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Motor Sizing

Motor Sizing

Motor Sizing

(OP)
I have a 1960's wound rotor motor that is used to open and close a swing bridge (the bridge rotates about it's centre axis to allow marine traffic to pass).  The motor is connected to a resistor bank; this is used for speed control and I assume it's also used to provide sufficient torque to overcome the stationary inertia of the bridge.

We'd like to replace this motor with a squirrel cage / vfd setup.  I'm a little new to this and am looking for a few pointers on how to properly size the squirrel cage motor so that sufficient torque is provided.  Any feedback or pointers to where I can find further informaition would be appreciated.

There is not much information available regarding the existing setup; below is from the motor nameplate:

3 phase, 60 cycles
Volts  440       Speed Full Load  670
Amp  37          
Sec Volts  104   Sec Amps  112
25 HP Continuous 40 degrees C   30 Min 55 degrees C

Sizing of the resistors is not know.
Will measurements need to be taken in order to determine the torque used for the existing setup?  If so what specific measurement should be taken, and how do I then relate this back to properly size a squirrel cage motor and vfd?

RE: Motor Sizing

1) Why are you going to  / wanting to change? In other words what do you perceive as your pain now and what do you hope to gain in going through this exercise?

2) Are you currently doing any braking functions with the WRIM and controller? This is very common on swing bridge applications.

3) Assuming you already know your benefits, are you aware of all the pitfalls of using a VFD in an application like this?

4) Have you considered leaving the WR motor and just adding a VFD to it?
 


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: Motor Sizing

Yes you will need to determine right load-torque, speed data to specify a motor. Work closely with your mechanical team members. There is a reason WRIM motor was used in this application to begin with.

Blindly applying VFD does not necessarily is a betterment. And Get someone, who knows about what jraef says, on board with your team.

Rafiq Bulsara
http://www.srengineersct.com

RE: Motor Sizing

(OP)
Thanks for the reply jraef; I've responded to your questions below:

1) Why are you going to  / wanting to change? In other words what do you perceive as your pain now and what do you hope to gain in going through this exercise?
- This is client driven; as part of their ongoing preventative maintenance program they are upgrading all electrical systems on the bridge.  They have capital budget for this year that they would like to spend; as the motor is getting on in years, they have chosen to replace it.
- Their maintenance staff would like to see a squirrel cage motor so that maintenance on brushes is eliminated (the motor is rather difficult to access)

2) Are you currently doing any braking functions with the WRIM and controller? This is very common on swing bridge applications.
- Yes, there is a single holding brake.  As part of this project we are upgrading the existing control system to a PLC. We'll be installing a new contactor for the brake and the brake will be controlled via the PLC.  If we go the VFD route, we'll look at adding resistive braking as well.

3) Assuming you already know your benefits, are you aware of all the pitfalls of using a VFD in an application like this?
- I'm aware of the reduced torque and I'm aware that maintenance is still required on the VFD, but I'm sure there are many pitfalls that I am not aware of - please enlighten me!

4) Have you considered leaving the WR motor and just adding a VFD to it?
- Yes, we have considered this and did suggest it to the client, but they cited my answer to #1 for wanting to replace it.

RE: Motor Sizing

(OP)
Rbulsara, what methods are there for determining the load-torque and speed data?  Measuring the speed/RPM seems trivial to me, but what about the load-torque?

RE: Motor Sizing

dj:
That depends on the load. That's why talk to your mechanical friends or mfr of the load mechanism.

Other way is to match the existing motor's characteristics, if you can get that from the original mfrs. Not sure nameplate will have all the data. But design code letters etc. could help.

Draw bridge is not continuous operation where you are going to save any energy by "upgrading" the starting method. I would stick to what worked for last 50 yrs and replace it in kind.

Some experts in motor design/repair may offer more insight in interpreting motor data.

Rafiq Bulsara
http://www.srengineersct.com

RE: Motor Sizing

If you must replace the existing WRIM, I would choose a 25hp 1800rpm inverter-duty TEFC motor coupled to a gearbox.  Pick the gearbox ratio so, at 670rpm on the output shaft, the motor is turning the input about 2700rpm.

That way, you get lots of low speed torque just like the original motor and the same top speed with reduced torque.

You may find that a little (maybe 10hp) of braking with resistors will give better control on decel.

