Motors & drives – testing issues

The design of a large electrical motor is an optimization involving various engineering disciplines. The main focus is the electrical design of the machine, which has a fundamental impact on the functionality and the performance.

With increases in the power, size, and speed of the electric motors, the mechanical and torsional designs become more important for the operation to remain within the tightened noise, reliability and vibration limits.

The usual technology for large electric motor drivers is to use an LCI converter associated with a synchronous two-pole electric motor.

However, VSI (voltage source inverter) technologies are becoming popular. Because of the reactive power consumption of its thyristor bridge, an LCI converter cannot properly power an induction motor. A VSI can power both induction motors and synchronous motors.

The LCI technology generates torque pulsations and a harmonic filter is usually required. LCIs have been used for decades and there have been many successful references.

There is a very low harmonic content when using a VSI system (no harmonic filter required) with a better ‘power factor’ (PF). A VSI solution could also offer a better cost, but references are limited. The selection between LCI and VSI depends on an application.

For large electric motors, the f lexible rotor concept is used (the first critical speed usually lies below the operating speed range). The rotor should be dynamically balanced. The field balancing would not be required (whereas it is often possible).

When passing the first critical speed, the local rotational centre changes from geometric to local mass centre, which means, the local unbalance in an elastic rotor varies with the speed.

Therefore, modal sets of unbalance weights should be used to balance each mode individually. As a minimum, “n+2” balancing planes (n= the number of modes to balance) are necessary for balancing.

A large electric motor should handle properly the thermal unbalances. Because of the inevitable use of various materials with very different thermal expansion coefficients, combined with non-uniform temperature distribution and large sizes, a symmetrical mechanical and thermally insensitive design should be achieved. A small asymmetry can cause an unacceptable dynamic load.

To reduce the risk of having nonperforming drive systems shipped, full-load, full-speed performance tests of entire drive system are mandatory. Usually following tests should be conducted: The testing a motor alone. The back-to-back test to verify the electric motor and the VSD performance. The string test for a complete compressor train system. The open-circuit and short-circuit

To reduce the risk of having non-performing drive systems shipped, full-load, full-speed performance tests of entire drive system are mandatory.

tests could determine the conventional motor losses. The no-load test, conducted at the rated speed can give the open circuit curve, which could indicate various loss contributions.

The majority of the losses come from the friction and the windage. Strong cooling air f lows produced by the cooling fans (an internal cooling) is associated with some losses. Typically efficiencies in range of 97-99 per cent could be expected.

When at least two similar VSD and motor systems are being supplied, the VSD-motor back-to-back test can be done (one in the motoring mode and another in the generating mode). It is theoretically possible just to supply the losses and the reactive power demands.

The bearing problems, excessive vibrations and oil systems issues are responsible for a considerable portion of failed performance tests of large electric motors. During a back-to-back test, observations are:

1) Assessment of the motor thermal performance: Heat-run tests should be performed to assess the motor full-load thermal behaviour at different operating and emergency modes.

2) Assessment of motor vibration performance. 3) Torque ripple measurement. 4) Assessment of motor torque overload capability: The torque required for the startup of the train (usually a pressurized compressor), which could be 130-145 percent of normal torque for around 90-130 seconds.

5) Motor voltage and current waveforms.

In a case study, during an electric motor test, suspicious noises and smokes were identified on the electric motor. The first observation (after the trip) were: The motor shaft drop by 1.6mm. The maximum temperature on the bearing reached 135oC. The sleeve bearings were damaged. The root cause was the lack of lubrication oil, because of the main oil pump failure (and also the failure of standby oil pump to start).