Even with similar specifications, the cost of low voltage ac motors can
vary quite substantially from manufacturer to manufacturer. So, in a
marketplace where competitiveness should be at the forefront of the
manufacturer's agenda, how is it that one motor is half the cost of
another? The difference is under the skin, according to Steve Ruddell
The cost and quality of materials is what determines the price of a
motor. Materials account for half the total cost of a motor, so even if
one manufacturer spends nothing on manufacturing, distribution, sales and
R&D, it still cannot explain the huge price difference between
manufacturers. conversely, a higher price is usually an indication of
high quality materials. So how will that benefit you, the user?
High quality components and manufacturing processes deliver durable
motors that will normally last ten to twenty years - not two, three or
four years. There is the argument that if a motor can be bought at very
low cost, you could buy a second spare unit and still be in profit.
However, the cost of downtime will soon surpass any cost savings achieved
by going the cheaper route. Downtime can cost as much as a new motor
every five minutes - a startling statistic to bear in mind when you next
compare your suppliers prices!
Bearings
Bearing problems are the most common causes of motor failure. The
longevity of the bearing, to a large extent, depends on the quality of
the bearing itself. It also depends on selecting the appropriate type of
bearing for the application and the load, and correct greasing for the
application. Bearings can either be sealed-for-life or require
lubricating from time to time, the latter generally providing a longer
service life.
In the uncertainty that maintenance can be carried out, however,
sealed-for-life bearings are preferable. It is worth noting that, for
serviceable bearings, re-lubrication must be compatible with the original
grease, while over-greasing must be avoided at all costs. The
manufacturer should be in a position to provide advice on correct
lubrication procedure.
Winding integrity
Winding problems are another cause of premature failure. Normally, after
many years of service, the insulating varnish will begin to break down,
eventually leading to a short circuit. A good quality motor can generally
be rewound without too great a reduction in efficiency; and while a low
durability motor can also be rewound, there are usually greater losses in
efficiency due to the lower quality steel of the stator laminations.
Windings can be insulated to different standards, Class F (155°C) and
Class B (130°C) being the most common. The cooler the motor runs, the
longer the service life for the insulation system and the longer the
motor will operate. The normal running temperature of high quality motors
at full load can be as low as 60-80°C, while lower durability motors tend
to run considerably hotter, resulting in premature failure. Winding
integrity is measured in 'withstand voltage' and this is generally in the
region of 1,200V. Motors are available with a withstand voltage in excess
of 1,400V, and these are occasionally used in electronic variable speed
drive configurations where there is a risk of voltage spikes.
The problem facing the buyer is knowing which motor is going to offer
good durability, as they all look pretty much the same on the outside and
may even be perceived as being of a respectable brand. In many cases,
engineers know a good motor when they see it, but are unable to put their
finger on what it is that makes it 'good'. Unfortunately, as they are
unable to quantify their requirements, their purchasing departments
simply buy the cheapest motor that fits the specification. Users
accountable for continuously operating machinery with downtime costs as
much as £10,000 per hour, or OEMs whose machines are destined for
critical duties, wi