Braking resistor for drive.

Hi, I've just had a new drive, the braking resistor I have had is very large to cope with my app but I'm worried where i should mount it, I know normaly you'd put it on top of the panel the drive is mounted in but I want to enclose it in a box. Would I need a fan etc for cooling, any other suggestions?

Braking Resistor

With ours we mount the resistor(s) in a "Cage" that is mounted to the top of the drive cabinet.

Its really just perforated metal that surrounds the resistor(no touch), but allows plenty of air circulation.

Rich

That's what we do, too. We have never had a problem with this, although dust will tend to burn if you decel hard.

Keith

To properly answer this question, I would need to know what kind of braking you intend to do.

For example, if the only use for the braking resistor is an E-stop on a very infrequent basis (12 hours or more between E-stops) then you can mount the resistor in the same cabinet with the drive and basically not worry about the heat. Even if the resistor gets very hot when E-stop occurs, there is no accumulation of heat and no resulting problems.

On the other hand, if braking is frequent or continuous, then the resistor must be mounted outside the drive enclosure where it can get its own air supply to carry away the continuous or near-continuous heat. An expanded metal enclosure is common and very practical.

Just make sure you dont mount them where someone can climb on them. I worked at a plant that had Danfoss drives with braking resistors mounted in a separate enclosure designed for it....lots of air inlets. These boxes were mounted on the side of block building that people walked on top of when a problem occured with vacuum, air etc. The lead guy on nite shift climbed down and stepped on the resistor enclosure which shorted out, took out the drive ... AND HIS SHOE lucky didnt take his foot.

I've started elevating (ie leaving an air gap) between the top of the panel and the braking resistor. I had an incident where the braking transistor in one of my drives "stuck on", this sent power to the resistor all the time causing it to ge extremely hot. Unfortunatley underneath this resistor was a SLC. The wiring for a devicenet card was tye wrapped on the top of the panel. The result was smoking burnt devicenet cable and several hours of downtime.

my 2 cents

Thanks for that, The braking resistor will be used constantly to slow down a big load and will operate every 15 seconds. The ramp down is for about 1.5 seconds.

I work in a concrete plant which presses wet concrete, the conditions are really dirty and dusty.

I'm glad somebody mentioned about somebody climbing on it because that is typical of what would happen here.

The resistor is in a perforated box but easy for dust to get on.

I was hoping to mount it in a box with holes for circulation, I think this it what I will have to do.

Braking Resistor for YOUR environment

I've worked in similar environments and know just exactly how corrupt the atmosphere can become in a short period of time. Make sure that your resistor has the proper power rating to handle what you are about to put it through! You mentioned that you were dealing with a large load involving frequent stops. The purpose of the resistor is, of course, to dissipate that energy in the form of heat. In the environment you describe the ambient temp is probably already uncomfortable to say the least. You might consider a series/parallel bank of six or so (depending on your load) to help ease the stress on that poor sucker. If you choose a cage just slightly larger than the resistor (or resistor bank) then you may have room to further enclose it (liberally, lottsa room to breathe) with a solid cover utilizing two ventilation fans about four to six inches in diameter. One blowing in (filtered), one blowing out. Of course the filter will have to be cleaned frequently so you may as well just forget the outer cover and blow off the resistor! Hahahahahhaa......Nevermind, please feel free to disregard this thoughtless thought!

This almost sounds like an application for a regenerative drive. You can get rid of the resistor altogether and pump that energy back into the grid rather than just adding more heat to your plant.

Just a thought, let me know if you need details

allscott said:

I had an incident where the braking transistor in one of my drives "stuck on", this sent power to the resistor all the time causing it to ge extremely hot.

Click to expand...

... Important safety tip kids... For ANY braking resistor, it is a hugely wise (and inexpensive) precaution to mount a simple NC thermal switch on or near the resistor assembly, (if one is not already provided. Many companies that make braking resistor assemblies include one) and incorporate that either into the drives coast-stop circuit, or an Emergency Stop string.

Unless one has an unlimited budget, the resistor power dissapation required for dynamic braking is a fraction (5%? 10%?) of the full rating of a drive output.

allscott said:

This almost sounds like an application for a regenerative drive. You can get rid of the resistor altogether and pump that energy back into the grid rather than just adding more heat to your plant.

Click to expand...

For simple braking, (not rapid cycling), a DB resistor and contactor/chopper are generally cheaper by far than a regenerative drive. If the application that requires a braking resistor is say for an AC drive, with an overhauling load, then regenerative braking is usually better.

For safety stops, we generally use both methods in systems. Using a two-stage safety relay, we usually regen down under drive power for 0.5 to 1 second, then the time-delay contacts on the safety relay open a DB/Coast stop circuit.

allscott makes a very good point here about using regen instead of snubber braking.

It would be wise to do a calculation on the payback for a regen drive considering the cost of recovered energy and, in this nasty environ-ment, the constant maintenance a resistor bank might require.

