Load sharing between motors
Load sharing between motors
(OP)
We've got a conveyor that we're upgrading from 1 x 350HP motor to 2 x 250 HP Motors. Both motors will run from a single starter with individual overload protection.
The problem is that the motors are not identical; close but not exact. They are both Tatungs and 2300V 250HP but one is 1775 RPM, 59.9Amps, 92.6% efficiency. The other is 1780 RPM, 57.0 Amps, 94.5% efficiency.
Does anybody have an estimation of how much imbalance there might be in load sharing? The motors couple through identical gearboxes into separate drive pulleys close together - the 48" conveyor belt does the coupling.
Thanks in advance.
The problem is that the motors are not identical; close but not exact. They are both Tatungs and 2300V 250HP but one is 1775 RPM, 59.9Amps, 92.6% efficiency. The other is 1780 RPM, 57.0 Amps, 94.5% efficiency.
Does anybody have an estimation of how much imbalance there might be in load sharing? The motors couple through identical gearboxes into separate drive pulleys close together - the 48" conveyor belt does the coupling.
Thanks in advance.





RE: Load sharing between motors
Your slips are 20 RPM and 25 RPM respectively. Expect one motor to be at about 80% load when the other is at 100% load. Total HP within rated current = (100%+80%)/2 = 90% = 450 Hp.
Approximately.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Load sharing between motors
Keith Cress
kcress - http://www.flaminsystems.com
RE: Load sharing between motors
(Problematical doesn't mean that it couldn't start loaded with a VFD on one motor The VFD could start one and when it could not accelerate any further, the second could be connected DOL.)
It may take some tweaking.)
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Load sharing between motors
RE: Load sharing between motors
This is assuming the use of a decent quality VFD. Of course, using an encoder requires a decent VFD too.
I'm not too sure on using 1 VFD and 1 full-voltage though. That presents a problem getting both motors to work together if the conveyor can be started loaded with both motors.
RE: Load sharing between motors
Have you considered the belt slip it will help balance out the load won't it?
If you find the load is too far out you have the option of modifying either pulley slightly to get them back into line.
I worked with a similar conveyor, 2 pulleys, 2 gearboxes, 3 x 900 HP motors, there was insignificant difference between the motor by itself and the pair.
Roy
RE: Load sharing between motors
I graphed the two motors assuming slip and amps were a linear function and, as Bill stated, came up with the same kind of load sharing values. I believe this will be adequate for our needs, even starting up fully loaded.
Thanks for everybody's help. It's nice to bounce some ideas around.
RE: Load sharing between motors
Nevertheless, the fraction of load sharing can be calculated based on the nameplate slip, by noting that torque starts at 0 torque with 0 slip and goes to full torque at full slip.
T1780 = Trated*Nslip/20
T1775 = Trated*Nslip/25
Ttotal = Trated*Nslip*(1/20 + 1/25)
T1780/Ttotal = 1/20 / (1/20 + 1/25)
T1780/Ttotal = 1 / (1 + 20/25)
T1780/Ttotal = 25/40 (slightly more than half)
Likewise you can find
T1775/Ttotal = 20/40 (slightly less than half)
Note that the nameplate speeds are ballpark numbers anyway (NEMA MG-1 allows 5 rpm deviation in these numbers). Also, losses cause increase in slip but do not contribute to output torque - was neglected in the above analysis.
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RE: Load sharing between motors
The 1780 rpm motor takes fraction of total load:
(1/20)/(1/20 + 1/25) = 25/45
The 1775 rpm motor takes fraction of total load:
(1/25)/(1/20 + 1/25) = 20/45
20/45 + 25/45 = 1.
Order is restored to the universe.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Load sharing between motors
RE: Load sharing between motors
The easy way to assess load-sharing, as has already been stated above, is to compare the motor currents.
RE: Load sharing between motors
Since the motors are high efficiency the rated speed is 1780 RPM (we can assume 1800 RPM no load) so only 20 RPM of slip. 20 RPM is 1.1%.
The existing conveyor drive pulleys are 30" with 3/4" lagging so a 31.5" diameter. If one the pulley diameter is worn by 1/8" that is a 0.4% difference so motor #1 will have to be at 36% load before motor #2 starts to do any work. Our case turns out even worse than that, so at full load with motor #1, motor #2 is only taking about 20% load.
The solution will be to put both motors & gearboxes on a single shaft and drive pulley.
The requirement for both pulleys to be identical is really basic in this setup, but nobody (including me) questioned it.
Interestingly, if we had a lower efficiency motor with higher slip (say a 1740 RPM motor) then the effects of the mismatched drive pulleys would be reduced somewhat.
So the conclusion is make sure you ask all the mechanical questions, even if your area of concern is only electrical. Don't trust the mechanical experts to consider all the factors.
RE: Load sharing between motors
Keith Cress
kcress - http://www.flaminsystems.com
RE: Load sharing between motors
Bill
--------------------
"Why not the best?"
Jimmy Carter