Part Winding operation of 3ph motors

[sacem1]

We have several machines which have 20+ HP motors used in machinery which is suposed to need all that power only on very special ocasions, most of the time the input power requiered will be less than half the motor power.

As a result from this we have to start and keep operating high HP motors with low power usage. We have added a second motor usually a 7.5HP conected to the back shaft of the 20HP and we start only the 7.5 and use the 20HP only as a intermediate shaft, when power is needed we simply start the 20HP and let the small 7.5 turn without any power conected, this works but sometimes its not easy to fit a second motor to the back of the main motor due to space limitations.

We start the 20HP motor using only one of the two windings that it has for each current phase and then switch to full operation after the motor has started, but could we just leave the motor running with the starting winding on and the second one with out power indefinitly.

The 20HP with only one winding connected is supposed to develop 45% of its torque and give 9HP at full speed wich should be more than enough to operate the machine and we could really have two motors in one (20HP/9HP) with full windings and partial windings conected.

Has anyone used this or has any experience with it, please share your experience with us.

Thanks

SACEM1

 

[waross]

That is an interesting idea.
There are two sets of losses in a motor. I^2R losses and mechanical losses. (for simplicity I include eddy current and hysterisis losses with I^2R loses). The mechanical losses are mostly windage.
It doesn't matter whether you use one winding, two windings or another motor, you have to pay for the mechanical losses.
Going from two windings to one will increase the current in the single winding. The amount of increase will depend on the power factor. There may or may not be a saving.
Likewise you may or may not have an actual saving with the small motors. You have the mechanical losses from two motors instead of one, and the I^2R losses in the small motor running loaded may be almost as great as the I^2R losses in the large motor running without load.
Using current measurements to estimate costs and loading is misleading. If you add capacitors to the 20 hp motor and improve the power factor to 100% you will be able to get an accurate estimate of power from the current measurement and may find that it is not worthwhile to even use the small motors. The current consumed by a three phase induction motor circuit at no load and 100% power factor is quite low.
respectfully

 

[sacem1]

We have found that even thou the theory says the same as you have stated, in the real world, starting the full load of the 20HP puts a strain in our power lines, and starting in partial winding gives them a more gentle load to cope with, what I was wondering is if letting the motor just work with a single winding would be harmful in the long run.

The Inertia of both motors is negligible if you take into account the big masses that we are making then move, slowly through gearing but big enough (usually 5ton+) so the additional loss of a 20 HP spinning freely is really not much of a burden.

In some case we have replaced the big motor with a smaller one and just leave the big one as a sideline backup so when a bigger power demand is requiered the change can be done in about an hour.

Logic tells me that if I work with only half the windings at their full Volt design I should not have any trouble if I do not try to get the motor to absorb more than the 45% torque I should be getting.

By the way I am planning on installing two separate overload protection heaters, one set for each starter line.

Regards

SACEM1

 

[sreid]

I don't think that running on one winding on the 20 HP motor is going to save you any current at all. The current requirement is primarilly the magnetizing current plus the load current. Running on one winding simply makes the current flow through wire with twice the resistance.

 

[edison123]

The part winding motors are used to reduce starting in-rush current. Once the motor reaches the speed, the motor current is dictated by the load, not by the windings. So leaving out one circuit will only increase the winding temp and higher copper loss and lower life as compared to both the windings sharing the current (whatever that may be).


* Basically, I would like a full-time job on part-time basis *

 

[itsmoked]

I would say amp out a motor in both modes and see. Also temp-gun the motor and look at its temperature during the "experiment".

Keep track of everything and tell us what happens!
Make a table and every few minutes document the results.

Abort the test if the case temperature exceeds reasonable or if the current readings steady-state above allowed name plate.

If the whole thing seems to work okay run it until thermal equilibrium is reached. (t>2hrs?)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi Keith;
That is a good idea for checking the one winding/two winding comparison.
As far as comparing the small motor with the big motor I have some doubts about checking amperage. The magnetizing current typically skews the results at light loading and makes the result meaningless. At no load the magnetizing component of the current will be larger than the load component of the current and as it is acting at right angles the magnetizing current effectively masks the load current changes. That is why I suggested capacitors to correct the power factor to 100%. Only then will the current reading on the lightly loaded motor be significant.
Respectfully

 

[jraef]

As others have said, it isn't going to make a significant difference if any at all to run it with only part of the windings. Current is determined by load and fixed losses in the motor. Using only part of the windings will not change the fixed losses enough to risk damaging the motor.

As much as I typically rant about Nola Energy Savers being pretty much a rip-off, I have to say that this sounds like EXACTLY the kind of rare application where it WOULD work. What they are good at is precisely the issue at hand here, reducing the fixed losses in the motor when running unloaded by reducing the voltage. If you can live with a ventilated controller, you should really look into them. Several brands of soft starters have this function built-in, which would also keep you out of trouble as far as starting voltage drop. My advice would be to go that route and stick to established brand names that will be around a while. Keep in mind though that any soft starter that uses a bypass contactor will defeat the energy saver function, so you must use one that does NOT rely on a bypass contactor to stay cool (hence the comment about ventilation).

There are however several "new kids on the block" selling them now to watch out for. They are making wild claims and setting up multi-level marketing schemes. They won't be around in a few years when they run out of ignorant distributors, so steer clear of them.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Marke]

Hello sacem1

The part winding system relies on reduced flux in the iron when only one winding is powered up. The reduced flux will reduce the start current and the start torque.
The reduced flux will also reduce the iron loss in the stator.

For a given shaft load, the reduced flux will cause the motor to operate at a higher slip. This will increase the slip losses in the rotor.

