变频器的平均无故障时间MTBF的概述以及计算方法

The following method is used for calculation of MTBF (Mean Time Between Failure) based on field failures for ACS 400. Only complete inverter module returns are considered in the calculation. The same method can be used for essentially all products where field failures can be measure by quantity and in time.

1. Basic Formulas.

The most common and simplest way of looking at reliability is to assume that an exponential

distribution applies. The reliability (R) or proportion of drives that remains functional after a specific time (t) can then be expressed as:

MTBF t e

R -=

This formula can be rewritten as:

R

MTBF ln 1-=

MTBF can now be calculated when reliability (R) is known. Reliability in this case is just a number that expresses how many units that are functional after a period of time. For example, if we are looking at 1000 drives for 1 year and find that 50 of the drives failed during the year, R = (1000 – 50)/1000 = 0.95. MTBF can be calculated to be 19.5 years using the formula above. The reason why the unit of

measure becomes years in this case is that we looked at the drives for 1 year. If the period of time had been ½ year, MTBF would be 19.5 ½ years = 9.75 years.

Note that the crude method of just dividing the number of drives looked at (1000) with the number of failures (50), yields a similar result (20 years). The difference is in that using the exponential

distribution takes into account the fact that the number of drives left in operation declines during the year.

2. Specific method used by New Berlin

When MTBF is to be calculated, the period of time during which failures are counted must be

determined. One major factor in selecting a time period is that failure rate has seasonal variations due to ambient temperature variations and variations in loading. For New Berlin this effectively means that the time during which failures are counted must be a multiple of 1 year. A 1 year period of time was chosen because if time is longer, the waiting period before an MTBF value can be calculated becomes too long.

Another consideration is what batch of drives to use for each MTBF calculation. In New Berlin we choose the drives produced in each month since the amount of drives is sufficiently large (approx. 3000) to produce a reasonable amount of failures during a year. The amount of failures affects the error in the MTBF calculation (see below) and it should therefore be chosen accordingly.

The third decision to make is when to start the year during which failures are counted. A dwell time after the production month is obviously necessary because it takes time for the drives to get into operation. In addition, the initial start up of drives and the initial period in operation is likely to have a higher failure rate (infant mortality) and an MTBF value should not include these extraordinary events.