Back to basics: single-phase motor theory

When examining single-phase motor theory, it is important first to clarify the key differences between a three-phase supply and a single-phase supply.

When examining single-phase motor theory, it is important to clarify the key differences between a three-phase supply and a single-phase supply.

It may be a little easier to think of a day-to-day similarity like this.

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 three-phase supply is like having three 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 GBP 20,000, depending on the area, accessibility, required cable lengths etc.

One example of an industry sector in which 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, while 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.

An electric motor uses basic magnetic rules of repulsion and 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.

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, as there would be no moment on 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 two main types of single-phase motor, and both use capacitors to shift the magnetic field on startup.

These are most commonly available in powers of up to around 3kW.

A PSC or "permanent split capacitor" motor has a running capacitor that 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 PSC type motors suitable for low-torque applications such as pumps and fans.

In CSR or "capacitor start and run" type motors the run capacitor is also in series with the auxiliary or start winding but there is 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 and 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.

The traditional method of removing the start capacitor from the running circuit on CSR motors has been to use a mechanical centrifugal switching device that caused that part of the connection to be open circuit on reaching the predetermined speed.

This device was reasonably effective but brought with it many maintenance and 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.

The centrifugal switching method was then replaced in the 1980s and early 1990s 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, ie 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.

The third and 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 two 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 and optimise the switching point based on changes in this supply, voltage drops etc.

Modern day electronics, particularly when encapsulated, can also cope far better with environmental condition, ambient temperatures and wider vibration levels.

This method can also help with increasing the starting torque even further with figures occasionally exceeding 300% of nominal torque.

The more serious motor manufacturers are also able to offer many electrical and mechanical variations of single phase motors for example, brake motors, double shafted, special mounting designs, dual voltage (eg 115/230V) combined etc.

Another telltale sign for quality-conscious single-phase motor producers is that their capacitors, be they single or double, are mounted within an enlarged terminal box to protect them and their starting devices from environmental and external mechanical damage.

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