How Do Dynamic Motor Brakes Work?

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iwire

iwire

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Lately I have been installing VSDs on a number of 5 HP 3 phase grinders. Along with the VSD we install an optional resistor that gets wired to terminals labeled dynamic brake on the VSD. The grinder without the brake would coast for at least a minute or two. With the brake it stops in a couple of seconds max. :cool:

What exactly is happening when the VSD changes from run to stop?

Is the motor being used as a generator and dumping the power into the resistor?

And if so why only one resistor for a three phase motor, I would have expected at least two or three?

Does the VSD put power into the motor to keep the field working as a generator?

Go easy, I am no electric motor expert. I can wire them fine, just don't know how they work. :)
 
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Bob.,,

I will explain real quick and simple way with the dyanamatic braking system


for most AC motor it is common to inject a small DC current in one of the two leads [ it will work both single and multi phase motors ] and what it do is make a magatanitc feild to slow down the rotor because of injected DC source act like counter electromagatic force to slow down the motor much quicker.

with the VSD it is easy to do this by feed little DC current to the AC motor but tricky part is to timed to how far you can inject it and when to stop injecting the DC current in the motor [ senice most VSD useally have somekind of feedback so they can able to read how far it will before it stop feeding the DC current.]

but with AC motors without VSD's this get little more trickier with this but it done often on it. they have a interlock switch on motor contractor to sense if the power is on or not when the motor is coasting it will inject the DC from power supply to slow down pretty fast but they have build in timer to stop suppling the power to the AC motor

for the DC motor it pretty straght foward it will become generator and used the load bank to slow them down pretty fast as long you are not doing the plugging it [ plugging mean reversing the connection very fast when still running ]

Hope it will help you.

If need more expain or other let me know i may find something to show ya

Merci, Marc
 
Mark,
I don't think that dynamic breaking and DC injection are the same thing. Most of the drives I have worked with can do both, but you have to add the resistor for dynamic breaking.
Don
 
Don, Yes that true you can add the resistor to adjust the amout you want to use on the dyamatic braking system.


I know the crane system often used this type of braking system.

[ i will find a link related to this info later and i will add to this fourm ]

Merci, Marc
 
Bob Lets see if I can help you understand..The analogy I will use is a diesel engine when they want to sow down they can use a device called a Jake break..It is a device that use the compression on the engine to slow down the engine that is generating the compression by changing the valve useage..Now the dynamic brake is very similar..the motor is spinning in a lets clockwise direction and you disconnect the electrical source and now you have a generator..the drive now takes the power and applies it to a different magnetic field and is injected back into the motor..This process generates allot of heat the resistor is how the excessive heat is disipated..The process is actually more complex but I think this is an easy and simple explaination..I am sure we will now hear more on the topic..

edited to add some more info..

I actually believe the style of braking you have discribed would be load braking..I believe you have described a machine that creates mechanical energy and the drive is probabully designed to do load braking..I believe that is why you are inserting the external resistor is to aid in the disipation of mechanical energy..I believe there will more in depth answers..I know Jraef has a very good understanding of this..
 
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Some of the dynamic brakes that I have worked on (printing presses) were even water cooled. The resistors darned near short the motor leads, and they can get quite hot. The bigger one's have a water cooled radiator.
 
You were right, Dynamic Braking is all about regeneration.

Any AC motor can become a generator under the following conditions:
1) The motor stator field is excited.
2) The motor is spinning faster than the applied power frequency used to excite it (overhauling).

When you have a VFD you are controlling the frequency to run at different speeds. But then when you want to shut down, you also now have the ability to alter the excitation field frequency as well. So during braking, the VFD is keeping the motor excited so that it keep generating no matter how much slower it is getting.

Then, the power it is generating is pumping back through the VFD transistors into the DC bus and charging the bus voltage higher and higher. At some point, the bus voltage would climb too high and damage the components. So a voltage monitor is put on the DC bus and when a threshold is reached, it fires a 7th transistor that pumps that excess DC power into a resistor to be burned off as heat. That is why you only need 1 resistor; it is DC going to it. The bigger the resistor the more power it can dissipate, but you are still limited to the power rating of the transistor used to fire into it. In mdshunk's example of a printing press where there is a lot of inertia to overcome and/or the braking can be constant in order to control web tension, the transistor is custom designed for the job. But for most common machine applications, the drive will come with a transistor sized the same as the power transistors, so that is why you will see the braking torque limited to 100% of the motor torque.

