Emergency stop for runaway large DC motor

[geshka]

Hello folks.
Looking for some bright ideas for the following problem. 

Large DC motor rated up to 20 Amp 100V can have two potential dangerous situations:
1. Motor can be suddenly detached from the load (torn belt)
2. Driving board - it's FET transistor with PWM control may have potential failure.  FET has become shorted and all the power can go to FET.     

Looking at some solution  - fast one, that will shut the motor down pretty much instant.  Current control is not really works as current is dropping if load is gone. Speed control with microcontroller comes to mind first , but it may take computational time and so on...

Any suggesitons?  Thanks. 

[themadhippy]

shorting the motor out will stop it pretty rapid

[mag_therm]

A common method would be with a change-over contactor to switch the armature to a dynamic braking resistor (DBR).
If separate field, ramp it up to field forcing level during braking.
The DBR value is chosen for controlled deceleration to minimize mechanical damage, and its power rating is based on the mechanical inertia.

You can search "DC motor dynamic braking"

[pcprogrammer]

shorting the motor out will stop it pretty rapid

Yep, with a big relay driven by a speed measuring micro. The micro should be able to detect a large raise in speed quick enough and put on the break to avoid any harm. The current might weld the relay shut, but since it is an emergency system it should be fine to have to replace it after resolving the original cause of the break.

[Gyro]

Yes, rotational system ultimate fail-safes are often sacrificial, eg SawStop... https://www.sawstop.com/why-sawstop/the-technology/

[geshka]

Thanks , folks , for the valuable input, so detection system is the microprocessor measuring rotor speed and then breaking relay/resistor. 
Is there any solution to detect a MOSFET failure ? When it is getting shortened and full power applied to the motor?  I am thinking about PWN "watchdog" that is reseting timer inside microprocessor,   when  it is continuous DC applied to the motor instead of, say, 10 PWM cycles - microprocessor will shut down main power.  is that viable solution ? 

[ifonlyeverything]

Do you have (or could you have) access to hall sensors or something to measure RPM?

[ajb]

How "dangerous" are we talking about here?  Is the danger to the equipment or to people around it?  Keeping people safe requires warrants much more care than keeping the equipment safe. 

Is there any solution to detect a MOSFET failure ? When it is getting shortened and full power applied to the motor?  I am thinking about PWN "watchdog" that is reseting timer inside microprocessor,   when  it is continuous DC applied to the motor instead of, say, 10 PWM cycles - microprocessor will shut down main power.  is that viable solution ? 

It's fairly common to build this sort of protection into the power stage, especially for half/full bridge topologies where you have high- and low-side transistors, where if both are turned on at the same time you short out the power supply.  'Shoot through' or 'desaturation' protection would be terms to look for.  This can be accomplished during the transition phase of a single PWM cycle, which is important because a dead short across a 2kW power supply can do a lot of damage very quickly. 

If your power stage is transistor + freewheeling diode, then monitoring the output output waveform would be the only way to detect a short in the transistor.  To detect a short in the diode side of the bridge you would need to monitor current or voltage at the output node, but be able to shut things down very quickly as above.

[geshka]

I have optical encoder that is measuring RPM.  it gives 200 PPR, if it matters

[geshka]

How "dangerous" are we talking about here?  Is the danger to the equipment or to people around it?  Keeping people safe requires warrants much more care than keeping the equipment safe. 


If your power stage is transistor + freewheeling diode, then monitoring the output output waveform would be the only way to detect a short in the transistor.  To detect a short in the diode side of the bridge you would need to monitor current or voltage at the output node, but be able to shut things down very quickly as above.

From the people it is only me   It is an equipment (lathe) with a lot of rotating masses that , when runaway can destruct itself 

I have power stage which is  transistor + freewheeling diode.  Is there any schematics examples to monitor waveform ? 

[mag_therm]

With dynamic braking on a lathe, take care not to spin the chuck off !

[Benta]

With dynamic braking on a lathe, take care not to spin the chuck off !

I think threaded spindle noses died out in the 1950s. Except for hobbyist machines, perhaps.

[geshka]

I have Hendey lathe from 1918 

[Benta]

I have Hendey lathe from 1918 

Wow! The top brand in lathes (Except for Monarch, but that's later).
Congrats.

[NiHaoMike]

Assuming it's powered from an AC power supply, have a contactor disconnect the AC power. Switching DC is far more difficult and shouldn't be necessary.

