Incorrect or missing data on cheap chinese VFDs and spindles

Hey Robert, hey all,

both the hand trim router and a spindle have advantages and disadvantages. Besides the Makita hand trim router first of all not being allowed by its manufacturer to be used in a stationary machine like a CNC and it was confirmed that you loose its warranty immediately as soon you do, I can try to list the pros and contras of both types of motor:

Hand router (universal motor)

Your hand trim router is a so-called Universal Motor (also called commutated series-wound motor, because it needs carbon-brush commutators to work and because its stator and its rotor coils are connected in series).

  • Pro: Such a motor has the advantage that you only need a simple cable to your domestic outlet to plug it in, because it is an alternating current (AC) motor for single phase electricity and its speed is independent of the frequency of the alternating current.

Besides of this, a universal motor has only disadvantages:

  • Contra:

    1. It is more complicated and therefore more expensive to be built as an induction motor
    2. It has a poor efficiency, 30–70% at its highest, which means it consumes more electric power and produces less usuable mechanical power (torque) as an induction motor
    3. It has a particularly unfavorable motor charachteristics, in particular for milling wood: When it has speed, it has no torque, but when it has torque, it has no speed. Unfortunately what you need for milling wood is high torque at high speed, which it typically can not deliver:

    – Source: Traute Meyer, CC BY-SA 3.0, via Wikimedia Commons (rotated and mirorred to reflect axes of Image 2 and with comments added by Aiph5u)

    Image 1: Motor characteristic (speed/torque diagram) of a Universal Motor (=the hand trim router): When you have torque, you have no speed, and if you have speed, you have no torque :frowning:.

    1. When you put a higher load on such a motor, it is slowed down.
    2. When such a motor runs slow, it gets hot (see warnings in your manual), up to the point where it begins to burn.

Spindle (induction motor)

A spindle, on the other hand, is an induction motor. The induction motor is wide spread in machinery and is considered as the workhorse in the industry. It has nearly only advantages:

  • Pro:

    1. It is simple to build, since it has only stator coils, but neither rotor coils, nor any carbon-brush commutators.
    2. It has an excellent efficiency, which means the biggest part of the current is transformed into mechanical power
    3. It is capable of a wide range of stable speed
    4. When put under mechanical load, it is not slowed down
    5. It will never be overloaded, or start to burn, because you always have to set the maximum current according to motor rating inside the VFD, so on emerging overload, the VFD will stop the spindle and if correctly wired to an emergency circuit, at the same time halt the g-code program in the CNC controller, so you can leave it alone without fear that it could burn your place down
    6. It has an excellent motor characteristic, which means, a constant torque over the entire speed range:

    – Source: Mechatron HFS-8022-24-ER20 Datasheet (with comments added by Aiph5u)

    Image 2: Motor characteristic (speed/torque diagram) of a frequency-controlled Induction Motor (=spindle):
    A constant torque available over a wide speed range, and with excellent efficiency

  • Contra: The disadvantage of a spindle is, in order to be able to run one, you need a three-phase current with a variable frequency. This means, you forcibly need a variable frequency drive (VFD), which is an AC-to-DC-to-three-phase-AC inverter that can produce three-phase electricity with a variable frequency.

What I find important to say in this context, is that between these two extremes, there exist a third category that would be the best solution for hobbyists and semiprofessionals who just want to plug in. Unfortunately, I have no information about whether Onefinity has them on their radar and will provide an appropriate Z slider to use them with the Onefinity CNC.

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