Reducing the Electrical Power Consumption of Troughed Belt Conveyors by Speed Control
Faculty of Mechanical Engineering – Section: Transportation Engineering & Logistics v
Summary
Belt conveyors are a cost effective option to transport large flows of dry bulk material over short and medium distances. The belt conveyors design and the belt speeds are determined for peak material flows. However, these peak flows do not often occur. When the actual material flow is lower than the maximal conveying capacity of the belt conveyor, the belt is not optimally filled. The conveying capacity of the belt conveyor can be adapted to the actual material flow by lowering the belt speed, effectively optimising the belt fill and reducing the belt speed. According to DIN 22101, this leads to a reduction of the required electrical drive power. This is called speed control.
Speed control can be performed passively and actively. Active speed control continuously adjusts the belt speed to match the belt’s conveying capacity to the actual material flow by means of a control system. Passive speed control employs a fixed belt setting based on the expected material flow, this is set prior to the conveying operation. Small and/or temporary variations in the material flow do not result in belt speed variation in such case. The belt load and speed are therefore semi-optimal and power savings are expected to be less compared to active speed control. Implementation of passive speed control is expected to be substantially easier and less costly compared to active speed control due to lack of a complex closed loop control system. Furthermore active speed control systems are prone to detrimental vibrations at certain speeds which can severely damage the belt conveyor. This possible negative impact can be avoided with passive speed control by correct speed selection. Active speed control has not been implemented anywhere in practice, passive speed control is being applied on a small scale worldwide.
If the receiving chute is not properly aligned, changing the belt speed alters the discharge parabola of the material flow and severe wear may occur. Suitable discharge chutes therefore have to be installed (variable geometry). Furthermore, frequency converters are required in case AC motor drive units are installed (which is generally the case). Frequency converters are priced competitively and are highly efficient when compared to alternative drive unit configurations which cannot facilitate speed control (fluid couplings). However, their employment can severely degrade the power quality of the local utility network if not sufficiently shielded. DIN 22101 can be used to predict the motional resistance of belt conveyors. Contitech [Alles, 1994] delivers data which can be used to correct the fixed friction coefficient of DIN 22101 to allow for varying belt loads and belt speeds (Contitech motional resistance model). In case material flows are lower than the belt conveyor capacity, both motional resistance models predict a reduction of the drive power if the belt speed is reduced. Other resistance models from literature indicate that speed reduction does not always reduce the required drive power. These motional resistance models describe that the resistance rises progressively at increasing belt loads. As speed control maximises the belt load, the power consumption may therefore also be higher compared to that of a fixed speed belt conveyor. These motional resistance models are however, not generic or incomplete, therefore they cannot be used to predict the power consumption and possible speed control savings of a generic belt conveyor. However, they do show that it is incorrect to assume that speed control will always lead to a reduction in the required electrical drive power. The theory on belt conveyor motional resistance does therefore not provide conclusive results on the possible reduction of the required electrical drive power by speed control.
Jan Hiltermann
Delft University of Technology vi
The electrical efficiencies of a speed controlled belt conveyor drive unit depend on the interaction between the AC induction motor and the frequency converter. Quantitatively little data are available on the dependency of the combined electrical efficiency on speeds and loads.
Because of the already mentioned inconclusive theory of the motional resistance of belt conveyors and the unknown electrical efficiencies at varying speeds and loads, it is only correct to assume that speed control will lead to savings of the electrical power consumption of a belt convey after validating measurements are performed at that particular belt conveyor or one that is very comparable.
Actual measurements of the required drive power on the EMO 171 belt conveyor were performed, they indicate that maximising belt load and reducing belt speed leads to a reduction of the electrical drive power of that belt conveyor. The predictions of the 171 belt conveyor drive power by DIN 22101 (f = 0.022) are representative for these results. However, the predictions of the speed control savings are lower than the measurement results. The Contitech motional resistance model predicts a required drive power which is too low compared to the measured results.
A calculation of the economic feasibility of speed control at the dry bulk terminal EMO has been preformed. It computes the power consumption of all 59 belt conveyor at estimated material flows and an assumed friction coefficient (f = 0.022). However, the calculation of the annual power consumption of all belt conveyors combined gives results 22% higher compared to the real power consumption. It is expected that validating the material flows at EMO instead of estimating them will increase the accuracy of this calculation. Lower material flows will result in higher speed control savings, a lower utilisation rate will slightly reduce the savings.
Currently, 15 belt conveyors at EMO are already equipped with frequency converters for other reasons than speed control. If only these belt conveyors are employed in speed control operation, speed control savings will be 5% (these belt conveyors are relatively short and/or have a low occupancy rate).
According to the current calculation, speed control savings currently justify the significant capital expenditures needed for speed control conversion of 4 belt conveyors at EMO. This is under the assumption that electricity prices at least remain constant in the future and that a return on investments of 7 years is acceptable. If however the cost of electricity rises with 18 % on an annual basis (as is the current expectation), 2 additional belt conveyors can be converted to speed control.
By applying speed control to the 15 belt conveyors already suitable for speed control and by converting the other mentioned 4 or 6 belt conveyors, the power consumption of EMO’s total conveying systems can be reduced by 11% or 16% respectively.
This economic feasibility calculation is only based on reducing the electrical power consumption by speed control. It is expected that maintenance related costs will decrease as well.
