Another motor question.

[Lancie1]

Rick Densing,

If I was the farmer, after paying for a new moter and getting zip for my money, at this point I would give the new motor back to whoever sold it to me and ask for my money back. Then I'd get the old one out of the scrap pile and take it to a motor rewind shop.

 

[Alan Case]

You will probably find that whoever purchased the motor just asked for a 7.5hP 230 Volt 1800 RPM motor. You cant blame the seller for this, he sold the guy what he wanted. Regards Alan Case

 

[ChuckM]

I assume that it has been checked that both are rotating the same direction.

 

[Vic]

Check the pulley diameters again, especially if one pulley was changed due to a different size shaft on the new motor. The pulleys must be exactly the same size.

If the motors are turning the same RPM, THERE IS NO WAY THEY CAN BE "FIGHTING EACH OTHER". The only way they can "fight each other" is if one motor is forcing the other to turn at a RPM greater than synchronous speed (1800 RPM). If the motors are turning the same speed but have different full load speeds (slip speeds), they will not share the load evenly. This will cause an overload on one motor and a light load on the other. The only way you can overload both motors is too high a load (more than 15 HP) or the motors are not turning the same speed.

 

[electric06]

"They replaced one of the motors. Same ratings but a different frame size. The pulleys are the same diameter."............................
7.5 HP 1800 RPM motor will have a 215T frame,7.5 3600 RPM motor will have a 213T frame,looks like this may be the problem,if frame size were changed

 

[rsdoran]

Can not use frame size to determine the RPM, its possible a 184T frame is available and both 1800 & 3600 can be a 213T, its more common for a 7.5 1200rpm to be a 254T.

 

[DickDV]

Rick, you state that the first motor is started and the current is 41amps. Is the motor able to get up to running speed before the second motor is energized? I would expect that the first motor's current would drop down to something less than 41amps when it gets up to running speed. Or maybe the load is so heavy that the first motor would stay in overload until the second motor begins to help out. Please explain the timing on this for me.

Now for some general comments. First, running two induction motors mechanically coupled together to share load is not a good idea under the best of circumstances. Induction motors simply don't share load very well when forced to run at common speeds.

Second, as long as both motors have the same number of poles (or base speed---1200, 1800, 3600, etc) they will not "fight" each other. An electric motor without any shaft load will run within one rpm of its sync speed--1200, 1800, 3600, regardless of what its nameplate rpm is. As you add load to the shaft, the motor will slow down slightly until, at nameplate hp and rated torque output, the speed will have dropped to nameplate speed. Depending on how the motor is built, one motor may have a full load speed of 1710 while another may have 1750. The important thing here is that this speed defines the motor torque output and is not enforced any other way. If you couple a 1710 rpm motor with a 1750rpm motor, the 1750rpm motor will reach its rated torque output at 1750rpm while the 1710rpm motor is producing somewhere less than half of its output. Clearly, this is not very good load sharing. Further complicating this, you can't believe the nameplate data when precision is required. There are some variations between motors as they are produced and the nameplates are made for the approximate average of all of the production. Good enough for most applications but not good enough for load sharing.

Third, single phase motors behave way out-of-character during their starting process. Asking them to load share during this starting phase is a real reach of faith. If the machine could be brought up to speed by the first motor and then the second motor added online before the load was applied to the machine, the odds of success would be improved. Even better would be decoupling the second motor from the machine until the second motor was fully accelerated and then coupling it back online.

Fourth, I'm not nearly as familiar with single phase as with three phase but, I believe there are different types of single phase starting. Isn't there part winding, capacitor, and maybe some others? I doubt that mixing these would be conducive to good load sharing during the startup process. Maybe Rick's motors are of different starting technologies.

Fifth, someone above mentioned inverters on single phase motors. Sorry, but that doesn't work, period. There are single phase input, three phase output inverters but the motor would always be three phase if downstream from an inverter. (There are minor obscure exceptions to this but nothing that would be useful in Rick's case)

Rick, would you also tell us what the machine is that the motors are driving and whether the machine can be brought up to speed without loading first.

Thanks.

