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Active solutions for high speed spindle application
June 2006
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Introduction
High speed spindles are used in a variety of industrial applications (for example, the wood, glass, metal, marble and granite industries). Whereas in the past high speed spindles were generally driven by DC converters, today, for technical and economic reasons, they are driven almost exclusively by frequency converters and asynchronous motors.
The spindle motor
Although spindle motors are asynchronous in nearly all cases, for reasons related to thermal load, dimensions, mounting and maximum speed they are almost never to IEC standards.
For these reasons, special care has to be taken during the motor's manufacture to achieve perfect rotor balancing and minimum rotor inertia. Only top quality bearings must therefore be used. The motor itself can be either air or liquid cooled.
The stator windings have to be designed to withstand dv/dt values of up to 5000 V/µsec since in most cases the frequency converters used to drive motors feature IGBT technology with trise & tfall times < 150 nsec. Cables between the frequency converter and motor can also be over 10 metres long, giving rise to PWM commutation wave reflection phenomena at the output from the frequency converter.
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In many cases the V/Hz ratio can be less than 1, and the deflux ratio of the c/n curve can be as high as 1:4.
Connections may also be needed for an encoder (an application that demands spindle orientation) or a zero speed sensor (as required by safety standards).
Frequency converter
A software version dedicated to spindle motors has been developed for the ACTIVE frequency converter range, combining the excellent performance and special features of the ACTIVE range with the necessary application-specific characteristics.
This software features special functions that satisfy all the principal requirements of high speed spindle motor control.
Characteristics
- Maximum output frequencies of up to 1000 Hz with stable functioning;
- Maximum carrier frequencies of up to 15 KHz with minimal loss;
- Excellent modulation index (normally obtainable only with SVPWM) to maximise torque in the deflux range;
- Excellent dynamics and “intelligent” acceleration and deceleration with low and high inertia tools;
- Rapid spindle motor stops in emergency conditions (less than 10 secs.);
- Controlled spindle motor stops in the event of mains power failure or an error or failure in the frequency converter (overvoltage, overcurrent, braking resistance failure, etc.);
- “On-the-fly” spindle motor control;
- Multi-spindle control from the same frequency converter (multi DataSet) with specific and optimised control parameters for each motor;
- Integrated braking module for powers up to 30 kW;
- Thermal imaging protection of external braking resistance;
- Spindle position and orientation control for high speed, precision tool changes (precision > 0.5°);
- Frequency converter control over a serial line or over any of the main field bus types currently on the market (CanOpen profile 4.0.2, Profibus DP, etc.).
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