Coupled 3 phase motor driving the same shaft

Are those V-belts coupling the motors to the shaft?

If they are timing belts or chains, and the shaft positions are carefully lined up, it might be O.K., though unconventional to say the least.

Yep v-belts

Assuming these are induction motors, it's probably not a big deal as long as they are fairly closely aligned.

If they are synchronous motors, then alignment becomes more critical because the "leading" motor will drive current rather than consume it.

https://en.wikipedia.org/wiki/Synchronous_motor

"Synchronous motors fall under the more general category of synchronous machines which also includes the synchronous generator. Generator action will be observed if the field poles are "driven ahead of the resultant air-gap flux by the forward motion of the prime mover". Motor action will be observed if the field poles are "dragged behind the resultant air-gap flux by the retarding torque of a shaft load".^[1]

There are two major types of synchronous motors depending on how the rotor is magnetized: non-excited and direct-current excited.^{[3] "}

These are induction motors, not seen any vfd on site. Probably the best thing to do here I guess is to loosen up v-belt to set just enough traction to the pulley, so the 2 motors will find its own synchronization, right? No wonder, v-belts are used instead of gears and chains. Kinda more like the below. It keeps tripping the generator. Anyhow, these motors are the drivers of a concrete mixing paddles- one you would find in a concrete mix batching plant. I was summoned to test some of these equipment lately. So yeah, if id find phase failure error on the error logs of the genset, prolly cause by unaligned poles.

Thanks man.

Last thing, how do you aligned poles of an induction motor? without dismantling the cage?

Hi,

At risk of answering a question with a question;

• do the two motors have the same rating and speed (what are they)?
• Are the motors direct-on-line started?
• Is the mixing vessel loaded when the motors are started?

This arrangement may simply be to avoid using more complex starting gear. the time delay flattens the peak current due to inrush

What type of starter do you have? What does the mixer supplier recommend? I would guess that load is close to constant torque, so with no VFD, needs DOL, and start both simultaneously. The generator needs to be rated accordingly, maybe it trips because it isn't big enough?

No idea about unaligned poles, or how to align them, but I doubt it's necessary.

I guess this is one way of reducing the starting current demand...I don't see a problem yet...

Thanks, Rixter

If the motors & belts are similar, they will share the load reasonably well. I’d expect a little more belt wear on one as opposed to the other. Obviously motors are wired for opposite rotation. If the full load is equal to both motors, it will stress the first motor during start, probably more than a reduced voltage starter, but not a whole lot more. I’d try starting both together, and see if your lights flicker too much. If not, best not to sequential start them, as it delays the up to speed for one of the motors, giving you unnecessary rotor heating.

This is known as a "pony motor" setup, which allows for two speeds of the driven shaft. The motors are interlocked so that only one can run at a time. There are time delays involved, especially when slowing down. The motors are of unequal horsepower.

That is another valid use of the term "pony motor", but is not the same as I described, nor applicable to the present situation.

“obviously in one direction with one motor actuated ahead by a delay timer seconds the other.“ from OP

What makes you think that is the arrangement? I can't see anything in OP's posts to suggest it.

I can't see anything wrong with it. I don't see a need to start one motor before the other, and depending on the type of load there may not be enough torque to get it away.

Many years ago I was involved with thickener mechanisms, where the rake was attached to a bull gear, which was driven by a pinion via a reduction gearbox. To get more torque at the rake mech, they tried adding an extra pinion/gearbox, but due to tolerance in the bull gear gear, first one gearbox, then the other, took most of the load, so both soon failed. To solve it, they used specially wound high-slip motors, 6-8% at full load, which worked. But I don't think the OP's set-up has a similar problem. If the motor slip characteristics are not identical they take slightly different loads, but not likely to be a problem.

Also if I remember right there are horizontal split-case centrifugal pumps with shaft coming out both ends, and a motor on each end.

I meant that more as a possibility rather than necessarily the case. What made me think of it was that I have seen and used the two-speed arrangement many times. It is/was common on fan drives for condensers and cooling towers. Today a single motor and VFD might be more likely.