[pcprogrammer]

Thanks , folks , for the valuable input, so detection system is the microprocessor measuring rotor speed and then breaking relay/resistor. 
Is there any solution to detect a MOSFET failure ? When it is getting shortened and full power applied to the motor?  I am thinking about PWN "watchdog" that is reseting timer inside microprocessor,   when  it is continuous DC applied to the motor instead of, say, 10 PWM cycles - microprocessor will shut down main power.  is that viable solution ?

Depends on the PWM frequency. But sure there are ways to detect if it is continuously on, only it might be that at times you have 100% power during normal functionality. So your micro needs to take action based on both speed and 100% power and decide what is needed to do. When speed is to high cut both power and short the motor. When speed is still ok, but power is 100% while your setting is below, just cut the power. This can be done on the mains side (AC) with another relay or a solid state relay.

[MarkT]

shorting the motor out will stop it pretty rapid

Very large DC motors will break from their mountings if you do this and destroy stuff, probably worse than the original problem!!

If the original question is about a safety circuit in an industrial setting, then there are laws and standards about hazard reduction/elimination.  Bascially design the issue out if you can, alternatively fit guards so no-one can be injured, and if that's not good enough use a safety circuit designed according to relevant standards/legislation.

I assume this is a series-wound DC motor?  The design-out philosophy would say "never use series-wound DC motors"....

[max_torque]

Just fit a normal manual E-Stop in reach of the operator, which cuts supply power seperately to the motor driver

You REALLY should have one of these anyway, because getting your head dragged into a lathe and being unable to stop it, really isn't very nice (don't, unless you really are not squeemish, google this, because the pictures are pretty horrible.........)

I'm going to assusme you are not silly enough to leave the lathe spinning un-attended btw!

[Muttley Snickers]

Just fit a normal manual E-Stop in reach of the operator, which cuts supply power seperately to the motor driver

You REALLY should have one of these anyway, because getting your head dragged into a lathe and being unable to stop it, really isn't very nice (don't, unless you really are not squeemish, google this, because the pictures are pretty horrible.........)

I'm going to assusme you are not silly enough to leave the lathe spinning un-attended btw!

That's exactly what I did by installing an additional E-Stop switch at knee height as I wasn't happy with the location of the factory fitted combination Start/ Direction/ E-Stop panel which looked like a tacked on afterthought. I recognised immediately that if I got caught up there was no way to stop the machine, and as a machining nut I have also seen those disturbing images.

Fortunately there was a cavity cast in the machine just below the direction lever which appeared to be where they originally intended to put the control panel, so it was an easy install with the motor and wiring directly behind the cavity and I already had the kill switch in stock.

The lathe is a Hafco AL320G, not a huge machine but heavy and plenty of grunt with both 160mm 3 jaw and 200mm 4 jaw chucks, and every edge on the machine and its accessories are like razor blades, I can post a picture tomorrow as a reference for others who need to do the same.

[Ground_Loop]

For both cases you mentioned, do you really need to stop the motor?  It seems that cutting power and letting it coast down would be sufficient.  As mentioned above, hard stops can result in problems of their own.

[ifonlyeverything]

I'm not really understanding how or why it's dangerous for the motor to disconnect from the load. Motor will run fine with no load. Operator won't be harmed unless it's an external belt drive with no guards?

[CaptDon]

Remember this very important safety item. If you sense overspeed the fault circuit must latch itself to prevent re-applying power when the RPM slows below the trip point during braking!! The circuit should be wired to self latch until manually reset!!

[moffy]

Remember this very important safety item. If you sense overspeed the fault circuit must latch itself to prevent re-applying power when the RPM slows below the trip point during braking!! The circuit should be wired to self latch until manually reset!!

An SCR into a brake resistor would do the trick.

[max_torque]

The most likely failure is going to be the drive belt braking or gross slipping.  Either of those, as long as someone is watching, is not really a problem in terms of safety.

You can use a sprung leaded mechanical belt tensioner "arm" that has a micro switch on it, that should the belt break or fly offf, the arm then springs down, opening the microswitch and estopping the machine.  For something being used by a person and hence monitored, i don't think i'd bother tbh, as long as that person can, as mentioned, easily stop the machine without risk to themselves etc

[geshka]

For both cases you mentioned, do you really need to stop the motor?

I can leave it coast free, sure.  No issues.  Just wanted to control power coming to the motor and react as quickly as possible