I would suggest that operating on one winding only should not be a problem provided that a) the current through that winding does not exceed the current through that winding when both windings are in circuit and the motor operates at full speed, and b) the slip does not exceed the normal full load slip of the motor.

Operation at light load on one winding only will increase the copper losses, and reduce the iron losses of the motor. There is a load level where these two losses are equal and the reduction will be zero. Below this load there will be a positive benefit, above this load, there can actually be an increase in losses.

An observation that I would make however, is that just because the motor is much larger than it needs to be, does not mean that it is costing more to run.
I have just done th comparison between two motors of the same type, one 7.5HP and the other 20HP. From the figures, the 7.5HP full load efficiency is 89.5% and the half l,oad efficiency of the 20HP is - 89.5%, in other words, the losses are not that different in the 7.5 - 10 HP range. So I have to ask the question, is there really any issue with operating the 20HP motor, using the part winding starter to start at reduced current and operating at low shaft load but both windings in circuit as normal. I expect that you will get very little difference in performance between this and the 7.5HP option. If the shaft load is low enough, you may get a benefit from reverting to single winding operation, but you would need to verify that you were infact below the threshold rather than above it.

Partly loaded installations often have higher losses at reduced voltage than at rated voltage.
The commonly heard claim that oversized motors waste energy is generally just not true for motors that are up to twice the required size. You need to do the sums.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

I do not recommend constant operation with half of the windings energized. Most of the motors will burn in short time. One of the reasons is that the motor will not develop full speed, then the current does not drop to normal levels. The typical noise developed when the first part of the winding is energized gives an idea of the stress imposed.

 

[jraef]

Actually, it will develop full speed if allowed to by the load, which he has already indicated is much smaller than what the 20HP motor is capable of. Risky yes, but not impossible. Speed is a simple relation betweeen frequency of the supply and the number of poles in the motor. Not developing full TORQUE is what is at risk here, and that may effect the final slip speed if the load is greater than what the torque can turn it at. if the load is extremely light however, it may end up at the same speed.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[aolalde]

Jraef.
Have you an idea of the low range harmonic content developed by that partial winding exitation? Then what is the resultant speed- torque curve? Do you have an spare motor to experiment? but be careful, you will probably loose it.

 

[sacem1]

Starting my replys from the last, it will not be the first motor I loose becuse of experiments and I am sure it will not be the last if that ocurrs.

However all of you have given me a lot of help and ideas to take into account so the actions I am going to take are going to follow this sequence:

I am trying this sistem in a 25 HP doble Wye wound motor which has a 37.5 Amp load at 440 and 75 Amp full load at 220V 60 Hz, we use 220V so the partial winding starting system will be installed in it.

I am using two motor starters instead of a motor starter and an auxiliary contactor for the starting set up, that way each set of windings is going to have a contactor for conecting the electrical supply AND a thermal overload relay.

That way if power demand from the lathe is higher than the permissible load of each partial winding then the relay will trip (both are series conected so if any one "cuts" then both contactors will cut the electrical supply.

A simple switc with 1-0-2 positions will select if the starting sequence stops in partial winding (45% torque) or goes all the way in the usual fashion to both windings connected.

In the circuit I need a JOG position in order to be able to turn partially/momentarilly the table (it is a verticla lathe the one we are dealing with) so the memory circuit for the first contactor will have to go thru an auxiliary contactor in order to be operated by the "START" button and the "JOG" button will connect power to the first contactor only when pressed, when the lathe operator releases the butto the energy will be cut to the first winding.

Also the auxiliary will power the timer for the second when the selector is on full power. Obviously the second contactor will have its memory on its NO aux contacts in the usual way.

Two amp meters are going in, one for each winding in order to check power absorved by partial/full windings connection.

Hopefully if this works I will save some money in electricity and in the alternative way of a second smaller motor fitted to the machines.

I will keep you posted on the results, we will have a few days delay because of a small oiling line problem in the lathe (we are just setting it up for work) and we have to access it from the underside of the machine.

Regards

SACEM1

 

[waross]

It sounds like you have all the bases covered.
respectfully

 

[jraef]

Please post your results, good or bad.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[sacem1]

Sure I will as soon as the oil problem is solved

Regards

SACEM1

 

[innov]

Just a thought... why not install two 10 HP motors in parallel (side by side). Connect the load to the first motor, and the second motor to the shaft of the first, with some kind of clutch (electric?) between them

 

[sacem1]

Usually the big (25 HP) motor comes allready installed in the machine in order to be able to cope with the maximum power requierements that the (lathe in this case ) will absorb, and as I stated earlier we have in some cases conected a smaller (10 HP) motor to the back side shaft of the big motor and start the motor that is requiered. Other times we have just installed a second motor by its side of the original one and change the driving belts from one to the other, usually when due to space restrictions the back conection is not possible.

However I do not know how well will TWO half power motors work when the high power is demanded by the lathe, if they don't synchronize themselves exactly, one would be dragging the other and robbing part of the power we need for the work being done, maybe someone else can elaborate on this.

Regards

SACEM1

 

[waross]

Induction motors should share the load as long as the drive ratio is exactly the same. A worn drive belt on one motor that runs deeper in the vee-groove than the belt on the other motor may be enough to cause one motor to "Hog" the load. I think your "Small motor-Big motor" solution is probably the way to go, particularly as the equipment is already supplied with the large motors.
I will mention again that if you add capacitors to a motor until the no-load current is at a minimum you will be surprised at how litle current the 25 hp. motors draw at no load.
respectfully

 

[sacem1]

I will start tests tomorrow and the next step if this does not work will be to follow your advice on adding capacitors.

Thanks

SACEM1