DC injection Braking

[dangnm]

We have been supplied with a fixed circular saw which, under regulation, must stop within 10 secs.
The supplier gave us a panel which uses a 30V tapping from a transformer that is rectified and used to inject the DC to stop the motor.
The problem is that every so often the braking fails and we can't find the reason!
I thought that the voltage may be fluctuating so i used the 50V tapping on the transformer but this caused the saw to stop too soon and loosen its nuts.
There's a varistor connected across the DC side of the rectifier - is this to regulate the supply to 30V? if so could it be breaking down and causing the problem?
Any info on DC braking welcome [sig][/sig]

 

[ELECTRICKEN]

Could you answer a few questions so we might help you better.
A- Is the brake on the saw motor a mechanical "an electric coil is mechanicaly causing braking"?
B- Is the D/C, put into the stator causing rotor lock?
C- How is the A/C to D/C transfer done is it mechanical relay,solid state,current sensing?

 

[jbartos]

Suggestion: Please add to the previous posting "request for additional information":
1. Manufacturer and its literature
2. Nameplate data

 

[GSI1]

Let try to help you
DC Injection braking is simply passing a DC current through 2 of the 3 motor leads when the run contactor is opened.
But keep in mind that the stopping torque will only be 30 to 50% of the motor torque this is usually enough to stop most loads. This should be enough to do what you need.
It may seam that it would be simple to just use the N. C. Aux Contacts on the motor starter to inject the DC current in motor windings but this will not work. First you must have some time for the AC line power to completely fall ( about 100ms ) before injecting the DC current into the motor, second the DC current must be removed after the motor is stopped or the stator windings will over heat. Keeping this in mind their must be a timer in the control somewhere. This time is most likely you problem if there is a question then replace the timer.
I think a better solution is to replace the motor starter with a good quality AFD (Adjustable Frequency Drive ) the new AFD’s have DC Injection stopping built in. The AFD controls have the timer fot the DC Injection built in. If that’s not enough they also have DB Braking ( Dynamic Braking ) standard on most AFD’s. The DB Braking torque can be as high as 150% of motor torque with Deceleration as short as less then 1 sec. And with an AFD you also get the advantage of smother acceleration ( the will offer cost payback in increased life gears, bearings , shaft couplings as well as the ability to match the cutting speed of saw to the work and increase performance.
I hope this helps you

 

[jbartos]

Suggestion: If the motor size is adequately large and start/stops more frequent, regenerative braking is more attractive since it reclaims the energy loss.