Yes, there are many types of braking systems:

1. Actuation: air, hydraulic, solenoid, electronic, electrical

2. Applicator: caliper-disk, band, hydro. retarder

That makes 12 combinations to name a few:

air-caliper/air band (2)

hydro.-caliper/hydro-band/hydro-solenoid/hydro-retarder (4)

solenoid-caliper/solenoid-band (2)

electronic - inverter (1)

electrical - AC reverse polarity/ eddy current- DC/ DC - Rectifier (3)

Lawrence Nordell

www.conveyor-dynamics.com ■

Sorry,

Motors & Eddy Current should have been added to the applicators.

LKN ■

Hello Eddie

I find that conventional brakes tend to be a pain, and wherever possible I think they should be avoided. (maintenance..heat...coal...fines ...fire...etc)

In this regard, the new breed electrical variable speed drive is great for ramping down as well as ramping up.

What I also like to do is let the material brake itself if I can.

I have found that the braking time varies dramatically with the amount of slack side tension.

If your conveyor does not have any nasty low spots in the vertical profile, and has a winch take-up

, you can winch out during stopping and let the material and belt sag do the stopping.

It is interesting that when you put a tail brake on for example, the braking tension in the belt goes up, thus removing the sag that does the stopping!

Rather like shooting yourself in the foot really.

LSL/Tekpro ■

Dear Eddie Seitei,

Mr. Nordell has already given detailed answer.

Popularly known & used brakes are:

1) Electromagnetic AC brake

2) Electromagnetic DC brake

3) Electro-hydraulic drum brake

4) Electro-hydraulic disc brake

5) Hydro-dynamic brakes (hydraulic retarders)

6) Electrical regenerative braking

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

REGENERATIVE BRAKES

You are describing an old method of quickly stopping a conveyor by applying reverse polarity to the AC motor. This is called plugging the motor. Don't know where the term comes from.

Reverse polarity is harsh and hard on motor and mechanical equipment. Initial application can produce more than 250% torque with respect to motor nameplate. The action is very non-linear and similar to the motor torque slip curve in reverse. Thus, it has high and low torque peaks and valleys.

It has been controlled by voltage regulation and inverter regulation. The later being costly but effective.

Regulated DC braking is used on inverter driven 4 quadrant control when power is lost. This is turning the AC motor into a eddy current brake. THis concept cannot hold the load at zero speed. It is used on Los Pelambres and other large inverter driven systems.

Regulated DC braking is also used on wound rotor motor drives.

Lawrence Nordell

www.conveyor-dynamics.com ■

Dear Shri Rekhawar,

The term regenerative braking is used to describe 3 phase A C Squirrel cage motor or slip ring motor operation beyond synchronous speed.

Squirrel cage (it also implies slip ring) motor on its own operates at speed less than the synchronous speed of 3 phase A C supply. This difference of speed (i.e. synchronous speed – actual speed of motor) is referred to as slip (in %).

If this motor is forced to rotate beyond synchronous speed, it resists such motion and applies negative torque (torque direction against rotation direction). This negative torque is synonymous to brake action, which also applies torque against motion. So, the term braking, but this braking torque only exists for motor speed say 1525 rpm etc. (as against synchronous speed of 1500 rpm). Hence the regenerative braking cannot be used for stopping conveyor but it can only be used to restrain the conveyor speed near synchronous speed (in negative mode).

The mechanical power to drive the motor beyond synchronous speed, gets converted into electrical power and thereby word regenerative. The motor torque speed curve in regenerative mode is mirror image of usual torque speed curve. Motor manufacturers will readily provide these curves. Present day availability of frequency variators enables to adjust synchronous speed and hence there is room for manipulating / creating restrain torque in wider speed range.

In short regenerative braking is not stoppage brake, but a ‘restraining electrical brake’ to keep equipment speed in specific range, by application of opposing force / torque.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

Mr. Mulani:

You are partially right in what you say, but, incorrect in addressing the question of Mr. Rekhawar regarding reverse electromotive force using reverse polarity.

