The Importance of Core Testing AC Motors.
As regulations on carbon emissions come into force, and more attention is focused on concerns about global warming and energy costs, more attention is being placed by industry on the efficiencies of AC Induction motors.
Although their basic design hasn’t changed for over a hundred years (as after all you can’t change the laws of physics), improvements in construction methods and materials has made them smaller and, more importantly today in a “green” conscious world, more energy efficient.
It has been said within industry that “You cannot rewind high efficiency motors”. This is a myth that is simply not true. You certainly can, but only if you have the right equipment, procedures and fully trained engineers working to the best industry practices. Here at Fletcher Moorland we do. We measure the core loss before any rewind work is carried and afterwards to compare and check for any deterioration. We the put this data in our comprehensive repair and failure analysis report. You can then be assured that your motor is as efficient as the day it was made.
High efficiency is gained by reducing the inherent losses. In an AC motor there are many sources of losses. However losses from the core are a major source of energy loss, about 25% of total losses in the motor. Significantly core losses still exist at low loads or with no load because they occur due to the alternating magnetic field in the core which is always present.
So why do we get losses in the core ?
Well to explain we need to describe what happens in an AC motor. The core is made from individual thin discs of high grade steel. Steel is used because of its excellent magnetic properties (its high permeability). These discs are pressed to form a hole in the centre that has a regular arrangement of teeth around its inner circumference. Each disc then has an insulated layer applied to both sides. These discs are called the “laminations”. Many layers of these shaped discs are stacked one on top of the other to form a long laminated hollow tube (the rotor runs inside the hollow centre section). This laminated stack is known as the Motor Core
.
Diagram of the core of an AC motor
There are coils of copper wire wound around the iron teeth, lying in the slots between them. An AC current is passed through the copper wire coils which results in an alternating magnetic field in the core concentrated at that tooth, forming a magnetic pole there. By arranging the coils in groups, and having three sets of these groups we can feed them with a 3 phase AC current and generate an alternating magnetic field that rotates around the magnetic poles formed inside the core. A moving rotor made of metal bars and lamination inside the core then follows this field around by magnetic induction (hence it being called an “induction motor”) and it rotates, making the motor shaft pressed into it rotate too.
This alternating field means that the core has to repeatedly be magnetised, demagnetised, then magnetised again in the reverse direction rapidly and continuously. This cycle is repeated typically 50 or 60 times a second (can be more or less than this if an inverter is used). Sadly this process takes energy which is lost as heat, the source of the core losses. There are many causes of core losses but there are two primary components ; Hysteresis Losses and Eddy Current Losses.
Hysteresis Losses. Some hysteresis losses are inevitable because of the way steel cores work. By passing a current in a coil around a steel core, the steel core concentrates the field from the coil and becomes an electro-magnet. Remove that current and the magnetism doesn’t completely go away, the steel remains partly magnetised. Electric motors run on AC current. To return the magnetic field to zero (demagnetise or de-gauss) then reverse the polarity of the fields direction requires more energy to overcome that stored magnetism. This effect is known as hysteresis. This extra energy needed is the source of the hysteresis losses. The degree of loss is a function of the peak flux density and the core material, and is also proportional to frequency. We can’t do much about the frequency as on an AC induction motor it determines our rotational speed. But we can choose the best materials, minimise the air gap between the rotor and inside of the core, and optimise the number of turns on the coils to get the optimum flux density to minimise the losses.
Eddy Current Losses. These are the major cause of all core losses. When we apply an alternating magnetic field to an iron or steel core, it induces an electric current inside it. This isn’t a bad thing in the rotor bars, because it’s needed to make the motor work! But it is a bad thing in the core. Because iron isn’t a perfect electrical conductor heat is generated by this current flow and power is lost. The core is made from thin laminations to reduce this. As each lamination has an insulated surface a smaller current circulates within in each one, not a large one through the whole core. So, by making the laminations thin with good insulation between them the eddy currents can be reduced hence the losses can be reduced. As Eddy Current losses are a function of the square of the supply frequency (and inverse square of the lamination thickness), effective laminations are even more important if the motor is run from an inverter above a frequency of 50-60 Hz, or is a high frequency motor such as a spindle motor or saw motor.
So what can go wrong with a core ?
Problems can occur with a motor core when the motor fails. One common failure is an electrical breakdown of the copper wires that make the electromagnet coils. When they fail a seriously high current flow can cause the copper to become molten and flow in between the laminations of the core causing short-circuits between them. The laminations thickness is effectively doubled at that point. As the eddy current loss is a function of the square of supply frequency and inverse square of the lamination thickness, a doubling of the thickness increases the loss by four times. If four laminations are shorted the loss goes up by 16 times. At the point of any short heat will be generated and energy lost. If we rewind the motor and don’t notice the lamination faults, then the motor losses will increase and a hotspot will occur at the site of the fault which could cause a premature winding failure.
Short circuits between laminations can also easily occur if a motor is badly repaired. Great care has to be taken not to overheat or distort the laminations. This would change the structure of the steel, destroy the insulation between them or increase the separation between them. This is especially critical on high efficiency motors were high grade steel and thin laminations are used and on motors operating on a high AC frequency.
So how do we prevent and detect core faults ?
Simply, choose a good motor repairer like Fletcher Moorland. A good motor repairer will employ the best industry practices, developed and recommended by worldwide experts to ensure there is no core damage during repair. A good repairer will use a “Controlled Pyrolisis” oven in the winding removal process to prevent overheating to the core. He will have a calibrated core tester and an Infra red camera to measure the core loss accurately and see hotspots where faults may have occurred between laminations. He will test the core for loss both before and after it has been repaired, especially after it has been rewound to make sure it is still as efficient as it was when it left the manufacturer.
How do we test cores correctly?
Basically we excite the core with a strong alternating magnetic field, one that is close to the operating flux density the motor will have in service. This can be done using a low current in many turns of wire around the core, or more practically a high current and a single turn through the core. Mathematically, by knowing the core dimensions we can predict the current that should flow in the coil. Higher core losses will produce a higher current than predicted by the calculations.
What is better is to use a properly manufactured and calibrated core tester like the one being used in the photograph.
A Core Test Being Carried Out With the Correct Test Equipment.
These testers are a development from IEEE standard measurement methods, and test methods recommended by industry bodies such as EASA to ensure that they give reliable results. Fletcher Moorland has this equipment. These testers use a transformer coupling principle to calculate the losses. We then calculate the weight of the iron so that the loss is expressed in Watts per Kilograms (or Watts per Pound in the USA). This way we can get a figure that gives a value representing the true loss irrespective of different motor sizes and designs. This can then be used for comparison and will give a good, reliable guide as to the quality of the core on any motor.
We also use an Infra Red digital camera (see the photo below) to look for hotspots and mark their position so the core can be repaired. Another important reason to check for hot spots is that they could overheat the coil or slot insulation at that point, and consequentially the windings will break down causing the motor to fail prematurely.
The photograph above shows a bad core, the hotspot where the laminations are shorting out is clearly visible as the bright orange-white area on the upper left of the picture.
The core is tested both before and after the rewinding process. The loss data in Watts per Kilogramme and a diagram showing the location of any hot spots is generated and stored on our computerised job tracking system. This can then be exported to the repair report you receive, proving that your motor core will still perform to its design standard after repair.
As more focus is put on you to ensure that you using energy wisely, you need to use a motor repairer that takes energy efficiency equally seriously and can prove that it does. Fletcher Moorland is that company.
