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Firstly, what is the difference between a 3 phase
supply
& a single phase supply ?
It may be a little
easier to think of a day to day similarity as below:
Using a single phase
supply is like one strong man pushing a car uphill. At some point, the work
is beyond what that one man can do whereas a 3 phase supply is like having 3
equally strong men each pushing the car in a relay system one after another.
The end result is that each man is not doing the work of the one strong man,
but together, the three of them push the car further. That is the total
distance travelled is much greater when added up.
However, there may be a
number of reasons that people are unable to use a three phase supply, the
main one being the cost of having it installed into a small workshop. For
example, this can be as high as £20,000 dependent upon your area,
accessibility, required cable lengths etc.
One example of an industry
sector that single phase motors are more commonly used in place of three
phase is where portable or mobile equipment is required, typically floor
polishing equipment, cement mixers, pressure washers, portable conveyors etc.
In these cases, single phase motors are widely used due to economics,
practicality and electrical safety, whilst still maintaining relatively high
starting torque and overall performance. In many of the above mentioned
mobile applications there is also a requirement for the driven equipment to
be powered using a stand-alone generator, this is again where economics and
practicality come in. Typically, three phase generators are very large and
cumbersome with a price tag to match whereas single phase compressors are a
comparably affordable solution and much more compact. This again encourages
the users of this type of equipment to follow the preference of single phase
over three phase.
Basic Electric
Motor Principal
An electric
motor uses basic magnetic rules of repulsion & attraction to twist a
rotating object (the rotor) around in a circle. Both the rotor and the
stationary structure (the stator) are magnetic and their magnetic poles are
initially arranged so that the rotor must turn in a particular direction in
order to bring its north poles closer to the stator's south poles and vice
versa. The rotor thus experiences a twist and it begins to rotate. But the
magnets of the rotor and stator aren't all permanent magnets. At least some
of the magnets are electromagnets. In a typical motor, these electromagnets
are designed so that their poles change just as the rotor's north poles have
reached the stator's south poles. After the poles change, the rotor finds
itself having to continue turning in order to bring its north poles closer to
the stator's south poles and it continues to experience a twist in the same
direction. The rotor continues to spin in this fashion, always trying to
bring its north poles close to the south poles of the stator and its south
poles close to the north poles of the stator.
Single-Phase
Theory 
Because it has but a
single alternating current source, a single-phase motor can only produce an
alternating field: one that pulls first in one direction, then in the
opposite as the polarity of the field switches. A squirrel-cage rotor placed
in this field would merely twitch, since there would be no moment upon it. If
pushed in one direction, however, it would spin.
The major distinction
between the different types of single-phase AC motors is how they go about
starting the rotor in a particular direction such that the alternating field
will produce rotary motion in the desired direction. This is usually done by
some device that introduces a phase-shifted magnetic field on one side of the
rotor.
There are 2 main types
of single phase motor as follows that both use capacitors to shift the
magnetic field on start up. These are most commonly available in powers of up
to around 3kW.
PSC or Permanent Split Capacitor
The running capacitor of
this type of motor is “permanently” left in series with the start
winding during its full operating cycle. Performance is maximised by matching
carefully the capacitance in relationship to the winding resistance but
typically the starting torque is quite low with a maximum of around 60%-70%
of nominal torque being the norm. This characteristic makes the
“PSC” type motors suitable for low torque applications such as
pumps and fans.
CSR or Capacitor
Start & Run
With this type of motor
it still has the run capacitor in series with the auxiliary or start winding
but then has a second starting capacitor in parallel to the running
capacitor. This is where the magic of a good quality high torque single phase
motor comes into play. The real trick is to be able to switch out of circuit
this additional starting capacitor at just the right moment. If it is
switched too late then damage can be caused to the capacitor and if it is
switched too soon the motor may not get up to speed and a cycling effect can
occur. With effective switching & accurate matching of capacitors to
windings, very high starting torques can be achieved reaching levels often
between 200% and 250% of nominal torque. This characteristic makes this type
of motor more suitable for machine builders with relatively high inertias to
move from standstill.
There are three main
switching methods for CSR motors.
Centrifugal Switch
- The old
traditional method of removing the start capacitor from the running circuit
on CSR motors was to use a mechanical centrifugal switching device that
caused that part of the connection to be open circuit upon reaching the
pre-determined speed. This device was reasonably effective but brought with
it many maintenance & reliability problems after short to medium periods
of use providing a serious amount of refurbishment work for motor repair
companies. Reliability was also found to be affected by environmental
conditions, ambient temperatures etc. This method is still fairly widely used
however, by many low cost motor manufacturers.
Switching Relay - The centrifugal switching method
was then replaced in the 80’s and early 90’s by some
manufacturers with an “SR” or “switching relay”. This
was thought to be a more reliable solution but at a much higher price. Unfortunately
it was found that this also was not the ultimate in reliability with similar
problems as the centrifugal device and added problems through vibration etc.
Unfortunately this method had zero refurbishment capability i.e. when it
failed it had to be replaced. The end result of this situation was that many
manufacturers then reverted back to the centrifugal switch solution.
Electronic
Switching Device - The third & ultimate switching solution is the
“ESD” or “electronic switching device”, this is the
modern day equivalent of the switching relay without the reliability or
performance problems experienced with either of the first 2 solutions. It
brings with it many other benefits not experienced previously as modern day
electronics can bring with them intelligence that older mechanical devices
cannot. For example the “ESD” can sense the voltage within the
auxiliary winding & optimise the switching point based upon changes in
this supply, volt drops etc. Modern day electronics, particularly when
encapsulated, can also cope far better with environmental condition, ambient
temperatures & wider vibration levels. This method can also help with
increasing the starting torque even further with figures occasionally
exceeding 300% of nominal torque.
Single phase
variations
The more serious motor
manufacturers are also able to offer many electrical & mechanical
variations of single phase motors for example, brake
motors, double shafted, special mounting designs, dual voltage eg. 115v/230v combined etc.
Another tell-tale sign
for quality conscious single phase motor producers is that their capacitors,
be it single or double, are mounted within an enlarged terminal box to
protect them and their starting devices from environmental & external
mechanical damage.
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