Hello-----We have a Reduced Voltage Motor Starter that needs replaced (1987). Since this is a motor[ 3ph/60HP/460V.60Hz ] that does not need its speed controlled, I suggested a soft start drive. I am not going to pretend to have any real world knowledge in this. Only things I have looked up. A quote came back that stated "vfd" so i have asked for clarification from the contractor. One thing that I am wondering is about shaft grounding. We have been doing the "aegis" ring on motors that now have a vfd but it looks that soft starts possibly do not need them...due to no freq change .....no inverter ? Any insight would be appreciated. Doug.
Soft Start--vs---VFD
- Thread starter zooby
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ron
Senior Member
- Location
- New York, 40.7514,-73.9925
We have found VFDs are a good value way of achieving a soft start since they are much more commoditized in the market.
The grounding ring is a good preventative method to avoid issues for VFDs.
The grounding ring is a good preventative method to avoid issues for VFDs.
sgtsparky
Member
- Location
- Dexter, Oregon, USA
I agree with Ron that a VFD can be used to ramp or soft start but a soft start only can be cheaper and have replaceable "transistors" where most of the VFD's when they go they are toast with modern VFD's. or that is my experience with multiple brands. the soft start or VFD will not last as long as an old fashion transformer based reduced voltage starter, where you may replace the contactors or timing relays not the entire thing. but both VFD's and soft starts are much smaller but 15-20 years they tend to blow the caps shorter life on cheep or undersized units.
Yes I seriously doubt anyone is getting 30+ years out of the vfds or soft start drives. ALOT of our old Westinghouse stuff is still chugging along since this place opened in 86'-87'. Lead times will impact our decision also-just like everyone else- and since we are a non-profit.....well you can guess what that means when the PO gets signed. I do appreciate your reply! if it had been a question about putting a loop in a wire on a receptacle screw, there would be a lot of reply's
Have a good day.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I am a vehement supporter of VFDs and they have been the focus of my career for almost 30 years now. But I do NOT to this day support the idea that they are “better” than other motor control methods if varying the speed is not part of the goal. VFDs require added technology that introduces with it an increase in sensitivity to disturbances in the line, and decreases the Mean Time Between Failures because there are far far more components inside.
In addition, even the best VFDs are 97% efficient, which means they introduce 3% losses (mostly heat). In most cases where varying speed is necessary, those losses are either more than overcome by other efficiency gains, or are an acceptable cost to accomplish the variable speed. But at a fixed speed, those losses are permanently part of the equation. So let’s say this 60HP motor is running a load that equates to 50HP on it. 50HP is roughly 40kW of power drawn from the utility (it’s actually more but I’m making it simpler). 3% losses means it is costing you 1.2kW in additional energy during every moment that motor is running. It doesn’t seem like much, but it adds up at the end of the year and that is extra energy that is serving no useful purpose. If this VFD is in an air conditioned space, then you also must add the extra burden it puts on the air conditioning to get rid of that heat. Again, IF variable speed was necessary, these would be non-issues. It only becomes problematic because you are NOT using the variable speed capabilities.
There are a FEW instances where you might need a VFD as a glorified soft starter, for example if the power source is so weak that it cannot handle any increase in starting power, or the acceleration ramp needs to be stretch out beyond 60 seconds. But because you ALREADY have been using a RV starter, we know that’s not the case.
Soft starters are FAR more simple than VFDs and do NOT use transistors or capacitors. The only suggestion I’ll add to that is to beware if the super cheap ones. Many of them cut cost my only controlling 2 of the 3 phases, one pole (usually the center one) is just a bus bar from line to load. This works, but is hard on the motor and adds in more risk of failure, not just of the soft starter itself, but now the motor too. Resist that temptation and make sure your potential vendors are selling you one that has 6 SCRs, not 4. If the vendor can’t answer that question, move in to someone else. Also, if there is ANY dirt or moisture in the surrounding air, get one with a Type 12 or 4 enclosure and a bypass contactor.
And no, you will not need the Aegis rings on the motor if there is a soft starter.
In addition, even the best VFDs are 97% efficient, which means they introduce 3% losses (mostly heat). In most cases where varying speed is necessary, those losses are either more than overcome by other efficiency gains, or are an acceptable cost to accomplish the variable speed. But at a fixed speed, those losses are permanently part of the equation. So let’s say this 60HP motor is running a load that equates to 50HP on it. 50HP is roughly 40kW of power drawn from the utility (it’s actually more but I’m making it simpler). 3% losses means it is costing you 1.2kW in additional energy during every moment that motor is running. It doesn’t seem like much, but it adds up at the end of the year and that is extra energy that is serving no useful purpose. If this VFD is in an air conditioned space, then you also must add the extra burden it puts on the air conditioning to get rid of that heat. Again, IF variable speed was necessary, these would be non-issues. It only becomes problematic because you are NOT using the variable speed capabilities.
