REPLACE DIESEL WITH ELECTRIC MOTOR

[JIMGEN]

WE NEED TO REPLACE A 375HP DIESEL ENGINE WITH A 460V,60HZ 1800 RPM,DESIGN A, SF 1.15 ELECTRIC MOTOR. OUR ENGINEERS PROPOSED A 250HP ELECTRIC. HOWEVER WE DID UPSIZE THE ELECTIC TO 300HP SINCE IT WILL OPERATE ON 380VOLTS, 50HZ.
THE DIESEL ENGINE HAS ABOUT 75HP OF LOAD WHICH THE ELECTIC WILL NOT HAVE SINCE AN ADDITIONAL ELECTRIC MOTOR WILL BE USED. APPLICATION IS A MATERIAL GRINDER WHICH HAS A HEAVY ROTOR WITH SOME "SHOCK LOADS" ENTERING THE GRINDER. TORQUE OF DIESEL IS 1100FT-LBS AT 1800RPM AND 1300 LB-FT MAX AT 1300RPM. IS THERE A "RULE OF THUMB" TO REPLACE A DIESEL WITH ELECTRIC?
ANY COMMENTS WILL BE APPRECIATED.

 

[aolalde]

The load speed- torque curve and the load constant of inertia WK^2 are the requirements to be matched by an electric motor.
From the information of the diesel motor that you provided; “TORQUE OF DIESEL IS 1100FT-LBS AT 1800RPM AND 1300 LB-FT MAX AT 1300RPM.”

HP = T*rpm/5252

HPnom = 1100*1800/5252 = 377 HP
HP@ T max = 1300*1300/5252 = 321.8 HP

Standard Induction motors will develop at least 175% Maximum torque.
For 300 HP,1485 rpm, 50 HZ, 380 volts, Induction SC Motor:

Tnom = 5252*300/1485 = 1061 Lb-Ft
Tmax = 1.75*1061 = 1856 Lb-Ft

Since actually the diesel motor supplies 75HP extra, you should no have problem to handle the grinder load with 300 HP at 1485 rpm.

I recommend you to buy a 50 HZ, 380 Volts, original design, not a 60 HZ, 460 Volts .

 

[MikeHalloran]

If memory serves, a Service Factor of 1.15 is not representative of a material grinder that produces shock loads.

It would be a real good idea to reveal to your electric motor supplier exactly what you propose to do with the motor, and exactly what supply you propose to run it on, if you have any expectation of warranty coverage.

-Mike-

 

[JIMGEN]

Aolalde and Mike H.
Thank you for your replies. As I recall the motor has 870 ft-lb full load torque, with breakdown torque ratio of 2.8 as a 300hp, 1785rpm unit. The manufacturer's sales engineer states that the 460v,60hz will be ok on 380v,50hz. The engineer has been fully informed of the intended use. I was just thinking if we should up size to a 350hp electric which shows full load torque of 1015 ft-lb with break down at 2.6. Our soft starter will handle either the 300hp or 350hp motors.

JIM

 

[JIMGEN]

aolade,
Also remember the electric is a 300hp,60hz, 1800rpm derated to 50hz, So I think the derated HP would be about 252. So Tnom =5252*252/1512 = 875ft-lbs. So Tmax = 1.75*875 = 1531 ft-lbs. I think this would compare favorably to the diesel with a net hp of about 300hp.
Appreciate your comments.

JIM

 

[jraef]

JIMGEN,
What you are proposing is very very similar to one that I just completed. We replaced a 167HP diesel engine on a rock crusher with a 200HP electric motor and VFD. When I did an analysis of the torque speed curves of both machines and adjusted for the operating speed, the electric motor FLT came out to be almost exactly the same output shaft torque as the diesel. The difference in HP was due to the fact that the diesel engine went through a transmission and gear reduction to get to the final speed, something that was eliminated in the electric power system. That is something to be careful of. The user missed that detail in the first 10 conversations I had with him, even though I asked point blank! It wasn't until I saw the machine that I caught it.
FYI, it works perfectly and in fact the user feels that the shock load (i.e. torque step change) response "feels" better with the electric motor than it did with the diesel. By using an open loop vector drive we can deliver 220% FLT to the shaft much faster than the diesel could (in fact, the dielsel NEVER could because it was at FLT already)

"Venditori de oleum-vipera non vigere excordis populi"

 

[Farkel]

Electric motor driven is the way to go; why do you think
the railroad has used diesel locomotives all these years.
They save on fuel, and have a better cost of operations.

