When is it worth the Cost to use a Soft Starter?

[kwired]

Wouldn't a soft start be a good idea where high power factors are a problem.
I can understand not using them on part winding starts. I would think a PW would have problems.
There was a young hvac tech that burned out a compressor that was part winding start because he held in a contactor.

I don't know what you may be getting at with high power factors. AC induction motors naturally have low power factor for one thing. Maybe you meant demand factor? Motor starting doesn't last long enough to be a problem with typical demand factor charges other than maybe for some cases with a high inertia load that takes a very long time to accelerate.

Part winding starting is nothing more than one possible equivalent of reduced voltage starting. Not exactly reduced voltage but you lessen the surge current by only energizing part of the windings for a brief time, once the surge current has dropped off and hopefully you have some rotation in the motor, energizing the second part of the winding won't have such a high surge effect either. Placing soft starter in line with PW controls is rather pointless when the soft starter alone would provide overall better control. The young tech likely held in part of the winding which accelerated the motor, maybe or maybe not to full speed depending on loading conditions and the second winding never was energized leaving the motor trying to drive the entire load on only part of the windings. The delay between part and full windings on normal startup is like only a couple seconds at the most but often much less than that, usually just a bang bang between the steps.

 

[a.bisnath]

Soft starters can help a lot with long conveyer belts as loaded belts will require very high starting currents that will lead to belt bursting, all depend on the need, especially where reduced voltage starting is required. Elevators for materials are another example , but then a momentary full voltage kickstart feature may be need to overcome initial high starting torque. Over the years they have reduced in size and cost. Bypass contactor is a good idea.
Depending on geographical location however it may be easier and faster to obtain electromechanical contactors than wait a long lead time
to get a replacement soft starter. I remember really large Westinghouse soft starters from 30 years ago now being replaced with much smaller ones
with a lot more features. one important thing to note is properly programming and setting the parameters of the soft starter as this may cause the starter to trip or stop when there no real motor or load fault

 

[Jraef]

Sounds similar to where I retired from. They had 4 roof top cooling towers with 75 HP motors. They changed a pulley on one tower to have it run at least 25% faster. Destroyed the gear box .

I have seen that way too many times. People mess with mechanical designs without understanding the physics. 25% faster on a fan results in the load increasing by the CUBE of the change in speed. So 1.253 is 195%, so that 75HP motor was trying to drive almost a 150HP load…

a.bisnath

one important thing to note is properly programming and setting the parameters of the soft starter as this may cause the starter to trip or stop when there no real motor or load fault

Yes, many digital soft starters (which is almost all of them now) will have a minimum load requirement of around 20-30% of their maximum rating, triggered after the ramp time has expired. An unloaded motor often can fall that low. Some allow you to disable that, others do not.

 

[dvcochran]

Steel chain drive in location that requires an explosion proof motor doesn't sound like a great idea

I think it would depend on how slow the chain is turning and how well it is (continuously) lubricated.

 

[herding_cats]

I'm a Yaskawa dealer and have gone down this rabbit hole many times. Whether you use a drive or a soft starter, the peak demand is going to be the same, unless the way the motor is loaded it pulls more kW on start than it would if it was on soft starter. One of the guys I worked with a few years back is certain that soft starters do not "save money" on the utility end. That they do not reduce demand costs. However it has been proven that installing a drive just for soft starts can. Drives are more linear. Soft starting gains are really more for reducing the abuse on belts, bearings, squirrel cage housings etc. I still have not put a real datalogger on certain loads to do comparisons on real active power, but I would like to at some point. In an area that bills on kVA (Amperage) then absolutely it would save on utility costs. Areas that bill kVA are in Puerto Rico, Bahamas, Hawaii. I don't know of any in the continental US. Island countries that use diesel generators bill in kVA to off set damage to the generators from peak power demand. That's their way of "encouraging" you to use drives or soft starts. By billing you insane demand charges.

 

[ptonsparky]

Demand is typically in 15 minutes intervals as I understand. Can a SS help in those few minutes?

 

[herding_cats]

Smart meters are taking samples 256 times a second. The 15 min intervals are taking the kW demand spikes over the 15 min period and average. Peak demand is the highest peak kW under any time period. Peak demand stays with you for 11 months until you can prove that it has been reduced.

