VFD braking options.

[JesperMP]

I need some clarifications from VFD experts.

I have a project with a vibratory conveyor that must be stopped relatively quickly. Stopping is once every 120 seconds. The application is otherwise simple. Fixed speed. VFD is a Siemens G120.

We have added a brake resistor for dynamic braking, but the problem is that its placement is not optimal for dissipating the heat.
I am therefore contemplating if I can do something else. I think these are my options:
1. Dynamic braking with brake resistor.
2. DC braking. I understand that it will cause heat-up of the motor instead of the brake resistor.
3. Compound braking. I am at a loss as to what is the difference to DC braking. Compound braking adds a DC component to the voltage supplied to the motor, and also heats up the motor.

I would also like to get some hints as to why I would chose one of the others - other than heating up the motor or the brake resistor.

 

[kamenges]

Of the three options you have listed, dynamic braking is the only one that allows the user to maintain torque control of the motor.

With DC braking you will get whatever braking torque the DC level will give you. The braking torque will also decrease with speed.

Compound braking, on the surface, would seem to be the best of both worlds. It allows for speed ramping to zero while still controlling bus energy. However, compound braking CANNOT be used when vector control is used. It will only function with U/f control. So if optimal torque control during decel is needed this is not the mode for you.

I have never done a test between DC braking and dynamic braking to see which one will produce better braking results. My gut feel tells me that a dynamic braking system with a VFD running vector control sized to allow for extended 200% torque output will produce the best decel results since it will generate high levels of braking torque all the way to zero speed. But I would think DC braking wouldn't be far behind.

I would say compound braking would come in where speed and torque control isn't critical but a semi-controlled decel of a high inertia load is required.

Keith

 

[GaryS]

First I assume that this is a motor driver vibrator
There are vibratory feeder that have a electronic controller
With DC injection braking the most braking torque you can get is about 60% and that at full speed as the speed goes down so does the braking torque so near zero speed you have 0 torque
So I don’t think this would be a good application.
I really don’t see where a Braking resister will do you much good you still need to control the ramp and really don’t see where you will have a large load pushing the motor on stop
From the short run time and the need for a quick stop it looks like this feeding into a weighting system of some.
You best choice would be to use the vfd to ramp the speed down.
Create a program block that will output speed command that is proportional to the actual weight and the target weight. That way when the vibratory feeder is commanded to stop it is already running at a low speed so it can stop quickly.
I have used this method many time over the years. Now you do have to play with the numbers a little to optimize the control. You will find that this give you greater accuracy on the weighting