 

[jraef]

Methinks the issue in propulsion drives is because of eliminating the transmission for speed and torque control. 4 speeds on a car is fine, but you need 18 speeds on a truck and it would be ridiculous on a train or marine vessal power system! Throughput power efficiency would probably favor direct drive from the engine if that were not an issue.

"Venditori de oleum-vipera non vigere excordis populi"

 

[JIMGEN]

We have decided to use the 300HP electrics and and not increase to a 350hp. ( Both of which would have been derated) We will test run in about six weeks. jraef, I agree with you ( and the torque curves ) the electrics do respond quickly to torque demands and maybe that is why electrics seem to do so well.
I will let you know how the electrics perform when we test the units.
JIM

 

[light1]

If the shock loads are more severe, NEMA Design Letter D motor may be considered. NEMA Design Letter A motor produces more torque than NEMA Design Letter B motor; however, NEMA Design Letter D motor produces a high start torque, about 250% of full load torque.

 

[JIMGEN]

The motor is a Design A with 4140 steel shaft which is called a crusher duty. We will soft start with a 400HP, 460v,60hz rated unit. (Derated for 380volts, 50hz). Soft start is rated 500% of FLA for 60 seconds. The torque curves are fairly impressive (at least to me). With a loss of 2.5% of synchronous rpm the torque is about 1305 ft-lbs (amps is 2 X fla = 798amp). With a loss of 5% loss of synchronous rpm the torque is about 1740ft-lbs and amps are about 1416. We have a PLC controlling the feed and will set it for about 110% of FLA to start the testing.

 

[jraef]

JIMGEN,
Hopefully you are still loging onto Eng-Tips and see this.
How did it work out? I am curious because on my similar application I ran into an unforseen snag yesterday... it works TOO well! The old diesel engine was never delivering anywhere near the torque it was rated for on paper. As a result, it would bog down if they loaded the machine too much and started spewing black smoke. The operators used that as an indicator and would cut back on the infeed conveyor. The new problem that cropped up is that the electric motor is so much better that we "found" the next weak link in the system, the V-belts. The torque response from the electric motor is so quick that a severe load step-change is now transferred immediately to the V-belts on the main drive, and they slip. This allows the crusher to slow down even though the electric motor is running at full speed and only 1/3 load! Through a long day of trial and error, I found that when I deliver more than 45kW of power from the electric motor (I have a VFD on it), the belts slip. Needless to say, we could have used a much smaller motor...

I am curious if you (JIMGEN) found the same to be true.

"Venditori de oleum-vipera non vigere excordis populi"

 

[busbar]

jraef — Is it fair to say the belts don’t spew their own version of black smoke?

 

[JIMGEN]

Hi Jraef,
This project has slowed a bid since we do not have to deliver the machines until Sep. We are continuing to design and have some building done. We should be testing in late Aug. We will not allow belt slipage as we will have enough belts to prevent slipage. We transmit up to 1000HP with belts off the diesel engines and do not have a problem.
As soon as we test the machine I will post some comments.
Thanks, JIMGEN

 

[jraef]

busbar,
Suprisingly, no! It actually took us a while to find out what was happening because we would have expected smoke, so we focussed on possible broken shafts, gears etc. Only when we removed the guard could we see that the sheave was spinning but not the belts, and there were 16 V-belts stretched tight on that sheave. From what I know of V-belt drive systems I'm sure they could have made it work, but it made them worry about actually finding the NEXT weak link and it might be more expensive, i.e. a shaft or journal. They decided to leave the belt spin as the "clutch" in the system, but had me adjust the VFD to not allow it to deliver more torque than the system could handle anyway.

PS, thanks JIMGEN. I look forward to reading of your experiences too.

"Venditori de oleum-vipera non vigere excordis populi"

 

[JIMGEN]

Hello All,

Just to let you know that the 300hp electric, 60hz, 460v,1800rpm that was derated to 50hz, 380volts, 1500rpm performed in actual testing very well (as compared to the 340hp net diesel engine (375hp less about 35 parasitic HP)Please note that I had a typo in my original post wherein I mentioned a 75hp parasitic load. During a full day of testing the unit was loaded heavy and maximum amp draw was just over 900amps. All components were within factory tolerances. We were confident in this application and signed off for final assembly.
Thanks for all your comments.