Question & Answer For WATTNOW Magazine February 2015


I have heard that high efficiency motors have a very high inrush current. Is this true, and if so what can be done about it?


To answer this question it is important to first understand the difference between locked rotor current (LRC), also known as start-up current, and sub-transient inrush current. The LRC is a constant value and its magnitude is determined by the design parameters of the motor, typically between 6 and 8 times the rated current. For example, when applying full voltage to the stator (DOL starting) the value of the LRC will be limited to the value stipulated on the given data sheet and will not exceed 20% of that value as per IEC 60034. Contrary to LRC, subtransient inrush current is a variable value and can be calculated mathematically by analysing the motor’s voltage and flux immediately after energising it. The maximum flux amplitude reached on the first half cycle of the applied voltage is dependent on the phase of the voltage at the time it is applied. The maximum flux can be twice as high as the steady state flux, which results in a large increase in magnetising current. Therefore the sub-transient current could peak at 3 times the LRC. This is true of all squirrel cage motors, regardless of efficiency class. As this lasts only for a few cycles (milliseconds), it is normally not detected by conventional meters. However, it could be detected by the instantaneous elements of protection devices (magnetics), which may result in nuisance trips during starting.

High efficiency electric motors generally have a lower X/R ratio than standard efficiency motors. Therefore the LRC is generally (not always) higher which, due to the phenomenon of all induction machines, could lead to higher sub-transient inrush currents. The percentage difference between standard efficiency and high efficiency motors is not constant but generally slight.

After many years supplying high efficiency motors to industry, the cases reported of nuisance tripping is so small that it is virtually unheard off. Although the inrush current is higher, it does not mean that it will affect all installations. In severe cases where the inrush current could cause concern, the issue is easily rectified by the correct selection and adjustment of protection devices used. The benefit of higher efficiency far outweighs the potential nuisance tripping that could occur due to incorrect selection and adjustment of protection devices or circuits.


Can motors retain their efficiency after being rewound and what can I do to ensure this?


Yes, motor efficiency can be retained after being rewound. To answer this question it is important to firstly understand where motor losses come from and what could increase the losses. Motor losses comprise the following: Joule losses, iron losses, mechanical losses and additional or stray load losses.

Motor efficiency will only be reduced or affected negatively if any of the associated motor losses were increased. Motor losses are not just increased, however. They could be affected during a motor repair or a rewind, as per the following example:

The Joule losses (also known as I²R losses) could be increased if the original winding specification was compromised, if different grades of copper or aluminium were used for the winding or when the quantity of copper was reduced by using a smaller diameter wire. Therefore if the resistivity or current flowing through the conductors was increased, then the losses will also be increased.

The iron losses are due to the magnetic field in the rotor and stator laminations and are dependent on the magnetic induction, frequency and the quality of the material used. If the laminations are being replaced, one should ensure that the composition and also the thickness of the steel were matched exactly to avoid additional losses. During the burn off process, one should take special care to follow the correct burn off procedure to ensure that the integrity of the steel laminations is not compromised.

The mechanical losses are due to the power requirement of the cooling fan coupled on the motor shaft and also due to bearing friction. These losses are directly affected by the motor speed. During repairs one should take special care to replace the fan or bearings with original equipment from the manufacturer to ensure that the losses are not affected. In addition, operating conditions such as ambient temperature, over greasing or misalignment also have an impact on mechanical losses.

Additional or stray load losses are caused by currents and magnetic flux resulting from high frequency components. Proprietary design techniques are used to reduce stray load losses. These developments are mostly unique to each manufacturer’s manufacturing process.

Reputable rewinding facilities are able to guarantee original design performance by incorporating OEM material and processes in the repair. The only way to ensure this is to use repair facilities that are pre-approved and accredited by the original equipment manufacturer.


When is it better to rather replace rather than repair an old motor?


It is important to realise that motors need maintenance and regular inspections. This hardly happens and is always questioned once the motor has failed. Once the motor has failed the next question is: “Should we repair or replace?”

So what’s the right answer? It turns out that the decision to repair, rewind or replace a failed motor is not always so simple and straightforward as you may have heard.

Yes, a lot of times the repair cost is a lot less than that of a new motor, but the impact of the running cost sometimes outweighs the additional cost of replacing the motor.

It is a known fact that the motor consumes the energy equivalent of the capital outlay within weeks. The motor cost in relation to its running cost over the full lifespan is so small that it should make no sense to repair a motor in the first place. However, the decision to never repair a motor is not viable. This is when a replacement policy should be established with the following highlights.
• Does the motor suit the application and is it correctly sized?
• Is it a critical application where reliability plays a big role?
• Is it a catastrophic failure with huge mechanical damage? For example, a damaged stator core.
• Is there evidence of a prior repair and do you have records of the repair?
• Does the motor run for long periods during the year?
• What is the failed motor’s efficiency rating? Now is a good time for a motor with increased efficiency.

Upon failure, it is worthwhile to replace all older, lower efficiency motors with a high efficiency option as the energy savings alone justify this. The cut-off point for replacement rather than repair is a hugely debated topic, but the general consensus of large users of electric motors seems to be to replace any motor that fails < 45 kW or when the ratio between repair and replacement is higher than 60%.

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