As long as there is room for the motor/gear combination, I believe this would make a very suitable replacement.  Use a drive with "sensorless vector" capability for good low speed torque performance.  Speed accuracy isn't all that important but you would improve the present speed stability considerably.

RE: Motor Sizing

If the speed at Full Load was 670RPM, I'd venture to say that was an 10 pole motor with high (7%) slip at full speed. I put the FLT as about 196 ft-lbs, and with a WR motor, you could have had 250-300% FLT at lower speeds virtually continuously, something difficult to do with a VFD unless you over size everything. If you directly replace it with an 1800 (1725) RPM motor, you would need at least 65HP (standard size would be 75HP). If you do the gear box thing as suggested by DickDV that would bring the motor size back down, but my experience on Swing Bridges tells me they already have a ring and pinion gear setup there, so adding another gearbox might be problematic. It might be better to have a new 10 pole motor made, but finding someone to make one that is inverter rated would be challenging too. Might be easier to go with a 75HP after all.

That's why you really need to work with an ME on this to determine EXACTLY what you need mechanically to do this right, then take it electrically from there. It could be that although they HAD that much torque but didn't really need it all, which may seriously affect the size and cost of your VFD system.

The bridge in my link above (I worked on retrofitting it in 1978) is being retrofitted with electronic braking this year. I'm trying to find out what that really means though. CalTrans can be a daunting place to get information from...

Curiosity note: In that video, the WR controller tap changer he is using came out of an old Westinghouse elevator when the bridge was built during WWII and was salvaged from a building in the infamous "Barbary Coast" area of San Francisco that had been torn down to clean up the city for the 1939 World Exposition; so it went from a "swinging [ill repute] house to a swinging bridge", or so the bridge operator at the time liked to put it. (Those weren't his exact words though). Still working to this day as far as I know.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Motor Sizing

(OP)
Agree with you that we'll need to get a ME on the team.

To satisfy my curiosity, I have a few further questions.  For roughly calculating the torque, I assume you used the following:

T = (HP x 5252) / RPM
T = (25 x 5252) / 670
T = 196 ft-lbs

And for a wound rotor motor at low speed we could see approximately 300%, or 588 ft-lbs.

Now, for the squirrel cage sizing, did you suggest 1800 RPM as that's a standard speed?
And to size it at 65HP for the replacement, how did you go about selecting that?  I understand that at 1800 RPM we have:

T = (HP x 5252) / RPM
196 = (HP x 5252) / 1800
HP = 67

But this is the torque at 1800 RPM, whereas we'll be running the motor at approximately 40% rated speed to achieve 670 RPM.  What is the typical operating characteristics of squirrel cage motors operating at 40% rated speed (in terms of torque, operating time, etc.) and why specifically do we need to oversize it to 67 HP?

Finally, for NEMA D design motors, isn't the low speed torque approximately 250% of rated?  Could something like this be used as a direct replacement?

Thanks again for your assistance.

 

RE: Motor Sizing

If the motor itself is not bad, it is possible to short the rotor windings, which would eliminate the brushes and control the motor with a VFD.  I expect that you would lose the overload torque, however.  To me, this sounds like an application for an off-the-shelf motor coupled to a variable displacement hydrualic pump-motor combination.

RE: Motor Sizing

1) I based the calculated HP on 1725RPM, the slip speed of a typical 1800RPM motor.

2) When using a VFD, torque remains constant throughout the speed change. That's why you use a VFD. HP goes down of course because speed is part of the HP calc.

3) A good Vector VFD can make the motor put out roughly 220% LRT (some claim higher), but only for a short time i.e. 30 seconds or so (again, some claim longer) depending on the VFD overload capacity, but it is also limited by the motor's OL capacity as well.

4) Because a VFD can control all aspects of a motor's performance, many people have had very good success with shorting the rotor of a WRIM and using as a SCIM with a very good quality Sensorless Vector drive. As potteryshard said though, that OL torque capability is more limited using the VFD than it was with resistance control.

More good reading from one of our other members, Marke, to help you understand.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Motor Sizing

Well, let's see here.  A 25hp four pole motor develops 75ft-lbs continuous torque.  Thru a gearbox 75 x 2700/70 = 302 ft-lbs on the gearbox output shaft continuous.  Sizing the VFD for 200% short-term overload gets the 302 up to 604ft-lbs  which is essentially the same as 196 x 3 = 588ft-lbs.

