Multishaft Disperser Motor Sizing

On the basis of your recorded measurements, I think the 37kw motor would suffice. The original might have been designed for even more viscous material than yours, or lower temperature, etc. Be sure that any overload protection is sized for the lower-current motor, just in case it ever does bog down.

(Should I change my name to Tomato when talking with a nightshade? )

Visocsity and Temperature are parameters that can not be controlled strictly at the plant. Should i go for motors with higher service factor?

If your history of ≤52A on an 80A motor is long standing and comprehensive, it doesn't seem that you would need a larger service factor. However, a larger service factor would allow for a wider range of viscosity/temperature, and perhaps give more peace of mind. It would need to be traded off against any extra cost, though; and also how big a problem some down time would be if you got an unusually viscous batch of ingredients.

You might also be able to contact the OEM and find out what their design assumptions were (might be difficult if the unit is old).

The units indeed are very old and it would be nearly impossible to obtain design parameters from the vendors (assuming if the company still exists or the contact info is the same as of now). Regarding the down time, any down time due to machine failure is not affordable. So i believe if we are to replace the existing motors with newer smaller motors then the motors have to have a higher service factor. Adding on, where can i get the torque vs current curves of my existing motors. Since the motors are very obsolete they are not in production any more. Also the motors have been rebuilt so that should cause a significant drift from the design torque-current curves. Would generic torque-current curves for 3 phase 4 pole indcution motors rated at 45KW 400V 80A suffice?

HP ≈ Torque_lb-ft x rpm/5252. Assuming same voltage, current is proportional to torque. Some additional detail depends on the design/torque type of the motor (A, B, C, or D); I think B is most likely, but it should be on the nameplate.

the motor may have been propertly sized earlier in the last century, due to starting times of less horsepower being too slow...due to starts and stops too often ... go modern first and then size the motor.

You say the motors have each been rewound twice, for what reason? If they had burnt out I would be getting concerned about down sizing. If it was general gradual insulation break down then go for it.

The motors indeed have been rewound however it was not due to overcurrents as necessary protection (overload/thermal) has been installed on every motor. The reasoon for rewinding is general insulation degeradation as you pointed out.

Cheers, for coming back with that information. Then I would agree with the rest and go for the more efficient lower rated drives.

It is nice when we get feedback.

The process will take whatever power it needs. If, at the agitation operating shaft speed, the process demand is, say 25kW depending on viscosity, density, impeller characteristics and dimensions, etc., then the power drawn will be about the same from a 37kW-capable motor and a 45kW-capable motor. So the savings based on motor capability and sizing are not necessarily there.

Just because the motor can produce a certain maximum shaft power doesn't mean that it operates at that power and presents that demand to the supply (reference a spinning, unloaded motor). Process conditions will determine that demand.

The correct answer to motor sizing and selection is currently imponderable, as the forum has no knowledge of local future product- and process-flexibility requirements that might dictate a different motor/agitator combination compared to the one(s) currently proposed.

What is the cash value of the process loss that results as the consequence of providing this equipment with a batch of new process material that it is undersized to agitate (rhetorical question)? The risk of a need to re-work and the waste of process materials and time might blow a hole in any justification calculation for smaller, more efficient motors.

Only local analysis can determine the correct route to take.

Indeed the motor sizing does not affect the process power requirement. However the benefit of undersizing the motors is make them operate at higher effeciencies and larger loading factors.

As for the batches for which the 37KW motor maybe undersized to disperse or agitate i thought of using motors with higher service factors. The process itself is of a nature which requires a higher torque initally and the then diminishes as the material gets dispersed. This i observed while looking at the current profile which showed higher currents initally and decreasing currents as the process neared completion.

Someone isn't listening.

PWS gets a GA for pointing out that there are often conditions that may come along that we have no way to anticipate. As a young engineer I often questioned why things were so overdesigned, it quickly became apparent when I had to a retrofit where things were designed for the current condition and could not handle a newer requirement.

You might want to look into why those motors required multiple rewinds, typically insulation fails due to overheating (assuming no abrasive or liquid ingress). What causes overheating, usually inadequate cooling air flow or overloading. Larger motors have typically have more iron, more copper and bigger cooling fans; all of which contribute to a cooler running machine under all conditions, and an extra measure of reserve under unanticipated loadings.

I had a similar experience where we replaced a 7.5hp motor to 5.5hp when the former unit had to be removed for PM. Not much problem with operation . But , we have to unload some of process liquid when ever the plant had a electrical or emergency trip.

I rather advice you to maintain new IE2 45kW unit.It sounds like you process fluid is much more viscous than our kaolin / water mix.