 

[retirede]

Smart meters are taking samples 256 times a second. The 15 min intervals are taking the kW demand spikes over the 15 min period and average. Peak demand is the highest peak kW under any time period. Peak demand stays with you for 11 months until you can prove that it has been reduced.

That’s not how it works here. The demand value used for billing purposes is averaged over 5 minutes, then the highest 3 consecutive 5 minute intervals are averaged to determine the demand charge.

 

[petersonra]

I think there is no question that soft starts save nothing on energy. As long as they have a bypass contactor they cost nothing extra. No bypass contactor means you eat energy losses continuously.

Soft starts reduce the inrush and make the utility happier. They also reduce the impact on your drive train so may reduce main and repair bills.

 

[Jraef]

Peak demand charges are complex, different among utilities and therefore widely misunderstood. In my years of integrating soft starts for different manufacturers, I have come across a few smaller utilities that had ratcheting instantaneous demand meters, but they were all what I call “secondary” power utilities, where they bought their power from larger generating utilities and resold it. Most utilities use a sliding 15 to 30 minute demand window, so any starting surge experienced is not seen, only the actual running load is. In all but those few cases, soft starters have no tangible direct effect on peak demand charges and the same can be said for VFDs. Unless you can stretch the addition of a load out to where the start time exceeds the demand window, it is not recorded.

But both technologies can have an INDIRECT effect on demand charges. VFDs can directly reduce the actual load if used on centrifugal machines like pumps and fans, and since demand chargers are a percentage penalty against actual load, reducing the load reduces the dollars associated with demand.

With soft starters, I have investigated a couple of claims of “peak demand charge” savings, to find a similar side effect. Both were lumber mills who installed soft starters on their major machines and realized some savings on demand charges. But as it turned out, it was the result of the OPERATOR behavior changes. Prior to adding soft starters, operators would leave machinery running idle during breaks or other unintended shutdowns, because they had been told an old myth that “it’s more expensive to restart it than to leave it running”. So once the soft starters were added, they were told it was OK to shut down even for 15 minutes. In one of them, this was especially true of the large (150HP) dust collection blowers that were attached to most of the saws, shapers etc (it was a millworks). Shutting them down when not needed again reduced the OVERALL energy consumption, which thereby reduced the bill, which was all attributed to the peak demand charges. Again, because the demand charges are a percentage penalty of overall kWh charges, it does appear to have that effect, and the MONEY is real, it’s just not what they thought it was.

 

[herding_cats]

And the complexity gets deeper when you start to look at demand charges vs. power factor penalties. Low power factor events are the same as peak demand events in some cases. I've seen it in real time with an AEMC 8600 Power Factor meter data logger. A motor start demand cost charge will get you on low power factor or peak demand. Or both depending on the quadrant billing system.

 

[garbo]

I'm a Yaskawa dealer and have gone down this rabbit hole many times. Whether you use a drive or a soft starter, the peak demand is going to be the same, unless the way the motor is loaded it pulls more kW on start than it would if it was on soft starter. One of the guys I worked with a few years back is certain that soft starters do not "save money" on the utility end. That they do not reduce demand costs. However it has been proven that installing a drive just for soft starts can. Drives are more linear. Soft starting gains are really more for reducing the abuse on belts, bearings, squirrel cage housings etc. I still have not put a real datalogger on certain loads to do comparisons on real active power, but I would like to at some point. In an area that bills on kVA (Amperage) then absolutely it would save on utility costs. Areas that bill kVA are in Puerto Rico, Bahamas, Hawaii. I don't know of any in the continental US. Island countries that use diesel generators bill in kVA to off set damage to the generators from peak power demand. That's their way of "encouraging" you to use drives or soft starts. By billing you insane demand charges.

