# DC Brake Motors



## chenley (Feb 20, 2007)

I'm needing some information on how DC brake motors work. Had to wire one in last night which I got to work by going with the nameplate data, although it was confusing being the first motor of this kind that I have wired in. 

I think I know the theory on how it works, although the nameplate connection data has bamfoozled me. Anyone have a good website to read up on them?

I know putting a VFD on a normal 3-phase motor would do the same thing, this was a factory with no panel space for the machine and they don't really want to change anything on it since the manufacturer comes in yearly and services it. 

Thanks in advance.


----------



## drsparky (Nov 13, 2008)

Are you are referring to resistive breaking? Then a large motor is shut off it will spin for a very long time. I bumped a 1000 hp motor to verify rotation and it took 25 minuets to stop spinning and it was not even up to full speed. By switching a large resistor pack onto the motor leads the spinning motor gets loaded down just like a generator under load. Diesel electric locomotives use this and call it dynamic braking.


----------



## Southeast Power (Jan 18, 2009)

The type I wired had a "brake" for safety. The brake is operated by a DC circuit. I had to fabricate a full wave rectifier to repair one several years ago.
I dont remember exactly how it worked.


----------



## Bob Badger (Apr 19, 2009)

You will need to be more specific as it could be many things, the other posters mentioned mechanical brakes and dynamic braking there is also DC injection braking which can stop a motor very quickly but heats the heck out of the motor.

http://www.claytonengineering.com/training/myweb6/Module16/Output/BrakingMotor.html


----------



## RIVETER (Sep 26, 2009)

drsparky said:


> Are you are referring to resistive breaking? Then a large motor is shut off it will spin for a very long time. I bumped a 1000 hp motor to verify rotation and it took 25 minuets to stop spinning and it was not even up to full speed. By switching a large resistor pack onto the motor leads the spinning motor gets loaded down just like a generator under load. Diesel electric locomotives use this and call it dynamic braking.


I probably would not "BUMP" a 1000hp motor unless I knew which way it was supposed to go, or it was de-coupled.


----------



## chenley (Feb 20, 2007)

It used DC injection braking. From the best that I can remember I had the three phases coming in. Had a positive and negative which two phases were tapped from the three phase. How it was wired within the motor, I have no idea. The equipment is called a "palletizer" and is just used to feed pallets for a process in the plant. So it was only a <5hp motor. 

All the other equipment that I have ever worked on we installed AB Powerflex's to stop motors for safety.


----------



## micromind (Aug 11, 2007)

DC injection braking is simple. Just remove the 3 phase AC power, and apply a DC voltage to any two of the motor leads. 

If the DC voltage is low, there will be little braking. If it's high, it'll stop the motor instantly. Obviously, the quicker it stops, the harder it is on the motor.


----------



## drsparky (Nov 13, 2008)

RIVETER said:


> I probably would not "BUMP" a 1000hp motor unless I knew which way it was supposed to go, or it was de-coupled.


De-coupled it was. It was part of the required commissioning procedure. 4180 volt 1000 hp motor coupled to a mean set of knives. Turns logs into "wafers" for oriented strand board. The soft start was the size of two large refrigerators.


----------



## John Valdes (May 17, 2007)

chenley said:


> I'm needing some information on how DC brake motors work. Had to wire one in last night which I got to work by going with the nameplate data, although it was confusing being the first motor of this kind that I have wired in.
> 
> I think I know the theory on how it works, although the nameplate connection data has bamfoozled me. Anyone have a good website to read up on them?
> 
> ...


Is this a DC motor with a DC brake coil? Or are you injecting DC into an AC motor to help it stop? I am not sure I understand the question.


----------



## chenley (Feb 20, 2007)

I'm not sure if it was DC injected or had a coil on the motor itself. The motor was a little longer in length than a standard 3 phase motor. The motors peckerhead had the lines for the three-phase power to be connected. Following how the previous motor was wired, two of the phases were tapped and lead to screw terminals that had (+) and (-). It worked that is about as much as I know about it. 

I'll see if I can snap a couple of pictures of it this weekend.


----------



## John Valdes (May 17, 2007)

chenley said:


> I'm not sure if it was DC injected or had a coil on the motor itself. The motor was a little longer in length than a standard 3 phase motor. The motors peckerhead had the lines for the three-phase power to be connected. Following how the previous motor was wired, two of the phases were tapped and lead to screw terminals that had (+) and (-). It worked that is about as much as I know about it.
> 
> I'll see if I can snap a couple of pictures of it this weekend.


It sounds like a standard brake motor by your description. Power on, brake off. Power off, brake on. Just remember to never wire the brake motor this way if you are using a VFD. In that case you control the brake coil independently from the motor.