What are the approximate diameters of all the pulleys?
What are the hp/kw and rpm of the two motors?
What sort of load is driven by the common shaft?
Does the shaft rotate the same direction in each case? (Just double checking--looks like yes.)

Been done before, it seems:
At first glance, it would seem the motors would just sync their speeds and there would be no problem. I'm guessing the motors are the same (brand, model, power, voltage, speed, etc.). If one motor shuts down, it would just be an additional load for the remaining motor.

I'm not sure why this is done (additional torque?). Why not just use a single, bigger motor?

This video is for a crusher. If one motor shuts down, the other motor can keep the crusher moving (at less power) until the incoming stream of rocks can be stopped. Having worked with grinders, I could see the advantage.

https://www.youtube.com/watch?v=iY4gwczyRj4

regards,

Vulcan

Before VFD's came in to the market it was standard practice to use "common field" configurations on twin motors.

Applications such as overhead cranes needed two motors driving at the same speed as they powered a crane up and down its rails with an identical motor on each end of the crane.

with the advent of VFD / VSD drives and shaft encoders the common field was not required.

as for the OP , we need more information about the intentions in that setup

I cannot for the life of me see a problem with this arrangement. It should work just fine......

regards

Andy

A lot of missing details in the posted question (the devil is in the details)

Several of the comments have possibly answered the question already.

I would like to offer up another possible reason for this configuration.

It was not indicated if both motors have the same number of poles.

If these motors had different number of poles .. this configuration would allow for two speed operation. (assuming 60 hz 3 phase.... ) often a specific motor "frame" (size) can be purchased with any of these pole configurations.

Minor point - your Speeds with rated load are for 60Hz, your synchronous speeds are for 50Hz.

Nice idea. There are two things to consider:

1. The lower speed machine would have to be capable of turning at the higher speed. In effect it would have to have a continuous mechanical overspeed rating. It would not therefore be a standard machine design. If we are considering 2/4/6 pole machines the overspeed would be very significant and would impact on the rotating elements of the design such as the squirrel cage;
2. The load is probably not linear so the rating of the high speed machine would be much greater than that of the lower. It would also have to overcome the windage losses of the lower speed machine rotating at the higher speed.

There is a lot we don't seem to know about this system so much of what is said is conjecture as well as speed(s) & rating(s) the age of the equipment and its location & environment would tell us a lot. Most of the contributors have implied that doing this today makes little sense when packaged motors and drives are available off-the-shelf; I can only agree.

A 3 phase induction motor, runs slightly slower than the fields, you might call it "slip".

If the load is constant, the amount of slip is too.

It happens smoothly and completely unnoticeable to the user.

Gutmonarch,

This is a reasonable application for the two motors, providing they have the same ratings (HP, speed). It is an indirect method for achieving "reduced voltage starting". Look up "part winding starting", in which a portion (typically 1/2) of the motor's windings are energized and after a short time delay the rest are energized. This will reduce the total inrush current while the load is getting up to speed. I believe the manufacturer knew what they were doing.

Continue learning--look up all the methods for reduced voltage starting (there are at least 6). See the differences between them and which ones are preferred for which applications.

--JMM

There is something that puzzles me with this proposed two-motor configuration. The mechanical load presented to just one motor during startup puzzles me. Look at this nominal torque vs slip curve.

If one motor's starting torque is sufficient to spin the mechanical load up to speed then why energize a second motor later? I guess this implies that the mechanical load will change significantly from time to time after reaching operating speed but as the curve shows the breakdown (aka pullout) torque of just one motor is nominally multiples of the stating torque. If that mechanical load does change enough that a single motor might not have enough pull-up torque to overcome the mechanical load changing a bigger motor instead of paralleling a second motor seems more cost-effective. No anticipated belt slipping, and no additional mechanical parts are necessary.

OP said 2nd motor starts within seconds of first, so more like reduced voltage starter, with lower accel torque, not up to speed either motor when energized