Standby Generator powering a Bridge Crane 1

[CuriousElectron]

Hello All,
I've got a scenario where a standby diesel generator would be powering an overhead crane. My concern is with reactive power flow between the genset and the crane motor. In scenario where the motor is plugged(breaking) would the power be flowing back into the generator and can be this powered be absorbed by the unit? There will be another loads connected to the generator, can some of this energy be absorbed by these loads? Should I expect the generator AVR system to be able to handle this energy transfer when the motor is in the three operating scenarios: forward, reverse and braking?

Thanks for your thoughts. The generator has a Y-connected winding, if that helps.

Thanks,
EE

 

[waross]

Don't worry about the reactive power, worry about the re-generated power.
This is a real concern when overhauling loads are fed from a small generator.
When a load is overhauling, the re-generated power is supplying the braking.
If the supply circuit goes open, you will lose your re-generative breaking.
The breaking system may or may not activate.
The brakes may be held off by the re-generated power.
If the brakes do apply they may or may not be able to hold the load.
With an overhauling load the motor becomes an induction generator.
This may easily drive the diesel generator over-speed and damage to the diesel engine may result.
One solution is a load bank that is capable of absorbing the maximum regenerated power, which is automatically connected when the crane is lowering a load.
You may consider delaying the overload trips while a load is lowering.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[CuriousElectron]

Waross,
Appreciate the reply. I understand when lowering the load, the crane motor turns into an asynchronous generator and returns active power(P) back to the source. If there are additional loads connected to the standby generator, would these loads be able to absorb this power in lieu of applying the load banks in the scheme?
With the solution utilizing the load banks, would they be connected in parallel to the crane motor, or would they be connected directly across the motor terminals and switched on during the lowering operation?
Thanks,
EE

 

[itsmoked]

The problem with native loads is.. can you guarantee their presence ALWAYS?

The load out of the crane appears at the motor's drive input terminals. Anywhere before that is where the load bank would need to be. I would imagine the crane maker to tell you exactly how to handle this for their crane.

Typically a drive will have a built in system for this eventuality and the load bank would become a specific resistor value hooked to specific terminals on the drive. This changes the drive system from being regenerative to non-regenerative. This would tend to cost more to operate the crane but would be the correct route for a generator fed case. They should have a solid plan for this situation. As they say, "Don't try this at home."

Keith Cress
kcress -

http://www.flaminsystems.com

 

[CuriousElectron]

I understand this is only a concern if there is failure with the re-generative breaking scheme? If the breaking circuit has resistors built-in would the resistors absorb this re-generative power regardless of the source of supply to the crane motor?

 

[waross]

By the way, plugging refers to abruptly reversing a forward running motor.
This was common with the old "U" frame motors used prior to the 60s.
Plugging a "T" frame motor is not recommended.
The point of connection of the load bank depends on local circumstances.
Other loads will absorb the power, but issues may arise if the other loads are not connected when you need them.
Considering that a runaway may load damage the generator engine on the one hand and cause property damage and possible injury on the other hand, it may be well to avoid assumptions concerning "other loads".

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

I understand this is only a concern if there is failure with the re-generative breaking scheme? If the breaking circuit has resistors built-in would the resistors absorb this re-generative power regardless of the source of supply to the crane motor?

Well no it's a failure of the 'energy dump' of any kind.

A modern crane is essentially a VFD running the lift motor. All VFDs will see deceleration regeneration occurring in them whenever they slow the driven motor. In that regard ALL VFDs are "regenerative" which goes without saying and often without recognition. But, when it becomes an overtly described VFD/system called specifically a "regenerative VFD" or "regenerative drive" then that actually means an entirely separate grid-tie inverter has been added to the system. This tends to double the price of the VFD or drive because it's a rarer bunch more of line voltage power hardware. All VFDs have to have the ability to absorb the inherent regenerated power. But, heavier regenerative loads like cranes, vehicles, and conveyors, require an external place to dump the energy. Large drives have provision for dumping large amounts of DC high voltage power into type-specific resistors. The 'resistor(s)' may be the size of a rolled-up newspaper or a washing machine or even the front top half of an entire locomotive. It depends. It's nothing you'd want to provide on your own. You'd want a certified solution provided by the crane company.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I should have asked;
DOL induction motors?
Wound rotor motors?
Multi speed DOL induction motors?
VFD driven motors?
Whatever, you don't want surplus energy making its way back to the generator and driving the generator as a motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

All good points Bill. I'm guessing VFD driven induction. Hard to see any technical advantage to the rest these days. But what do I know. LOL

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FacEngrPE]

Plugging of travel motors is usually possible, unless they are driven by VFD's breaking resistors are not usually provided. The breaks will always engage when the control is in the stop position. Plugging of modern motors, leads to short lives, so crane controls usually have time delay on reverse. Setting the time delay up correctly requires a good crane electrician.

Hoists are a different matter.
Not all VFD;s are hoist duty rated. VFD hoists must have ample breaking resistors even if line regeneration is available, to avoid possible loss of load control from a supply interruption.

All of the hoist drives have some sort of energy disapator.
[ul]
[li]Mechanical load breaks - common on small hoists[/li]
[li]Eddy current breaks = Common with wound rotor and older non VFD induction motor hoists[/li]
[li]Breaking resistors - provided with DC resistor control hoists, and AC hoists with VFD / Inverter drive control[/li]
[/ul]

ECM (Hubble) has a crane control catalog with useful wiring diagrams

https://www.hubbell.com/ecm/en

Dont scrimp on hoist controls, you can get in trouble fast.
Using inappropriate drives can result in loss of load control.

 

[itsmoked]

Thanks Fac.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FacEngrPE]

Your welcome Keith. - note the minor edit. I could say more but I get wordy and start repeating myself.

 

[CuriousElectron]

Thanks all for the replies. It's a wound-rotor motor.
It seems like these types of motors were common for the hoist applications in the past.
Regards,
EE

 

[waross]

For a wound rotor, definitely a load bank.
The load bank should be capable of absorbing all of the energy that the crane is capable of regenerating.
If the crane is in constant use, and the fuel to power a full capacity load bank may become an issue, you may consider a control system that monitors the real power out of the gen-set and ramps up the load bank in steps to match the regenerated power.
An alternative may be to initially connect the full load bank whenever the crane lowers the hook, and then use a control scheme to step down the load to match the regenerated power.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Gads what a PITA!! Depending on the size you might want to upgrade away from the wound rotor.
How big is it?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FacEngrPE]

Most of the overhauling load on wound rotor hoists will wind up in the eddy current break if the crane is set up correctly. The eddy current break will not eliminate regeneration.

Keith has a good idea, changing the hoist drive to inverter style might be simpler than dealing with line regeneration to a generator or load bank. You do not "have to scrap" the wound rotor motors, to run them with a VFD. The rotor circuit just needs to be set to a constant value. Consult with the drive manufacturer to determine the fixed resistance needed.

Wound rotor / eddy current hoists are mostly obsolete for new cranes, but are common for older cranes. Here is the patent from 1958.