----------



## Introyble (Jul 10, 2010)

chenley said:


> I'm needing some information on how DC brake motors work. Had to wire one in last night which I got to work by going with the nameplate data, although it was confusing being the first motor of this kind that I have wired in.
> 
> I think I know the theory on how it works, although the nameplate connection data has bamfoozled me. Anyone have a good website to read up on them?
> 
> ...


Since you mentioned a variable frequency drive ~ Are you referring to dynamic breaking?

The whole thing acts as a voltage regulator. The control circuit monitors the DC bus voltage level
and turns the switching device on and off at the appropriate level. In the case of a 460 volt drive,
the nominal DC bus voltage is about 650 volts. If the DC voltage reaches about 810, the drive will
protect itself with an over-voltage trip. A 460 volt dynamic brake will regulate the DC bus voltage
to about 750 volts. This provides enough voltage margin to prevent the over-voltage trip but does
not interfere with the 650 volt nominal level observed during a “motoring “ condition. When
regeneration pushes the DC bus up to 750 volts DC, the switching device turns on allowing
current to flow in the resistor. Regenerative energy is then released into heat. This causes the DC
voltage to drop. The switching device will stay on until the bus voltage reaches about 735 volts
DC and then turn off. This difference between turn on and turn off voltage is called histories. For
this reason one can observe DC voltage bouncing between 735 and 750 while the dynamic brake
is active. The dynamic brake function is the same for drives rated at 575 and 230 volts. The turn
on and turn off voltage


----------



## John Valdes (May 17, 2007)

Introyble said:


> DC and then turn off. This difference between turn on and turn off voltage is called histories.


Its not "histories" its called "hysteresis". So your a motor and drive guy? Good to have you around.


----------



## Redfish31 (Nov 23, 2011)

*Motor Brake on a VFD*

I have recently installed new control cabinets on Aircraft Hanger Doors. The old control cabinets did not have VFD's but the new ones did. On the old cabinets the motor brake leads were tied in to the motor leads and not to a set of contacts that would release the brake when the push buttons were engaged. We did not change anything with the wiring on the motors but left them as is and hooked the motor leads up to the new VFD in the control cabinet. The doors seem to be working fine right now but I have a feeling that over time this could cause a problem with use. Will this be the case or should the motor brake function as it is now?


----------



## micromind (Aug 11, 2007)

Redfish31 said:


> I have recently installed new control cabinets on Aircraft Hanger Doors. The old control cabinets did not have VFD's but the new ones did. On the old cabinets the motor brake leads were tied in to the motor leads and not to a set of contacts that would release the brake when the push buttons were engaged. We did not change anything with the wiring on the motors but left them as is and hooked the motor leads up to the new VFD in the control cabinet. The doors seem to be working fine right now but I have a feeling that over time this could cause a problem with use. Will this be the case or should the motor brake function as it is now?


One possible issue; a lot of brake coils are 120, and they're connected to motor leads so that a 230 or 460 volt motor acts like an autotransformer. 

The way to tell is the brake is connected to T1 & T4, T2 &T5, or T3 & T6. A wye connected motor (most small ones are) will give 120 this way regardless of the incoming voltage.

If the brake coil is fed by a VFD, it very likely won't last long.


----------



## nolabama (Oct 3, 2007)

Why will a VFD kill a motor brake?


----------



## McClary’s Electrical (Feb 21, 2009)

nolabama said:


> Why will a VFD kill a motor brake?


 

A "motor brake" in it's simplest form is nothing but a coil actuating a brake pad against the rotor. The coil will probably not like the waveform produced by the VFD, but I don't know that it would kill it. Although the coil is probably rated at 60 htz and 60 htz only.

What kind of brake?


----------



## ilikepez (Mar 24, 2011)

Don't most VFDs have a set of auxiliary contacts controlled by the VFD but powered by whatever you choose? Seems that would be the best way to control your breaks instead of trying to control them with the motor output of the VFD.


----------



## micromind (Aug 11, 2007)

ilikepez said:


> Don't most VFDs have a set of auxiliary contacts controlled by the VFD but powered by whatever you choose? Seems that would be the best way to control your breaks instead of trying to control them with the motor output of the VFD.


That's the best way; you can't supply the brake coil with the VFD output. 

But more often than not, you'll need to run two more wires from the VFD to the motor, and the VFD relay contacts might not be able to handle the current of the brake coil. 

Also, most brake coils are 120 volt; this voltage might not be available at the VFD. Even if it is, if it's derived from a control power transformer, it might not be big enough. 

In short, powering a brake motor with a VFD is often not as easy as it seems.


----------



## JRaef (Mar 23, 2009)

It's clear that you are referring to what John Valdes was talking about, a "Brake Motor" is what we call them, but what it really should be called is an "AC motor with a DC held brake". But unlike the Germans, we don't like long drawn out names like that so "Brake Motor" it is.

