| Energy Saving - An introduction | | | | Mechanical control methods such as inlet guide |
| Many systems use constant speed motors and | | | | vanes, throttling valves, discharge dampers do not |
| control process flow rates or pressures by | | | | take advantage of the affinity laws. |
| mechanically regulation using throttling valves, | | | | With mechanical flow control methods the motor |
| dampers, fluid couplings or variable inlet vanes etc. | | | | always runs at full speed and the flow is |
| These devices generally do not control flow or | | | | mechanically restricted. |
| pressure efficiently because energy is dissipated | | | | A variable speed drive saves energy by reducing |
| across the throttling device. | | | | the actual speed of the motor when full flow is |
| Running a motor at full speed while throttling the | | | | not required. |
| input or output is like driving a car with one foot | | | | Example A fan is running at fixed speed (50Hz) |
| on the accelerator and the other on the brake; a | | | | and the output from the fan is restricted by a |
| part of the produced output immediately goes to | | | | discharge damper to restrict airflow to the |
| waste. | | | | correct level for the process. The input power is |
| A variable speed drive can save over 60% of the | | | | typically 95% of full load power. |
| energy. This is possible as it controls the energy | | | | A variable speed drive is fitted to the system and |
| at source, only using as much as is necessary to | | | | the discharge damper removed so there is no |
| run the motor with the required speed and torque | | | | restriction to airflow. The speed of the motor is |
| - much in the same way as the accelerator in the | | | | reduced to 40Hz which gives the same airflow as |
| car controls the engine revs and without the foot | | | | before when the motor was run at full speed and |
| on the brake. Types of loads - which are suitable | | | | a discharge damper used. Now the input power is |
| for energy saving? | | | | typically 50% of full load power. |
| Drive applications are categorized with respect to | | | | Therefore by using a variable speed drive, the |
| power and torque changes in response to the | | | | power being consumed is reduced by typically |
| motors speed. It is important to understand the | | | | 45%. |
| type of load for a particular application because | | | | Centrifugal Fans Massive potential energy savings |
| not all are equally good energy saving | | | | using a variable speed drive compared to the two |
| opportunities for the application of a variable | | | | most common methods of flow control for fans: |
| speed drive. In fact, if a variable speed drive is | | | | - Inlet guide vanes require about 60% power to |
| used on some loads there will be little or no | | | | give a flow rate of 50% - A discharge damper |
| energy savings. | | | | requires a huge 90% power to give 50% flow |
| Variable speed drives and the loads they are | | | | Centrifugal Pumps - Operating at 75% flow |
| applied to can generally be divided into 3 groups: | | | | requires less than 50% power, whilst the throttling |
| - Constant power | | | | valve requires around 90% power. |
| - Constant torque | | | | Centrifugal fan - Typical input powers |
| - Variable torque | | | | The following table shows the typical input power |
| Constant Power Loads | | | | to a motor when run at full speed with flow rate |
| In constant power applications, the power | | | | is restricted by an outlet damper compared to |
| requirement remains constant at all speeds, and | | | | the typical input power when the same motor is |
| the torque requirement varies inversely with | | | | run at reduced speed from a variable speed drive, |
| speed. One example of this type of load would be | | | | achieving the same air flow rate as with the |
| a lathe. At low speeds, the machinist takes heavy | | | | outlet damper. It can be seen that if an outlet |
| cuts, using high levels of torque. At high speeds, | | | | damper reducing the air flow rate to 80% uses |
| the operator makes finishing passes that require | | | | 95% input power, a variable speed drive achieving |
| much less torque. Other examples are drilling and | | | | the same air flow rate uses 50% input power. |
| milling machines. | | | | Other advantages of variable speed driveso A |
| Typically, these applications offer no energy | | | | variable speed drive can also make it possible to |
| savings at reduced speeds. | | | | stop a motor completely when it is not required |
| Constant Torque Loads | | | | as re-starting with a variable speed drive causes |
