Wound Rotor Dynamic Braking Control

[JGFB]

I am a junior engineer working on the dynamic braking circuit of a 3PH 1000HP 440RPM 4160V (Stator) 590V (Rotor) Wound Rotor Hoist with 8 steps in the rotor circuit for speed control.

The operator panel uses a two direction joystick with a push button on the handle. One direction is for hoisting, the other for lowering and when the push button is depressed, the dynamic braking circuit is enabled. As the lever is actuated farther in either direction, the rotor contacts are closed; the final step being short circuit.

The dynamic braking circuit applies DC voltage to two of the motor leads. I understand that this reduces the synchronous speed of the motor to zero and produces negative slip and hence negative torque for braking.

My task is to control the rotor circuit during dynamic braking using a PLC based on the operator's input via the joystick and the speed of hoist using the given resistor steps. As this hoist will be transporting people, it is critical to know the deceleration rates and ensure that they are within acceptable limits.

I'm looking for help in understanding the speed control aspect of dynamic braking. I've been looking around the net for some good theory with no luck. Could somebody point me in the right direction. Dynamic braking seems like a broad topic these days, but I can't find much with regards to wound rotor.

If there is a typical graph showing braking torque against rotor resistance at different speeds, it would be helpful. Or a formula that could help me make my own graph. Or even some useful foresight from someone with any experience would be much appreciated.

Thank you for your help with my first post.

 

[itsmoked]

Gads...

People.

You haven't mentioned the mechanical brake.

This is for a mine shaft?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[JGFB]

The main brake is on the hoist drum and is hydraulically released, spring applied. The pinion brake is hydraulically released and applied by gravity (i.e. weights).

Since the mechanical brakes are only supposed to be used in emergency situations and once the conveyance comes to rest it didn't really apply to my questions.

All I'm really looking for is some information on how rotor resistance affects braking torque at different speeds.

As I understand it now, I just use the same concept as for accelerating except backwards: if the speed is higher, more braking torque is required which means more amps in the rotor circuit. As the speed diminishes, I'll decrease the current in the rotor circuit to maintain the deceleration rate until the hoist nears zero speed and the mechanical brakes are applied. Is this correct?

To answer your question, yes this is a mine hoist. An eight foot single drum sinking hoist.

 

[LionelHutz]

If you really want to get into the fine details then I think you're looking for something more involved, likely simulating the application using a program like EMTP. Do you have the electrical model of the motor?

Determining the braking torque of a 3-phase motor caused by applying DC to 2-phases is not an easy task without the electrical model of the motor. Very roughly, the speed-torque curve will "flip" but that is a very rough approximation.

Still, not something to tackle when you're not sure especially since this involves human life.

 

[JGFB]

Thank you for the post.

As a junior engineer, I'm not solely responsible for this. I have many engineers above me to check this work. I am just trying to gain a better understanding of the this type of system so that I can at least be in the same frame of mind as my superiors throughout the design process.

This is not a new hoist. They are simply looking to improve the DB system to gain more control while braking. This hoist has been functional for many years and is just in need of some upgrading.

I will be working through this task in detail eventually. I am simply looking for some basics so that I don't have to ask the silly rookie questions when we get started. I would like to be prepared.

Your comment about the speed-torque curve "flipping" is helpful. This makes me think that I am on the right track. I'm not looking for anything in great detail, just a general understanding.

To me, this is a very interesting task and I thought that by sharing it with you, I might spark some interesting insight that I could bring to the design table.

Thanks