34 Fans EC motors come under scrutiny
Fan efficiency has become a central issue in ventilation and air conditioning technology. Geoff Lockwood, ebm-papst UK technical director tells us why.
FAN EFFICIENCY is encouraged not only by the statutory basis created by the Energy Conservation Directive, but also by increased environmental and cost awareness on the part of users. Against this background, it comes as no surprise that energy-efficient EC technology is increasingly being employed in more and more areas, in large evaporators and heat exchangers and in control cabinet cooling.
In recent times, users of modern EC technology have been unsettled by reports of current reverse transfers caused by intermediate circuit capacitors. This is the case in particular for large systems which are equipped with a large number of EC fans and which are not switched off from the power supply when out of use.
However, there is no cause for alarm, the switching technology employed in EC motors has been successfully established in a vast range of different applications for many years. The power electronics component needed for the variable- frequency output voltage adjustment is in principle identical in design in EC motors as it is in a frequency inverter for asynchronous motors. The only difference is in their dimensions.
With ebm-papst’s GreenTech EC motors, for example, the complete control electronics are integrated into the drive to form a compact unit. Any problems that might occur with idle power or current reverse transfers are already known from speed-controlled asynchronous motors. It is easy to recognise that the intermediate circuit capacitor cannot cause any current reverse transfer when the drive is stationary.
What about the capacitor? An EC motor is a permanently excited, brushless synchronous motor which is operated by a DC voltage through a control and power electronics unit – or commutation electronics.
With mains-powered commutation electronics, the DC voltage is generated by rectifying the AC mains voltage. The rectified voltage – or intermediate circuit voltage - is smoothed by a capacitor – the intermediate circuit capacitor. Depending on the position and speed of the rotor, the commutation electronics use the DC voltage to generate a three-phase, speed-synchronous AC voltage of variable frequency and amplitude.
The motor currents flowing through it generate the rotating field in the stator winding, causing the engine to turn.
ACR News January 2014
As a rule, the intermediate circuit capacitor is permanently connected to the 400VAC or 230VAC power supply. A control input on the controller is used to control the motor speed. Because the power electronics are not disconnected from the power supply when the motor is stationary, it could be concluded that the intermediate circuit capacitor remains active, causing an unwanted capacitive current reverse transfer.
However, in the case of the GreenTech EC motor, if no current is drawn from the intermediate circuit, the capacitor will remain charged. No charging current flows through a charged capacitor, in this operating state there can therefore be no phase shift between voltage and current, no idle power, no harmonic waves and thus no current reverse transfer. It is therefore not necessary to disconnect the motor from the power supply when the speed is zero in order to avoid negative current reverse transfer in the intermediate circuit capacitor. Because air-conditioning equipment usually works continuously, this situation is in any case rather exceptional.
The capacitors in the power line filter When the fan is running, the switching principle, which has been established for many years, is of course not free of reverse transfers from the power supply. When current is drawn for the motor, the – non- linear – charge current in the intermediate circuit capacitor loads the power line above all with higher-frequency impulses, so-called current harmonics.
By contrast, the phase shift of the charge current is practically negligible – just a few degrees capacitive.
In the case of the GreenTech EC motors, current harmonics are reduced by active or passive harmonics filters in the intermediate circuit, while higher frequency interference emissions at the power input of the commutation electronics are reduced by power line filters, anti-interference capacitors located between the live and the neutral wires or between the phases. The size of their capacitive values depends on the interference spectrum and the necessary degree of damping.
The overall capacitance of the power line filter is usually in the range of a few 10nF to a few µF. The capacitors, which are essential for interference suppression, cause a capacitive idle current, with a typically dimensioned power line filter. So long as the power voltage is applied to the commutation electronics input, this idle current will flow, which in the above example causes a total idle power of around 160VAr. The effective power draw in standby mode is approximately one per mil of the nominal output for a 3kW GreenTech EC motor. This effective power draw is buffered by the intermediate circuit capacitor, but the charge current for this is so small that it can be neglected compared to the idle current of the power line filter. The idle current and idle power in standby therefore have nothing to do with the intermediate circuit capacitor, but rather are caused by the necessary use of power line filters.
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