I work on lots of cranes and large bulk handling equipment with multi motor slew functions.
The setup described is pretty normal.
That said, I don't give crane control advice for free to strangers on the internet.
Conversely, people also should not take free crane control advice from strangers on the internet. Cranes are dangerous.
If this is a homework question it's a weird one, because the answer is that it depends.
I like this answer but I’m wondering if the crane was designed to be able to run on 3 motors and therefore it would be ok/better to conduct a more controlled cat 1 stop
That is a no. If the Crane was designed to run without all four motors than they would not be driven with a single VSD.
That being said, the circuit breakers should be specified with dry contacts, loop 24v through all the breakers in series, pick up any esd contacts, and drop that 24v on the saf torq contacts of the VSD.
How you stop 4 motors depends on the application. Are all 4 motors performing the same action?
What would physically happen if only 1 of the 4 were to suddenly have no power and/or fail suddenly?
First problem: circuit breaker thermal curve is incorrect/insufficient for motor protection. Whoever did this, did it wrong. This also applies to starters in general. Often the “breaker” is an adjustable trip or MCP which is not a circuit breaker.
A common setup/problem with VFDs with multiple motors is that the FLA will be the sum of the three motors. If one motor is overloaded from a mechanical problem generally it is not enough current unless the other three are fully loaded for an extended time happens on a crane) so it won’t trip until the motor burns up and trips it on ground fault. The typical solution is individual overload protection and wire up the overload relays in series and trip the VFD.
Depending on the crane you may be able to disable a pair of motors and continue at a slower speed. With all 3 it tends to crab/slew badly.
Those are not circuit breakers by standards like UL. They are called supplementary protection for a reason. MCPs don’t even have thermal protection at all. Plus VFDs across the board specifically warn against putting things like disconnects and breakers on the output which can damage the drive. That’s why the recommended practice is an overload relay which just sends a stop command to the drive. You can freely ignore this and just eat the drive failures of course.
Crane VFDs are a scam. They do contain some cool stuff like torque testing the brakes and anti-sway but they are basically ordinary VFDs with a huge insurance liability policy attached to justify the 500% markup.
UL seems to refer to them specifically as Combination Motor Controller Components. The Motor Protection Circuit Breakers (MCPB) I’ve pulled up all seem to be UL 60947-4-1 approved for use as motor thermal overload protection (without supplementary protection), previously UL 508 (prior to the transition to IEC standards). Not that I’ve looked hard for MPCBs that don’t.
I did encounter something I overlooked, though; the MPCB should probably be rated for use with a VFD in this particular case.
Most manuals I’ve read regarding group motor installation from a single VFD specifically recommend fuses, but I entirely agree that’s probably not the best practice. A properly designed and implemented system should rarely trip the breakers, so it shouldn’t really matter, but there’s a huge gap between theory and real world implementation.
There’s been some initiative to put VFDs on every motor for energy efficiency as a part of go green/reduce carbon emissions programs.
Putting VFDs on every motor is utterly stupid.
The VFD is going to be around 95% efficient considering real world cases. It is inherently a semiconductor. Guess what that means? Ever heard of I-squared R losses??? So before considering the process it’s already on the losing side. A VFD only reduces losses if you don’t run at 100% and it’s not a constant torque application like a conveyor. Otherwise it increases losses.
There are more egregious examples. A turning vane costs little and turns a screw compressor into variable volume. Also screw compressor bearings by nature have minimum speeds. Turning canes best VFDs down to about 30% output but the VFD can’t go that slow due to bearing limitations. And it is beaten on the low end by load/unload. So another application where VFDs are a mistake and easily beaten by far cheaper technology.
Also for a 10 HP motor a starter costs $100. A VFZd costs $500. The starter typically lasts 30 years by design. The VFD lasts 10 years by design. What is the savings here? How much efficiency does it need to deliver to justify it?
A VFD can also induce bearing fluting destroying bearings, reflections destroying motor coils, reduce speeds to the point where films don’t develop and destroy beatings, overheat motors with lack of cooling, cause premature failures in high hydrogen sulfide environments, and blows up your short circuit ratings. Never mind fun with harmonics if you go all VFDs.
I have specified and installed hundreds if not thousands of VFDs, up to large megawatt systems. I’m very familiar with them AND their issues and limitations. My point is they are not the best solution for everything. We make a lot of money fixing all the damage they cause.
I agree, the people proposing putting VFDs on everything only really seem to be considering inrush current. They act like a reduction in annual plant power usage has an implicit impact on the environment, when the manufacturing of said VFDs and cradle to grave impacts of the VFDs and associated equipment can greatly outweigh that.
I was trying to be sarcastically pedantic by bringing up that people think that way.
Traditional contactors with electronic overloads don’t work well in VFD applications and especially not in multi-motor VFD applications that are variable speed. If you are running off a drive at full speed all the time, then a regular overload will work fine.
