How much detail do you need to spec a motor/drive in an RFP? 1

[KM]

I'm writing an RFP for a replacement stoplog lifting machine on a dam. I don't have to spec everything out in detail, but I am describing the machine operation required. I want to spec out a few key points to make sure we have some quality control over the final product.

The existing motor is a 20 HP 3 phase working from a 60 Amp supply of 250 VDC / 600 VAC power on the dam deck. The motor probably dates from the 1950s and the nameplate reads as follows:

GE Electric Motor 20 HP Phase 3 MR 404
55581-20-840-64-220-100
1/2HR 55 - 93.5 - 442359

I will leave the horsepower sizing up to the designer of the new machine, but suspect I want to call for the new motor to be TEFC enclosure (because of outdoor use). I figure manually-reset thermal overload protection is most appropriate. The machine may be abused so I want to call for a service factor of 1.15 But after that, I have questions.

Duty Cycle: At most, the machine operates for a few hours at a time, but the motor would see it as bursts of 5-7 minutes, break for a couple, another burst, and so on, for about a 4 hour stretch. Does this count as "Continuous Duty"?

We will likely have this thing for another 50 yrs so I'm looking at NEMA Premium Efficiency motors. But since the use pattern of use is start-stop-start-stop, would such a fancy motor be worth spending the extra money? Doesn't that efficiency stuff apply only to truly steady-state continuous use?

Load: Getting a log moving in the sluiceway involves friction of the log ends against the check under water pressure and current. This is higher as you go deeper or water is faster, so it varies. Similarly, when you place a log you want to go slower as you get to the bottom so you don't jam the machine. I can see some variation of torque with speed, does this mean I need a "constant horsepower" motor? Do you spec this out for the motor, or is it part of the way the controller is configured?

And speaking of controllers, is it enough to just describe the operation we want for the machine and say to the designer: "select VSD to suit?" Or do I need to keep reading and figure out how to spec one up better.

Sorry for the vagueness of the questions. Electrical stuff is quite new to me.

 

[aolalde]

If an electro-mechanical engineering firm is solving the whole project they should look for all the requirements; since ambient, safety codes, operational, liabilities etc. to the specific ratings of the motor requirements as HP, Speed, Time rating, Starting method, Torque design,Insulation system, allowed temperature rise, etc.

 

[rbulsara]

I have generally placed the burden of selecting right motor and its drive on the manufacturer of the equipemnt being driven and have them provide it as a package. You may want to specify the voltage and prefererred manufacturer and any special features for monitorinng and controls. Since the machine manufacturer have the best knowledge of the load -torque characteristics of their equipent and also the past experience with similar devices.

 

[jbartos]

Suggestions to KM (Mechanical) Mar 9, 2004 marked ///\\I'm writing an RFP for a replacement stoplog lifting machine on a dam. I don't have to spec everything out in detail, but I am describing the machine operation required. I want to spec out a few key points to make sure we have some quality control over the final product.
///There are standard specs available on the market, e.g. MasterSpec by AIA, certain manufacturers have generic spec forms, etc. It is difficult to start an important spec from the scratch.\\The existing motor is a 20 HP 3 phase working from a 60 Amp supply of 250 VDC / 600 VAC power on the dam deck. The motor probably dates from the 1950s and the nameplate reads as follows:

GE Electric Motor 20 HP Phase 3 MR 404
55581-20-840-64-220-100
1/2HR 55 - 93.5 - 442359
///Contact GE for the motor spec sheet, e.g. Fort Wayne, Indiana, USA.

http://www.geindustrial.com/cwc/ultra/TechPubs/e-GET-M1006.pdf

http://www.ge.com

\\I will leave the horsepower sizing up to the designer of the new machine, but suspect I want to call for the new motor to be TEFC enclosure (because of outdoor use). I figure manually-reset thermal overload protection is most appropriate. The machine may be abused so I want to call for a service factor of 1.15 But after that, I have questions.

