Wye-Delta Starters vs Softstarters

[mull982]

I'm looking for some pros and cons with using a wye-delta starter vs. a soft starter for 480V motors.

First I know that wye-delta starters require a 6-lead motor so if the motor is not a 6-lead motor then a wye-delta starter is not an option.

In terms of cost I would imagine that a wye-delta starter is significantly cheaper and would guess that both take up roughly the same real-estate in an MCC or enclosure?

I think the one negative of the wye-delta starters is the open transition between wye and delta and the current surge that comes with this where a sotstarter is able to provide a controlled ramp in order to avoid this current surge.

Are there any HP limitations where for larger HP's one would be better than the other?

 

[Jraef]

The difference in cost is less than it once was, and in MCCs, some mfrs no longer offer Y-D as an option any more. It costs a lot of money to get something tested and listed under UL845 in an MCC, above and beyond the initial UL 508 listing of a starter by itself. So with all the changes in things like circuit breakers and OL relays taking place causing everyone to re-do their UL845 listing of MCC units, several mfrs chose to not re-list their Y-D and RVAT starters in MCCs. Soft Starters only, which makes the decision easier.

I stopped recommending Wye-Delta probably 20 years ago now, I've just seen too many problems and costs associated with the long term ownership. The nail in their coffin for me came when I had to get replacement contacts for a 350HP Westinghouse Y-D starter and the entire set (9 contacts) cost me as much as a replacement soft starter. Y-D starters are only cheaper when you first buy them.

Then any difference in initial cost is wiped out the first time the transition blows a fuse or twists off a shaft from the torque spike, or damages some other piece of electronic equipment from the voltage spike caused by the transition. The other big issue I still see with them is that newbies in our field often have never seen one and have no idea how to connect them correctly, so one connection error wipes out all possible savings as well.

 

[petersonra]

I don't think there are any pros anymore. The cost of soft starters has come down enough that you are not saving much money, if any at all, so why would you want to use a solution that provides no operational advantages?

 

[Ingenieur]

the only reason the Y-D were used was that better tech was not available (it was the BAT at the time)
SS is reliable and much more flexible

 

[mull982]

Thanks for the feedback!

So it sounds like softstarters are the way to go and wye-delta has become an antiquated technology.

I guess I should have mentioned that I'm looking at a specific application related to starting centrifuge motors (200HP). I have seen some information where there can be issues with starting centrifuges with soft starters due to long starting/acceleration times. Can anyone shed some light on what some of these specific issues are and how they can be avoided. Perhaps oversizing of softstarters is required?

There are centrifuge motors currently on wye-delta starters that are being considered for replacement with sofstarters but I want to evaluate any potential issues with softstarters on these motors and loads.

 

[retirede]

High inertia loads can have issues on Y-D as well. If the motor cannot accelerate the load on starting in wye, it may begin to stall, then essentially start across the line when the transition to delta occurs.

 

[Ingenieur]

What is a long accel time?
and why a long time? They would seem to be a well balanced low inertia load
once spinning at a certain speed transition to a bypass contactor

 

[Fnewman]

It has been quite a few years since I dealt with centrifuges, but as I recall, you are talking about a high mass that has to be accelerated to a relative high speed. Acceleration times of at least a minute would not be surprising. Primary air fans for pulverized coal mills are a similar situation. All that said, most of the time the problems arise because of insufficient information sharing between the equipment and starter suppliers.

 

[Jraef]

So yes it's true, some soft starters cannot be used at all on centrifuges, others have limited application, but there are many that can be used very successfully. The tricky part is finding someone you trust to give you a straight answer. Salesmen don't like to tell you "no" when it means losing a sale. You need to talk to applications engineers. If a supplier will not hook you up with one, move on immediately.

The issue has a lot to do with design. It's a competitive world and soft starters are struggling with staying relevant in a world filling with inexpensive VFFs. So some mfrs have responded by moving the price point down and down, but do so by removing capacity for tough duty, like centrifuges.

So first off, eliminate ALL soft starters that use SCRs on only 2 of the 3 phases. Not only will that never work on a centrifuge, you may damage the motor trying. Next eliminate anything that says "pump and fan duty" or anything that sounds remotely like that. They typically reduce price by reducing (or in one car even eliminating) heat sink. Heat sink is a VERY critical aspect of making yours successful. So what you want is a true "heavy duty" soft starter, rated st a MINIMUM of 500% overload capacity for 30 seconds, the more the better. In most cases that is going to mean over sizing the starter, but every mfr will have their own formula, there is no universal rule on that because it's all about the heat sink design. Next you want one that allows current limit starting. Most do, but not all. You are going to want about a 350% current limit and you are likely to need that to hold for at least 45 seconds, maybe more. Some will not allow more than a 30 second ramp time, eliminate those too because what happens is that at the end of 30 seconds, they go across the line no matter what, even if your motor is far from full speed. So current still surges, but now you exceed the thermal damage curve and the starter trips on OL before it finishes. And that's another issue: you want one that will allow you to at least set it for a Class 30 OL curve for starting, or allow bypassing the OL during starting altogether. You will need that. I'd give you some brand names but I work for one of them, do it wouldn't be kosher to the forum rules. So do your due diligence. There are at least 4 viable products I know of, you can find them too with a little effort.

