Jump to content

15 Hp Softstart W/brakes ?


Matt

Recommended Posts

We are building a Custom machine for a customer that has requested Softstarting as well as Dynamic braking. These are simple 15HP Induction motors that will be 460VAC..

 

 

I have a considerable amount of experiance with VFD's and No experiance with Softstarts ! Does the Softstart also include a DC Brake ? Or would the braking need to be seperate ??

 

My question then is, Why not just use a VFD and ramp the starting, As well as Decell the Stopping. Granted this app will require the use of braking resistors. The Speed control may come in handy but is not required.Cabinet Space may become an issue if the Softstart and brakes are seperate ! From What I can tell the cost difference isn't that great either !

 

 

Any opinions ??

 

Thanks

Matt

Matt

 

"Its easier to get Forgiveness than Permission"

 

Never ever say to a customer "It Can't Do That" Because It CAN !!

Link to comment
Share on other sites

You have hit upon one of the dilemmas going on in the soft starter industry. Small starters, especially those with any kind of add-on feature such as reversing or braking, are becoming less competitive against VFDs of the same size. The cost to build a soft starter may have gone down in the high volume mass-produced arena, but as soon as you have to have someone touch it in an assembly shop to add braking or reversing, the added labor erodes the cost difference against a low-end VFD, especially in the 15HP range.

 

Some soft starters do include a form of braking as standard, either 1/2 wave DC injection, or a form of cyclo-converter braking. These are not as effective or desirable as full wave DC injection but they have the advantage of being relatively low cost options, i.e. they often don't require additional components. You need to evaluate what you want to accomplish when it comes to braking. If what you want is to just cut down on coast-to-stop time so as to improve productivity, those versions should be OK. If you need braking for avoiding over travel of heavy conveyor loads or long time braking of high inertia loads, Full wave braking would be better, but that is where you bump in to the cost difference against a VFD.

 

Keep in mind also that a VFD is more complex than a soft stgarter, more to go wrong, more issues to deal with such as harmonics etc.

"He's not dead, he's just pinin' for the fjords!"
Link to comment
Share on other sites

Hello Matt

 

Many soft starters include DC brake functions, but not all are equal.

Generally, the DC brake functions included in soft starters are single phase half wave in operation. This is not as good as a three phase full wave braking system, but in many cases it is adequate.

 

The problem with the type of braking typically used in soft starters, is that it is made up of pulses of DC. This still has a strong line frequency component and so must not be applied at a high level at full speed as the motor will synch onto the line frequency component rather than the DC component.

 

Adding a contactor to short two terminals on the motor will force the current to continue to flow in the motor windings for longer, reducing the line frequency component and increasing the braking current without synching on the line frequency.

 

Another option is to use a cyclo-converter technique to create a lower frequency for the first part of the decelleration. This does not require any extra componentary, it is just a parameter setting.

 

The negative of DC braking for induction motors, soft starters or otherwise, is that during the DC braking cycle, the kinetic energy of the load is being transferred to the rotor, exactly the same as during start. Thus a Stop is the same as another start and the maximum starts per hour must be halved.

 

DC braking that is included in soft starters, does draw higher pulse currents on two phases of the supply than a full wave DC brake system, but much the same as a half wave DC brake system. A weak supply can be affected by the application of DC braking.

 

Best regards,

Link to comment
Share on other sites

Thanks for the Info Guy's !!

 

I am really leaning towards the VFD solution here. The reason for the braking is just to reduce coast /stopping time. Not a safety issue and very little inertia on the motors. It will reduce changeover time for the operator and thats always a plus.

 

This same machine will have 14 AXIS of servo control and a PC for the Operator interface, So the VFD may afford me the opportunity to extract fault, load, etc. data from the drive if I ever need to.

 

Thanks for the great Information on Softstarts as well. I have not had any experience using them and really wasn't sure what was even available as far as Starting / Braking in one device.

 

I have used DC injection brakes in the past, And found them to be quite a pain though...

 

 

 

Thanks a Million !

 

MATT

 

 

 

 

Matt

 

"Its easier to get Forgiveness than Permission"

 

Never ever say to a customer "It Can't Do That" Because It CAN !!

Link to comment
Share on other sites

Hello Matt

 

I would question the use of a variable speed controller if the speed does not need to change.

The advantage of the soft starter is i) it is cheaper, ii) it is more efficient, iii) it is more robust, iv) it will last much longer, v) No EMC issues.

The VFD is usually designed to last 5 - 7 years. I have soft starters in the field that have been running for more than 25 years.

The VFD has losses of 4 - 5%, the soft starter is more like 1%.

 

The fault and load information you refer to can be downloaded from a good soft starter, exactly the same as a VFD, and the brake is not a problem, especially if it is a low inertia load. - all you do is set one parameter to enable it.

 

If the speed is constant, I use a soft starter, if the speed changes, I use a VFD.

 

Best regards,

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...