If you are thinking about connecting an electric motor to a variable frequency drive, there are three aspects that must be taken into consideration. These aspects are insulation stress, bearing stress and thermal stress.
Operating a motor with frequency control imposes higher stress on the motor winding than direct mains operation This is primarily due to the steep pulse edges (du/dt) and the motor cable, depending on cable length, type, routing, etc.
The steep pulse edges result from the rapidly switching semiconductor devices in the inverter stage of the frequency converter. They operate at a high switching frequency in the range of 2 to 20 kHz with very short switching times in order to reproduce a sinusoidal waveform.
In combination with the motor cable, these steep pulse edges are responsible for the following effects at the motor:
- High pulse voltages Ull on the motor terminals put additional stress on the interwinding insulation
- Higher pulse voltages between the windings and the laminations Ûle put additional stress on the slot insulation
- Higher voltages between the windings Ûwdg put significantly higher stress on the insulation of the wire in the windings
Under unfavourable conditions, frequency-controlled motors may fail due to bearing damage caused by bearing currents. Current flows in a bearing when the voltage across the bearing lubrication gap is high enough to penetrate the insulation layer formed by the lubricant. If this happens, imminent failure of the bearing is signalled by increasingly louder bearing noise. Bearing currents of this sort include high-frequency eddy currents, earth currents and EMD currents (spark erosion).
Which of these currents may lead to bearing damage depends on the following factors:
- The mains voltage at input of the frequency converter
- The steepness of the pulse edges (du/dt)
- The type of motor cable
- Electrical shielding
- System earthing
- Motor size
- The earthing system of the motor housing and the motor shaft.
Bearing currents can be reduced by the following measures:
- Fitting output filters (output chokes, du/dt filters, or sine-wave filters)
- Fitting electrically insulated bearings
- Good earthing of all metallic system components with low-impedance connections
- Shielded motor cables
- Fitting a DC suppression filter
Operation with a frequency converter increases the power dissipation in the motor. The additional harmonic content causes iron losses and current heat losses in the stator and rotor. The magnitude of the losses depends on the amplitude and frequency of the harmonics of the drive frequency. The additional current heat losses in the rotor depend on the slot geometry. Iron losses and current heat losses in motors are not load-dependent. The additional losses in the motor cause higher thermal stress on the winding insulation. However, with modern frequency converters the additional heating of standard motors (up to frame size 315) is comparable to the additional warming due to mains voltage tolerances and is therefore negligible. Manufacturers sometime specify a derating factor for transstandard motors (frame size 355 and above).
If the converter is not able to generate the full mains voltage at the rated mains frequency, it is advisable to select a motor with Class F insulation. Operating a motor at a voltage lower than with direct mains operation increases the motor temperature by up to 10 K.
If you are not sure about whether your motor can be operated in combination with a variable frequency converter then ask the motor manufacturer to confirm that the motor is designed for operation with a frequency converter. And check the allowable operating speed range (minimum and maximum rpm).
Article by Gregers Geilager
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