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Old 11-27-2019, 09:18 AM   #1
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Default Braking a motor not on a vfd

Hey guys, we have a situation with a new machine that was built which has 2 10 hp motors on it running auger bits. It is a custom machine, and when it was built it has conventional motor starters running the motors. The problem is when the auger bits are done drilling and the operator hits the off button, the motors keep spinning for quite a while without any way to stop them. Is there a braking system (resistor or otherwise) that can be integrated to slow these puppies down quickly without adding a VFD and DC braking module? Maybe some kind of mechanical brake on the shaft? This is in the brainstorming stage still since the machine was just tested yesterday and I didn't hear about this issue until last night. I haven't installed any new stuff with braking components so I am looking for some direction.
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Old 11-27-2019, 09:27 AM   #2
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Its too bad they are not hydraulic.
10HP is a decent size motor, I don't know if they like to be plug reversed at that size or if a typical mechanical brake could stop it.
Have you considered some kind of a clutch?
That way, you wouldn't have to screw around with the motors that much.
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Old 11-27-2019, 09:36 AM   #3
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there is add-on brake kit that fits motor if there is some place to move the motor (often goes between motor and output shaft)
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Old 11-27-2019, 09:56 AM   #4
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We've wired mechanical brakes to starters for downhill conveyors and bi-folding airplane hanger doors. So it can be done.


Someone will just have to sort out how to add a brake to what you have there.
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Old 11-27-2019, 11:41 AM   #5
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What about an electric brake using DC injection to create a stationary field? Get yourself a good sized resistor, a full wave rectifier and a good relay to insure both ac and dc voltages cannot be sent to the motor at the same time. Then the coil of the relay can be run using a stop button. Heat is killer though. That resister for instance is going to need a giant heat sink and heat sinking paste applied to the mounting means. All this can be done outside of using a drive, and the parts are pretty cheap. Obviously it would be best if the augers can be removed from the load before braking is applied or else you are gonna need one hell of a big resistor.
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Old 11-27-2019, 12:11 PM   #6
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Why all the coast on an auger? Yes all kinds of braking options, but they all add a maintenance and production cost. Most augers use gear reduction so that is good most of the time. If it is inertia coast that will wear parts or like mac said a lot of heat to burn off.
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Old 11-27-2019, 01:11 PM   #7
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https://www.electriciantalk.com/show....php?p=5243342

Here is a suggestion that I thought looked interesting.

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Old 11-27-2019, 09:37 PM   #8
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I checked into an similar application a few years ago. The least expensive fix was to install VFDs and breaking resistors. This also saves all of the guessing and improvising. By the time that a brake is purchased, a structure and supports are fabricated and installed, and power and/or electric controls are configured, installing two $1200-$1500 VFDs is a cheap option.

Most mechanical brakes for motors are more to hold position after stopping rather than to stop a rotational load. Mechanical brakes are another item that requires regular maintenance.

If you were to install a mechanical brake, will enough force to stop the load quickly, it would be likely to cause drive train and equipment damage from the repeated mechanical shock loading .
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Old 11-27-2019, 10:38 PM   #9
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Quote:
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I checked into an similar application a few years ago. The least expensive fix was to install VFDs and breaking resistors. This also saves all of the guessing and improvising. By the time that a brake is purchased, a structure and supports are fabricated and installed, and power and/or electric controls are configured, installing two $1200-$1500 VFDs is a cheap option.

Most mechanical brakes for motors are more to hold position after stopping rather than to stop a rotational load. Mechanical brakes are another item that requires regular maintenance.

If you were to install a mechanical brake, will enough force to stop the load quickly, it would be likely to cause drive train and equipment damage from the repeated mechanical shock loading .

I had to rebuild from scratch the dynamic brake for my Greenlee 555 SBC. The parts cost me less than $30 bucks , but I had relays galore already laying around.
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Old 11-28-2019, 01:21 AM   #10
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If you're going to build your own DC injection braking system (I've done several), be careful.....it doesn't take much DC to stop the motor quickly and it'll have the ability to bust stuff if too much is used.

DC injection can stop the motor MUCH faster than it can start.
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Old 11-28-2019, 06:41 AM   #11
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Just thinking out loud here, could you use a reversing starter reconnected to the motor leads to create a induction generator to slow the motors down? The energy could be wasted off in a electric unit heater or such.
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Old 11-28-2019, 07:03 AM   #12
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Quote:
Originally Posted by 460 Delta View Post
Just thinking out loud here, could you use a reversing starter reconnected to the motor leads to create a induction generator to slow the motors down? The energy could be wasted off in a electric unit heater or such.
Wouldn't that be plugging?
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Old 11-28-2019, 07:52 AM   #13
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Quote:
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Wouldn't that be plugging?
I was thinking you could connect the motor leads to the line side on the reverse starter and the use the load side to go to a resistance bank [heater].
The reversing starter would be used for the mechanical and electrical interlocking aspect.
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Old 11-28-2019, 08:28 AM   #14
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Quote:
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I was thinking you could connect the motor leads to the line side on the reverse starter and the use the load side to go to a resistance bank [heater].
The reversing starter would be used for the mechanical and electrical interlocking aspect.
Ah I see, that sounds good. I think that would be dynamic braking. Seems like a pretty nice way to do it, you convert that mechanical energy back to electrical through the motor and get rid of it in a heater or other load.
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Old 11-28-2019, 08:31 AM   #15
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Quote:
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I was thinking you could connect the motor leads to the line side on the reverse starter and the use the load side to go to a resistance bank [heater].

The reversing starter would be used for the mechanical and electrical interlocking aspect.

Thats a brake chopper minus the chopper. Not plugging just generator stopping. So the contactor pair is just a pair. Since resistors don’t care about direction.