You are stating the obvious action of all AC motor torque slip curves whether they be AC squirrel-cage induction with only primary windings or, the same AC motor with secondary windings to control torque fed by external electrical resistance using slip rings of a wound rotor motor. Both types apply negative torque beyond synchronous speed into the super-synchronous region.

This torque-speed curve is an 180 degree inversion plus rotation of the well known zero to synchronous torque-speed curve. It must be noted that this all occurs in the first electrical quadrant.

I believe his question is: can the motor function as a brake below synchronous speed to near zero speed by:



a) reversing the polartity - inverting the electro-motive force direction of rotation (ie shifting the torque speed curve from the first to the fourth quandrant) or we add the followings,



b) injecting and regulating DC voltage into the AC rotor via the slip ring of a wound rotor drive - converting it to a controllable eddy current brake in the first electrical quadrant,



c) by pre-programmed or feedback controlled dropping the apparent AC line frequency of the AC motor with a speed (Hz) feedback loop via an inverter tailored for the task - this has multiple types such as flux-vector controlled inverters or not, and is also in the first electrical quadrant - as stated in the third paragraph, but, where the apparent synchronous speed is reduced by the inverter control,



d) using an inverter to regulate motor braking torque with reverse polarity via a speed feedback loop - extremely difficult due to the motor signature torque curve's high degree of non-linearity.

A mechanical holding brake in still needed at near zero speed.

Lawrence Nordell

www.conveyor-dynamics.com ■

It should also be noted that there are many other electrical and mechanical types of brakes not listed, which require some explanation and which are not in common use. Hence, they are omitted from the common list. I admit some common brakes may have been left out - its my failing memory that does, from time to time, forget me.

LKN ■

Mr. Nordell,

Happy to read your reply regarding regenerative braking. The original question pertains to regenerative braking and my answer relates to only that topic. So, there is a difference in interpretation of the question. Also, Mr. Rekhawar says that during regenerative braking the motor acts as a dynamo (generator) and generates the power and feeds back into the system i.e. stator.

Only difference is, you are referring to plugging of the motor and I am referring to regenerative braking. So Mr. Rekhawar should be happy to get double the information!

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

Maybe I will confuse some on the nomenclature of the first and fourth quadrant. I do confuse myself as well.

In rectalinear coordinates, the first quadrant has motor torque positive, from zero up the Y-axis, and RPM from zero, positive out the x-axis, in the conventional way. The fourth quadrant has the conventional values of torque negative from zero down the Y-axis if the polarity is reversed.

Although the standard positive powered synchronous motor range is in the first quadrant, it reverses beyond sychronous as it crosses into the fourth quadrant with an inverted and rotated image of its first quaradrant torque-speed curve. What makes this confusing is that the X- axis (RPM) continues to extend in the positive direction. while the torque or Y-axis transitions from positive to negative torque.

My appologies for being to quick from the hip.

LKN ■

Mr. Mulani:

We are beyond the original question of the thread starter.

I interpreted Mr. Rekhawar's third paragraph operative words: "stops the drive".

Both super-synchronous and plugging retard higher speeds. Your comments refer to retarding speed above synchronous RPM, but does not "stop the drive", while plugging, among the other noted methods, retards RPM below synchronous speed to "stop the drive". I note again it does cannot hold this state at zero speed (RPM).

LKN ■

Magnet brakes and clutches are getting some play today with the ability to regulate the variable controllable strength of new materials.

LKN ■

A side bar:

I originally spell checked on Google the word "rectilinear". It came back and asked did I mean "rectalinear". I changed it. I then went back and found both have been used. The vast majority is "rectilinear" which agrees with Microsoft.

Therefore, I request again to petition the "powers that be" for a spell and grammar checker.

Humbly,

LKN ■

By the way Eddie,

It is also necessary to look at the overall conveyor system before you design the brake(s).

For example, if you have a long (high inertia) horizontal overland conveyor feeding a short inclined (low inertia) stockpile conveyor, it may appear that significant braking must be applied to the longer one.

Not always true.

We have found it far better to simply put a flywheel on the shorter one, so it gets approximately the same coating time as the longer overland with the high inertia.

Flywheels do not need the maintenance that brakes do.

Good luck

LSL Tekpro ■