Good luck.

 

[mull982]

So yes it's true, some soft starters cannot be used at all on centrifuges, others have limited application, but there are many that can be used very successfully. The tricky part is finding someone you trust to give you a straight answer. Salesmen don't like to tell you "no" when it means losing a sale. You need to talk to applications engineers. If a supplier will not hook you up with one, move on immediately.

The issue has a lot to do with design. It's a competitive world and soft starters are struggling with staying relevant in a world filling with inexpensive VFFs. So some mfrs have responded by moving the price point down and down, but do so by removing capacity for tough duty, like centrifuges.

So first off, eliminate ALL soft starters that use SCRs on only 2 of the 3 phases. Not only will that never work on a centrifuge, you may damage the motor trying. Next eliminate anything that says "pump and fan duty" or anything that sounds remotely like that. They typically reduce price by reducing (or in one car even eliminating) heat sink. Heat sink is a VERY critical aspect of making yours successful. So what you want is a true "heavy duty" soft starter, rated st a MINIMUM of 500% overload capacity for 30 seconds, the more the better. In most cases that is going to mean over sizing the starter, but every mfr will have their own formula, there is no universal rule on that because it's all about the heat sink design. Next you want one that allows current limit starting. Most do, but not all. You are going to want about a 350% current limit and you are likely to need that to hold for at least 45 seconds, maybe more. Some will not allow more than a 30 second ramp time, eliminate those too because what happens is that at the end of 30 seconds, they go across the line no matter what, even if your motor is far from full speed. So current still surges, but now you exceed the thermal damage curve and the starter trips on OL before it finishes. And that's another issue: you want one that will allow you to at least set it for a Class 30 OL curve for starting, or allow bypassing the OL during starting altogether. You will need that. I'd give you some brand names but I work for one of them, do it wouldn't be kosher to the forum rules. So do your due diligence. There are at least 4 viable products I know of, you can find them too with a little effort.

Good luck.

Thanks for the background. Its been a while since I evaluated motor starting but it sounds the issue with a centrifuge is the high initial torque requirement to overcome the load inertia. With reduced voltage during starting the available torque will be reduced by the square of the voltage so I guess with the initial reduced voltage upon starting there needs to be some way to still provide the available torque to the load? I guess that's the part I am trying to understand regards to providing this initial starting torque with the soft starter and what this means for the soft starter rating/requirements. Or is it simply the fact that the load will take longer to accelerate with reduced torque and therefore the soft starter needs to be capable of handling the starting current for a longer duration of time?

These centrifuge loads sound similar to a large Ball Mill load we had at a plant I used to work at. The ball mill when loaded had a very large initial intertia and it needed a high initial starting torqure which was provided by a Liquid Rheostat. Depending on the rate at which the resistance in the Rheostat would change would affect the starting time of this motor and load.

 

[kwired]

Wye - delta is primarily for reducing the surge current upon initial energizing of the windings. Transition from wye to delta is within a second or so, the motor won't likely have sufficient torque to accelerate a high inertia load at all when connected in the wye configuration. You will likely need a motor that is "derated" or is otherwise specially designed to take the high current it will see while starting. It will also probably have a limit of number of starts in certain time period.

My only experience in this area was a cream separator in a dairy plant, it was only a 30 HP motor. It was across the line started, but had a start and run contactor, no overloads when in start mode. Seems it took at least a couple minutes before it switched to the run contactor (been over 20 years since I had anything to do with it), and would take maybe half hour or longer to coast to a stop.

 

[zbang]

These centrifuge loads sound similar to a large Ball Mill load we had at a plant I used to work at. The ball mill when loaded had a very large initial intertia and it needed a high initial starting torqure which was provided by a Liquid Rheostat.

How common are LRs now, either for new installs or in existing setups? I see they're still being made and look useful for specific circumstances.

 

[Jraef]

How common are LRs now, either for new installs or in existing setups? I see they're still being made and look useful for specific circumstances.

Liquid rheostat starting requires a Wound Rotor motor, and WR motors are no longer popular, so LR starting isn't used much other than in existing WR installations. Even then, soft starters and VFDs are easier to implement now. There's nothing really wrong about LR starting, it's just an old technology that few people still understand. Those that still insist on it are generally people who are uncomfortable with power electronics, but those guys are aging out of the work force now and the millennials who take their place are used to VFDs and look at that stuff like it should be in a museum.

 

[Besoeker]

Liquid rheostat starting requires a Wound Rotor motor, and WR motors are no longer popular, so LR starting isn't used much other than in existing WR installations. Even then, soft starters and VFDs are easier to implement now. There's nothing really wrong about LR starting, it's just an old technology that few people still understand. Those that still insist on it are generally people who are uncomfortable with power electronics, but those guys are aging out of the work force now and the millennials who take their place are used to VFDs and look at that stuff like it should be in a museum.

We made variable speed drives for wound rotor motors. Slip recovery drives. These were usually for large fans and pumps - centrifugal cube law loads.
Normally these operate from about 70% to 100% speed. They had dry stepped resistance starters. Five stages kep the current peaks to about 150% of full load.

These are application specific of course. For SCIMs we used electronic soft starts with a bypass contactor. As others have mentioned they are much more flexible than star-delta starters and you don't have that horrible transiend when you switch from star to delta.