Linde T20 AP, Service Training

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Service Training
Section 6
Page 8
High speed calculation microprocessors are used to make complex mathematical calculations required to arrange this
operation in sequences. It generates a revolving magnetic field variable at three phases. The synthesised AC supply
applied to the motor windings is thus an AC supply with variable frequencies and voltages. The motor rpm and the
available torque are controlled very accurately during the drive and also while braking through recovery.
6.1.3.2 Speed control and direction of travel
The truck speed is proportional to the frequency of the power supply applied to the motor. The direction of travel depends
on the conducting path through the three motor windings, one with relation to the other.
When the AC frequency applied is low, the average AC voltage applied to the motor windings is also low. On pressing
the accelerator initially, the voltage and the AC motor frequency will be low. Gradually, as the truck speed grows, the
frequency and AC voltage also grow simultaneously.
With the AC asynchronous type of motors used in the truck 131, the maximum torque is obtained when the motor
rotating speed is slightly less than what is determined by the AC voltage generated by the controller, i.e. the
“synchronous speed”. The minor difference is called “creep speed”. The controller regulates the voltage and the motor
frequency to ensure that this creep speed is maintained at the optimum value to have efficient performances.
The controller monitors the motor current and to ensure that it does not become excessive, it automatically assumes
priority on the accelerator loading to reduce the frequency and the AC voltage when necessary.
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