Soft Starters

[philly]

Do all soft starters have internal bypass contactors, or are there some that require external contactors. I know most newer softstarters have internal byass contactors but was curious if there were older ones that required an external bypass contactor.

With an internal bypass contator the softstarter must be sized for the full load of the motor that it is being connected to since it will run through the soft starter during starting. But what if you are using the soft starter to start several motors using external bypass contactors? Can the softstarter then be rated smaller than the motors since it will be switched out by the bypass contactor on each motor once it gets up to speed?

 

[Jraef]

1. Not all SS designs include internal bypass contacts. The trend is headed that way, but it's probably about 50% still. The idea is somewhat controversial. If you have the integral bypass contacts, that foregoes the possibility of using the bypass to start the motor in an emergency if the SS is down. So if you want that and the SS has integral bypass, now you have to buy ANOTHER bypass starter, and you already paid for one. For that reason some people will always want the non-bypassed option. but some mfrs, like Siemens, have completely gone away from having a non-bypassed version already and I think that's a mistake.
2. Technically, one CAN cut down on the size of the components inside of a soft starter if you know there will be a bypass contactor. That's why manufacturers have started going to that design philosophy in spite of the other issues it raises; they can make it cheaper and smaller. But that does not translate to the field, it only counts for the manufacturer. The UL listing and therefore the nameplate will be based on a maximum motor size, you can't play games with that in the field. Lets say for example that you need a 250HP starter and if you used the Integral Bypass version it uses the same SCRs as one rated for 150HP, if you bought a 150HP unit and rolled your own bypass, you are stuck with a unit that says 150HP max on the nameplate connected to a motor that is 250HP. An inspector is going to bounce that. There may also be internal problems with the maximum settings on the overload programming, current limit etc.

 

[philly]

1. Not all SS designs include internal bypass contacts. The trend is headed that way, but it's probably about 50% still. The idea is somewhat controversial. If you have the integral bypass contacts, that foregoes the possibility of using the bypass to start the motor in an emergency if the SS is down. So if you want that and the SS has integral bypass, now you have to buy ANOTHER bypass starter, and you already paid for one. For that reason some people will always want the non-bypassed option. but some mfrs, like Siemens, have completely gone away from having a non-bypassed version already and I think that's a mistake.
2. Technically, one CAN cut down on the size of the components inside of a soft starter if you know there will be a bypass contactor. That's why manufacturers have started going to that design philosophy in spite of the other issues it raises; they can make it cheaper and smaller. But that does not translate to the field, it only counts for the manufacturer. The UL listing and therefore the nameplate will be based on a maximum motor size, you can't play games with that in the field. Lets say for example that you need a 250HP starter and if you used the Integral Bypass version it uses the same SCRs as one rated for 150HP, if you bought a 150HP unit and rolled your own bypass, you are stuck with a unit that says 150HP max on the nameplate connected to a motor that is 250HP. An inspector is going to bounce that. There may also be internal problems with the maximum settings on the overload programming, current limit etc.

Thanks Jraef

We are retrofitting and old conveyor that had an existing softstart long ago but for whatever reason the softstart was removed from the circuit and conveyor is now just started across the line. I looked at the softstart that is sitting there now and it does not have an internal bypass contactor but has a contactor in the same enclosure that is being used now for across the line starting. The literature for the softstarter says that an external bypass contactor is not required for the rating of the softstarter is sufficient to handle the full load running current of the motor once up to speed.

We really dont need this softstarter for any electrical reaons (pretty strong power system with no voltage drop and plenty of current capacity) so I was wandering how beneficial this would be to put on a conveyor for mechanical reasons? Would there be alot less wear and tear on a conveyor extending its life a good amount? I figure with a conveyor we would use the torque controlled ramp?

If we install another conveyor in the proximity to the softstarter,(same size motor 150hp) and want to softstart it as well, is it typically cheaper to buy and additional softstarter or add additioanl bypass contactors to be able to start both conveyors in sequence off of the one softstarter.

Electrically is a softstarter to eliminate voltage drop at the motor itself, or on the bus the motor is being served from to eliminate issues with other motors. I would think that for voltage drop during starting at the motor itself, then cable size can be increased during installation as opposed to a softstart.

 

[philly]

Anybody have any good links for programming a softstart for a toruqe ramp based off the speed vs torque and speed vs current plots for a given motor.

 

[Jraef]

Thanks Jraef

We are retrofitting and old conveyor that had an existing softstart long ago but for whatever reason the softstart was removed from the circuit and conveyor is now just started across the line. I looked at the softstart that is sitting there now and it does not have an internal bypass contactor but has a contactor in the same enclosure that is being used now for across the line starting. The literature for the softstarter says that an external bypass contactor is not required for the rating of the softstarter is sufficient to handle the full load running current of the motor once up to speed.

