Understanding how to save motor running costs for ever

Electronic variable speed drives (VSDs) – also known as inverter drives – find uses throughout building, industrial and process applications. The main reasons for their popularity are to save motor running costs or to improve motor torque control. They have been around for over forty years and have become smaller, smarter, and less expensive. But they are not cheap, so how do users justify the investment?

The lifetime cost of an electric motor comprises the initial cost, running cost and maintenance cost. According to the Carbon Trust, the cost of buying an electric motor is only the start. In one year a running motor can cost up to ten times its original cost in energy. Typical running costs for a loaded motor are in the range £2,300/year for a 2.2kW motor. This increases to £39,000 a year for one rated at 37kW (assuming an electricity price of £0.12/kWh).

Without speed control, electric motors are either on or off, and when they are on they are running at full speed. This full speed is a function of the supply frequency and the motor construction (number of poles). For a standard 4-pole induction motor that means 1440 rpm assuming a 4% slip factor. Motor efficiencies continue to improve in line with IEC regulation and IE3 motors or better are now standard.

In fan and pump applications, the motor’s fixed speed means that the fan or pump is always going flat out. To provide some control over the airflow, fan applications use dampers to restrict the flow. Similarly, pumping systems use valves to control the flow rate. A fair analogy to these would be driving a car with the engine running at full speed and controlling the speed using the brake. Clearly not an efficient or quite solution, so replacing valves and dampers with motor speed control has many benefits.

Save motor running costs

For fan and pump applications where speed regulation is permissible inverter drives offer a self-funding energy-saving option. This is because the relationship between motor speed and power consumption is governed by the cube law. Controlling the flow by changing motor speed means that a relatively small speed change produces a large fall in power.

Using the cube law, a 10% speed reduction means the motor needs only 73% of its maximum power (100% x0.9×0.9×0.9). Likewise, a 30% speed reduction (100% x .7 x .7 x .7) used only 34% of maximum power.

Using the earlier example of a 37 kW drive, a modest 20% speed reduction would reduce the energy use from £39,000 to under £20,000 and deliver a payback period of under one year. Curiously, to save motor running costs of this size is almost unbelievable in the mind of the users.

Other benefits

In addition to energy savings, inverter drives offer a range of benefits including noise reduction, improved flow control and reduced wear. They are simple to install or retrofit and replace normal motor switchgear and starters. Finally, they provide a soft-start function and protection for the motor.

So, how many motors are you running and what could you be saving?

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