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Motor options for hoisting application

Motor options for hoisting application

Motor options for hoisting application

(OP)
Greetings!

I am an end-user writing a Request for Proposal for the design and fabrication of a lifting device for my employer.  I'm not designing it, just trying to describe what I want the hoisting machine to do so that a fabricator can design and build what we need.  

I require some assistance in describing narrowing down the almost infinite options for the electric motor and drive in the RFP so that we can compare apples with apples when looking at the Proposals.

The load is to be lifted on two stiff arms fitted with gear racks that are driven by pinion gears.  The pinion gears's shaft will be driven by the motor through a gearbox. The items to be lifted weigh 21 kN (4781 lbf). There is a fair bit of starting and running friction.  So it will take a force of 325 kN (73 kips) to start and once it breaks free the running load is about 100 kN (22.5 kips).  We want it to start lifting fairly slowly at about 0.75 m/min (0.5 inch/sec), accelerate to a running speed of 2.5 m/min (1.6 inch/sec), then slow down again before being stopped by a positive mechanical brake.  

The machine is outside and subject to freezing winters and high-humidity summers.  The motor and drive will be somewhat sheltered to keep it out of the rain and snow but subject to wide temperature fluctuations. The machine will be connected to 600 volts, 100 Amps, 3-phase power.  

For what it's worth, the existing (old) machine's motor nameplate says:
General Electric (GE) Motor
20 hp, 3 phase
MR 404
55581-20-840-64-220-100
½ HR 55-93.5-442359

The new machine will probably be a bit bigger than the old one, but arranged pretty much like the old one.  So most likely we'd be looking at another motor in the 20 to 25 hp range again.  

Questions:

1.  Shall we carry on with another 3-phase AC motor, or at this point should we switch to a DC motor (which are supposed to be better for hoisting applications, or so they say).  If so, is the inverter and associated do-dads involved with converting 3-phase AC to DC particularly awkward, complicated or fussy?  Would they mind terribly being outdoors in freezing winter or in high humidity hot summers?

2.  Someone with a similar machine had an AC motor with a VFD and told me to avoid VFDs because they are complicatd, awkward, fussy, and don't like being frozen, hot, or at high humidity.  Is this a general consensus here, or is my contact prejudiced against VFDs for no good reason?  

3. Either way I am going to need a gearbox to lower the motor RPMs to achieve the required movement speed.  If I ask for a 2-speed AC motor, do I get to specify what those two speeds are? Ideally they would be in the same ratio as my two load speeds (i.e. 1 : 3.333), with the gearbox converting between them.  I'm afraid of being stuck with the two speeds on a 2-speed motor being in some other ratio, like 1 : 2 or 1 : 4 or something, and thus requiring something expensive and complicated for gearing.

I don't want to design the thing---the fabricator we hire will do that.  But in the RFP I want to at least direct everyone towards the same family of solutions.

Any and all advice appreciated.

We would all be better off if we tried to be better, instead of trying to be better-off.  --Aristide Pierre (Peter) Maurin   

RE: Motor options for hoisting application

You would want to keep with and AC motor. They are more reliable and are used in hoisting applications all the time.  You would want a vector drive type VFD.  They can hold a load in one place, start and stop it, and accelerate it under close control.

You would need an additional mechanical brake too, of course.

If this unit isn't used daily you probably don't need a regenerative drive - which puts returned energy (load lowering), back to the power supply - but instead would need a large power resistor for dumping the energy of a lowered load.

You'll likely need an auxiliary blower for the motor running at low speeds.  And perhaps an external auxiliary fan for the VFD/enclosure.

Others with more experience will no doubt chime in momentarily.

I might suggest you hire someone, who's done this numerous times, to write your specification. IT WOULD BE MONEY WELL SPENT.

Keith Cress
kcress - http://www.flaminsystems.com

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