| In constant torque loads, the power is directly | | | | far less stress than starting direct on line - soft |
| proportional to the operating speed. Since torque | | | | start is an inherent feature of the drive.o |
| is not a function of speed, it remains constant | | | | Regulating the motor speed has the added benefit |
| while the power and speed vary proportionately. | | | | of easily accommodating capacity rises without |
| Typical examples of constant torque applications | | | | extra investment, as speed increases of 5-20% is |
| include conveyors, extruders, mixers and positive | | | | no problem with an AC variable speed drive as |
| displacement pumps. Usually these applications | | | | long as there is enough spare capacity in the |
| result in moderate energy savings at lower | | | | system.o Reduced maintenance compared to DC |
| speeds. | | | | systems (brushes and commutators)o Reduced |
| In variable torque load applications, both torque | | | | motor/application noise levels.o If the variable |
| and power change with speed. Torque varies with | | | | speed drive has an internal PID loop, it will be |
| speed squared, and power varies with speed | | | | possible to automatically control flow or pressure |
| cubed. This means that at half speed, the power | | | | based on feedback from a sensor within the |
| required is approximately one eighth of rated | | | | system. This can make further energy savings as |
| maximum. Common examples of variable torque | | | | the motor can slow right down if very little flow |
| loads are centrifugal fans, blowers and variable | | | | or pressure is required. |
| discharge pressure pumps. | | | | Another method of saving energy |
| The use of a variable speed drive with a variable | | | | Most companies forget about the motors when |
| torque load often returns significant energy | | | | considering energy saving. As well as saving |
| savings. In these applications the drive can be | | | | money by installing a variable speed drive, installing |
| used to maintain various process flows or | | | | high efficiency motors can also save energy and |
| pressures while minimizing power consumption. In | | | | money. Please see the enclosed document for |
| addition, a drive also offers the benefits of | | | | further details on high efficiency motors. Motor |
| increased process control, which often improves | | | | Control Warehouse can supply EFF1 accredited |
| product quality and reduces scrap. | | | | motors. Please take a look at our website for |
| Effective speed ranges are from 50% to 100% | | | | further details. |
| of maximum speed and can result in substantial | | | | Example of energy saving using a variable speed |
| energy savings. | | | | drive |
| How do variable speed drives achieve energy | | | | A 30kW pump operating for 16 hours during |
| saving with variable torque loads? | | | | weekdays and 12 hours during week ends, total |
| Variable speed drives regulate the speed of | | | | of hours per week = 92 hours. |
| motors and in turn the speed of the fan or pump | | | | Energy Cost at constant speed Energy |
| by controlling the energy that goes into the | | | | consumption per week - 30kW x 92hours = |
| motor rather than restricting the flow of a | | | | 2760kWh Assume electricity rate is 10p per kWh |
| process running constantly at full speed. | | | | Energy cost per year - 2760kWh x £0.10 |
| A variable speed drive can save over 60% of the | | | | x 52 weeks = £14352 |
| energy as it controls the energy at source, only | | | | Energy Cost at variable speed Assume average |
| using as much as is necessary to run the motor | | | | speed is 75% which corresponds to 42% power |
| with the minimum speed and torque. | | | | consumption Energy consumption per week - 0.42 |
| Large amounts of energy can be saved on fan | | | | x 30kW x 92 hours = 1159.2kWh Energy cost |
| and pump systems, because of the affinity laws | | | | per year - 1159.2kWh x £0.10 x 52 = |
| for pressure and flow rates. | | | | £6027.84 Value of energy saved per |
| The Affinity laws state -Flow is directly | | | | annum by using a variable speed drive |
| proportional to speed Torque is directly | | | | £14352 - £6027.84 = |
| proportional to speed squared Power required is | | | | £8324.16 |
| proportional to speed cubed Therefore, this | | | | NOTE: This calculation is just an example using a |
| means that if 100% flow requires full power 75% | | | | figure of 10p/kWh but gives a good guide as to |
| flow requires 0.753= 42% of full power 50% flow | | | | what can be saved by using variable speed drives. |
| requires 0.53= 12.5% of the power | | | | |