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The Characteristics of Clock Motors

By Elina | Published on Dec 09,2015
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Note that all synchronous motors are rated for a specific mains frequency (50 or 60Hz), and will require a modified gear train to be able to use a different frequency. They cannot be rewound to suit a different mains frequency - the rotational speed is determined by the number of poles and the applied frequency, not the coil.

Most synchronous clock motors use multiple "claws" to create a large number of poles. As can be seen from the above photos, it isn't actually necessary to maintain perfect spacing or to even make sure that all poles are present! The above motor is 24 pole, so rotates at 250 RPM (with 50Hz mains frequency).

The general principle of the multipole synchronous motor is shown in Figure 5. The coil is held between two plates, each with radial sections that are then bent upwards to form a ring of poles around the rotor.

The rotor has multiple magnets embedded in a (usually) plastic disc, and these will align with the stator poles when no power is applied. When AC current flows in the coil, each stator pole alternates between North and South polarity, in sympathy with the applied AC. With 50Hz mains, this changes 50 times per second (60 with 60Hz mains).

When the power is first applied, the rotor typically just jiggles around for a short time, until a N on the rotor aligns perfectly with a S on the stator. As the poles alternate between N and S magnetic polarity, the rotor follows the field and turns, jumping from one pole to the next to maintain attracting magnetic polarities. It may choose to run in either direction at this time.

By adding a direction-sensitive pawl, the motor is usually set up to ensure the pawl engages with the rotor should the motor spin in the wrong direction. This stops the rotor, but adds sufficient "bounce" to force the motor to run the right way. The lug on the rotor can be seen in Figure 4 - the pawl is attached to the first "wheel", driven by the rotor pinion. A simple viscous (oil) drive pushes the pawl into the rotor should it spin in the wrong direction. Correct rotation keeps the pawl against the right hand red post and out of the way.

This type of synchronous motor does not actually spin smoothly. Its rotation is in steps, and this can be felt if one gently touches the rotor or the first wheel. There is a slight vibration that is quite evident, and this is similar to the behavior of stepper motors (as used in printers, fax machines and many other computer driven motor applications). Although there may not appear to be any similarity whatsoever, a synchronous clock motor is very similar to the motor used with quartz oscillator driven movements.

A significant disadvantage of synchronous motors is that they are frequency sensitive. A clock designed for the Australian or European market will run 20% fast in the US, and a US motor will run 16.7% slow elsewhere. On the other hand, they are more accurate (long term) than most quartz movements, because the power utilities worldwide maintain exceedingly accurate control over the number of AC mains cycles per day. In Australia, that means 4,320,000 cycles per day (50Hz mains).


Timer Synchronous Motor

Above, you can see the evolution of the simple synchronous motor. The mechanism shown is from an electrical time switch of fairly recent vintage. Gone are the defined poles using claws, and the motor has been simplified down to the bare minimum. It still needs the automatic reversal mechanism, and the motor shown is a 6 pole type (so will rotate at 1,000 RPM). If you compare the above motor with a quartz clock motor (see below) the similarity is immediately obvious - in fact there is hardly any difference except physical size.

This motor doesn't have enough turns to support the full 240V mains voltage. Within the housing of the time switch, there is a resistor to reduce the maximum current flow. Without this, the motor would burn out in only a few minutes. This is a similar arrangement to that used in the rewound Telechron motor described above, and while it is cheaper than a capacitor, the resistor uses more energy and generates heat (although neither is troublesome).


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