# DC injection braking on soft starter.



## John Valdes (May 17, 2007)

Bump.

I'm surprised Benshaw did not have a suggestion. Maybe a mechanical brake? Not sure about one that big though.
The soft start must have digital outputs to control more stuff.

You installed it and you bought it. IMO Benshaw should step up and provide an answer.


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## Forge Boyz (Nov 7, 2014)

John Valdes said:


> Bump.
> 
> I'm surprised Benshaw did not have a suggestion. Maybe a mechanical brake? Not sure about one that big though.
> The soft start must have digital outputs to control more stuff.
> ...


Thanks for the bump.
They did say that we could add a brake to it. And yes it has all the I/O it needs to do so. It is in the manuals. My question was, based on someone's experience, is it a good idea to use a DC brake in this application and what would the downsides be.

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## just the cowboy (Sep 4, 2013)

*eddy current brake*

On large loads we used eddy current brakes. They have no parts to wear out and you can control the ramp time. It don't inject DC it put a field around the brake drum/shaft.

https://www.thomasnet.com/products/eddy-current-brakes-7781206-1.html

Cowboy


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## paulengr (Oct 8, 2017)

You just add the extra contactor as per the MX2/3 manual connected through a spare output relay and set everything up as per the manual for standard braking.

I’ve done tons of work on chippers in the Carolinas and Virginia and lots of conversions. We are the factory service reps for Benshaw among others. Nobody thinks twice about how long it takes to stop. First time I’ve even looked at DC injection in years which is sort of an old VFD technique from the 90s on free wheeling diode front ends for VFDs.

The upside of DC injection braking is it’s cheap and no pads to replace. On a VFD it is essentially free. On a soft start you add a small contactor.

The downside of DC injection braking is that as it implies you are putting DC into a normally AC motor which induces a braking torque on the rotor. Think of it like holding a big magnet to the rotor to stop it. It makes a strange sound because whenever the rotor is out of phase (50% of the time) it creates a braking torque at each pole so it makes an audible racket like a soft start at slow speeds.

The downside is that the motor is eating all the inertia stored in the chipper as it stops. Since that energy has no place to go, the motor eats it as heat. Worse still the cooling fan gives you cooling proportional to the square of RPM so every 10% reduction in speed cuts cooling by 20%. So...get my drift? Right at the point where we need that fan to cool down the motor, it isn’t putting out any air, This is hard on motors unless you let them cool off for a while after they stop, like changing knives.

The thing to remember is that you will not be able to restart as often and you really especially need to follow the NEMA max starts per hour charts without an external blower, If you had an AFE VFD you could do regenerative braking which converts the inertia back into line power but you don’t so all braking options generate heat.

You can do mild DC braking without damage. Some VFDs do it automatically by default. It is harmless as long as you are not aggressive or trying to quickly stop high inertia loads like chippers. So I would have no problems using it to slow done faster but it’s not going to stop a chipper quickly and you may fire it long enough to bring the speed down but that’s it.

In practice most chip mills have to remove guards, do lockout, turn the water off, empty conveyors, move the knife boxes across the floor, and other things so the time spent slowing down is not wasted time. If it has much in the way of wood built up on it at all it will brake pretty quickly anyway. It’s only when I’m testing an empty mill that it takes 30+ minutes to come to a full stop. We often just take a stringer, lay it on the shaft, and stand on it to make it slow faster. I’m sure it’s not an OSHA approved brake though.



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## Forge Boyz (Nov 7, 2014)

@paulengr Thanks. Lots of good info. Are you saying that for this application you would use the standard braking? There is also the heavy duty brake option that uses a 7th SCR and semiconductor fuse. I did see in the manual that a temp sensor on the motor is required if you are going to brake it. I don't think they would do a restart very soon as they change the knives at the end of the day. The exception to that could be if they hit something and dull them during the day.

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## JRaef (Mar 23, 2009)

Think of it this way; every motor has what's called a "Starts-per-Hour" rating. When you are braking a motor electrically, you are going to create just as much heat in the motor as if you are starting it. So in effect, you cut your motor Starts-per-Hour rating in half, because every start implies a stop of equal thermal stress. 150HP motors may, depending on the inertia involved in the load, be capable of only 1 or 2 starts per hour normally, so cut that in half. The reason they want you to add a thermal sensor is because when you are using DCIB, you are heating the motor up BEFORE it stops, vs when starting, you heat it up, then run it to where the fans cool it off. So if someone tries to re-start it too soon, the OL protection is unaware of how hot the motor is and you can damage it before the OL algorithm figures it out.

If it's a relatively new Benshaw soft starter, it likely has a programming feature in there that will allow you to set the Minimum Time Between Starts, something like that. Get an assessment of what the motor can take from the motor supplier and program that into the soft starter. It will force operators to be careful about how they use it. Assuming you have an MX3 soft starter (because that's the one that can have the DCIB option added after the fact), it does have that feature (Starts-per-Hour). Turn it on.

I've done dozens and dozens, if not maybe over a hundred, production chippers with DC Injection Brakes, including a 900HP 460V whole log chipper (12' diameter). Typically they run non-stop except when the sawyers have to change out the knives. Going from 45 minutes spin-down to 90 seconds adds a couple of hours of production time to each day. Well worth it. 

You will want the Heavy Duty option. The one with the contactor is for light duty applications, defined as the load inertia being less than 6x the motor inertia. I can GUARANTEE that this will be the case on a 58" chipper... probably in the neighborhood of 15x the motor inertia. The light duty option ain't gonna cut it.


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