Sometimes a bypass contactor is used just because the soft starter needs to be in a sealed enclosure (i.e. NEMA 12 or 4). Soft starters reject about 4.5W of heat per running load amp, so for a 150HP 460V motor that's pulling 200A, that's a 900W heater in the box, it can get pretty hot inside.

We really dont need this softstarter for any electrical reaons (pretty strong power system with no voltage drop and plenty of current capacity) so I was wandering how beneficial this would be to put on a conveyor for mechanical reasons? Would there be alot less wear and tear on a conveyor extending its life a good amount?

Plenty of reasons to use them for mechanical issues, one mfr I used to work for used a marketing slogan that was "Electrical solutions for mechanical problems" because they were better justified for those reasons rather than strictly electrical. Belt conveyor's see extended belt life. I once had a belt salesman tall me they stopped promoting them for that very reason...

I figure with a conveyor we would use the torque controlled ramp?

Torque ramp is mainly useful when you must have a more predictable acceleration and don't care about current, because torque ramp usually precludes the ability to do current limit. In order to maintain a specific torque, it may be necessary to allow current to go where it will. But in your application, where you don't care about current because you could start across-the-line, then it might be a good choice.

If we install another conveyor in the proximity to the softstarter, (same size motor 150hp) and want to softstart it as well, is it typically cheaper to buy and additional softstarter or add additioanl bypass contactors to be able to start both conveyors in sequence off of the one softstarter.

No, it rarely works out cheaper unless the soft starter is exceptionally expensive, as in Medium Voltage. It takes more isolation contactors than people think it does at first glance.

Electrically is a softstarter to eliminate voltage drop at the motor itself, or on the bus the motor is being served from to eliminate issues with other motors. I would think that for voltage drop during starting at the motor itself, then cable size can be increased during installation as opposed to a softstart.

A soft starter INCREASES the "voltage drop" to the motor, that's how it works. the purpose, from that standpoint, is to reduce the VD on the supply.

Anybody have any good links for programming a softstart for a toruqe ramp based off the speed vs torque and speed vs current plots for a given motor.

I'm not sure anyone does it that way, but the current plot, as I said earlier, would be forgone anyway. The motor plots in general would be meaningless as well, they have nothing to do with what is connected to the motor; you would need the torque / speed curve plots of the load. You can sometimes get them from the machine supplier, but they too would be relatively meaningless in that they are based on across-the-line starting. All you really need to do it ensure that at any given moment, you always have more torque available from the motor than the load needs to accelerate.

 

[philly]

All you really need to do it ensure that at any given moment, you always have more torque available from the motor than the load needs to accelerate.

Is this usually done by trial and error or is there a calculated way of doing this even without the load torque curve?

 

[Jraef]

Is this usually done by trial and error or is there a calculated way of doing this even without the load torque curve?

I always do it by trial and error, not too much error... but I've been doing it a long time. The only time I would plot something would be if I knew I had a problem to overcome, but that was usually a current restriction issue, not a torque problem.

 

[Ragin Cajun]

Soft starting reduces the starting shock to mechanical systems, conveyors and similar loads in particular.

RC

 

[weressl]

1. Not all SS designs include internal bypass contacts. The trend is headed that way, but it's probably about 50% still. The idea is somewhat controversial. If you have the integral bypass contacts, that foregoes the possibility of using the bypass to start the motor in an emergency if the SS is down. So if you want that and the SS has integral bypass, now you have to buy ANOTHER bypass starter, and you already paid for one. For that reason some people will always want the non-bypassed option. but some mfrs, like Siemens, have completely gone away from having a non-bypassed version already and I think that's a mistake.
2. Technically, one CAN cut down on the size of the components inside of a soft starter if you know there will be a bypass contactor. That's why manufacturers have started going to that design philosophy in spite of the other issues it raises; they can make it cheaper and smaller. But that does not translate to the field, it only counts for the manufacturer. The UL listing and therefore the nameplate will be based on a maximum motor size, you can't play games with that in the field. Lets say for example that you need a 250HP starter and if you used the Integral Bypass version it uses the same SCRs as one rated for 150HP, if you bought a 150HP unit and rolled your own bypass, you are stuck with a unit that says 150HP max on the nameplate connected to a motor that is 250HP. An inspector is going to bounce that. There may also be internal problems with the maximum settings on the overload programming, current limit etc.

Follow the money...

Integral contacts allow a more economical solution. In that case neither the SCR's nor the contacts need to be fully sized.

The SCR's don't need to be sized for continous duty, they only have to carry the current for the duration of the ramp-up and ramp-down period and they are duty limited, e.g. sized based on yhow many starts you wish to execute in one hour.

The contacts need not be sized to break or make any substantial current as they open and close at fully open SCR bypass.

Since the SCR's are 'open' during full power run, you do not experience the associated heat loss.