Have to disagree with your statement that " Whether you use a drive or a soft starter, the peak demand is going to be the same ". Large hospital / research centers that I retired from had over 500 VFD's from 2 to 1750 HP & only one soft starter that was for a 250 HP fire pump. All of the probably over 100 fans ( AHU'S, supply & exhaust ) had a ramp up time of at least 30 seconds and often watched a 100 HP VFD that I PM'ed fan & pumps with a 60 second ramp up time only draw less then ten amps first few seconds then gradually to what speed it was set to run. If my old memory serves me correctly a plain Jane four pole 100 HP 480 volt NEMA frame motor draws 124 amps. Majority of AHU'S had two supply fans and always ran both motors and seldom over 75% of maximum speed. A soft starter with only a short ramp speed to motor stated RPM'S will always have a higher peak demand then VFD'S with longer ramp up speeds. On AHU'S that were installed back in the 1970's they had me set ramp up speed to 10 minutes in the winter to avoid tripping out one of the 3 or 4 Freezestats. Could only imagine how low peak demand would be on a 100 HP Motor that took 10 minutes to obtain maybe 75% of full speed.

 

[herding_cats]

Have to disagree with your statement that " Whether you use a drive or a soft starter, the peak demand is going to be the same ". Large hospital / research centers that I retired from had over 500 VFD's from 2 to 1750 HP & only one soft starter that was for a 250 HP fire pump. All of the probably over 100 fans ( AHU'S, supply & exhaust ) had a ramp up time of at least 30 seconds and often watched a 100 HP VFD that I PM'ed fan & pumps with a 60 second ramp up time only draw less then ten amps first few seconds then gradually to what speed it was set to run. If my old memory serves me correctly a plain Jane four pole 100 HP 480 volt NEMA frame motor draws 124 amps. Majority of AHU'S had two supply fans and always ran both motors and seldom over 75% of maximum speed. A soft starter with only a short ramp speed to motor stated RPM'S will always have a higher peak demand then VFD'S with longer ramp up speeds. On AHU'S that were installed back in the 1970's they had me set ramp up speed to 10 minutes in the winter to avoid tripping out one of the 3 or 4 Freezestats. Could only imagine how low peak demand would be on a 100 HP Motor that took 10 minutes to obtain maybe 75% of full speed.

It depends on the motor/load relationship.

At Yaskawa we see about a 20% reduction in inrush current by using a drive as opposed to no drive or SS at all when starting, but again depends on the nature of the load and whether it’s variable torque or constant torque. However, a motor that is running at full speed under load, has about 2% less current than a motor on a drive. Accounting for the losses.

 

[garbo]

It depends on the motor/load relationship.

At Yaskawa we see about a 20% reduction in inrush current by using a drive as opposed to no drive or SS at all when starting, but again depends on the nature of the load and whether it’s variable torque or constant torque. However, a motor that is running at full speed under load, has about 2% less current than a motor on a drive. Accounting for the losses.

My old mind can not follow that inrush current is only reduced by 20%. Was told that motors draw locked rotor current on start ups and a VFD with at least 10 second ramp up time would not produce inrush current greater then motor nameplate ampere. I have looked thru several old drives power usage ( KWH ) that had over 100,000 running hours then looked at motor nameplate ampere. Because our buildings seldom had motors running at full speed especially at night & weekends drives only averaged 60 to 70% of power compared to same motors on a starter. Drive tech told us that at full speeds to aspect A 2 to 3% power lost. Had 100 HP VFD'S in air conditioned mechanical rooms ( 78 degrees in summer ) that heat sink temperature was over believe 140 degree F with the cooling fans running. Back in the 1980's we had a machine where the 5 HP Reeves drive motor / belted speed control mounted to a gear box was in dead center of the machine.Took 8 hours to replace it every year because could no longer adjust speed. Had to take our a shaft and other things to crawl inside to gain access. After we replaced this speed drive with only a motor bolted to a new gear box and a drive ran great for next ten years. Even for 40 to 75 old belted driven exhaust fans & Air handlers installing drives greatly extended V belt life.

 

[kwired]

It depends on the motor/load relationship.

At Yaskawa we see about a 20% reduction in inrush current by using a drive as opposed to no drive or SS at all when starting, but again depends on the nature of the load and whether it’s variable torque or constant torque. However, a motor that is running at full speed under load, has about 2% less current than a motor on a drive. Accounting for the losses.

Power factor isn't passed through the drive. Reactive current is still present between drive and motor though.