 

[Rick Densing]

They are runnining a bucket conveyor up on an angle. The first motor will be in overload until the second kicks in. I don't think that it can be started w/o load all the time. No precision, just hauling grain up an elevator. It moved about a half a million bushels before crapping out.

I had figured that the two motors are supposed to work like Dick said, I just wanted some confirmation.


I was thinking about single phase. Instantaneous power is on constant like 3 phase. I wonder if this has anything to do with it.

I guess I will just have to wait until Tuesday to find out what they did.

 

[DickDV]

Rick, I've been thinking some more about how this kind of situation could be handled better.

The best that comes to mind is to use a single 15hp single phase motor and connect it to the conveyor thru a variable pitch pulley system. Starting the motor/conveyor at minimum speed would also be starting the motor at minimum load and minimum inrush current. After the system is started, it can be run up to full speed which would place full load on the motor. The difference is that the motor is allowed to start with light load and then, after getting the motor up to speed, load it up. Single phase motors just hate to start under full load.

The only problem I see with the v.p. pulley system is that the system would need to be run down to minimum speed when stopping or you won't be able to restart at minimum speed. It's one of the bad aspects of any v.p. pulley system.

Otherwise, that's my recommendation. That the earlier motors worked ok in tandem is probably more luck than design elegance!

 

[LadderLogic]

If they are fighting each other, why?

Because only synchronous motors can be precisely synchronized. Asynchronous ones will always be slightly different and will produce slightly different torque at the same speed (this is assuming the mechanical linkage between the two is absolutely rigid - which, in this case, is not).

Number of phases is an additional factor here. When several thre-phasers are connected to the same power source (and all the phases match), the rotating magnetic field inside them will be exactly the same at any given instance. The single phasers apparently use capacitors to generate the field, so the variance in the capacitance and the motor winding impedance will produce fields out of sync. Not much, but could be a problem...

Having two async motors to work in parallel is not a very good idea to begin with, but in this situation, one would have to try matching - maybe the local motor vendor could help.

 

[Tom Jenkins]

I can't give the theoretical basis for the behavior, but I was told years ago by guys that learned the hard way that load sharing is not a good idea. I'm sure the variations in torque/speed curves are the key, but I don't know the details.

A non-electrical answer would be to put a fluid coupling on the motors. FMC Stearns used to make such a device that was designed for direct mounting of pulleys, and I suspect a search of Thomas Register's site will display many more. The clutch will allow for transferring torque while permitting differences in speed.

 

[DickDV]

Actually, load sharing with induction motors wired ACROSS THE LINE is troublesome. If you have some way to measure motor torque and control it, as in the use of an inverter for each motor, then load sharing is not only possible but very handy is some difficult applications.

In fact, loosely viewed, whenever you have draw control or web tension control, you have a specialized example of load sharing.

Even with all the clever modern technology, I am still in awe of the old line shaft paper machine drives with the tapered sheaves for draw control. Not only did it work quite well but, in some cases, it was very hard to replace with electronic controls.

I also tend to get awestruck at antique engine and tractor shows but that's another story and off topic as well!

 

[randylud]

Line shaft drive, ahaaaaaaa!

I to remember with some fondness, the days when line shaft drive were the order of the day for large multi-speed sectioned machines. My first project as a new engineer was to install a stretch line (staple fiber making machine) in a plant in Virginia. The machine was German made by NeuMag. I was awestruck as well with the massive size of the component pieces of the machine. The line shaft was connected about midway the machine to a 300KW DC motor with roughly a 4-5" dia line shaft. Off of the line shaft were electrically controlled PIV's to vary the speed of each section of the machine. I will give the German companies one thing, they sure do not scrimp on the amount of steel they use in machinery. This bad boy was some kind of beefy. But when the mechanical erector from Germany and I tested the line E-stop from full speed, I was really impressed. From 1200 FT/Min to zero in about 9 seconds. That is really quick considering the massive amount of rotating inertia. The entire braking system was DB on that hunk of a 300KW motor. The DB resistors would glow red for a few seconds near the end of the motor's rotation. WOW, for a young engineer, that was impressive. The paper mills I worked in used steam turbines in similar line drive operations before the advent of electronic drive controls.

 

[Terry Woods]

Why would they fight each other?