Duty Cycle: At most, the machine operates for a few hours at a time, but the motor would see it as bursts of 5-7 minutes, break for a couple, another burst, and so on, for about a 4 hour stretch. Does this count as "Continuous Duty"?
///Yes, it does (over 3 hours). The heavy-duty service motor may be preferred.\\We will likely have this thing for another 50 yrs so I'm looking at NEMA Premium Efficiency motors. But since the use pattern of use is start-stop-start-stop, would such a fancy motor be worth spending the extra money? Doesn't that efficiency stuff apply only to truly steady-state continuous use?
///It depends on the motor duty classification. Heavy-duty motors have it accounted for.\\Load: Getting a log moving in the sluiceway involves friction of the log ends against the check under water pressure and current. This is higher as you go deeper or water is faster, so it varies. Similarly, when you place a log you want to go slower as you get to the bottom so you don't jam the machine. I can see some variation of torque with speed, does this mean I need a "constant horsepower" motor? Do you spec this out for the motor, or is it part of the way the controller is configured?
///The motor controllers, VFDs, do have options in their programming menu.\\And speaking of controllers, is it enough to just describe the operation we want for the machine and say to the designer: "select VSD to suit?" Or do I need to keep reading and figure out how to spec one up better.
///There are various VFDs and various requirements around them, e.g. harmonics, etc. It is better to have a motor application specialist to assist you with this spec since there are many subtleties not yet addressed in the above postings.\\

 

[kassad]

Couple of points on jbartos reply:
///Duty Cycle: At most, the machine operates for a few hours at a time, but the motor would see it as bursts of 5-7 minutes, break for a couple, another burst, and so on, for about a 4 hour stretch. Does this count as "Continuous Duty"? ///
By what i understand from the post that the motor will run for 5-7 min, stop (or running at no-load) for 2-3 min, and the cycle is repeated for 4 hours. This is not continous duty. A continous duty (in layman terms means that the motor is running 24/7/365) means that the motor is properly de-energized before it is again started. IEC refers to this as cold start and even this is limited per hour.

A 20HP motor to achieve thermal equilibrium, takes about 15-20 minutes and the cooling down period is longer than that. Starting a hot motor is harmful to the windings (Hot Start). According to IEC, this is per S6 duty cycle (provided the motor is not stopped but is running at no-load during the 2-3 min). A motor manufacturer should be consulted for this duty cycle. (NEMA does not have such sub sections on intermittent duty. It considers all non continous duty as intermittant).
///Load///
The variation in load should be specd out both for motors and as well as for VFD's. But most VFD's by nature are variable torque machines. And do remember to specify an inverter duty motor when using a VFD.

I agree with jbartos that a lot of smaller subtleties are missing.

 

[KM]

Thanks, folks. I think you've all confirmed my suspicion that I'm out of my depth here. I'll be getting an electrical eng'r to look at this with me.

 

[macmckim]

Few Points about the existing motor.

a) The GE motor is pre-U-frame, meaning mfg before 1956.
Since you have noted 600V, motor would have been built
in Canada in Peterbough, Ont.
b) frame size as noted is 404 series. Today 100HP come in
this frame series.
c) Motor is a 30 min duty cycle motor, and most likely
motor built for elevator services, so I think its a
design D rotor design, high torque, high slip.
The motor design was used due to possible ice in the
sluiceway, freezing some the machinery and log.
You did not note motor's RPM, so based on experience
I think this is the type of motor you have.
d) Make sure your engineer compensates for the oversize
frame you have, Rotor intertia will be alot higher than
todays premium efficiency motors.
e) If motor is a design D - you cannot buy a premium
efficiency in this design.
f) If motor is a design D, best company for this is
Imperial Electric, I think they are in Ohio area of
USA.

 

[jbartos]

Suggestion: I agree with the previous posting, Imperial Electric, for the elevators motors.
Continuous Motor Duty Definition: Operation at substantially constant load for an indefinitely long time.
This will comply with:
IEEE Std 100 "Dictionary"
NFPA 70 National Electrical Code
IEEE Std 45
etc.
In some instances, 3 hours or a longer time is considered the continuous duty.