I have learnt loads from this topic and I'm very greatful.
When I say big load it's probably not big to you guys.


The output of the drive is only 2.8Kva, I have choosen a 600watt 100ohm braking resistor.

My application is simply moving a bogey car from A to B in 2 secs. The car weighs apporx 250Kg. So has to accel very quickly then reach speed and start slowing down almost straight away.

I think regen drives may be an overkill for this app.


Can anybody explain what a regen drive does and in what situaction is if suitablty to use one. I have heard of them but never understood what they do.

One thing I learnt recently is that if you only use the motor as the brake then you cannot slow a drive quicker than it can be accelerated.
That is if you attempt to stop the load with a small motor really quick then the load will just overdrive the motor, you have to use a bigger motor. Regards Alan Case

AndEdtec said:

Can anybody explain what a regen drive does and in what situaction is if suitablty to use one. I have heard of them but never understood what they do.

Click to expand...

Regenerative Drives are drives that operate in all 4 quadrants of power.

Quadrant I - Drive delivers forward torque, motor rotating forward. ('Motoring Mode' of operation)
This is the 'Normal' condition, providing power to a load, or accelerating. Similar to using a simple motor starter.

Quadrant II - Drive delivers reverse torque, motor rotatating forward. ('Generating Mode' of operation)
This is a 'Regenerative' condition, where the drive itself is absorbing power from a load, such as an overhauling load or decelerating.

Quadrant III - Drive delivers reverse torque, motor rotating reverse. ('Motoring Mode' of operation)
Basically the same as Quad I, except we are going backwards. Similar to using reversing starters.

Quadrant IV - Drive delivers forward torque, motor rotating reverse. ('Generating Mode' of operation)

The 'Other Regenerative' condition, where again, the drive is absorbing power from the load in order to bring the motor towards zero speed.

A Regenerative drive accomplishes all of the above purely electronically, without having to use contactors to switch leads around.

DC Motors/Drives are simpler, so I'll go into them. Assume a 3 phase, fully controlled bridge, drive here.

A Single quadrant DC Drive would have one power bridge of six controlled switching devices, (usually SCR's) to control the applied voltage level to the armature of the DC Motor. This type of drive can only run in a motoring mode, and would require physically switching armature or field leads to reverse the torque direction

A Four quadrant DC Drive would have two complete sets of power bridges, or twelve controlled switching devices. One bridge controls Forward Torque, and one controls Reverse Torque. During operation, only one set of bridges is active at a time. For straight motoring in forward, the FWD bridge would be in control of the power to the motor. For straight motoring in reverse, the REV bridge is in control.

What happens though, when we want to decel from forward rapidly, or follow a controlled ramp for example? If the drive just shuts off the FWD bridge, the motor would coast down to zero based on the system inertia. This is an uncontrolled stop. In the Regen (4 Quadrent, 4Q) drive, the controller switches off the FWD bridge, and begins turning on the REV bridge, in order to actively generate a negative torque to the motor, allowing for a controlled rate of decel. Since the motor is now acting as a generator, with the drive as the 'load' power is going somewhere. In the case of the DC Drive, the power is dumped directly back onto the incoming supply lines, and is thus mostly recovered (except for wire/switching heat losses).

AC Drives can be regenerative also, but are generally move expensive, as AC Drives require two full "DC Drive Type" bridges between the incoming AC Source, and the drives internal DC Bus. The internal DC Bus is used to provide power to the six output devices that actually create the rotating field for an AC motor.

Non Regen AC Drives most commonly have a simple 3 phase full wave diode bridge to generate the DC Bus. Regen drives must have instead the equivilant of an entire regenerative DC Drive in place of the simple diode bridge, hence the added cost.

Non-regen AC drives, almost without exception, can always operate directly in quadrants I and III (Motoring forward or Reverse).

AC Drives that use a chopper and resistor to enable Quadrant II and IV operation aren't regenerative in the sense of delivering power back to the line, but are regenerative in that they can actively remove power from the driven load.

Regen drives are most often used where there are many and rapid reversals of torque direction, or for overhauling loads, or say, for unwinders, where you are holding back against the rest of the machine.

I hope this helps explain some things

Just a couple of comments to add to rdrast's excellent post on regen.

First, in the simplest form, a regen AC drive has two inverters, one for the usual purpose of converting DC bus power to three phase variable frequency power for control of the motor and a second inverter literally facing backward to invert DC bus power back into the AC supply synchronously so the braking energy is recovered for use elsewhere. That's where the extra costs are.

Second, the decision to use regen AC is primarily one of costs and energy recovery. The added cost of the regen drive has to be compared to the value of the recovered braking energy.

As a general rule, you have to do a lot of braking to justify the added drive cost.

Occasionally, the regen/snubber decision is made on the basis of how hard or hazardous it is to get rid of the heat of the snubber resistor. The regen drive generates very little excess heat when braking and that can be beneficial especially in explosive atmospheres and other difficult environments.