It's called "Dynamic Braking" (DB) because the amount of braking torque that can be applied is dynamically changing as the load decelerates. In other words, the braking energy is a function of the kinetic energy in the spinning mass and as it declines, so does the braking capacity. So the faster it is spinning or the more nertia it has, the harder you can apply the brakes to it, but as it slows, you run into the law of diminishing returns and at some point, you actually have NO braking power left. That is why in almost all VFDs they will use DC Injection Braking (DCIB) at the very end. The problem with DC injection is that it traps the kinetic energy inside of the motor, specifically in the rotor where it has trouble getting out as heat. So the ideal package is to use the DB to get most of the energy out of the load, then finish it off with DCIB when it's just creeping along.

An alternative to using DB resistors is what is called Line Regenerative (a.k.a. Regen) Dynamic Braking. It is the same, except instead of firing the excess DC power into a resistor as heat, it pumps it through another inverter on the front end as power back into the utility supply. That's great for those continuous load situations like printing presses, but is a quantum leap more expensive because you are essentially using 2 VFDs to do one job.

Hope that helped.
 
Thank you all, I have a much better understanding of how it works. :cool:


Jraef said:
Hope that helped.
Click to expand...

Yes very much, I appreciate the added info on the DC injection.

The machines are actually large belt grinders and do not roll that easily due to the soft wheels the belts run on. They seem to rely on only the dynamic braking. When stop is pressed they rapidly slow down and then when almost stopped coast to a stop.

They also have other large conventional grinders that are just grinder wheels connected directly to a the motor shaft, I have to assume they are using DC injection as they stop very quickly, almost like a mechanical brake has been applied.

The motors also groan pretty loud during the braking, I assume the motor could be damaged with short cycles?
 
DC injection?

DC injection?

I have dealt with numerous dynamic braking systems for motors with VSD (including a 30 MW motor), but I am not familiar with DC injection. It has to saturate the core or back steel, but how does this dissipate the energy from the load inertia?
 
iwire said:
Thank you all, I have a much better understanding of how it works. :cool:
...
The motors also groan pretty loud during the braking, I assume the motor could be damaged with short cycles?
Click to expand...

Typically if they are groaning loudly that is DCIB, and yes, short cycling is a definite problem. The rule of thumb is, count each braking cycle as if it is another starting cycle. So if a motor is rated for 10 starts per hour, that is 5 start/stops with DCIB. I typically recommend less, because in DCIB the kinetic energy is trapped mainly in the rotor and it takes longer for the heat to pass through the shafts that if it were in the stator. But there are different types of DCIBs out there, some are harder on the motors that others. A lot of it has to do with how fast you ant it to stop. What are called "1/2 wave" brakes are more gentle on the motor heating issue, but of course can't stop the motor as fast.
 
sdv said:
I have dealt with numerous dynamic braking systems for motors with VSD (including a 30 MW motor), but I am not familiar with DC injection. It has to saturate the core or back steel, but how does this dissipate the energy from the load inertia?
Click to expand...
See above. It doesn't, that's the problem. The kinetic energy is turned into heat in the motor and the motor has to dissipate it. Not the case with DB, but you need a VFD to do DB (for the most part) and that cost can be too high for people who don't need to vary the speed. DCIBs are relatively cheap.
 
Thanks everyone for the information here and the timeliness of this thread. It allowed me to ask some detailed VFD questions at an interview this morning:smile:.
 
Jraef said:
Typically if they are groaning loudly that is DCIB, and yes, short cycling is a definite problem. The rule of thumb is, count each braking cycle as if it is another starting cycle. So if a motor is rated for 10 starts per hour, that is 5 start/stops with DCIB. I typically recommend less, because in DCIB the kinetic energy is trapped mainly in the rotor and it takes longer for the heat to pass through the shafts that if it were in the stator. But there are different types of DCIBs out there, some are harder on the motors that others. A lot of it has to do with how fast you ant it to stop. What are called "1/2 wave" brakes are more gentle on the motor heating issue, but of course can't stop the motor as fast.
Click to expand...

Thanks I did not know you had to count the braking cycles as starting cycles..that is a good piece of knowledge to have..:smile: :)
 
cschmid said:
Thanks I did not know you had to count the braking cycles as starting cycles..that is a good piece of knowledge to have..:smile: :)
Click to expand...

It really depends on _how_ you do the braking cycles.

When you start a motor 'across the line', you have a temporary situation where the rotor is stationary but the rotating field is turning at full speed. The motor is in a high slip condition, will draw perhaps 6x full rated current, but the mechanical output is zero. Lots of electrical power going in to heat the motor (not 6x rated power, because of power factor, but a damn lot). As the motor comes into synchronism, the heating rate goes down.

