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First, the end goal is to run a power tool (namely a Dewalt DW734 thickness planer) designed for 110V at 60Hz at a country where the power is 220V at 50Hz. The motor is a brushed universal motor, constant speed (i.e. not variable speed, but also doesn't seem to be electronically controlled).

I've read a lot on the Internet about running motors (universal and induction) at different frequencies (50 and 60 Hz, which are the common frequencies around the world), but there are conflicting opinions.

Naturally I will use a step-down transformer to get the voltage to 110V. The question is how will the frequency change affect the motor?

Some sources claim that a universal motor doesn't care about the frequency at all (and can run on DC as well), while others claim that the motor will run at 50/60=83% speed (but this might be the case only for induction motors, not universal brushed motors).

Other places claim an issue not directly with frequency, but with the voltage-frequency ratio (110/60 vs. 110/50) causing the motor to draw more current and over-heat.

Another issue comes from electronic speed control and variable speed control (the DW734 doesn't have them, but other power tools do). Will these require the correct frequency, or do they operate on DC anyway and the line frequency is not important?

There are power supplies that convert the frequency as well (VFD invertrs), but they are more expensive than regular step-down transformers and not all of them produce a perfect sine wave (which is another question - how important is a perfect sine wave vs. a PWM signal?).

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Brushed universal motors are largely independent of AC frequency, and as you have heard, will also run on DC. Their maximum speed is normally way above any synchronous speed referenced to line frequency. In their construction, the phase angle between the rotor and stator fields is set by geometry, not line phase. This is also why they run at high speeds - they just don't care about the line frequency. This is ideal for appliances such as vacuum cleaners and tools such as routers and planers, which don't need to maintain constant speed under varying loads. On the down side, they are typically not very efficient at low speeds, but for an application where they can run fast, that's not a problem.

As long as you get the voltage right, you'll be fine.

Variable speed controllers will also work well.

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  • \$\begingroup\$ Then why aren't all universal motors in power tools rates as 50/60 Hz (only some are labeled as such)? Or is this just for marketing reasons? \$\endgroup\$ – Eli Iser Jul 6 '14 at 10:35
  • \$\begingroup\$ First off, I don't know for sure. I'd guess that, for tools built by companies which don't market outside the US, it just never occurred to them. \$\endgroup\$ – WhatRoughBeast Jul 6 '14 at 12:20
  • \$\begingroup\$ Thank. And another follow-up question - you said "variable speed controllers" should work; what about constant speed under load controllers? \$\endgroup\$ – Eli Iser Jul 6 '14 at 12:25
  • \$\begingroup\$ That's much harder, and for universals requires that it be built in to the tool. Basically, it has to sense the motor speed (or a proxy) and change the drive accordingly. It's not something that can reasonably be done as an add-on. \$\endgroup\$ – WhatRoughBeast Jul 6 '14 at 12:32
  • \$\begingroup\$ No, I meant would a tool with constant speed designed for 60 Hz run correctly on 50 Hz? \$\endgroup\$ – Eli Iser Jul 6 '14 at 13:20
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The variable speed module that keeps the speed of a universal motor constant whilst under load is called "variable speed with electronic feedback".

This type of variable speed control module is a transistor based circuit and 'senses' the load on the motor and automatically adjusts the amps and voltage supplied to the motor within a typical 10% preset tolerance to accommodate the load placed on the universal motor in order to maintain the set speed of whatever speed the universal motor is set at.

A typical example is an old Black & Decker BD533E Corded Electric jigsaw pictured below, this is a common Black & Decker domestic power tool (manufactured from the late 1980's to mid 1990's) - I have this saw, it is 230 volts ac 350 watts and 50hz, the variable speed module in this saw does a pretty good job at keeping the motor speed constant at low speeds, even when the saw is under heavy load! enter image description here

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  • \$\begingroup\$ Your driver will not adjust the current in any way. The motor will. \$\endgroup\$ – winny Apr 11 '17 at 18:49
  • \$\begingroup\$ In phase angle control method Speed control is achieved by varying the firing angle for the TRIAC. Phase angle control is very cost effective solution but not very efficient .In PWM method rectified AC line voltage is switched at a high frequency by a Power MOFSET or IGBT device to generate time varying voltage for the motor. In this method to control the motors by providing stable speed control, preventing large currents and drawing minimum harmonic current from ac mains supply are required. To meet these requirements using AC chopper with current and speed feedback is preferred. \$\endgroup\$ – Joseph Apr 11 '17 at 19:36
  • \$\begingroup\$ The AC universal motor drive controls the rotation speed by means of phase-angle partialization. This method consists of changing the RMS voltage applied to the motor. In this case, the voltage is a function of the firing angle of the Triac. \$\endgroup\$ – Joseph Apr 11 '17 at 19:36
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Nothing significantly changes 50 or 60 Hz for universal motors. Just adjust voltage 110 to 220V. Reminding: a 110V rated universal motor may run in 220V but a 220V rated universal motor should not be runned in 110V since probably wire gauge will not be sufficent for almost double current.

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