Part 3

Cost

What cost issues arise with medium voltage drives?

The industrial market is changing. New projects are now demanding a quick payback as developers demand a more rapid return on their investments. As a result, the purchase of every component is highly cost driven. Typically, if you meet the performance specifications at the lowest cost you get the order.

Downtime in critical industries like petrochemical, water, pulp and paper, metals, power generation and mining seriously impacts on customer profitability with thousands of dollars lost even for short process interruptions.

Today's medium voltage drives need to take these commercial constraints into consideration and as such drives are being introduced which waste no space by having fewer parts but while still using proven technology.

 

How important is the initial purchase cost of a medium voltage drive?

Initial purchase price will always be an important consideration. But it is not the only consideration. You need to look at other aspects such as running cost and service, each of which adds to the lifecycle costs.

 

What is meant by lifecycle costs?

There is a change in purchasing attitudes throughout industry. Initial capital expenditure is not the only driver. Industry is becoming more aware of the need to consider lifecycle costs - that is the cost of running a drive from cradle to grave, often referred to as the total cost of ownership.

Cost issues to be considered include:

• floor space
• installation and commissioning
• training
• energy consumption
• reliability
• maintenance
• spare parts
• after-sales support
• disposal

 

When will the initial purchase price be recovered?

Medium voltage drives based on current technologies are energy efficient and allow very attractive pay back times. Payback times of 12-18 months are typical, for applications where energy saving alone is considered. When the drive is used to improve product quality or reduce re-work, the payback times can be even shorter, depending on cost of energy, application and duty cycle.

As standard drives are pre-engineered, the initial purchase cost can be lower as any engineering investment is spread over a larger customer base.

Summary Pad -

• Initial purchase price is not the only consideration.
• Consider running costs and service, each of which adds to the lifecycle costs.
• Take into account transformer losses into the room and the subsequent need for air conditioning; cost of floor space; cost of support and service; energy consumption; training, disposal and reliability.
• Payback times of 12 - 18 months are typical, depending on cost of energy, application and duty cycle.

 

Efficiency

How efficient are today's medium voltage drives?

Users should strive for efficiency levels for medium voltage drives of about 97%, including the transformer, power factor correction equipment and harmonic filters (if used).

Care should be taken to consider the total drive system efficiency, not just the efficiency of the converter. A drive system, therefore, comprises transformers, power factor correction equipment and harmonic filters (if used), converter, output filter (if used) and motor.

 

What energy savings can be achieved?

The energy savings alone in a drive rated at even less than 1 MW may be so large that the drive will pay for itself in a few months. For example, take a 746kW fan at a speed of 870 rpm and following a given duty cycle. Then comparing a medium voltage drive with a fluid coupling during one year's operation reveals that the energy consumed by the fluid coupling is about 800,000kWh more than that consumed by the AC drive.

Dividing the actual energy consumed by the amount of hours in a year gives average losses for the fluid coupling drive of 121kW compared to 25kW for the AC drive. Thus, the fluid coupling drive's average losses are nearly 100kW larger than those of the medium voltage drive.

 

What other efficiency related issues are provided by medium voltage drives?

Electronic speed control offers much more than just energy savings. Less maintenance is one benefit, since electronic speed control often replaces a mechanical transmission sytem.