Rick asks... "Why would they fight each other?".

The name of the game is "relative motion" (and relative position!).

The magnetic field flying around the stator moves at a particular constant speed - based on number of poles and frequency.

Fewer poles = faster rpm for a given freq.

More poles = slower rpm for the same freq.

The rpm of the stator fields in both motors are the same. (Although, not necessarily physically in sync!)

When the first motor is started, the rotor is resting and loaded. When the power is applied the stator field is up to speed immediately. Meanwhile, the rotor sits there for a fraction of a second... and says... huh?

While the rotor sits there, a great number of flux lines from the stator field are "cutting" through the conductor bars in the rotor. A huge EMF is developed in the rotor. That develops the field in the rotor. The field in the rotor and stator are "attractive". They try to get in sync with each other.

As the rotor groans its' way up to speed, the rate of "cuttings" reduces. The EMF in the rotor reduces. The energy demand on the stator (like the primary of a transformer) is reduced. The motor settles in at normal speed. There will indeed be slip - slip is required to maintain the field in the rotor. If the rotor came exactly in sync with the stator there would be no relative motion and thus no "cuttings" at all and there would be no induced EMF in the rotor - the rotor would tend to stop. At that point, there would again be relative motion between the stator-field and rotor-field - thus producing EMF in the rotor. The rotor eventually "finds" that speed which keeps it going - the slip speed. If the load is heavy then it takes more current to maintain that speed. If it's a light load, it takes less current to maintain that speed.

BTW, the slip speed is dependent upon physical characteristics as well as electrical. For example, the size of the air-gap between the rotor and stator is a major factor in the "transformer coupling" between the stator and rotor. Smaller the gaps produce greater the electrical couplings. The "lamination" method in the stator also contributes.
It's the "little things" that kill ya!

So... the first motor is running.

Since both motors are connected to the same shaft by pulleys, the rotor in the second motor is already somewhere near up-to-speed.

Start the second motor.

The field in the second stator comes up to speed immediately. Since the rotor is already near speed, there won't be the normal in-rush current.

Now...

If the slip-speed of the second motor is slightly different from the slip speed on the first motor, then one of those motors is going to try to pull the other motor faster than its' slip-speed.

Initially, one of the motors is going to have a change in "relative motion" between the stator-field and rotor-field. That motor will try to restore the relative motion to what it was.

Eventually, both motors will be moving at a speed other than its' slip-speed. One will be faster, one will be slower.

The faster one ends up acting like a generator. The slower one ends up acting like an increased load. In both cases, the current level increases. You end up with an effect called "circulating current". This is very common in parallel generators. If one generator is operating at a slightly different frequency than the other, the faster one is "motoring" the other. The current circulates between the generators.

I said that the speeds of the fields rotating in the stator are the same... this is true... however... the physical position of the rotor field in one motor, relative to its stator field, might be as bad as 180-degrees out of sync with the other. This is determined by the physical coupling between the two motors.

If you are using simple belts they should probably tend to slip into sync. During the course of slipping, there will still be a considerable amount of excess current. That really depends on how tightly the belts are tightened.

If, on the other hand, you are using belts and pulleys with teeth... they will not tend to slip into sync. You will end up with a great deal of excess current.

You might suggest to your friend that he try to acquire another motor from the same batch - matched motors as it were. If he is using a toothed belt, then get the key-ways aligned between the two motors. If that doesn't work then use an ordinary set of belts - not toothed belts. And maybe not so tight.

 

[rsdoran]

What Terry said

I know it was said that the slip "shouldnt" be a factor but from ALOT of experience with mechanically coupled motors etc...I KNOW for a fact it can create a multitude of chaos.

I have worked with systems that were 20 years old (or older) and then something failed and had to be replaced...the system never worked the same again.

I have seen many devices used to assist in eliminating these issues. I have seen fluid clutches, transmissions and of course in recent years inverters used to assist in the starting load factors.

Technically the best way is to use a properly sized motor. If the dual motor system is the only possilbe way then maybe look at 2 things...determine a way to make sure the system starts under minimum load OR look at the possibility of creating a lower load start...transmission..fluid clutch etc.