Motor life cycle costs - what's in it for the designer?
Motor failure is one of the most common causes of downtime. If you are
selecting motors as an OEM, select with care - it will be your company,
not the motor manufacturer, that gets the blame when the system goes down
Why should a designer care about the life cycle costs of motors? That
depends on the application. For machines with long running hours, many
users insist on using high quality motors. This is because downtime costs
in many continuous applications are forbiddingly high and, also, because
running costs can be much more significant than the purchase cost of the
motor. Most motors in continuous operation use up their own capital cost
in energy in the first three months of operation. After that, a low
efficiency motor will continue to waste energy for the rest of its life.
A 90kW motor in continuous operation can use £12,000 worth of energy
during a 10-year life.
Many OEMs select motors solely on the basis of cost. While they may be
able to pass some of the savings on, it is not necessarily in the best
interest of their customers. Motors are only as good as their constituent
parts and about 55% of the cost of motors goes towards materials. This
means that you get what you pay for.
The greatest single cause of failure in a motor is the bearing. Always
make sure that the motor manufacturer uses bearings from a reputable
manufacturer. Also, ensure the bearings get adequate lubrication.
Re-greasable bearings will give longer service life, providing that
somebody bothers to re-grease them. If the end user does not have a
suitable maintenance infrastructure, sealed-for-life bearings are better.
Over-generous greasing can be just as big a problem as insufficient
greasing. When too much grease is added, the bearing elements do not have
enough room for heat expansion, which causes friction and overheating.
Some motors have a grease relief valve to avoid this happening.
If you really want to get on top of bearing reliability, ask your motor
manufacturer about L1 bearings. L1 and L10 are statistical measurements
for calculating bearing life. L10 means that 90% of bearings achieve or
exceed the calculated bearing lifetime; L1 means that 99% of bearings
achieve or exceed the calculated bearing lifetime. The improved
reliability is achieved by more frequent re-greasing intervals.
The second most common cause of failure is the winding. There is a
connection here with energy efficiency. If the energy efficiency is good
(eg Eff1), it means that more of the electrical energy is converted to
useful work and less vented off as heat. This makes for a cooler running
motor, which will increase the life of the winding. Theoretically, a
reduction of 10-15ºC in the running temperature of the stator will double
the life of the winding. The normal operational temperature in high
quality motors running at full load can be as low as 60-80ºC, while lower
quality motors frequently run in excess of 90ºC. There have been
instances where motors have been observed running at temperatures as high
as 130ºC. At these temperatures, the motor is only likely to last a
couple years at best.
The life of the winding also depends on the production techniques used. A
compact winding with a good slot-fill gives a high density of copper and
good performance. Automated winding equipment gives a higher density
slot-fill than hand winding and gives a repeatable quality time after
time. This is mostly used for small motors, with hand winding coming in
as motor sizes go up. Consequently, you will want your motor manufacturer
to use automated equipment as high up in the frame sizes as possible.
The integrity of the winding is measured in withstand voltage and is
usually in the region of 1,200V. Motors with a withstand voltage of
1,400V and above are available and offer increased reliability. This is
especially important to help withstand the voltage spikes produced by
some variable speed drives.
For further information, order a copy of ABB's motor quality guide.