Motor Current Improved and Load of Motor Insulation Reduced
Three-level technology versus LC filter
For the operation of electric motors, manufacturers often use LC filters between motor and frequency converter. These solutions are individual combinations of passive electronic components that either smooth the switching edges of the pulse pattern supplied by the converter (dV/dt filter) or even provide almost sinusoidal motor voltages and currents. An LC filter is required when a pulse-controlled operation of the frequency converter causes inadmissibly high loads on the motor insulation or when low-quality motor currents occur. However, frequency converters with three-level technology – like SD2M by SIEB & MEYER – can eliminate the need for LC filters in both cases.
There are relevant standards and guidelines on the insulation of stator windings. IEC 600034-25, for example, contains detailed information on the maximum limit values for voltage peaks and voltage rise rates at the motor terminals. The basic problem when operating converters with the well-established pulse-width modulation (PWM) is the fact that the square-wave output voltage of the converter does not arrive with the same shape at the motor terminals. Depending on the physical construction of the whole system or machine and the used motor cables, voltage peaks of more than 2,000 V may occur at the motor. The motor cable length in particular affects the voltage peaks – and that from a length as short as five meters.
Avoiding voltage peaks
High voltage peaks put much load on the stator windings and can cause insulation failures that might damage the stator windings permanently. This in turn leads to short circuits between the windings. Permanent damage may occur within seconds or later after a few months. LC filters can prevent this effect. Using them, however, means additional costs, additional installation space and weight as well as losses in efficiency. Apart from that, the LC filter must be dimensioned for the individual application in advance, which is time-consuming and requires flexibility.
For the evaluation of voltage peaks, the following two values are determined: the maximum voltage amplitude (Vpeak) and the maximum voltage rise rate (tr). Today's standard converters in the power range above 2 kW usually have a three-phase 400 V power supply and work with two-level PWM. Modern and fast semiconductor switches generate voltage peaks at the motor that can reach values far above 1,000 V. Therefore, the criterion of the voltage rise rate (tr) becomes even more important, although adhering to the standard values is often not possible. This makes the use of LC filters necessary – but they are not the only option: Converter technology with three-level PWM is a valid alternative. The maximum voltage jumps amount to only half of the two-level technology. Usually, this keeps the maximum voltage pulses measured at the motor below 1,000 V and the corresponding voltage rise rate at a permissible value – even when state-of-the-art semiconductors are used. The conclusion: LC Filters are not required.
Improving the quality of motor currents
The quality of the motor currents applied to three-phase current motors must be very good. Even a small variation of the ideal sine-wave form causes losses in the motor. Approximately 90 % of these losses are generated in the rotor and lead to unwanted heating. The non-sinusoidal motor current is the output of the converter due to its PWM operating mode. It is called ripple current and overlays the sinusoidal motor current. The generated ripple current depends on the switching frequency, the DC voltage of the converter and – most crucial – the motor inductance. Small inductances cause great ripple currents: This is especially problematic with high-speed synchronous motors, as they require small inductances because of their physics. The generated rotor heat can have a great impact on rotor stability, permanent magnets and bearings. These problems occur especially at high rated currents of the motors. To avoid them, standard converters with two-level PWM and low switching frequency are often used in combination with LC filters.
Another potential solution, however, is to increase the switching frequency for PWM. Doubling the switching frequency reduces the ripple current usually by half. There are technical as well as economical limits to this solution, though. For one thing, fast switching power transistors are more expensive for higher voltage ranges. In addition, the switching losses in the output stage increase drastically, which has a very unfavourable effect on the efficiency and thus also the cooling effort. Apart from that, not all motors can cope well with increased switching frequencies. With some constructions, increasing the switching frequency hardly reduces the motor losses. This is often the case with synchronous motors using permanent magnets without segmentation.
Three-level technology in combination with higher switching frequencies
Three-level technology represents a valid alternative here as well. Compared to two-level technology, only half the voltage is supplied to the power semiconductors of the output stages. This makes the use of power semiconductor designed for much lower voltages possible. Better yet, these semiconductors can switch faster (for technological reasons). The result: There are fewer switching losses in the output stage, which enables significantly higher switching frequencies. In addition, the motor is loaded with only 50 % of the voltage jumps compared to two-level technology. By using three-level technology only, the losses generated in the rotor drop by about 75 %. Three-level technology in combination with higher switching frequencies can reduce the losses by up to 90 %. LC filters can then often be omitted.
The conclusion: Using converters with three-level PWM can save LC filters in many applications. Long motor cables do not pose an obstacle and the quality of the motor current is considerably better. There are fewer losses in the rotor (heat) which is especially beneficial for high-speed motors. The required space and the weight of the complete system decrease whereas the flexibility of the application increases.