There are a FEW instances where you might need a VFD as a glorified soft starter, for example if the power source is so weak that it cannot handle any increase in starting power, or the acceleration ramp needs to be stretch out beyond 60 seconds. But because you ALREADY have been using a RV starter, we know that’s not the case.
Soft starters are FAR more simple than VFDs and do NOT use transistors or capacitors. The only suggestion I’ll add to that is to beware if the super cheap ones. Many of them cut cost my only controlling 2 of the 3 phases, one pole (usually the center one) is just a bus bar from line to load. This works, but is hard on the motor and adds in more risk of failure, not just of the soft starter itself, but now the motor too. Resist that temptation and make sure your potential vendors are selling you one that has 6 SCRs, not 4. If the vendor can’t answer that question, move in to someone else. Also, if there is ANY dirt or moisture in the surrounding air, get one with a Type 12 or 4 enclosure and a bypass contactor.
And no, you will not need the Aegis rings on the motor if there is a soft starter.
wow....huge thanks for taking time. Good Things....
I have 20 soft starts AB (30 to 40 hp) located in a climate controled MCC put in 1998. Loads were large cooling fans and air compressors. These are still working in 2023 and all have never had any issues with these soft starts. Air compressor motors were removed and rebuilt after 10K hours. No winding issues at all on the air compressor motors. The fans have not had any soft start issues. The biggest issue we have is mechanics who over tighten belts and take out bearings.Yes I seriously doubt anyone is getting 30+ years out of the vfds or soft start drives. ALOT of our old Westinghouse stuff is still chugging along since this place opened in 86'-87'. Lead times will impact our decision also-just like everyone else- and since we are a non-profit.....well you can guess what that means when the PO gets signed. I do appreciate your reply! if it had been a question about putting a loop in a wire on a receptacle screw, there would be a lot of reply's
Nice! Thank you "Motormuff"for the input.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
I have seen issues with SS where the power source can’t handle the increased amps when SS is turned on.
The first VFD I installed was a AB 1336 in 1994. Great drive, but like all vfd and plcs, they become obsolete and can’t get parts.
The first VFD I installed was a AB 1336 in 1994. Great drive, but like all vfd and plcs, they become obsolete and can’t get parts.
would you be able to give me an example of that scenario? so i can make sure i am clear with that. Thanks!
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
The way a soft start works basically is reducing the voltage when the motor starts and gradually increasing it up to full voltage. This greatly reduces the starting current but does not eliminate it.
With across the line starting you will get starting current that is maybe 6 to 10 times what the full current eventually ends up being.
With a soft start it's much less than 6 to 10 times but it's not one times. It's possible that if you had a very limited supply a soft start might not work all that well but an across the line starter wouldn't work even worse.
RumRunner
Senior Member
- Location
- SCV Ca, USA
- Occupation
- Retired EE
Since you did not mention the motor application that this motor is used for . . . I would assume that the motor is not that critical in its operation in your industrial environment .
The VFD is highly useful and beyond comparison when your operation requires subtle (soft) deceleration as well as gentle starts when handling products.
Soft Starters are highly recommended and almost trouble-free during its life compared to VFDs.
As opposed to VFDs--soft starters don’t do anything during decelerating stage.
My experience on this was when I serviced equipment of an egg-packaging facility --where even the forklifts operate (they were electric driven) are operated “gingerly” (more cautiously) and avoided sudden movements.
Essential for various conveyors.
One thing that VFDs differ from Soft Starters is the incidence of HARMONICS.
Soft Starters don’t experience this anomaly.
Note:
Since you stated that you’ve had this motor since 1987-- Soft Starters were already widely used.
I have mine installed for my wife's clothes dryer around that time and is still humming along. Its the plug-in model.
I bought it from my electrical supplier when I was doing contracting.
Reiterating MURPHY’S LAW.:
Anything that could go wrong will.
I also support OCCAM”s RAZOR philosophy
Yep I forgot .... it is one of two pumps for an aquatic exhibit (sea lions) that runs most of the time. The motor itself has been replaced at some point but the RVAT starter is from 1987. clothes dryer eh....Since you did not mention the motor application that this motor is used for . . . I would assume that the motor is not that critical in its operation in your industrial environment .
The VFD is highly useful and beyond comparison when your operation requires subtle (soft) deceleration as well as gentle starts when handling products.
Soft Starters are highly recommended and almost trouble-free during its life compared to VFDs.
As opposed to VFDs--soft starters don’t do anything during decelerating stage.
My experience on this was when I serviced equipment of an egg-packaging facility --where even the forklifts operate (they were electric driven) are operated “gingerly” (more cautiously) and avoided sudden movements.
Essential for various conveyors.
