Thursday, December 8, 2022

Electric Motor Speed ​​as a Diagnostic Tool

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Engineer Oscar Núñez Mata from Motortico shares an interesting article on the “Electric Motor Speed ​​as a Diagnostic Tool” as we can see below:

Characteristic curve of Torque vs Induction motor speed

The three-phase induction electric motor is defined by its characteristic curve Torque (Torque) vs Speed. This reflects the torque capacity as a function of speed. The figure above shows a typical case of a general purpose motor.

Note that at startup (Zero Speed) the motor develops a starting torque, which is used to start moving the load. Gradually the engine accelerates and stabilizes at one point of operation, enough to keep the load moving. Once started, the load will cause changes in speed. Typically the speed levels are as follows:

  • Nominal Speed: At Nominal Torque, and is the one that appears on the Rating Plate.
  • No-load speed: Without load, it does not appear on the nameplate, but it does appear on the technical sheets.
  • Temporary Minimum Speed: Normally it occurs at 150% of the Nominal Torque, it is transitory, and if it remains for a long time it can cause permanent damage to the motor.

The normal thing is that the speed oscillates between the empty speed and the nominal one, due to changes in the load. For example, The table below presents a real case of a motor with a nominal speed of 1750RPM, depending on the mechanical load, corresponding to 4 poles in a 60Hz system.

[table caption=»Características de Carga» width=»600″ colwidth=»200|50|50|50|50|50|50″ colalign=»left|center|center|center|center|center|center|»]
% Loading, 25,50,75,100,125,150
Speed ​​(RPM), 1789,1776,1762,1750,1733,1714
Current (A), 7.1,8.62,10.9,13.3,15.9,19.3
[/table]

This engine rotates at 1750RPM at full load, and at idle the engine speed would be greater than 1789RPM. Note that speed decreases with increasing load. The motor current has an inverse behavior: It increases with the load. It is clarified that these data are at plate voltage.

Using speed as a diagnostic tool

The induction motor changes its speed with the load, this variable could be used to determine if it is overloaded or not. On many occasions the motor presents a current above the nominal (from the nameplate) and there are doubts if it will be due to mechanical overload or another problem. When this happens it is recommended to take a tachometer (contact type or not, or a strobe light) and measure the shaft speed. It should be compared with the plate: If it is less, the motor is overloaded.

Three additional variables to consider during your analysis:

  1. The supply voltage: Variations in the motor voltage with respect to the plate voltage influence the developed torque. This causes the speed to change as well. So it is recommended to verify that the voltage is within the allowed ranges (NEMA standard motors ± 10%, and IEC motors ± 5%).
  2. Motor connection: Make sure that the motor is connected correctly, according to the manufacturer’s design.
  3. The motor is repaired: Make sure the motor was repaired correctly. Measurement of the no-load current can serve this purpose.
Case study:

A newly installed mixer motor shows a current surge, above plate. The maintenance man assures that the engine is damaged, and that it must be replaced. Before proceeding to disconnect it from the machine, take a tachometer and measure the speed.

Result: 1600RPM, when the board is 1760RPM.

Another piece of information: the voltage is within the range. Production department staff confirm that they made a change to the product formula. Before the mixture was less viscous.

Conclusion: The Engine is OVERLOADED, due to changes in the product formulation.

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