Duplex Motor Control for Crane Bridge Drive 3

[nchrista]

I have a question concerning the motor system for a crane bridge drive. I'm writing a spec. for a 15 ton upgraded to 25 ton (by mechanical means with reconditioned motors) overhead electric crane. Presently the bridge has a duplex drive (for 2 DC mill type 800 series motors), series wound, magnetically controlled system. A spec. that I downloaded from the Army Corps of Engineers states that they must be connected in series. Does this mean that the motors must be electrically connected end to end to form one large motor, i.e...25hp +25hp = 50hp? Or does this mean something else...perhaps a misprint, where series connected was used in place of series wound?

Also, what is meant by duplex control?

Nick

 

[niallnz]

Nick

I think that in this case duplex refers to the two drive motors on the bridge, one on each side. The Corp spec requires the motors to be electrically connected in series so that each motor has the same current the there for the same speed.

15ton up to 25ton, what are you doing to the bridge structure to carry the extra load?

 

[nchrista]

There's a structural engineer taking care of that part, my job was to take care of the electrical portion. From what I read in his portion of the spec. the bridge structure is nearly adequate to handle the uprate, but will need upgraded wheels, bottom plate reinforcement, and web stiffeners. A preliminary study was done by a crane manufacturer, and they provided some idea of what might need to be done. Most of the mechanical/structural stuff was quite foreign to me and it took some time to get acquainted with. The main idea is that a new 3 sheave, 25 ton lower block will be put into place with extra ropes to reduce the crane lift. Therefore, the motors and drives will only need to be reconditioned.

So, in your opinion, the bridge motors operate at the same time? Is this to keep the bridge going straight or not developing twisting torques?

 

[niallnz]

Absolutely, if there are two bridge travel motors with no mechanical link between them, then they must operate at the same time and at the same speed to prevent the bridge skewing and jambing up on the wheel flanges or worse.

 

[nchrista]

How much time should I alot for supplier to train plant crane operators?

 

[niallnz]

The operating context of the crane shouldn't have changed much so your operators should still know how to drive it, unless the controls have been radically changed, in which case a day of familiarisation would be reasonable.

If you have upgraded the controls with a PLC and variable speed drives then your service tech's may need a couple of days to get their head around the new systems.

 

[badasselectric]

with dc motors volts=speed amps=torque. dc bridge motors are always connected in series. at least on the dc cranes I have worked on P&H, and Alliance. as one motor see's more load vvoltage across that motor drops, the balance of the voltage shifts to the other motor increasing its speed,and balancing the load on the bridge again.

 

[mc5w]

What you would need is to have both series fields connected is series with each other. You would then need to have tyhe rotors connected in parallel. This will assure that both motors operate at the same speed and have nearly equal torque.

Connecting both motors just in series will produce equal torque in each motor but would still allow 1 motor to turn faster than the other motor. If 1 motor turns a little faster the bridge could derail. The pieces of steel plate that you see keep the bridge from falling off of the runway rails if there is a derailment.

Streetcars that ran off of a 1200 volt catenary usually had the traction motors connected in series pairs with the fields connected in series and the rotors connected in series. There was no serious consequence if one traction motor tried to run a little faster especially if both motors were on the same truck.

 

[DickDV]

mc5w, that doesn't sound right. Don't you want the shunt fields to be wired in parallel and both armatures to be in series?

That would provide the load sharing discussed above, it seems to me.

 

[mc5w]

The application is for series motors, not for shunt motors or compound wound motors.

For compound wound generators that have independent prime movers the wiring diagram is quite different because you want equal load division in spite of slightly different speeds.

In this application, you want torque sharing but you also need equal speed so that the bridge does not derail. Best way to do this is to have equal voltage on both rotors and equal current in bothe field coils. This means that the series field coils need to be in series with each other. The rotors need to be in parallel with each other and then in series with the series field coils.

If the motors were compound wound, the shunt fields would need to be in series with each other and then operated from the same field resistor and power supply so that each shunt field has theoretically equal current.

In this application you might need a shunt resistor around one of the series field to tweak equal torque output. This is because each motor will have slightly different air gaps among other things that make the motors slightly nonidentical.