How to compute the drum rotation speed for a Giesen W15.

A Giesen roaster controls the speed of all its motors through frequency inverters. An inverter raises or lowers the frequency of the electric current depending on the settings that the user makes.

An electrical motor powered by lower frequency spins its shaft at a lower speed, just as a motor that is powered at a higher frequency spins its shaft at a higher speed.

The rotational speed of a single-phase electric motor shaft is calculated with the following formula: N = 120 x F / P, where F is the frequency of the electric current and P is the number of poles of the motor. The value obtained (N) is called the synchronous speed because the motor is not in load, ie it only spins its shaft.

When the electrical motor is under load, the speed decreases slightly due to a “slip”. The higher the load, the greater the slip. For example, when a motor is blocked, ie the shaft does not rotate, the slip is 100%. The slip calculation formula is s = (Ns – Na) / Ns, where s is the slip, Ns is the synchronous speed and Na is the shaft speed.

All electric motors have an identification plate attached to the motor with rivets. This plate should tell you everything there is to know about your motor. Here is an example.

Electric Motor ID Tag

In the example above we have a 4-pole motor, powered at a frequency of 60 Hz. In load, this engine generates a rotation of 1710 RPM.

Theoretically, this engine should rotate at a speed of 120 x 60/4 = 1800 RPM but the manufacturer specifies a nominal speed of 1710 RPM. In this case, using the above formula, the slip is s = (1800 – 1710) / 1800 = 0.05 so 5%. In practice, this slip will be smaller and I explain below why.

The connecting element between the electric motor shaft and the drum shaft is called a gear motor. Each gear motor has an identification plate attached. Here is an example.

Through a system of gears, this gear motor slows down the speed of the electric motor shaft by 20 times (the ratio).

The 5% “slip” we talked about above is valid when the conditions are met, namely when the motor is powered at a 60 Hz frequency. If the motor is powered with less than 60 Hz, it will practically have a smaller slip that I estimated at 2.5%. I perform the estimation and then verify the calculation with a tachometer like this.

To use this tachometer, remove the round cover from the faceplate. It houses one of the bearings on which the drum shaft sits, a bearing that you should lubricate monthly. Now you have access to the drum shaft and you can measure and check its speed very easily using the tachometer above.

Now let’s calculate an example.

Let’s say you’ve set your Giesen to 44 Hz. What will be the drum RPM?

Powered by an electrical current of 44 Hz, the motor will rotate at 120 x 44/4 = 1320 RPM but we must also take into account a slip of 2.5%, so the motor shaft speed will be 1287 RPM.

In the example above, the de-multiplication factor is 20:1. This means the rotation speed of the electrical motor shaft will be reduced 20 times, respectively 1287/20 = 64.35 RPM and, although it is theoretical, you can check this value with the tachometer. The practical speed could differ a little due to aged bearings or electrical tension fluctuations.

Smaller batches, higher speed.

Bigger batches, lower speed.

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