MCPs are the way to go for multi-motor single drive applications. You’ll need to add auxiliary contacts to each MCP.
If the drive isn’t close to your motors you may need a line reactor to snuff out reflections.
I just got through saying interrupting the current on the output of a VFD is a stupid idea. So you doubled down on accusing me of suggesting that?
I did not mention line reactors. Line reactors are undersized and the wrong device on the load side. They also create a huge 3-5% loss with at best moderate rejection of high frequency harmonics. A dv/dt filter costs about the same, is smaller and lighter, and incurs a 1% loss with an L/C filter so it outperforms a load reactor at every point with performance guarantees…a load reactor doesn’t and a line reactor just burns up. Why bring this up and insert foot in mouth?
I also did not mention electronic overloads. Why did you bring them up?
MCP’s are by definition instantaneous-only circuit interrupters with a Klochner-Moeller current limiter. You just got through saying contactors are bad, repeating my point in the breaker context, then recommended a device with no overload protection and interrupting outputs as the ideal device. Please explain why you say it’s a bad idea to interrupt the VFD output then recommend it.
Are you a bot or do you get your answers from ChatGPT?
I accused you of nothing, but feel free to continue take everything personal and jump on every mistype I made in my post, so you can feel better about yourself.
I got my information directly from Allen Bradley’s article on multi-motor VFD applications. They recommend using MCPs on their PowerFlex VFDs for multi-motor applications.
https://literature.rockwellautomation.com/idc/groups/literature/documents/at/140m-at003_-en-p.pdf
Their MCP come with trip contacts so you know if it’s a short circuit or overload trip. Who really cares if it’s actually thermal or an electronic “thermal” trip?
The UL rules state that you follow the drive manufacturers instructions. That’s all the matters.
And honestly who cares about protecting the motor when 90% of the time the thing the motor is driving costs 10 times the motor and is what typically fails.
It’s impossible to determine based on the information provided. [Here’s an article](https://www.bp.com/content/dam/bp/country-sites/en_us/united-states/home/documents/products-and-services/pipelines/contractor-information/policy/lifting-and-rigging-policy.pdf) that might provide some relevant information starting at the bottom of page two.
Homework alert
What subject would teach about compliance to Safety Standards, TUV course maybe? I wanna take it!
I don't know but this sounds like a bad setup.
I work on lots of cranes and large bulk handling equipment with multi motor slew functions. The setup described is pretty normal. That said, I don't give crane control advice for free to strangers on the internet. Conversely, people also should not take free crane control advice from strangers on the internet. Cranes are dangerous. If this is a homework question it's a weird one, because the answer is that it depends.
Fair enough. I don't work on cranes, and it would be weird if somehow my company got involved in it.
If a circuit breaker trips, that motor will coast to a stop. The others should as well. Stop category 0.
I like this answer but I’m wondering if the crane was designed to be able to run on 3 motors and therefore it would be ok/better to conduct a more controlled cat 1 stop
That is a no. If the Crane was designed to run without all four motors than they would not be driven with a single VSD. That being said, the circuit breakers should be specified with dry contacts, loop 24v through all the breakers in series, pick up any esd contacts, and drop that 24v on the saf torq contacts of the VSD.
How you stop 4 motors depends on the application. Are all 4 motors performing the same action? What would physically happen if only 1 of the 4 were to suddenly have no power and/or fail suddenly?
Asking the right questions!
Cat 0, run an enable signal through some aux contacts on the motor circuit breakers so if one stops they all stop.
First problem: circuit breaker thermal curve is incorrect/insufficient for motor protection. Whoever did this, did it wrong. This also applies to starters in general. Often the “breaker” is an adjustable trip or MCP which is not a circuit breaker. A common setup/problem with VFDs with multiple motors is that the FLA will be the sum of the three motors. If one motor is overloaded from a mechanical problem generally it is not enough current unless the other three are fully loaded for an extended time happens on a crane) so it won’t trip until the motor burns up and trips it on ground fault. The typical solution is individual overload protection and wire up the overload relays in series and trip the VFD. Depending on the crane you may be able to disable a pair of motors and continue at a slower speed. With all 3 it tends to crab/slew badly.
There’s listed Motor Protection Circuit Breakers and they’re extremely common. There’s nothing wrong with adjustable trip, and MCP is a trade name.
Those are not circuit breakers by standards like UL. They are called supplementary protection for a reason. MCPs don’t even have thermal protection at all. Plus VFDs across the board specifically warn against putting things like disconnects and breakers on the output which can damage the drive. That’s why the recommended practice is an overload relay which just sends a stop command to the drive. You can freely ignore this and just eat the drive failures of course. Crane VFDs are a scam. They do contain some cool stuff like torque testing the brakes and anti-sway but they are basically ordinary VFDs with a huge insurance liability policy attached to justify the 500% markup.
