Need some tips for selecting the optimum motor & drive combination for your latest high-speed spindle application? Read on to find out why this oft-misunderstood process is far simpler than you think.
You will have almost certainly noticed that more and more high-speed electro-spindles on modern machine tools are built using permanent magnet synchronous motors. Why so? Well, quite simply they not only help to reduce the machine footprint, but improve surface finish accuracy and quality as well.
In the first instance, the application must be studied in continuous mode and the design engineer has to define the desired maximum speed, along with the maximum continuous torque to be applied for low speed deep-cut roughing operations, or large diameter machining applications.
However, if the power (torque x speed) is calculated at this early stage, the result will often be so huge that it will exceed what is needed – the drive will be oversized and of course far more expensive than it need be. For this reason, the maximum continuous power required by the spindle must be defined, running with a field weakening mode to limit the drive size
With this data clearly defined, motor choice becomes easy and should comply with the following properties:
Field-weakening mode manages the phase advance between the current and the back electromotive force (BEMF) of the motor in accordance with the power requirement.
Select the drive size that provides enough rated current (Io) to the motor in order to generate the maximum torque needed. To calculate this current, divide the desired torque by the torque constant (Kt) of the motor. This is expressed in Nm/Arms.
Take note that as the size increases, the output frequency has a nasty habit of decreasing. For this reason you should double-check the drive’s ability to provide sufficient frequency. Here, you should calculate the minimum frequency requested by the motor to reach maximum speed.
The result must be less than or equal to the maximum output frequency of the drive. This is not to be confused with the PWM frequency of the drive that corresponds to the switching frequency.
The next step only applies to synchronous motors with magnets, largely because of their potential to reach high voltages. During high speed operations, the BEMF of the motor must be below the maximum DC voltage of the drive’s power bus. Typical power bus values are 540 VDC for a 400 VAC power supply. As a general guide, avoid exceeding 800 VDC. Check the voltage value of the motor’s BEMF at maximum speed. This limit is given by another motor constant Ke expressed in Vrms / 1000rpm:
The results are best plotted in vector diagrams showing axe mode, field weakening mode below the maximum voltage of the drive, and field weakening mode beyond the maximum voltage of the drive.
The latter will show if the BEMF is over the drive limit due the high maximum speed. In this case a protection module has to be added. This is because, in the event of an incident or power supply failure, the motor can become a generator (driven by the load inertia). Here, the BEMF is on the terminals of the drive and, at extreme speeds, the motor and the drive will be damaged by excessive voltage. A protection module will short-circuit the motor or clip the motor voltage accordingly. Of course, the protection module and the drive must be capable of withstanding the short-circuit of the motor current to ensure integrity. The motor, the drive and the protection module have a maximum voltage limit (around 2000V).
The following chart will guide you through the process:
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