Cranes and hoists

Status
Not open for further replies.
C

cs_88

Member
Location
Riverton, Ut
crane guy here hoping to get some clarification from the gurus.
In my industry we commonly see two motors powering a bridge or trolley and being controlled by one vfd.
The debate of overload protection for these motors seems to come up occasionally and seems to be based on the vagueness of article 610.43(C).
If you read this article, it seems that the code begins to address this configuration but doesn't seem to finish the thought.
They begin the article with "Where two or more motors drive a single
trolley, truck, or bridge and are controlled as a unit and
protected by a single set of overload devices with a rating
equal to the sum of their rated full-load currents,"
But after that the article simply copies the second half of 610.43(A)(3) word for word.
The article just seems incomplete especially because the first half of the article ends in a coma not a period. (copy and paste error)?
The debate is whether the vfd can be programmed to provide overload protection for both motors or if each motor has to have it's own.
Any thoughts.
 
The VFD has no idea which motor its output current is going to at any moment or in what proportion it is being split between two or more motors.

The VFD can be set to provide overcurrent protection to the multi-motor circuit to protect the wires, assuming that all the wiring is sized for the total current of all motors. But it cannot be used as overload protection for the motors. There has to be a dedicated temperature sensor for each motor (which could I imagine be set to control the Stop or Enable input of the VFD rather than controlling separate contactors for each motor.
You have to decide whether, as a design feature, you want to lock out all motors when one of them goes into overload.

PS: I agree that (A) was probably a defective cut and paste after a change was made in (C).
 
In our manufacturing process when one or more motors is being controlled by a vfd we do make sure each motor is protected individually, either through motor klyxons or by overload relays.
If either motor faults it disables the drive.
On the other hand, one of our vendors says to simply program the drive to the sum of both motors to provide proper overload protection.
I'm just curious about the intent of 610.43(C).
 
cs_88 said:
In our manufacturing process when one or more motors is being controlled by a vfd we do make sure each motor is protected individually, either through motor klyxons or by overload relays.
If either motor faults it disables the drive.
On the other hand, one of our vendors says to simply program the drive to the sum of both motors to provide proper overload protection.
I'm just curious about the intent of 610.43(C).
Click to expand...
To me the intent of (C) is very clear.
The VFD can be used to provide overcurrent protection (as in the function of a branch short circuit and ground fault breaker) to both motors as long as there is independent overload protection for each motor.

PS: I am not sure that a Klixon meets the language of section (A) that the temperature sensor shall be "thermally in contact with the motor winding(s)."
An integral temp limit switch that is part of the motor construction would be what I see the section as requiring.
The same wording does not occur in (C) but the way it that paragraph is written it appears that the additional wording in (A) is assumed to apply.
All in all rather badly constructed.
 
Last edited:
No, it's not a cut and paste error, it's not EXACTLY the same. The last sentence shows that it is specific to MULTI motor applications.
(C) Multimotor. Where two or more motors drive a single
trolley, truck, or bridge and are controlled as a unit and
protected by a single set of overload devices with a rating
equal to the sum of their rated full-load currents, a hoist or
trolley shall be considered to be protected if the sensing
device is connected in the hoist’s upper limit switch circuit
so as to prevent further hoisting during an overtemperature
condition of either motor.
Click to expand...

The issue is, both motors are connected to the SAME shaft ("... a single trolley, truck, or bridge..."). So in effect, the motors are ACTING as if you have just split the windings of a larger motor, they are not really two separate LOADS. One motor cannot really overload without the other motor overloading too, because they are on the exact SAME load. It's a unique exception allowed ONLY for cranes, trolleys and hoists that are connected like this. On all other multi-motor applications behind a VFD, you must have separate OL protection for each motor. This is the only exception I am aware of.

That last part about the upper limit switch circuit is an important part of this by the way, not a typo. It means that if you want to do this (single OL relay), not ONLY does the OL relay have to drop out the starter coil, it ALSO must disable the entire control circuit AS IF you had reached the end of travel limit. That's because the motor may have a brake coil wired to the controls (rather than powered by the motor power), and if only the starter is disabled by the OL relay, but the other controls still work, someone might be able to engage the brake coil separately and drop a load. That's because especially on hoists, you don't immediately disengage the brake when you energize the motor, you disengage it slightly AFTER ward, so that the load doesn't start dropping before the motor develops torque. That means the brake coil circuit is separate from the motor circuit, and needs to be disabled too.

None of this would apply to a VFD driven system that has OL protection built-in, because it is not a separate OL relay that can be wired into the upper travel limit switch circuit. You still need the OL relay down stream of the VFD, or at least every one of these I have seen is done that way.
 
So you think that it is not a typo that the single motor section also refers to "either motor"? Or that the multi motor section does not fully describe the thermal sensor?
 
Status
Not open for further replies.
Top