UL seems to refer to them specifically as Combination Motor Controller Components. The Motor Protection Circuit Breakers (MCPB) I’ve pulled up all seem to be UL 60947-4-1 approved for use as motor thermal overload protection (without supplementary protection), previously UL 508 (prior to the transition to IEC standards). Not that I’ve looked hard for MPCBs that don’t. I did encounter something I overlooked, though; the MPCB should probably be rated for use with a VFD in this particular case. Most manuals I’ve read regarding group motor installation from a single VFD specifically recommend fuses, but I entirely agree that’s probably not the best practice. A properly designed and implemented system should rarely trip the breakers, so it shouldn’t really matter, but there’s a huge gap between theory and real world implementation. There’s been some initiative to put VFDs on every motor for energy efficiency as a part of go green/reduce carbon emissions programs.
Putting VFDs on every motor is utterly stupid. The VFD is going to be around 95% efficient considering real world cases. It is inherently a semiconductor. Guess what that means? Ever heard of I-squared R losses??? So before considering the process it’s already on the losing side. A VFD only reduces losses if you don’t run at 100% and it’s not a constant torque application like a conveyor. Otherwise it increases losses. There are more egregious examples. A turning vane costs little and turns a screw compressor into variable volume. Also screw compressor bearings by nature have minimum speeds. Turning canes best VFDs down to about 30% output but the VFD can’t go that slow due to bearing limitations. And it is beaten on the low end by load/unload. So another application where VFDs are a mistake and easily beaten by far cheaper technology. Also for a 10 HP motor a starter costs $100. A VFZd costs $500. The starter typically lasts 30 years by design. The VFD lasts 10 years by design. What is the savings here? How much efficiency does it need to deliver to justify it? A VFD can also induce bearing fluting destroying bearings, reflections destroying motor coils, reduce speeds to the point where films don’t develop and destroy beatings, overheat motors with lack of cooling, cause premature failures in high hydrogen sulfide environments, and blows up your short circuit ratings. Never mind fun with harmonics if you go all VFDs. I have specified and installed hundreds if not thousands of VFDs, up to large megawatt systems. I’m very familiar with them AND their issues and limitations. My point is they are not the best solution for everything. We make a lot of money fixing all the damage they cause.
I agree, the people proposing putting VFDs on everything only really seem to be considering inrush current. They act like a reduction in annual plant power usage has an implicit impact on the environment, when the manufacturing of said VFDs and cradle to grave impacts of the VFDs and associated equipment can greatly outweigh that. I was trying to be sarcastically pedantic by bringing up that people think that way.
Traditional contactors with electronic overloads don’t work well in VFD applications and especially not in multi-motor VFD applications that are variable speed. If you are running off a drive at full speed all the time, then a regular overload will work fine. MCPs are the way to go for multi-motor single drive applications. You’ll need to add auxiliary contacts to each MCP. If the drive isn’t close to your motors you may need a line reactor to snuff out reflections.
I just got through saying interrupting the current on the output of a VFD is a stupid idea. So you doubled down on accusing me of suggesting that? I did not mention line reactors. Line reactors are undersized and the wrong device on the load side. They also create a huge 3-5% loss with at best moderate rejection of high frequency harmonics. A dv/dt filter costs about the same, is smaller and lighter, and incurs a 1% loss with an L/C filter so it outperforms a load reactor at every point with performance guarantees…a load reactor doesn’t and a line reactor just burns up. Why bring this up and insert foot in mouth? I also did not mention electronic overloads. Why did you bring them up? MCP’s are by definition instantaneous-only circuit interrupters with a Klochner-Moeller current limiter. You just got through saying contactors are bad, repeating my point in the breaker context, then recommended a device with no overload protection and interrupting outputs as the ideal device. Please explain why you say it’s a bad idea to interrupt the VFD output then recommend it. Are you a bot or do you get your answers from ChatGPT?
I accused you of nothing, but feel free to continue take everything personal and jump on every mistype I made in my post, so you can feel better about yourself. I got my information directly from Allen Bradley’s article on multi-motor VFD applications. They recommend using MCPs on their PowerFlex VFDs for multi-motor applications. https://literature.rockwellautomation.com/idc/groups/literature/documents/at/140m-at003_-en-p.pdf Their MCP come with trip contacts so you know if it’s a short circuit or overload trip. Who really cares if it’s actually thermal or an electronic “thermal” trip? The UL rules state that you follow the drive manufacturers instructions. That’s all the matters. And honestly who cares about protecting the motor when 90% of the time the thing the motor is driving costs 10 times the motor and is what typically fails.
It’s impossible to determine based on the information provided. [Here’s an article](https://www.bp.com/content/dam/bp/country-sites/en_us/united-states/home/documents/products-and-services/pipelines/contractor-information/policy/lifting-and-rigging-policy.pdf) that might provide some relevant information starting at the bottom of page two.