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2.3. MOTOR CONTROL PRINCIPLE
2.3.1. INTRODUCTION
The SORVALL T3 / SORVALL RT3 family of centrifuges is fitted with an asynchronous 3
phase motor. During centrifugation, the microprocessor generates a system of sinusoidal 3 phase
voltages variable in amplitude and frequencies. During the braking phase the energy from the
rotating parts is commuted across a resistance.
2.3.2. THE MOTOR
The motor is comprised of a 3 phase stator and a short-circuited rotor.
When correctly powered, the stator creates a magnetic field rotating at the synchronised speed
Ns :
Ns = F/p
F : supply frequency
P : number of pairs of poles of the motor
The rotating field generated in the motor induces e.m.f.'s. These e.m.f.'s produce currents in the
short circuited rotor.
These induced currents found in the magnetic field create forces opposed to those which induced
them.
It is the relative speed between the rotor and stator which is the cause of these induced currents,
thus the forces will reduce this relative speed by driving the rotor at a speed close to that of the
fields created at the stator.
The difference (s) between the synchronised speed Ns and the actual speed N of the rotor is
called the slippage.
s = Ns - N
Ns
At a given frequency, the slippage value depends on the resisting torque (i.e. that of the rotor).
Rotor
Service Manual 11290099
Stator
page 12 of 44
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