Only issue with this is your grid resistor is going to see a lot of heat. Need to size it for 6x FLA amps at 10 seconds since that’s what you will get.

Looking back at OP who said “DC module” what is that? No such animal on a VFD. Only two things I can think of . One would be a DC choke which is just a filter and for a totally different purpose (cheaper alternative to line and load reactor) and the other possibility is a brake chopper resistor which is not DC injection but regeneration but burning off the excess power as heat instead of into the line.

Not understanding why someone would avoid buying a cheap 10 HZp VFD. It’s just not that much money.

Let’s review stopping methods in order of effectiveness.

Decelerate on VFD, not regeneration. As slow as you want. This is not coasting. Often used on pumps to avoid things like slamming a check valve where on its own the motor stops abruptly,

Coast to stop. Easy on everything but very slow with good bearings and high inertia. It’s the default and what you get just cutting everything off.

Regeneration. Fairly fast. Requires a 4 quadrant drive where both the front end and inverter are full transistor systems. On small drives this is cost neutral (under 25 HP). On larger ones a lot of pump/VFD drives and virtually all medium voltage drives use free wheeling diodes so regeneration is not possible. I’m regeneration the VFD merely runs backwards where the motor is a generator, output up to same torque as motor name plate is possible. Usually very inexpensive to do. No extra thermal load on the motor.

Brake chopper. VFD has one extra transistor which it uses to connect an external resistor to the DC bus. Usually much higher power output than regeneration so can do up to around 175-250% of name plate (motor stall torque limit). Can be combined with regeneration. No extra heat load on motor.

DC injection. Turn the motor into a magnet by putting DC on the stator, Can be fairly fast up to 300% of name plate torque but since this heats the motor to destruction fairly quickly it’s limited in how much can be done.

Plugging. Done normally with reversing contactors. Reverse the line connection via contactors. Motor stalls for an extended time so again 175-250% of name plate stall torque but at 6x FLA current on motor. Motor cooling, wiring, and contactors have to be grossly oversized due to extended stall time. This is very rough on motors but is done in some crane applications.

Mechanical brake. Typical brake motors have brakes with 25% of the torque of the motor but they can easily exceed motor torque. Very simple to use. A Stearns brake just bolts onto the back of a motor with an extended shaft (past the fan). Brake voltage can be different from motor. Common voltages are 120 V (control voltage) and.motor voltage. With voltage same as motor often brakes are wired in parallel to motor at the peckerhead so it releases at start and applies at stop with no extra contactors, wiring, etc.

The last item is the cheapest, easiest way to brake if you don’t have a VFD, etc.
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Old 11-28-2019, 08:46 AM   #16
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Quote:
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Thats a brake chopper minus the chopper. Not plugging just generator stopping. So the contactor pair is just a pair. Since resistors don’t care about direction.

Only issue with this is your grid resistor is going to see a lot of heat. Need to size it for 6x FLA amps at 10 seconds since that’s what you will get.
...

Brake chopper. VFD has one extra transistor which it uses to connect an external resistor to the DC bus. Usually much higher power output than regeneration so can do up to around 175-250% of name plate (motor stall torque limit). Can be combined with regeneration. No extra heat load on motor.

...

Mechanical brake. Typical brake motors have brakes with 25% of the torque of the motor but they can easily exceed motor torque. Very simple to use. A Stearns brake just bolts onto the back of a motor with an extended shaft (past the fan). Brake voltage can be different from motor. Common voltages are 120 V (control voltage) and.motor voltage. With voltage same as motor often brakes are wired in parallel to motor at the peckerhead so it releases at start and applies at stop with no extra contactors, wiring, etc.

The last item is the cheapest, easiest way to brake if you don’t have a VFD, etc.
The mechanical brake does seem pretty cheap and easy. Does the chopper (dynamic brake?) without a drive stress or strain or wear the motor? (Doesn't seem like it would be hard on the mechanism.) Any down side other than the big wiring for the heater?
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Old 11-28-2019, 10:07 AM   #17
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There are lots of options but the original post said bits and drilling which could mean a lot of dust or wet conditions which would make braking resistors a less attractive options against a sealed braking unit.
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Old 11-28-2019, 12:38 PM   #18
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When using a mechanical brake, the coil voltage can often be different than the motor voltage and still be connected to the motor leads.

If the motor is wound Y (the internal stator connection, not the power supply connection) then a 120 or 240 coil can be used at either motor voltage (230 or 460).

If it's wound ∆, then 240 will work but not 120.

This will work only if the motor is started across-the-lines, not with a VFD or a reduced-voltage starter.
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Old 11-28-2019, 06:59 PM   #19
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The mechanical brake does seem pretty cheap and easy. Does the chopper (dynamic brake?) without a drive stress or strain or wear the motor? (Doesn't seem like it would be hard on the mechanism.) Any down side other than the big wiring for the heater?

A brake chopper circuit is VFD specific. It modulates the output to a resistor to maintain a maximum DC bus voltage. With contactors it’s just on 100% or off.

As a 100% brake (not chopping) you effectively have a generator at least for a short period of time but since current is rapidly decreasing so is your braking torque so it fades on you before you get to zero speed. No extra stress on the motor. You can’t exceed stall torque so no extra strain. This isn’t like plugging.
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Old 11-29-2019, 12:06 AM   #20
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At another job I had I redid an impact crusher that had a large rotor with a huge amount of rotational mass. The whole shebang was ran off of a diesel genset, and I showed the operators how to use the engine as a dynamic brake by just shutting off the fuel and letting the motor run as a generator and the generator run as a synchronous motor. It cut the coast down time by 2/3 and pumped cool down air through the engine to boot.
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