Advice on crane, 2 speed motor running with contactor, want to change to VFD

[alibabaaaa]

Hello

I have a requirement from plant, we have a crane working with a two speed motor (nameplate attached to the post) right now we control the motor with two AC contactors (low and high speed) and brake. I do not have the electrical drawings

We want to remove the AC contactor control adding a VFD to control the high speed winding for the motor. Is this possible?

Im thinking on using a AB drive and wire the control buttons to trigger fixed speeds on the drive, high speed at 60 Hz (to get 3450 RPM) and low speed at 14.7 Hz (to get 850 RPM) I will wire the fast speed winding to the drive output and keep the low speed winding open.

I know how to do the drive part, that's not a problem. I want to know if the torque of the high speed winding will be the same at low speed (14.7 Hz) or will this be a problem

Thanks in advance!

Javier

 

[LionelHutz]

The HP/RPM ratio is constant so this means the HP increases linearly with speed and also means that each speed produces the same torque. I wouldn't expect you to have a problem with torque. If you use a decent VFD then the motor will be able to produce a constant torque throughout it's full speed range. The only issue you should have is motor cooling when running the high speed winding at a reduced speed.

 

[alibabaaaa]

Sorry for the dumb question... how do you know its HP/RPM ratio constant? Im not an expert for motors... I've heard there are motors which the HP/RPM ratio isn't constant

Thanks!

 

[TugboatEng]

VFD'S running on hoists and winches need to run vector control to get maximum torque at low speeds. You're going to need a motor with a shaft encoder.

 

[alibabaaaa]

Would it work if I run the VFD in sensorless vector mode? I guess this is the same than vector control with encoder, having less accuracy for the speed, am I right?

 

[jraef]

Yes, you can. You have a 2 speed 2 winding motor, notice that the HP rating varies at (roughly) the same ratio as the speed difference. That means the design is expecting the same torque at the lower speed. That's exactly what the VFD will do for you.


" We are all here on earth to help others; what on earth the others are here for I don't know.
" -- W. H. Auden

 

[waross]

The torque is determined by the slip frequency. The slip frequency is the frequency associated nominal speed minus actual or rated speed.
Example: A 1760 RPM motor has a slip of 3600-3450=150 RPM.
With the VFD set to 1000 RPM the motor will produce rated torque at 850 RPM. (16.7 Hz.)
If you leave the mechanical brake in place you may be able to run two speeds in sensorless vector mode.
Interestingly the 3450 winding has 150 RPM slip. The 850 RPM winding has %0 RPM slip.


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

 

[alibabaaaa]

waross: thanks for the answer, my plan is engage the mechanical brake when the drive is not in running (So when the motor speed is 0 RPM), to be sure the motor does not move with the load.

So all the AC motors can develop full torque from 0 RPM using a VFD in vector mode?

 

[ScottyUK]

Induction motors paired with a decent VSD can hold zero RPM under load for a short period of time, but most motors won't be able to sustain this mode of operation for long without forced cooling.

 

[edison123]

Are the original two windings rated for VFD duty? If not, you might have to rewind the motor. If you rewind, you can rewind for only for one speed with more copper area in it since the other winding is eliminated. This would help in cooler running of the motor.

Muthu

http://www.edison.co.in

 

[Skogsgurra]

A few observations:

1. A 4Q drive is absolutely necessery if you are running a crane. You need controlled speed both hoisting and lowering. You have not mentioned that you are going for a four-quadrant drive.

2. As mentioned by others, you need a vector drive with an encoder. I would NOT try a sensorless drive. There are a few sensorless drives capable of handling hoist applications. But they are few and far between and you really need to know what to look for.

3. The torque produced by the low speed winding is around four times the torque that the high speed winding can produce. So you cannot use a VFD on the high speed winding. At least if you want to use the crane in the same application with the same load as you have today. That answers your last question: Yes you will have a torque problem.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

HP/RPM is a ratio, so calculate it by dividing HP by RPM for each speed. You could also do RPM divided by HP, same difference just a larger answer.

The point is that the HP varies linearly with rpm. If you look at the HP formula, it's torque x rpm divided by 5250. Since HP is varying directly proportional to rpm, the torque must be constant. You could calculate the torque at both speeds to confirm this.

Does this motor actually run in reverse as well? You posted there are 2 contactors to control it which would indicate the motor only runs in 1 direction. Given that information, I would assume the motor is only driven to lift the load, so a standard VFD would work. However, a standard VFD can't handle regenerating energy (basically lowering where you're trying to keep the motor from running away to a higher speed). So, if you run the motor in reverse and/or the load tries to over-speed the motor then you need a regenerative VFD or a VFD with a fully rated braking resistor.