When you stop a motor with DCIB, then you have a temporary situation where the rotor is spinning at full speed, but the 'rotating field' is stationary. From the point of view of the rotor, this situation is exactly the same as starting.

But when you use a VFD, for starting you _can_ adjust the supply frequency so that the motor is in synchronism right from the get go. The rotor is stationary and the rotating field is turning at low speed. You develop full torque at _normal_ current and reduced voltage. As the rotor accelerates you increase the drive frequency and rotating field speed, maintaining normal current as the motor accelerates. Such a start produces no more heating than normal full load operation.

Similarly, with proper dynamic braking, the motor will operate at normal currents over the entire speed range. Eventually you will reach the point where the rotor speed is too low to permit regenerative braking, but you can still modulate the frequency to minimize the power input for braking the motor.

Of course, this will depend upon the VFD having good speed feedback, and good control algorithms, and it is probably more cost effective to allow more heating during both starts and stops then to try to be maximally efficient during these operations.

-Jon
 
I think even if you did load braking which is self exciting braking if I am correct it still causes heating on the rotor..DCIB is separately excited braking which also causes heating on the rotor..Does using and external power source create excessive or larger amounts of heat on the rotor??So if you have a large fan on a VFD and you slow it down using the dynamic braking feature it causes excessive heat on both the motor and VFD especially in smaller VFD's were external resistors are not used..does this have an effect on the VFD as well??And have I done a proper job of explaining my thoughts??
 
The issue of creating additional heat in the motor from braking applies to DC Injection, not usually Dynamic Braking with a VFD. DB puts the heat conversion out of the motor and into the resistors. The motor is fine and you can do that all day (or to the limits of the resistors).

Small cheap VFDs without external DB resistors are still doing the same basic thing as those with it, but they are trying to dissipate the energy through their built-in pre-charge resistor and it is highly ineffective. That's why you will see specs on small drives that say you can have "20% braking torque, 10% duty cycle", because it can't take any abuse. What they do then if the resistor is getting overheated is to revert back to coasting and/or DCIB, often without letting you know. You get what you pay for.
 
Just wanted to toss a few things into this thread - and say Hello!!! :smile:
(it's been a long time since my last post!)
The information is in addition to the great information already posted by the other members.

Anyhow, as to Dynamic Braking, there are two definitions:
  1. Regenerative Braking: When a DC Motor has Electric Braking applied to it, via regenerative actions,
  2. Dynamic Braking: When an AC Motor has Electric Braking applied to it, via DC Current injection.
The first method - DC Regerative Braking, is when the DC Motor is used as a Generator, typically when a large dynamic load is connected to the Motor, then Line Current is removed from the Motor's Terminals, but a small value of Current is driven into the Field Windings, to keep the Magnetic Field established.
That "Braking Current" may be regulated per the intensity of the Braking required.

With the DC Regenerative Braking, the DC True Power generated by the Motor is sent to a "Braking Resistor" (or an array of Braking Resistors for heavy usage situations).
The True Power developed by the DC Motor is converted into Heat Energy, at the Braking Resistor(s) - thus placing a low Resistance load across the DC Generating device.

The Motor slows down because the Kinetic Energy built up in the rotating load is converted to Heat at the Resistor(s) - and continues to slow in a linear fashion per the speed of the rotating load, the remaining Kinetic Energy in the rotating load, and the fixed Resistance of the Braking Resistor(s).

Railroad Locomotives with DC Traction Motors employ this technique for an alternate braking mechanism - used in conjunction with the Locomotive's Independent Air Brakes, and the "Trainline" Air Brakes.
Still termed "Dynamic Braking" by the builders (GE and GM), the Locomotive has a separate throttle for Dynamic Braking - which is segmented into Eight (8) individual set points - ranging from "0" to "8", for control over the complete Braking application.

The effective range for Dynamic Braking is between 15 and 30 MPH, and so the Dynamic Braking is typically used on grade descents and when stopping a Train.

Locomotives with AC Traction Motors (Motors are driven via VFDs), typically employ an Injected DC to the Traction Motors - which results in a non-rotating magnetic field produced in the stator windings, then induced into the rotor - which quickly retards the speed of the moving rotor (or places a "binding load" on the rotor).

Since the AC Traction Motor will generate some VARs / True Power, a Resistor (or array of Resistors) is placed across the Motor(s) during the Braking application.

That's all for now!!!
 
Scott,

A nit-picking point:

My book, which is rather old, says that dynamic braking dissipates the kinetic energy in a resistor while regenerative braking pumps the energy back into the supply lines. Would not be surprised though to find different definitions in different sources.
 
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