can think of them as single speed motors, car speed is controlled by hydraulic checks/valves. For years hydro's were across the line or more common wye delta contactor start. Now they all come with this siemens soft start unit. The controllers are manufactured by many different companies (Motion Control Engineering, Virginia Controls, Otis etc) and almost all use this soft starter. I'm sure the engineers who designed the controllers know how they work, but the support people I deal with have no idea. 

Micromind, your description of a soft start is exactly what I expected to find. However what i find is line voltage on the load side of the starter with the motor at rest. I don't know what frequency it is since my meter is not that sophisticated. It could be (i guess it has to be) micro amps of whatever it is or my motor would be running. Basically that unexpected reading at the load side of the starter is what started my quest to figure out how these things work. I've investigated the siemens website but cant find the model of the soft starter. 

I still bet it is a ramped voltage soft starter, the unexpected voltage tricks me into thinking it may be frequency drive.


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## nap (Dec 26, 2007)

can you post any data and nameplate references found on the starter?


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## nick (Feb 14, 2008)

yes dennis thanks for the lesson on elevators and dc , i was thinking that the only difference between the vfd and the soft start is freq and some power switching circuits in the power drive section ,the soft start is simple ,but they both are scrs to function meaning switch on or off . one freq one just ramps up slowly with a bridge these are both controlled by a pwm circuit on the gates , the gates on the scr must have a positive to turn on and a negative to trun off if not it will stay on when turned on . just thinking if you touch you hand on a gate of a scr it will turn on dont do this just a example of how sensitive the gate is , when you shut off or rest that motor the gate has still a static charge on it and the dc power supply for the drive electronics thats why i think your false voltage is present from the dc power supply from your electronics is not dischaged yet by the cap filters . best to ya


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## John Valdes (May 17, 2007)

Are you sure you are reading the output terminals. Make sure you are not reading millivolts. This is very common with digital meters.
One other thought: Is there a contactor somewhere between the motor and the SS. As you are aware the elevator starts as soon as the button is pushed. The contactor may be the master here and the SS the slave?


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## nick (Feb 14, 2008)

*solstart*

go to this web page solstart read the manual ,warning when the mains voltage is connected to the starter even if start signal has not been initiated , full voltage will appear on the load terminals t1 t2 t3 go to just about any soft start manual and it tells you power is on load side even when start is not applied looking into this i found they must have a contactor to switch motor off if not, it seems to be off but it still has voltage due to the scrs and the circuits internal connections to these soft start starters like the power dc supply stays on due to the dc capacitors they keep it going like the everready bunnie ,,[danger warning] its a pdf solstart web site .:thumbsup:


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## DennisR (Sep 6, 2008)

nick said:


> go to this web page solstart read the manual ,warning when the mains voltage is connected to the starter even if start signal has not been initiated , full voltage will appear on the load terminals t1 t2 t3 go to just about any soft start manual and it tells you power is on load side even when start is not applied looking into this i found they must have a contactor to switch motor off if not, it seems to be off but it still has voltage due to the scrs and the circuits internal connections to these soft start starters like the power dc supply stays on due to the dc capacitors they keep it going like the everready bunnie ,,[danger warning] its a pdf solstart web site .:thumbsup:


That's awesome, thanks nick. I'm embarrassed to say I don't remember if there was a contactor between the SS and motor. I guess there would have to be. I'll do some further investigating next time I'm on a hydro job. I'm sure that must be the answer though, and it's nice to know. Thanks everyone :thumbup1:


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## nick (Feb 14, 2008)

ok thanks dennis now we have a question who has the last rule on elevator installs look at the post on this site ,we had a debate last week right here .is it the state elevator inspector or the fire marshall ? meaning if theres a electrical problem .the post was elevator shunt trip location in machine room . as far as the contactor theres two one for overloads one for bypass . actually John had it before my post i just looked at the manuals and the circuits he gets the credit . good one JOHN!!!


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## DennisR (Sep 6, 2008)

I'll check out the thread tomorrow. I just got back from another night call, this weekend has been crazy. In general inspections seem to be a case by case situation here in NYC, but I'll let you know what i've experienced with shunt trips/fire marshall's etc... 

PS. Good call john. I think you have it figured out.


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## JAW (Jun 29, 2011)

*Soft Start*

If it's a Siemens starter installed inside the "DELTA" then you have main line supply voltage at the T1-3 taps when idle because the fault contactor is energized (normal state unless faulted) which connects the main line to the T4-6 motor leads. You are reading the main line potential that is present at the opposite end of each motor winding just as if the starter were an open contactor. If you measured from L1-T1 you would also see full potential since the starter is "off".
The starter ramps up voltage/current in the time set by parameter that limits peak starting current at which point there would still be line voltage potential present at the T1-3 terminals but no voltage drop between L1-T1, L2-T2, L3-T3. You would also then see full line potential across the motor windings T1-T4, T2-T5, T3-T6, which you won't see when idle since there is starter is off and there is no difference in potential across the windings until it's energized.
Hope this helps.


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## oliquir (Jan 13, 2011)

Most multimeters are not able to read output from vfd or soft-starter since signal is very noisy (sinusoidal with 3-10khz pwm) the best way is to use an oscilloscope to see actual output signal


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