Protection of a motor

Hi everyone,,,

I was wondering why the order of the elements to control/protect a motor are the following, suposing I have only a contactor and a thermomagnetic motor breaker:
Example1:
PowerSource----->Breaker---->contactor----->motor

why not : Powersource---->contactor---->breaker----->motor ???

Example2:
Now supose i have a VSD : the diagram should be as follow:

PowerSource---->Breaker---->VSD---->motor

why not: PowerSource----->VSD---->Breaker----->Motor ??

Supose in the second example with the VSD, i have a short between the VSD and motor, if I choose option 1 the VSD will be the first device to see the short, however they are not intended to protect against shorts.. and they might get damaged, so there is my question, shouldnt the breaker be down the VSD ?? and in this case the first device to see the short will be the breaker ??

I know it doesnt , but cant understand why..

Thanks in advance!

If it’s going to fail there is nothing you can do about it. The short circuit protection (weather it’s a breaker or fuses ) are both there to isolate the faulted elements from the rest of the system and prevent the failed element from damaging other items in the system.
To that end hey must be between the power source and the device that you want to protect. Otherwise a shorted transistor in a VFD could trip the mains to the system or with the available current it could actually catch fire and burn up your panel.
While there in no rule against placing a breaker or fuses between the VFD and the motor it not advised.
Some VFD’s don’t react well to the output opening while it is running and I don’t know of any that will allow you to switch in a load after it is running. That’s the quickest way I know of to blow up a VFD.

Placing the motor contactor before the breaker would protect the motor and the field wires in the event of a short or ground fault. The contactor itself would not be protected and there would be no way to isolate it if you have to work on it later if necessary. And here again if you should have a failure in the contactor it could cause the mains to open up or cause a fire in the panel. Not something you want to do.
Not to mention that the electrical code states that the first thing from the source must be a disconnect device and then short circuit protection then a control device.

Just a side note circuit breakers can and do fail closed meaning they are not disconnecting the power source. When I first got into the trade I was told if you want to be absolutely sure the circuit will open then always use fuses. Even if there is a bad one it will always open at some point they never fail closed.

PowerSource>-----Disconnect>---ShortCircuitPrtection>------ControlDevice>-------OverloadRelay> ----Load

Any other way is inviting trouble
The disconnect and short circuit protection can be one device
The control device and overload (if used) can be the same device. A VFD combines both.

What do you mean by breaker, actual circuit breaker, or an overload?

Guess it doesn't matter.

For example 1, I've seen configurations with the O/L before or after the contactor. Most starters that I've encountered the L side is the contactor and T side is at the bottom of the O/L.

Just looked at one of our new panels for reference, and the leads go directly through an O/L and into the bottom side of a drive, which I would guess has a contactor inside that pulls in when the safety circuit is complete.

For example 2, I've never seen a disconnect between the motor and the drive terminals, though I'm sure there are situations where it might be deemed necessary. In school I remember being told that if you suddenly dropped the load off a VFD while it was running that it could cause component failures. Hopefully we can get a drive expert to chime in.

This doubt came because I was told that a residual breaker to protect persons, should be before the circuit breaker, because if there was a short downside, this one wouldnt damage the residual breaker, as the circuit breaker would open un first,

I mean if I have this situation:


PowerSource---->Residualbraker---->CircuitBreaker----->Load

And i have a short between the circuit breaker and the load... Which device will see the high current first ??

I supose the residual breaker right ?? As soon as the short happens, the high current coming from the powersource through the phase, will see first the residual braeaker, then circuit breaker, then the short,come back with the neutral through circuit breaker then residual breaker and to power source

Asuming this is a phase-neutral instalation

All breakers are designed with the capability of breaking high current flow
from the power source and would normally be the point where the
distribution is separated from the load so that means breaker first.

Read the label on your breaker.

The prospective fault current rating of breakers are many times higher
than that of other equipment in the panel, and needs to be, as a panel
with a 500 kilowatt transformer right next to it needs to be able to deal
with a prospective fault current of 25,000 amps.

( think of the damage of a lightning strike in your panel )

A residual breaker also has a high Fault current rating, and
in our area protects the load on a max of 3 other breaker
it needs to be first for that reason

The current-limiting device should come first in a branch circuit so it protects the whole circuit. It's just that simple.

The statement "VFD's are not designed to protect against short circuits" is false. VFD's do a superior job of protecting against shorts because of their clearing time being far shorter than fuses or breakers.