So, how does this hoist work? Would the hoist drop when the brake is removed or is it geared so the motor can't be back driven by the load?

 

[waross]

Speed ratio 850:3450 ~ 1:4
HP ratio 8:32.5 ~ 1:4
Torque ratio 1:1?
Low speed 20/8 = 2.5 Amps per HP
High speed 42/32.5 = 1.29 Amps per HP
Given the high current per HP of the low speed winding I would tend to use the high speed winding.
However I would try to compare the breakaway torque of each winding before making a final decision.
I misspoke in a previous post in that I did not consider regeneration. I apologize.

A question for the gurus; I understand the need for an encoder in typical hoisting operation. However in this instance the drive is not asked to stop and hold the load. The motor will be run at one of two fixed speeds and stopping and braking will be done by a mechanical brake. Is this not more like a downhill conveyor application? Regeneration is an issue but does an overhauling conveyor need a shaft encoder to run at a fixed speed?


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

 

[Skogsgurra]

My take is that the hoist could work very well with the existing contactor control. Not worth changing it if it weren't for a need to get a better control. And then, it becomes critical near zero and most drives cant handle that without an encoder. There is one that I have used with great success in such application. But I hesitate to mention the brand, mainly because I'm not sure if it is available overseas.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

Gunnar - Invertek, right? I haven't personally seen one work on a hoist, but from the success stories I've read it's the manufacturer I'd look at first. It can do the job without an encoder which just makes it simpler.

 

[Skogsgurra]

Yea, Invertek.

I worked with the now deceased Ragnar Jonsson, who developed the algorithm. He was active in his lab until pancreas cancer got him.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Thank you for the information Gunnar.
I a guessing that it may be time for replacement of the old contactors and an upgrade to a VFD is being considered.
Another question;
With DOL starting there is a torque surge as the motor accelerates past the breakdown torque speed at about 20% to 25% slip.
Will ramping up the speed with a VFD possibly avoid subjecting the machinery to breakdown torque? Does it matter?
Thanks.

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

 

[jraef]

I think it is critically important to differentiate between this being used for the HOIST, or for the TRAVEL, and that has never been established by the OP.

If used for the hoist, I never recommend using a drive that does not provide "torque proving" capability so that the drive KNOWS that the shaft torque is available BEFORE allowing the brake to be released. I'm not aware of any VFD capable of doing this without an encoder feedback. ABB claims their DTC can do it, A-B claims they can do it with their "Encoderless Flux Vector" control, I wasn't aware of Invertek being able to do it, but apparently so? Still, when I was doing crane work for Boeing on the facility where they make 777s, ABB came in to challenge my assertion with their DTC drives and I challenged back that if they were so certain it was good, to go stand under our 40ton test weight while I release the brake. They declined. But in reality we were releasing brakes on hoists that were carrying entire aircraft fuselages, with workers almost underneath them every day. We used FVC (FOC) drives with encoder feedback on those hoists and never had a single problem.

Now if these motors are for the travel trolleys, not the hoists, I would not hesitate to use Sensorless Vector Control.

And there are PLENTY of very good reasons to add VFDs to both hoists and trolleys. Simple contactor control introduces the specre of load swinging which, in a lot of applications, can be very dangerous, if not deadly (think molten metal pot line). So for years, the solution was to have ballast resistors to dampen the surging and later, soft starters. But compared to those, a properly applied VFD adds a LOT of control to any aspect of a crane.


" We are all here on earth to help others; what on earth the others are here for I don't know.
" -- W. H. Auden

 

[iop95]

FOC sensorless need a very good motor model and much updates with temperature (rotor resistance variation is biggest problem) to have a good/real orientation of rotor flux.
It use phase currents, but at low speeds and even worse at zero speed, accuracy is very bad... so cam't "know" rotor flux angle and can't have a correct control.
Maybe an advanced algorithm (predictive)can handle a torque control at zero speed also, but is no so "sure" and robust as encoder version.
With VFD, acceleration mean increasing output frequency (and voltage coresponding) by a n steps, that mean motor pass through n mechanical curves, so can't use mechanical curve that is valid for DOL.
Motor parameters varies with VFD frequency/speed and mechanical curves also. More, with VFD may modify mechanical curves as you need but keeping motor temperature below limit (short times and/or additional cooling).

 

[roydm]

A very interesting topic, lots of traps for young players.