AC And DC Motors Reviews

Many systems use constant speed motors and control process flow rates or pressures by mechanically regulation using throttling valves, dampers, fluid couplings or variable inlet vanes etc. These devices generally usually do not control flow or pressure efficiently because energy is dissipated throughout the throttling device.

Running an electric motor at full speed while throttling the input or output is much like driving an automobile with one foot about the accelerator and the opposite on the brake; a part in the produced output immediately would go to waste.

A variable speed drive can help to save over 60% from the energy. This can be done because it controls the energy at source, only using around is necessary to own the motor with the required speed and torque - much within the same way as the accelerator inside car controls the engine revs and without the foot on the brake. Types of loads - that are ideal for energy saving?

Drive applications are categorized with respect to power and torque changes in reaction on the motors speed. It is very important to see the kind of load for the particular application because not all are equally good energy saving opportunities for that application of the variable speed drive. In fact, in case a variable speed drive is used on some loads there will be little if any energy savings.

Variable speed drives as well as the loads they may be placed on can generally be divided into 3 groups:

- Constant power
- Constant torque
- Variable torque

Constant Power Loads

In constant power applications, the power requirement remains constant at all speeds, and the torque requirement varies inversely with speed. One example of this type of load could be a lathe. At low speeds, the machinist takes heavy cuts, using high amounts of torque. At high speeds, the operator makes finishing passes that require a lot less torque. Other examples are drilling and milling machines.

Typically, these applications offer no energy savings at reduced speeds.

Constant Torque Loads

In constant torque loads, the ability is directly proportional on the operating speed. Since torque is not just a function of speed, it remains constant even though the power and speed vary proportionately. Typical types of constant torque applications include conveyors, extruders, mixers and positive displacement pumps. Usually these applications result in moderate energy savings at lower speeds.

In variable torque load applications, both torque and power change with speed. Torque varies with speed squared, and power varies with speed cubed. This implies that at half speed, the ability required is approximately one eighth of rated maximum. Common samples of variable torque loads are centrifugal fans, blowers and variable discharge pressure pumps.

The use of an variable speed drive having a variable torque load often returns significant energy savings. In these applications the drive may be used to maintain various process flows or pressures while minimizing power consumption. In addition, a drive also supplies the advantages of increased process control, which regularly improves product quality and reduces scrap.

Effective speed ranges are from 50% to 100% of maximum speed and may result in substantial energy savings.

How do variable speed drives achieve energy saving with variable torque loads?

Variable speed drives regulate the pace of motors along with turn the pace of the fan or pump by controlling the power that goes to the motor rather than restricting the flow of the process running constantly at full speed.

A variable speed drive can conserve over 60% in the energy because it controls the energy at source, only using up to is important to perform the motor using the minimum speed and torque.

Large numbers of energy might be saved on fan and pump systems, because with the affinity laws for pressure and flow rates.

The Affinity laws state -Flow is directly proportional to speed Torque is directly proportional to speed squared Power required is proportional to speed cubed Therefore, what this means is that if 100% flow requires full power 75% flow requires 0.753= 42% of full power 50% flow requires 0.53= 12.5% from the power

Mechanical control methods such as inlet guide vanes, throttling valves, discharge dampers usually do not make the most in the affinity laws.

With mechanical flow control methods the motor always runs at full speed and also the flow is mechanically restricted.

A variable speed drive saves energy by reduction of the particular speed in the motor when full flow just isn't required.

Example A fan is running at fixed speed (50Hz) and the output in the fan is fixed by method of a discharge damper to restrict airflow on the correct level for that process. The input power is usually 95% of full load power.

A variable speed drive is fitted towards the system and the discharge damper removed so there exists no restriction to airflow. The speed of the motor is reduced to 40Hz which provides exactly the same airflow as before once the motor was run at full speed as well as a discharge damper used. Now the input power is usually 50% of full load power.

Therefore by utilizing a variable speed drive, the energy being consumed is reduced by typically 45%.

Centrifugal Fans Massive potential energy savings employing a variable speed drive compared for the two most typical types of flow control for fans: - Inlet guide vanes require about 60% power to provide a flow rate of 50% - A discharge damper requires an enormous 90% power to give 50% flow

Centrifugal Pumps - Operating at 75% flow requires lower than 50% power, whilst the throttling valve requires around 90% power.

Centrifugal fan - Typical input powers

The following table shows the normal input power to some motor when run at full speed with flow rates are restricted by an outlet damper compared for the typical input power when the identical motor is run at reduced speed from the variable speed drive, achieving a similar air flow rate as with the outlet damper. It could be seen when an outlet damper reducing the air flow rate to 80% uses 95% input power, a variable speed drive achieving a similar air flow rate uses 50% input power.

Other advantages of variable speed drives

o A variable speed drive can also ensure it is possible to avoid a motor completely in the big event it isn't required as re-starting with a variable speed drive causes far less stress than starting direct online - soft start is surely an inherent feature from the drive.
o Regulating the motor speed gets the added benefit of easily accommodating capacity rises without extra investment, as speed increases of 5-20% is not an issue having an AC variable speed drive so long as as there is enough spare capacity inside system.
o Reduced maintenance in comparison to DC systems (brushes and commutators)
o Reduced motor/application noise levels.
o If the variable speed drive has an interior PID loop, it is gonna be possible to automatically control flow or pressure based on feedback from a sensor inside system. This can make further energy savings since the motor can slow right down if almost no flow or pressure is required.

Another way of saving energy

Most companies ignore the motors when referring to energy saving. As well as saving money by installing a variable speed drive, installing high efficiency motors may also save energy and money. Please see the enclosed document for further information on high efficiency motors. Motor Control Warehouse can supply EFF1 accredited motors. Please take a have a look at our website for further details.

Example of energy saving employing a variable speed drive

A 30kW pump operating for 16 hours during weekdays and 12 hours during week ends, total of hours per week = 92 hours.

Energy Cost at constant speed Energy consumption weekly - 30kW x 92hours = 2760kWh Assume electricity minute rates are 10p per kWh Energy cost each year - 2760kWh x £0.10 x 52 weeks = £14352

Energy Cost at variable speed Assume average speed is 75% which corresponds to 42% power consumption Energy consumption per week - 0.42 x 30kW x 92 hours = 1159.2kWh Energy cost per year - 1159.2kWh x £0.10 x 52 = £6027.84 Value of energy saved per annum by employing a variable speed drive £14352 - £6027.84 = £8324.16

NOTE: This calculation is merely an example utilizing a figure of 10p/kWh but gives a fantastic guide as to what may be saved through the use of variable speed drives. For a far more accurate price of possible energy savings, a complete survey including tests can be required