So, if the mechanical issues are not too ugly, the 25hp motor would have it covered.

Probably the key issue tho is jraef's statement about not how much torque can be developed but rather, how much torque is actually NEEDED.  600ft-lbs may be way more than actually is getting used and the sizing could be reduced somewhat if that is the case.

RE: Motor Sizing

(OP)
Thanks for the patience guys, as I said I'm a little new to this and am trying to learn as much as possible.

jraef - I guess where I'm getting confused is the following:

We worked out that the FLT of the existing wound rotor motor is around 196 ft-lbs.  With the external resistors, at LRT, this could be up to 300% or 588 ft-lbs.

We then went on to say that a 75HP 4-pole motor w/VFD would be suitable.  But this only provides continuous torque of approximately 228 ft-lbs, which is not very close to the 588 ft-lbs currently achievalbe, unless we go into overload, but this would still only get us to around 456 ft-lbs (at 200%).  Depending on the load characterisitics (which are unknown at this time) this may not be enough.

An aside regarding running in overload - what % overload is typical for motors, how long can they be run in overload, and how does one go about properly sizing a VFD so that it can drive a motor in overload?

DickDV - thanks for the suggestion with the gearbox; this certainly appears to be an attractive option.  Do you have any suggestions for gearbox manufacturers so that I can do some research into size, etc.?

I read here (http://www.kilowattclassroom.com/Archive/VFDarticle.pdf) that with a VFD you'll produce full torque up to motor base frequency (60Hz), then after that the torque begins to decrease.  Running the motor at 2700 RPM would give approx 80% torque, so the 75 ft-lbs would only be 60 ft-lbs; if I up size to 30HP, then at this 80% I'd have the needed 73 ft-lbs - is my logic correct here?  

RE: Motor Sizing

(OP)
DickDV, as you and jraef have illuded to, ultimately we need to determine the load characteristics so that we can figure out EXACTLY how much torque is needed.

Would this be a mechanical exercise, or can we take electrical measurements and work it out from there (i.e. measure the size of the resistors and gather voltage, current, and rpm measurements as the bridge completes a movement)?

RE: Motor Sizing

The most accurate method would be direct measurement if you have sufficient space to mount a sensor. Something like the devices made by http://www.sensors.co.uk/ for example. The rental service might be worth looking at.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Motor Sizing

The most basic principle of sizing motors, gearboxes, and VFD's is to start with the load requirements.  In many cases, especially existing cases, this can be difficult to obtain.

In your case, since you have a reasonably good understanding of the motor's torque behavior, you may be able to get close enough by simply measuring motor current.  I would be careful with that tho as WRIM's can be tricky to evaluate unless you have detailed data sheets and know exactly what the rotor circuit resistances are under load.  But, I think it is safe to say that under no circumstances could the motor deliver much more than the previously stated 300% of nominal.

There are lots of suitable gearbox manufacturers out there and many good ones.  I've always trusted Sumitomo, Stober, Dodge, and Winsmith.  One benefit of Sumitomo is a very compact parallel shaft or concentric shaft design.  That is one of the key design issues you will need to deal with---parallel shaft, concentric shaft, or right angle gearing.  It's well outside my expertise so I'll leave that to your experts.

As to short-term overload capabilities, if you buy a NEMA motor in Design A, B, or C, (probably C would be best here but B would be ok), you can figure at least 200% for one minute in ten minutes repeatatively.  That's the motor.  You will need to size the VFD for continuous running amps and also, in this case, for 200% one-minute overload to cover the amps the motor will need in overload.  That will probably mean the drive will have to upsized to 30hp with a heavy duty rating.

Your question about torque in overspeed can be answered like so.  A 25hp 1800rpm motor is good for 75 ft-lbs of torque continuous from zero to base speed.  At 2700rpm the torque will fall by the inverse of the overspeed 1800/2700 to 50ft-lbs.  However, since the gearbox ratio is now 3/2 higher than without overspeed, the net torque at the output of the gearbox will be the same.  With the higher ratio tho, at all slower speeds the torque will be 3/2 higher including starting torque.  That's just what you will need to get a heavy swing bridge moving.  Once its moving, the torque would be expected to drop to somewhere under nominal on the motor.

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