Technical Papers

"The speed of the economic motor has increased further [...]", Dr. Wolfram von Fritsch, CEO of Deutsche Messe AG (German trade fair company), concludes at the Hannover Messe in April. This increase in performance is deliberate and normally proves to be unproblematic. It is different with speeds of high speed electric motors. In this field proper prevailing conditions must be established; otherwise "exodus" might follow. Why and how to prevents this is explained below.

In the field of standard frequency converters the pulsed power output stage almost without exception comes with fixed intermediate circuit voltage and pulse-width modulation (PWM). This design meets the technical demands of many applications and is convincing in its simple and therefore cost-effective construction. But when used in particular applications, these devices can not reach optimum operating conditions. One example is driving high speed electric motors. For this purpose SIEB & MEYER developed a powerful and feasible solution taking the economic viability into account.

Drawbacks of Conventional Devices

Besides the motor performance the motor heating caused by the frequency converter is of decisive importance in high speed applications. Compared to slow-running motors the rotor volume of motors used in this field of application is very small. Thus, heat absorption and emission is correspondingly lower. In consequence, the rotor temperature becomes much higher, whereas the motor losses are the same. Such rotor heating can easily lead to a thermal destruction of the motor.

The additional losses caused by the converter mostly come from higher frequency harmonics in the motor current, the current ripple. They cannot be prevented because of the pulsed power semiconductors. Different investigations have shown that the active motor inductance decreases with increasing frequency; although the amplitude becomes smaller there are considerable losses mainly in the rotor. When a standard device as mentioned above is used with high speed electric motors, the resulting current ripple is usually very high because the inductance of these motors is comparatively low and an increase in the switching frequency of the output stage can not be proportioned to the compensation. Therefore large external motor chokes or sine-wave filters must be implemented which cause the motor voltage to drop significantly; consequently the performance is reduced. Another problem with standard frequency converters is the limited frequency of the output stage. The ratio of maximum switching frequency to maximum rotating field frequency drops with increasing rotating field frequency. Thus, the output voltage can not be controlled by PWM anymore. 

Application-oriented Solution for High Performance

SIEB & MEYER offers a functional alternative with a special range of converters featuring a controlled intermediate circuit and pulse amplitude modulation (PAM) in the product series SD2S and SD2T. Compared to conventional converters these devices provide an additional closed-loop DC/DC converter allowing variable control of the intermediate circuit voltage. Thus, in combination with PAM the provided output voltage is independent from the rotating field frequency.

This solution offers fundamental advantages: The block-shaped output voltage supplies only a small share of low-frequency harmonic current, the motor inductance is in full effect. This way the losses caused by the converter are reduced to such an extent that motor chokes are not required. Virtually, there is no limit to the possible rotating field frequency because the output stage pulsing matches it. Only the voltage determined for the operating point takes effect on the motor causing lower harmonics especially at lower speeds. Thus, the motor heating is reduced. Thanks to the low switching frequency problems related to electromagnetic compatibility (EMC) or motor insulation are unlikely. Instead of either motor or power electronics now both reach an equally good efficiency.

Depending on the used motor there are specific advantages: HS asynchronous motors provide a sensorless control via simple but robust U/f parameterization. The PWM control with an intermediate circuit voltage suited to the operating point allows above average performance and optimal rotation characteristics in the lower speed range (< 500 Hz). HS synchronous motors offer a robust sensorless control featuring high dynamics in a wide speed range as of approx. two percent of the rated speed. In addition only a few parameters are necessary for initial operation and the control characteristics remain well even for greater parameter tolerances.