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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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up vote 4 down vote accepted

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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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? – Eli Iser Jul 6 '14 at 10:35
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. – WhatRoughBeast Jul 6 '14 at 12:20
Thank. And another follow-up question - you said "variable speed controllers" should work; what about constant speed under load controllers? – Eli Iser Jul 6 '14 at 12:25
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. – WhatRoughBeast Jul 6 '14 at 12:32
No, I meant would a tool with constant speed designed for 60 Hz run correctly on 50 Hz? – Eli Iser Jul 6 '14 at 13:20

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