In terms of visocisty Kaolin and putty are more or less the same however what bothers me is an avg current of 37A for a 45KW motor and currents as low as 22A for rated current of 80A. Then again it might make sense to replace existing 45KW motor with an IE2 45KW motor however i believe it would be justified to replace 45KW motors on the other 2 MSD units with 37KW motors which process emulsion paint which has a viscosity much much lower than putty

Momentum of fluid in motion could be the reason for the low running amps, and amps fluctuation. A full tank process fluid may require higher starting amps. You mentioned that 45kW unit has shorted previously. I suggest that it is better for you to have discussion with your operators and investigate for a more logical reason: 1) 1) Why 45kW drive? and 2) Why it shorted previously? 3) Operating Cost saving with new IE2-45kW,4) Cost to install the 37kW . And make your own judgement .

Indeed that is the approach i followed. Investigation revealed that tripping of the 45KW motors was caused due to operator negligence by switching the motor on with a locked rotor, standard procedure dictates that shafts are to be raised from flluid and then switched on so that locked rotor condition does not occur.

Moving on to 'why 45KW motors'. I went through the design files supplied by vendor but i could not find any reasoning as to why 45KW motors were selected and since the files are very old i doubt i can contact the designers about it as i pointed out earlier.

Finally if i install a 37KW motor indeed the savings are tremendous and payback drops to a mere 1.5 years but the question remains. Will the motor suffice?

Also regarding the current drop due to fluid momentum seems misfit. The reason bieng that there are 3 shafts, the agitatir and 2 dispersion shafts. The agitator shaft makes up for the fluid flow where as the shape of the dispersion blades is such that sheer stress is generated in fluid with adjacent layers rubbing against each other to result in maximum grinding/disperiosn of the raw material particles. The diameters of the dispersion blades are much smaller than the agitator blades.

This is what i think. Correct me if iam wrong.

If operators can burn out a motor by starting it under the wrong conditions, it is my experience, and obviously your experience, that they will. A limit switch tripped in the raised position, and wired in series with the start push button, would seem to be a useful and inexpensive addition to the circuit. With maybe a key switch to over-ride the limit switch in exceptional circumstances. The key to be held by a shift supervisor and or maintenance engineer.

Making a system idiot proof, sailor proof, monkey proof or gorilla proof, is relatively easy compared to making it operator proof.

That indeed is the case. Handling operator negligence is a tiresome task. The simplest solution is installing a limit switch in series with the start button. Infact we are in the process of installing limit switches on our machines however key switches as over-ride on the limit switches was not planned. It is indeed a wonderful fail-safe for exceptional circumstances. I will make it a point to do the same on the MSD units.

I wish i had posted my question sooner.

Do you know a standard motor used 5-6 times it's current to get started. This spinning reserve must be generate by power companies at all times. It also made our operations expensive and hard on motor life. Six times currents results in 36 times the heating at rated voltages. No wonder a rewind or two. Especially if the motor was started often or took too long to come up to speed.

You have described processes that with proper protection should never have to be rewound. Motors however are stupid. 60 hp Motors will draw approximately 75 amps at 460 volts and run with out a problem for the life of the bearings. Then it willl need servicing. If the motor is slowed by load, it's current must increase. If this happens life is reduced. That's why motors are stupid. They don't know when they are asked to produce more than they are designed. Two rewinds seems too much ...

Your application can be updated to modern technology with a huge benefit. A slower speed motor (6 pole rather than 4 pole) driven by a variable speed drive will give you benefits of speed (you can still achieve the higher speed) and horsepower based on viscosity demands and allow for the better mixing of your product. But I digress...your solutions take a little physics, a knowledge of speed, horsepower and torque and better controls. If you solve these parameters properly, you'll increase your efficiency, mix better product, and lower your costs. Maintenance costs too will be lowered as your electrical and mechanical systems employed will be lowered considerably. Stress causes heat to increase and heat is the enemy of all processes.

You need a 21st Century "Motor Mother"

Indeed. I looked into the speed and torque parameters. The RPM of the motors is governed by the demands of the process and as such the process has to be carried out at 1450rpm. So speed is not a variable here. It has to remain constant. The torque however varies. For that we have installed VSDs on these motors with Direct Torque Control mechanisms. IF the VSDs are configured properly then the VSD should regulate the current as per the torque requirement. However what i miss is that motors intrinsically have this property of lowering current when torque requirement is lower and vice versa. Then what is the point of Direct Torque Control?

Hello Everbody.

I am back with an update on my previous query and a new problem. As discussed previously regarding downsizing the 45KW motors it was decided to replace these motors with 37KW motors with a service factor of 1.25. Which brings me to another question;

To recapitulate the motors under discussion are directly coupled to dispersion shafts on a MSD unit. As pointed out earlier at process initiation the motor draws in a reasonable current ( Load factor approx 0.7 - 0.8) however as the process progresses the currents drop drastically. Now to remind you, the process has fixed speed requirements (1500 rpm in this case) also the motors are driven by VFDs. Now my question is if i were to run the motor on a PWM signal ( say motor runs for 50 seconds and stops for the remaining 10 seconds a minute) after the process has progressed reasonably and the loads are low, what would be the effect of this on my motor and the process itself. Would the vortex formed in the fluid sustain for these 10 seconds or not. Secondly would this achieve any savings on energy?. And if there is some potential in this idea how do i go about investigating it?