One common reason for putting a disconnect in the drive-motor leads is when the motor is a long way and out of sight of the drive as in an air handler on the roof with the VFD inside the building. It is true that opening this switch under load will result in a nasty flash and possible damage to the VFD altho some manufacturers have recently claimed that they now have protection built into their IGBT networks to protect the IGBT's. In my view, it's still not good practice. Such switches should have pre-action switches which are wired into a Run Enable circuit in the drive so the VFD shuts off its output before the contacts open. The drive should always be stopped before the switch is reclosed or the drive will fault on overcurrent due to the starting inrush current of the motor.

I hate to disagree but VFD’s are not designed to handle short circuits. The better ones will be a little more tolerant of a short on start up. But that’s because at start up the output voltage and current are very low (less then a volt and less than 1 amp) and sense circuit is quick enough to turn off the output transistors before there is enough current to cause damage. But if the VFD is running at full speed 60Hz 480VAC, then a sort on the outputs will destroy the output transistors. There is just no way to sense the high inrush current and shutdown the transistors in time to prevent damage. Some systems have inductors connected in the output circuit to filter the current inrush. But as we know once a fault starts the damage is cascading the longer It remains connected to the source. With a vfd when it faults you have all the energy stored in the caps as well as the line source that is going someplace, out through the faulted transistors.
As I stated before the job of the fuses or breaker it isolate the faulted circuit from the source as fast as possible but the damage is done. You can only limit the amount of damage. A lot of VFD’s fault on power up when the power first hits the caps if one of them is bad the inrush current is only limited by the precharge resister and then the short circuit protection on the line. I have seen the precharge fail and the caps explode on power up you don’t want to be in front of it when that happens.
That’s why when I power up a new vfd the first time the panel door is always closed. I have seen more than my share blow up on power up.
The new IGBT vfds will tolerate an open circuit while running fairly well in most cases they just trip an alarm and shut down. But on the larger or older vfd’s with SCR outputs they don’t do well on an open circuit after they are running. Without the load connected there is no place for the current to go that means no commutation to turn off the scr’s so when the phase rotates to the next it is a phase to phase short on the output and things go bang.

One thing to remember with the new vfd;s that have the safe torque function. Do not confuse that with the vfd off and safe to touch, the safe torque only turns off the output transistor drive, transistors leak and there is still lethal voltage on the output leads with reference to ground. Isolate and ground to ne safe. I have seen many people get socked on the outputs of a vfd that was turned off.

You need to apply vfd’s with care and select the best one for the application. If you have questions take the time to call the manufacture application engineers. Tats what they pay them for.

As for a contactor or disconnect between the vfd and the motor that’s common in my world. I have system running for over 30 years with the original vfd and contactors switching motors to output many times a day no problems at all. You just need to be careful with the programming is all.

What about this?


Main breaker: 63A

wire: 10 mm²

breaker: 20 A

wire: 2,5 mm²

contactor

wire 2,5 mm²

If you reverse this sequence you need to use more thicker wire untill after the breaker.

hmm another question ! supose i have this situation, its unreal but supose:

PowerSource---Breaker1-----Breaker2----Load

Supose breaker 1 and breaker 2 are exactly the same, and there is a short between breaker 2 and the load,

Theoretically Which breaker will open first ?? breaker1 or breaker2??

Supose they are exactly the same model,

Flip a coin no one can tell and them try it a again and the other one will trip
But why the 2 breakers in series in the same circuit they would not give you any better protection than the single breaker

Gary I know that in real world what you said would happen, but thats why I said, what Theroetically what would happen, and of course is an unreal situation, there is no point of putting two breakers in serie

In the real world it could be no 1 or no 2 or most likely both

But in theory the one in the top of the panel because heat rises
and it will start off being the hottest so that means the grease
in its linkages will be the lowest viscosity so it will trip first.

The one closest the source in this case 1
Not because of the heat but it will see the inrush current first. Heat takes time to build up

Breakers are made to meet the specified characteristics, but due to production tolerances and mounting dependent variables like environmental heat you can't tell for sure which one will open first even if both are from the same batch.

Maybe you gain some insight reading this, kept me interested for a while:
http://testguy.net/content/197-Characteristics-of-Circuit-Breaker-Trip-Curves-and-Coordination

The breaker should be first - I do not use circuit breakers at all but us motor circuit breakers on all motor circuits - higher kA rating and higher magnetic trip point for motor starting.
The breaker must protect all the wiring and therefore the wiring to the contactor, VSD and the like.
Additionally when calculating fault levels if the contactor was first the fault level would be terribly low.
You have to have a look at fault level calculations for a complete system when doing the design.
You also have to look at cascading from one switchboard to another - the upstream circuit breaker should never trip unless there is a fault in the connecting wiring - the downstream circuit breaker should always be the one to trip.
Yes, I am a switch board manufacturer.