Effects of semiconductor power converters.

Effects of semiconductor power converters

The control of electric motors by power electronic converters has a number of significant effects. These are primarily due to the introduction of harmonic components into the voltage and current waveforms applied to the motor. In the case of ac machines which are normally considered to be fixed speed there are additional implications of variable speed operation, including mechanical speed limits and the possible presence of critical speeds within the operating speed range.

Effects upon dc machines

The effects due to deviation from a smooth dc supply are, in general, well understood by drive and motor manufacturers. The impact of ripple in the dc current increases the rms current, which leads to increased losses and hence reduced torque capacity. The harmonics associated with the current ripple lead to the now universal practice of using laminated magnetic circuits, which are designed to minimize eddy currents. With the chopper converters used in servo amplifiers and traction drives, frequencies in excess of 2 kHz can be impressed on the motor and special care is needed to select a motor with sufficiently thin laminations.

The ripple content of the dc currents significantly affects commutation within a dc machine. The provision of a smoothing choke can be extremely important, and a recommendation should be made by the motor manufacturer depending upon the supply converter used.

Besides the thermal and commutation impacts, the ripple current also results in pulsating torque, which can cause resonance in the drive train. Laminating the stator field poles not only improves the thermal characteristic of the motor but also its dynamic behaviour by decreasing the motor time constant.

Effects upon ac machines

It is often stated that standard ac motors can be used without problem on modern PWM inverters and while such claims may be largely justified, switching converters do have an impact and limitations do exist. The NEMA MG1:1987, Part 17A gives guidance on the operation of constant-speed cage induction motors on ‘a sinusoidal bus with harmonic content’ and general purpose motors used with variable-voltage or variable- frequency controls or both.

Machine rating: thermal effects

The operation of ac machines on a non-sinusoidal supply inevitably results in additional losses in the machine, which fall into three main categories:

Stator copper loss. This is proportional to the square of the rms current, but additional losses in the winding conductors due to skin effect must also be considered,

Rotor ‘copper’ loss. The rotor resistance is different for each harmonic current in the rotor. This is due to skin effect and is pronounced in deep bar rotors. The rotor ‘copper’ loss must be calculated independently for each harmonic and the increase caused by harmonic currents can be a significant component of the total losses, particularly with PWM inverters having higher harmonics for which slip and rotor resistance are high.

Iron loss. This is increased by the harmonic components in the supply voltage.

The increase in iron loss due to the main field is usually negligible, but there is a significant increase in loss due to end winding leakage and slew leakage fluxes at the harmonic frequencies.

The total increase in losses does result in increased temperatures within the motor, but these cannot be readily represented by a simple de-rating factor since the harmonic losses are not evenly distributed throughout the machine and the distribution will vary according to the design of the motor. This has special implications for machines operating in a hazardous atmosphere, and this is covered further in section 15.7.5.4.

Many fixed-speed motors have shaft-mounted cooling fans and operation away from the rated speed of the machine results in reduced or increased cooling. This needs to be taken into account when specifying a motor for variable-speed duty.

Machine insulation

The fast-rising voltage created by a PWM drive can result in a transiently uneven volt- age distribution through a winding. For supply voltages up to 500 V, the voltage imposed by a correctly designed inverter is well within the capability of a standard motor of reputable manufacture, but for higher supply voltages an improved winding insulation system is generally required to ensure that the intended working life of the motor is achieved.

There can also be short-duration voltage over-shoots because of reflection effects in the motor cable, which is a system effect caused by the combined behaviour of the drive, cable and motor. The length of the motor cable can increase the peak motor voltage, but in applications with cables of 10 m or less, no special considerations are generally required. Output inductors (chokes) or output filters are sometimes used with drives for reasons such as long-cable driving capability or radio frequency suppression. In such cases no further precautions are required because these devices also reduce the peak motor voltage and increase its rise-time.

The IEC 60034-17 gives a profile for the withstand capability of a minimum standard motor, in the form of a graph of peak terminal voltage against voltage rise-time. The standard is based on research on the behaviour of motors constructed with the minimum acceptable level of insulation within the IEC motor standard family. Tests show that standard PWM drives with cable lengths of 20 m or more produce voltages outside the IEC 60034-17 profile. However most motor manufacturers produce, as a standard, machines with a capability substantially exceeding the requirements of IEC 60034-17.

Bearing currents

The sum of the three stator currents in an ac motor is ideally zero and there is no further path of current flow outside the motor, but in practice there are conditions which result in currents flowing through the bearings. These conditions include:

Magnetic asymmetry. An asymmetric flux distribution within an electrical machine can result in an induced voltage from one end of the rotor shaft to the other. If the bearing breakover voltage is exceeded, a current flows through both the bearings. In some large machines, it is a common practice to fit an insulated bearing, usually at the non-drive end, to stop such currents.

Supply asymmetry. With PWM inverter supplies, it is impossible to achieve perfect balance between the phases instantaneously, when pulses of different widths are produced. The resulting neutral voltage is not zero with respect to earth, and its presence equates to that of a common mode voltage source. This is sometimes referred to as a zero sequence voltage. It is proportional in mag- nitude to the dc link voltage in the inverter (itself proportional to the supply voltage), and has a frequency equal to the switching frequency of the inverter.

The risk of bearing currents can be minimized by adopting a grounding strategy which keeps all system components grounded at the same potential. This needs to be achieved for all frequencies and high inductance paths must be avoided, for instance keeping cable runs as short as possible. In addition, a low impedance path should be defined for the common-mode currents to return to the inverter. As the common-mode current flows through the three phase conductors in the supply cable, the best return path is through a shield around that cable. This could be in the form of a screen. Such measures are well defined by most reputable manufacturers in their EMC guidance and general guidelines are provided in Chapter 14.

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