Braking Resistors

Why can't the crane manufacturer answer all these questions over the telephone, please?

You should not make any alteration to a cranes control system without consulting the manufacturer.

...as to do so may invalidate any indemnity insurance cover on the equipment.

Thanks JRaef that was really helpful.

One more thing i need to ask which keeps me confused

if a freely rotating induction motor is stopped suddenly after the power being cut off what will happen to the energy stored or voltage generated.? where will it gets dissipated , across the windings or across the windings and the cable connected and if so will the motor winding gets damaged..??

You always have the best answers on motors and their control.

I will add the reason for the contactor. Its purpose is for safety.

The contactor is energized before the vfd is commanded to run the motor and it is deenergized after the motor is stopped. It is a secondary means to control the motor. It will keep the motor from turning if there is a fault in the control system or vfd which could cause an unintentional start. It could also be used as a secondary means to stop the motor again in the event of a fault.

Yep and always on the incoming side of the VRD. This also serves as a safety device if there is a fault in the VRD then power is removed from it.

I don't know how it is possible to run a VRD in this application without an incoming contactor. This contactor would normally be operted by the HMI which in turn enables the VRD and supplies power.

It's a braking resistor not intended as a circuit control device. It's there to control deceleration of the motor it's applied to. That's why it's called a braking resistor. In a bridge crane in the drive for the bridge the purpose is to slow the motion of the load to a stop to keep the load from swinging to much.

Correct me if I am wrong but i have had many years of experience in the EOT industry.

1. The contactor should always be on the input of the VFD. If it is on the output then it is nothing more than a soft starter.

2. Depending on the size of the motor a braking resistor may not be required. All VFD for this type of application will have a brake relay which will de-energise the motor brake once it has reached a pre-determined ramp down speed, normally 0.

3. Braking resistors are required where there are high torque/ load applications. If not used in this application it will destroy the VSD due to the regenerative energy produced by hte motor during ramp down. And will also be fitted with brake relay for final stop. In this instance the motor brake is a park or holding brake and plays no part in the braking of the crane.

3. Braking resistors do indeed dissipate the regenerative energy produced by the motor during ramp down. This energy is dissipate in the form of heat generated in the resistor.

Motor size has nothing to do with the use of a braking resistor. It's the application. Example is The new miter saws use of one to increase safety. It reduces the amount of time it takes for the blade to spin down to a stop. Most application that I have seen the resistor is switched in and out of the circuit. If not switched it would leave a high potential on the output of the VSD.

If you use a four quadrant active front end VFD, the regenerative energy available during deceleration can be fed back to the system provided there are other consumers in the network to absorb that energy. However, in absence of other loads at that moment, the bus voltage will rise and the energy can be dissipated through the braking resistor.

In case of power failure, however, the above methods of braking will no longer be available and the hydraulic operated mechanical brakes will stop the mechanism immediately.

Wow. Hydraulic brakes!! Just how old are your EOT's? EOT brakes are always fail to safe, electrical or mechanical.

These are not hydraulic thrustor type drum brakes, but hydraulic disc brakes allowing greater braking torques while equipment remains relatively compact. Disc brake inertia is much less than drum brake.

The braking resistors are there to absorb the energy created by regeneration when the load on the motor continues to cause rotatation after the voltage is removed.

Also;

When the applied voltage is removed, the magnetic field surrounding the motor lead conductors, motor winding conductors, and the motor laminations will collapse back into the conductors inducing very high voltage levels that will exceed the insulation breakdown voltage rating of the conductors. (The steeper the angle of collapse up to a limit of 90 degrees, the higher the induced voltage.)

The resistors provide a path to shunt or short this induced current through motor winding and associated conductors to keep the voltage level from exceeding the voltage rating of the system electrical insulation.

This also prevents the dangerously high voltage from getting back into the output circuitry of the VFD and causing damage there.

The operation of the braking resistor contactor is very fast and is more than capable of providing the needed motor protection if working properly.

In some cases the VFD has a separate filter network on the DC buss circuitry that includes resistors which perform the same way as the braking resistors to prevent excessively high voltage from being induced in the buss and conductors from the collapsing magnetic field(s).

This is the same principal taken advantage of by automotive ignition coils to provide high voltage to the spark plugs.

Keep in mind that in many cases such as with a crane, the engineering design incorporates the braking and VFD resistors into one package to save money.

Lots of good answers here already, but nobody appears to have said that you appear to be misunderstanding the job of the brake resistors completely.

The motor would be "braked/stopped" fastest with a short across the windings after power is removed, but a considerable current would be drawn from the back emf in the motor. This could possibly/theoretically even damage a motor not designed for dynamic braking.....

To drop this current to a reasonable level AND to slow the motor down more slowly/gently, braking resistors are used......

They are an integral part of the way the braking works AND inbuilt safety arrangements, you should leave well alone. No mods.

Stopping once or twice will warm up the motor only slightly. Stopping very often using dynamic braking may eventually over heat a motor not designed for such braking as quite large currents flow, albeit only for very short times....

NOTE.

Any electric motor (that I have heard of!), when spinning, generates a voltage when power is removed. (it is actually there all the time and it "opposes" the flow of current from the supply, which is why it is called "Back" EMF!! but you can ignore that in this situation if you wish).

Shorting this back EMF in some way stops the motor very quickly, like overloading a generator for example. You see this even in good quality hand tools, whether battery or mains driven, in the way the motor stops almost instantly. Cheap tools run on and can be a danger to the user.

If the short remains on the hand tool motor, you will find it difficult to turn at any sort of speed by hand. Remove the short and the motor is then far easier to turn by hand.

Here is a short video using dynamic braking and then only power removal braking on a motor driving a plastic disk via a gearbox, but you can distinctly hear and see the difference.

http://www.youtube.com/watch?v=n9PWwmQJfIs