Drives, Motors for Shipyard Hoists

[rajsiyer]

Hi friends, I am participating to bid for a job at a shipyard, where a few existing EOT cranes and winches are to be retrofitted from DC to AC. AS these motors are in the range from 50 to 100HP and we have to be extra careful in offering the right things and despite my age and experience (47,24 yrs) I seek help to be able to ask the right questions... Are motors for winch use materially different from the NEMA-B design? what would be an appropriate NEMA design(B,C,D.. etc) for winch/crane use ? . What are typical overload ratings and service factors?. So also, in the matter of selecting a proper VFD for the winch motor, what are the issues which I've to be extra careful about. ? I am quite curious as to why DC motors still are widely used in winch applications. Is some kind of braking utilised ? I have seen some smaller EOT cranes with motors which are started up using rotor resistances. Anyone who has been through a retrofit from DC to AC as described above may kindly share their experience i shall be most grateful. Regards, & best wishes to all who read this Raj S. Iyer

[paulengr]

<quote>I am participating to bid for a job at a shipyard, where a few existing EOT cranes and winches are to be retrofitted from DC to AC.</quote> At least in the U.S., there's a huge amount of liability here. Cranes and hoists have all kinds of special safety codes and liabilities that you don't have to deal with on other stuff. This is from experience maintaining/running dredges and in foundries (which have dozens of cranes typically). That being said, there are some really nice prepackaged drives on the market that will do the job nicely. <quote>Are motors for winch use materially different from the NEMA-B design? what would be an appropriate NEMA design(B,C,D.. etc) for winch/crane use ? . What are typical overload ratings and service factors?. So also, in the matter of selecting a proper VFD for the winch motor, what are the issues which I've to be extra careful about. ?</quote> There are 2 issues to be concerned with here. The primary concern when it comes to cranes and winches is first variable speed control and second being able to start/stop under a load. Obviously DC motors cover this since the maximum torque is at rest. So plan on increasing horsepower by 150% to 200% to compensate. You will need to work this out in terms of torque, not horsepower, since the conditions coming in and out of dead stop are what matter. Plan on the extra expense of a constant torque drive too instead of a less expensive speed-based drive. One of my winches where I work at now has a class D motor specifically for the starting torque but an AC drive can overcome this as long as I increase the motor (double the horsepower rating) It can be done! Second when you see older AC cranes, they always have huge banks of resistors to deal with operators "plugging" the motors (reversing them while the motor is still running forwards!) along with mechanical brakes. EVERY AC crane is going to have to have a brake because you can't hold a load with an AC drive. These things are a maintenance problem but at least with an AC drive the only time you are going to use it is while holding a load (it will be relatively light duty). The third issue is heat. Watch how a crane operates for a while. Plan on maximum service factors (you can look them up in a book). I might even consider an externally cooled motor simply because I'd say 90% of the time the motors are accelerating or decelerating with very little "full speed" time. Best to take actual measurements for this (put an encoder on the crane and measure speed) if you don't have a ready-made crane design. For that reason, I'd personally call a crane service company (Kone Cranes in the U.S. for instance) and let them deal with it and the liability. <quote> I am quite curious as to why DC motors still are widely used in winch applications. Is some kind of braking utilised ? I have seen some smaller EOT cranes with motors which are started up using rotor resistances. </quote> If you don't have a DC motor, how are you going to deal with a load when the speed goes to ZERO? There is no torque in an AC motor at ZERO speed (shaded pole fractional horsepower motors excepted). The advantage of DC over AC for a crane is that the motors are smaller, the drives are less expensive, there's no resistor bank or brake to deal with, and on a horsepower basis alone, you get more torque per horsepower. The downside is that the costs (drive and motor) are much higher, and the costs to maintain them (drive and motor) are much higher. About the only way to make an AC motor more expensive is to use a multispeed motor; the contactor bank is more expensive than a drive, changing contact tips adds up very quickly, and you will spend more time replacing brake shoes since coasting to zero speed is not as predictable. The third possibility here is hydraulics over AC. This eliminates almost all the disadvantages of the AC system. I'm not sure about costs but my guess is that they will be very competitive with an AC drive system because you can get rid of all the expensive stuff and just go with a simply AC motor+contactor and you only need one drive for all axes of movement. The costs to maintain it are higher, and hydraulic systems are always messy even when you look at the marine systems which are usually "spotless". You will commonly see hydraulic systems on any crane/winch system that operates on the water. The only reason that they are going to AC drives in maritime applications is for low maintenance and to get rid of the pesky oil leaks that cause such a huge environmental problem. The winches are simple and very long lived. It is trivially easy to get a winch with a brake option built in. I've had plenty of experience maintaining all 3 types of systems. For the money, I'd really be torn between the extra cost of an AC-only system and a hydraulic system, but I'd lean towards the AC-only system because I've gotten tired of having everything around me smell of hydraulic fluid, but that's a personal issue.

[rajsiyer]

Hi Paul, Thanks a lot. I really appreciate that you took the time to reply at length. As matters stand now, I've involved the motor experts from M/s Cromton greaves Ltd., and the Drive experts from Schneider. What I feel will result is, a crane/hoist application motor from crompton, rated for S4 or S5 duty, with a Continuous duty factor of 40% and thermal rating for max 150 start/stops every hour. This will most likely be powered by an Altivar 71 torque vector controlled drive, which is for 100 HP rating but suitably de-rated for a crane/hoist motor of 75 HP, and capable of more than 250% starting torque. The technical specifications are being discussed at various fora, and it will be a while before they are finalised. As I understand, the base speed of the existing DC motor is 600 RPM would have the same torque as a AC motor of also 600 rpm, with the same power rating. However as the 600 RPM machine is disproportionately larger or expensive, similar torque could be had from an 8-pole or 750 rpm motor, at about 95 HP. The drive would be configured however to a max speed of 600 rpm. The existing brakes would be maintained. These are DC 110V and shall remain that way. Same as you, I'll share my experiences at Mrplc.com so that other engineers may benefit. Regards, & best wishes for your continued success and happiness, Raj S. Iyer