HAMMOND ORGAN

The synchronous motor of the Hammond is very simple. In fact, it’s so simple that it has no means of starting by itself. Once brought up to speed, it can continue to run at 1200 RPM indefinitely and drive all of the tone wheels, but it can’t get off zero RPM without external help no matter how much AC you apply! A shaded pole induction motor is a simple motor which is self-starting. However, its speed is not constant enough to control the tuning of a musical instrument. But, by virtue of being self-starting, it can start a Hammond generator main shaft and accelerate it to and even slightly above its rated speed. Once the main shaft of the Hammond tone generator is running slightly over 1200 RPM, you can supply the synchronous motor with 60 Hz power, and it will immediately lock in step with the incoming AC, slow down, sync in and continue to run at 1200 RPM. The synchronous motor of the Hammond runs with a series of pulsations, one pulsation for each half cycle of the incoming 60 Hz. power. Just below is a flash animation that shows how the Hammond synchronous motor works.

Flash Animation, left. This is an elementary diagram of the Hammond synchronous motor. The rotor is a roughly six pointed star shaped piece of mild steel attached to the shaft. A laminated rectangular stator surrounds the rotor and has two pole projections which are wrap-ped with wire coils. Stan-dard 120 volt, 60 cycle alternating current power is applied to both coils ma-king the pole projections become magnets. The red arrows on the ends of the wires which lead into each coil denote which direction the AC is flowing at any moment.

     The letters N and S denote North and South magnetic poles which form on the motor elements as a result of the AC which flows in the coils. As the AC reverses its direction, indicated by the small red arrows, the polarity of the stator poles changes (red letters N and S). The stator poles also induce opposite poles on the arms of the star-shaped rotor. Because opposite poles attract, the stator poles pull the rotor poles around, causing rotation of the star. As the rotor poles pass the stator poles, the stator poles reverse polarity, attracting the next two poles of the rotor. Notice that once the rotor is turning, the poles on the rotor become "permanent" as long as the rotor is turing. Such a motor will not start by itself, but once started will run at a constant speed from then on until we shut off the supply of 60 cycle AC power to the stator coils. Such a motor will continue to run in whatever direction it is first started. This type of motor is called a "reluctance" synchronous motor. It is extremely simple and has the distinct advantage of running exactly in step with the AC that is supplied to its coils. If the frequency is constant, the rotational speed will likewise be constant. In the USA, utility-supplied 60 cycle (60 Hz) power is extremely accurately controlled and may be considered a standard reference for any circuits which require critical timing or constant speed operation. In order to show all of this clearly, the animation is greatly slowed down. At 1200 RPM, the star would appear only a blur and the sequence of events would not be visible.

Figure 5. This is the shaded-pole induction motor at the back end of the Hammond tone generator. Its only purpose is to start the main shaft and accelerate it up to about 1450 RPM. Because the synchronous motor is coupled to the main shaft, it likewise gets pulled up to 1450 RPM. Then we can apply power to the synchronous motor which imme-diately slows to 1200 RPM, locks into step with the in-coming power, and continues to turn the main shaft at exactly 1200 RPM as long as power is applied. As soon as the synchronous motor has pulled into sync and stabilized, the power to the shaded-pole starting motor is cut off, and a spring disengages the pinion on the start motor shaft.

     This means that it introduces a strong 120 Hz mechanical vibration into its output shaft. If coupled directly to the tone wheels, its 120 Hz vibration would carry over into the generated tones of the instrument and also make a loud 120 Hz mechanical buzzing noise. To prevent this, the synchronous motor is not rigidly coupled to the drive shaft, but rather drives it through a combination of three springs and two small flywheels. (See figure six.)

Figure 6. The somewhat elaborate coupling and mechanical vibration filtering between the synchronous motor and the input shaft of the Hammond tone gen-erator. The roughly rectangular assembly to the right is the synchronous motor. Immediately following it are two flywheels with two springs at a forty-five degree angle between. Next you see a helical spring on the shaft just before the small mechanical coupling. All of these smooth out the output of the synchronous motor and deliver steady, vibration free rotation to the tone gen-erator main shaft. If you roll your mouse over this picture, individual titles of the parts will appear.

     The springs absorb the pulsations and the flywheels further smooth out the rotation, just as the flywheels of engines smooth out the pulsations that result from the intermittent action of their cylinders. After this mechanical filtering, the precise 1200 RPM rotation of the main shaft is very smooth and contains no extraneous mechanical buzzing to make extra mech-anical noise or introduce a spurious 120 Hz vibration into the tone wheel system.
     But wait! There’s more. The tone wheels are driven by gears from the main drive shaft, which itself is made up of a number of sections which are flexibly coupled together. The main shaft spins at 1200 RPM, but the tone wheels are geared from the main shaft at twelve different speeds.
     Gear teeth also introduce extraneous mechanical frequencies into any machinery where gears are involved in power transmission. Therefore, the tone wheels are not rigidly coupled to their driving gears, but are arranged in pairs, two wheels to a single tone wheel shaft. Between each pair of tone wheels is a bakelite gear which drives the tone wheels through two helical springs. The bakelite gear can spin freely on the shaft; the pair of tone wheels being driven entirely through the springs. (Figure seven, next page.) The two tone wheels act also as flywheels, and the springs absorb the gear tooth ripple.
     The Hammond tone generating unit is divided into a series of compartments, with four tone wheels (in most cases) in each compartment. All four wheels in any compartment run at the same speed, but have different numbers of teeth. Thus they all generate the same note, but each produces a different octave of that note. As is true in any system where alternating currents flow through coils, there is some transformer action whereby the AC in one coil induces a small AC of the same frequency into adjacent coils. The Hammond tone gene-rating system is no exception. One of the ways to eliminate this problem in the Hammond is to keep wheels and coils of unrelated frequencies magnetically shielded from each other. This is the main reason why the tone generating unit is divided into compartments separated by steel plates. Steel plate provides excellent magnetic shielding.

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