Cyndi Nyberg
EASA Technical Support Specialist 

When a motor is test run without a load after it has been rewound, one of the questions we are asked is why the amps are too high, even higher than the nameplate full load. Here are a few of the common factors to consider. 

First, if there is a problem with the data, it is im­portant to realize that not every service center has access to EASA’s extensive database and technical knowledge. Of course, some motor failures are so catastrophic that it is impossible to determine the connection or even the turns per coil for a particu­lar winding. Without access to the EASA database, if factory data cannot be obtained, the service cen­ter is left to make assumptions. If this is the case, you may have inherited someone else’s problem if they misinterpreted the data. 

Effects of stator core damage 
The stator core has possibly been damaged, either when it failed (e.g. ground failure), or if it was inadequately repaired. These previous failures may have caused damage to the core, and you may be inheriting the prob­lem. If the core has significant damage, either to the laminations or to the interlaminar insulation, the core losses will increase. If the rotor has rubbed on the stator bore, and the laminations are smeared, the shorts will increase the eddy current losses in the core, which will in­crease the current and heating.

If the burnout process was not properly con­trolled (refer to EASA Tech Note 16), the lamination coating may have been damaged. If a core loss test is not done prior to and after burnout, the damage may not be detected until the motor is test run. By then it is too late. If this is the case, the motor will not only draw high current without a load, but it may also heat up very quickly. 

Winding data may be wrong 
Another reason that a motor might draw higher than expected no-load current is if the winding data is wrong. There are two different situations when the data is wrong. First, the data may have been changed from the original factory data during this or previ­ous rewinds, causing the core iron to become saturated. When the flux densities of the design drive the core to the point of saturation, the motor will draw high current even without a load. If the original turns per coil are reduced, the flux densities will increase. Too many “dropped” turns will drive the core into saturation. 

For larger motors that have only a few turns to begin with, even one dropped turn can saturate the core. This is especially true if the original de­sign had somewhat higher flux densities at the beginning. The design values will vary depend­ing on the manufacturer, vintage, frame size, and energy efficiency, among other things. The problem is not only limited to motors with very few turns. 

Small motors that have a large number of turns are often designed with high flux densi­ties, close to the saturation level, so even a drop of one or just a few turns can drive the core into saturation. Another factor that can drive up the flux densities, and possibly satura­tion of the core, is if the span is reduced. Similar to a drop in turns, if the coil span is reduced, the densities will also be higher. The correct chord factor and its effect on the per­formance of the motor is an entirely different issue. However, a drop in the coil span can in­crease the flux densities, and can saturate a core if the densities are high already. 

If you are not sure that the data you took out of a motor, or the data you want to put back into it, is the original, be sure to either confirm it with the original motor manufacturer, check it with the EASA Motor Rewind Data on CD-ROM, or at least calculate the flux densities. 
 
Possibility of wrong connection 
The second situation where the data might be wrong on a rewound motor, causing the amps to be much higher than expected is if the turns and coil span are correct, but the wrong connection is used. If the original connection should have been a wye, and now it is delta, the flux densities will be 1.73 times the original, most likely saturating the core. On the other hand, if the original connection was one circuit, and was connected for two circuits, the densities will be double what they should be. Again, this will most likely saturate the core. 

Increase in horsepower rating 
Customers often ask members to increase the horsepower rating of their motors. Most of the time, this can be done with no problem (an in­crease of one hp rating is normally okay). Many motors are designed with low flux densities, and therefore can be strengthened. However, if the re­design is not done properly, the winding could be too strong and saturate the core. In this case, the no-load amps will likely be quite high. 

On the other hand, if a motor is redesigned, and there is enough iron to make it stronger, of course the no-load amps will be higher because of the de­sign change. However, if this redesign was done previously, and the nameplate was not changed to reflect the increase in horsepower, the data might be perfectly fine and not saturate the core, but the amps will still be considered too high. 

While in this case, the amps may not be ex­tremely high (above the original rated full load current), they will still probably seem too high. No-load amps for a particular design can vary based on 
the design, number of poles, etc. but they will usually follow certain rules. Again, the only way to know for sure that the data has been changed, if your customer does not tell you or does not know, is to check with the manufacturer or the EASA database. 

Check actual applied voltage 
One more item to check for if a newly rewound motor is drawing ex­cessive current without a load is the actual applied voltage. Unfortunately, the power company cannot supply ex­actly 460v to everyone, all the time, every day. It is not uncommon to see voltages much higher than the “nomi­nal” voltage level. Some motors will perform without a problem at much higher than nameplate voltage—it may even make them perform better. 

However, if a winding is designed with rela­tively high flux densities, then it may not have as much “room” for over-voltage as some other de­signs. Stated simply, more voltage equals more flux, and the relationship is linear. The high sup­ply voltage can push the core into saturation, if the design is close to maximum flux densities. This would be especially true if any changes have been made to the winding in the past, such as dropped turns. 

One possibility that is often overlooked is that high no-load current could be caused by a mechanical problem. If the stator and rotor stack are misaligned axially, the current can be high. 
When a motor runs with high amps without a load, there could be a number of reasons why. It is important to pin­point the reason because it could be generating a lot of heat. Otherwise, the motor may be back for rewind within a short period of time. 

Categories: Testing, Motor testing

Tags: Motor testing No-load current No-load test

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