The motor part is just a standard AC 3 phase motor. On the back (non-drive) end of that motor is a SPRING LOADED brake assembly that has plates and disks, just like a clutch on your car. The plates are attached to the stationary frame, the disks are attached to the motor rotating shaft. When there is no power applied to the motor, as in the motor is off, the spring assembly clamps the plates to the disks and the shaft of the motor cannot turn, it is locked in place. 

When the motor gets powered, a part of the motor power is siphoned off to a rectifier, converted to DC and used to pull in a solenoid plunger that will pull the brake plates and disks apart. The reason they use DC is because once the solenoid pulls in, DC uses a lot less power (and stays cooler as a result) than if they used AC for the coils. Remember, this will need to be held ON for the entire time the motor is running, and the motor is ALREADY going to be hot.

They do it this way because it is "fail safe" in that a loss of power, whether intentional (hitting the Stop button) or unintentional (utility power outage), results in the motor coming to a safe stop. Think of a hoist motor on a hillside tram. If the power fails while you are going up the the hill, a standard motor will just spin backward and you crash at the bottom. But with a brake motor if the power fails, the springs clamp down and the shaft stops, you get out of the tram and buy a lottery ticket. For this reason, you cannot *ass*ume that you can replace a brake motor with a standard motor and a VFD with any kind of electrical braking option, because if the power fails, NO BRAKES! You have to carefully evaluate WHY a brake is specified and if you don't know, don't *ass*ume.

You absolutely cannot power a motor brake from a VFD, the VFD is changing the frequency AND THE VOLTAGE. So if the brake coil is designed to accept 460V fro the motor terminals and the VFD is running at 1/2 speed, it is only getting 230V. The brake solenoid may not release, which means the brake engages, the VFD sees a locked rotor and hopefully trips, but you are also overheating that brake solenoid coil. If you have a Brake Motor and you need to power it from a VFD, you must run a SEPARATE power source to the motor pecker-head that is dedicated ONLY to the brake, not the motor, and use a relay output of the VFD to drive a small contactor to switch the brake coil on and off, in the same logic as the VFD (meaning what John Valdes said; motor on, brake powered and released, motor off, brake de-energized and set).


----------



## joethemechanic (Sep 21, 2011)

I am not a big fan of mechanical braking for motors. Adds expense, and complexity, and it seems rare that I find them properly adjusted. But there are applications that you really have no choice like on a hoist.

Lots of times it seems that DC across the armature is more practical because it never needs adjustment and there are no friction parts to wear out.

1/2 wave rectified DC provides a bit of a different deceleration curve from what I have found. Not sure what you might need the different curve for, but try it on a little motor some time. Some day you might find an application for it


Oh BTW, dc injection braking will work on single phase applications too


----------



## nolabama (Oct 3, 2007)

I got my first real experiance with a brake motor this past weekend. I know I do not like them. They are a PITA but we have no choice but to use them. I like machinery brakes over brake motors.


----------



## McClary’s Electrical (Feb 21, 2009)

nolabama said:


> I got my first real experiance with a brake motor this past weekend. I know I do not like them. They are a PITA but we have no choice but to use them. I like machinery brakes over brake motors.


 

120 volt release, spring applied hoist motor on a crane










400 HP dynamic brake









2500 HP 4160v air brake motor


----------



## John (Jan 22, 2007)

joethemechanic said:


> I am not a big fan of mechanical braking for motors. Adds expense, and complexity, and it seems rare that I find them properly adjusted. But there are applications that you really have no choice like on a hoist.


Another way to brake a 3 phase motor that is real old is called plugging. Basiclly what you do is slam the motor in reverse until it stops turning. AB made(still makes?) a plugging switch. all you need is a reversing starter and the plugging switch. It's the most sure way that I have ever seen to really stop a motor. This metod has the stopping power to stop a motor in a fraction on a second.

There are also 3 phase friction brakes that don't use a solenoid as an actuator. These are almost as good as the plugging method.


----------



## nolabama (Oct 3, 2007)

the ones I have looked exactly as jraef described - PITA to remove and repair


----------



## Motorwinder (Dec 30, 2010)

John said:


> Another way to brake a 3 phase motor that is real old is called plugging. Basiclly what you do is slam the motor in reverse until it stops turning. AB made(still makes?) a plugging switch. all you need is a reversing starter and the plugging switch. It's the most sure way that I have ever seen to really stop a motor. This metod has the stopping power to stop a motor in a fraction on a second.
> 
> There are also 3 phase friction brakes that don't use a solenoid as an actuator. These are almost as good as the plugging method.


Very hard on the rotor. This will severely shorten the life of the motor.

The 3 phase brakes are the best. There is also tapered rotor brakes.


----------