One thing that VFDs differ from Soft Starters is the incidence of HARMONICS.
Soft Starters don’t experience this anomaly.
Note:
Since you stated that you’ve had this motor since 1987-- Soft Starters were already widely used.
I have mine installed for my wife's clothes dryer around that time and is still humming along. Its the plug-in model.
I bought it from my electrical supplier when I was doing contracting.
Reiterating MURPHY’S LAW.:
Anything that could go wrong will.
I also support OCCAM”s RAZOR philosophy
....sweet. Thank you!RumRunner
Senior Member
- Location
- SCV Ca, USA
- Occupation
- Retired EE
Forgetfulness is human. . . . and its not a mistake.
Forgetting something is the outcome-- when there is a lot of things that needs to be remembered
Not admitting a mistake is a bigger mistake.
The notion that you only need a Soft Start is a decision that is commendable.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Jraef does that a lot!
alexfox1986
Member
- Location
- Manchester
- Occupation
- Electrical Engineer
A soft starter is a good choice for a motor that does not require speed control, as it provides a smooth start-up while reducing the high inrush current that can occur during start-up. A soft starter can also help to extend the life of the motor and the driven equipment by reducing mechanical stress during start-up.
In terms of shaft grounding, the need for an Aegis ring or other shaft grounding device depends on the application and the type of drive being used. As you noted, a soft starter does not typically produce the high-frequency electrical noise that can damage bearings in a motor, so a shaft grounding device may not be necessary.
However, it's always a good idea to consult with the manufacturer of the soft starter to confirm whether or not a shaft grounding device is recommended for your specific application. The manufacturer can also provide guidance on any other installation or wiring requirements that may be necessary for proper operation of the soft starter.
As for the contractor's use of the term "VFD" in their quote, it's possible that they may have used the term interchangeably with "soft starter" or mistakenly assumed that a VFD (variable frequency drive) was required for your application. It's important to clarify with the contractor exactly what type of device they are proposing and to ensure that it meets the requirements of your application.
mbednarik
Member
- Location
- central iowa
- Occupation
- Electrician
There are other risks when concerning a VFD as well. Such as do the load side conductors share a raceway with other loads? The high carrier frequency can cause issues on other circuits if so. You already mentioned shaft grounding, and a load reactor may be needed if there is significant length between the vfd and the motor. If you do not require speed adjustment, stay with a soft start.
Timpi Electric Solutions
Member
- Location
- Indiana
- Occupation
- Electrician
I'm late to join the party, but I will bring up one note regarding VFDs vs. reduced voltage soft starters. VFDs have no in-rush current; none. Reduced voltage soft starters (RVSSs) have a reduced in-rush than a direct on line starter, but the in-rush is still significant. Some commercial and light industrial companies that I work with have begun transitioning from RVSSs to VFDs on some of their larger applications that don't actually need speed control. The reason is utility costs.
Starting large motor loads with a DOL starter or RVSS causes a tremendous starting current. In my area, the local utility charges customers a hefty surcharge for large current draws, and especially during peak hours. Starting large motors without a VFD during peak hours (especially early morning when all the factories are starting up) is an extremely costly practice. They've found that it is cost effective to install much more expensive VFDs rather than pay the insane utility rates for in-rush currents during peak hours.
One other note that I'll add about RVSSs is that they have the possibility of being inefficient, too. RVSSs typically use a semi-conductor - much like a solid state relay - to gradually increase the voltage to keep the in-rush current relatively low. Semi-conductors like that generate heat, which creates an inefficiency similar to that of VFDs. This can be eliminating by using an RVSS that utilizes semi-conductors to START the load, but then switches over to electromechanical contacts once the motor has REACHED full load amperage.
Starting large motor loads with a DOL starter or RVSS causes a tremendous starting current. In my area, the local utility charges customers a hefty surcharge for large current draws, and especially during peak hours. Starting large motors without a VFD during peak hours (especially early morning when all the factories are starting up) is an extremely costly practice. They've found that it is cost effective to install much more expensive VFDs rather than pay the insane utility rates for in-rush currents during peak hours.
One other note that I'll add about RVSSs is that they have the possibility of being inefficient, too. RVSSs typically use a semi-conductor - much like a solid state relay - to gradually increase the voltage to keep the in-rush current relatively low. Semi-conductors like that generate heat, which creates an inefficiency similar to that of VFDs. This can be eliminating by using an RVSS that utilizes semi-conductors to START the load, but then switches over to electromechanical contacts once the motor has REACHED full load amperage.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
I'm surprised by your comment because I had been getting the impression that utility demand metering was typically done on a 5 or 15 minute interval basis, and I wouldn't expect in-rush currents to have a meaningful effect on average power over that kind of timescale. Are you referring to a typical utility demand charge, or a different type of metering for inrush specifically?
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