VFDSPINDLE VFD & CNC spindle wiring, sizing and setup

Unit converter

Power, torque, speed, length and temperature — the five conversions that come up when you are reading a spindle nameplate against a drive datasheet. Type into any box; the rest follow.

The converters

Power

Imperial mechanical horsepower. If your nameplate says PS or "metric hp", see the note below.

Torque

Type into whichever unit you have; all five update.

Speed, and the frequency that produces it

Synchronous speed, no slip. Set the pole count to match the nameplate — most router spindles are two-pole.

Length and temperature

Where each constant comes from

None of these are looked up in a table. They are either definitions or one line of arithmetic, which is why the page can show them:

Conversion constants used by the boxes above
QuantityConstantWhere it comes from
Horsepower to watts1 hp = 745.699872 WDefinition: 550 ft·lbf/s, with the foot, the pound and standard gravity all defined exactly
Metric horsepower1 PS = 735.49875 WDefinition: 75 kgf·m/s. About 1.4 % smaller than imperial hp
Foot-pound to newton metres1 ft·lb = 1.355818 N·mDefinition, from the same three constants
Newton metres to inch-pounds1 N·m = 8.85075 in·lbThe above, divided by 12
Inch-pound to ounce-inches1 in·lb = 16 oz·inDefinition of the avoirdupois ounce
Kilogram-force centimetre1 kgf·cm = 0.0980665 N·mDefinition: standard gravity times one kilogram over one centimetre
Inch to millimetres1 in = 25.4 mmDefinition, exact since 1959
Frequency to speedrpm = 120 f / pOne revolution per cycle pair: 60 s per minute, two poles per cycle

The two that catch people out

Horsepower is not one number

An imperial horsepower is 745.7 W. A metric horsepower — PS, sometimes written "hp" on a European or Asian nameplate — is 735.5 W. The difference is about 1.4 %, which is small enough that nobody notices and large enough to matter if you are sizing a drive against a marginal nameplate. If the plate gives watts or kilowatts, ignore the horsepower figure entirely; if it gives only "hp" and the machine was not made for the US market, assume PS and check the current against the plate.

Rpm and Hz are only linked through the pole count

There is no single "Hz to rpm" factor. rpm = 120 f / p, so the same 400 Hz is 24,000 rpm on a two-pole spindle and 12,000 rpm on a four-pole one. This is why a spindle rated 24,000 rpm at 400 Hz and one rated 12,000 rpm at 400 Hz are not contradicting each other — they simply have different windings. Set the pole count before you read anything off the box, and take it from the nameplate rather than from the maximum speed, because the maker may be driving the spindle above its base frequency.

These are synchronous numbers. An induction spindle always turns slightly slower than the rotating field — a few per cent at rated load. The converter gives you the field speed, which is the number that goes into the drive; the measured speed will be below it.

Parts

Spindles

Questions

Is a spindle hp the same as an imperial horsepower?

Not always. Imperial horsepower is 745.7 W; metric horsepower (PS) is 735.5 W, about 1.4 % smaller. If the nameplate is not from a US-market machine and only says hp, assume PS and check the current.

How do I convert rpm to Hz?

Through the pole count — there is no single factor. rpm = 120 f / p, so 400 Hz is 24,000 rpm on a two-pole spindle and 12,000 rpm on a four-pole one.

Why is the measured speed lower than the converter says?

Because the converter gives synchronous speed, and an induction spindle always runs slightly slower than the rotating field — a few per cent at rated load. The synchronous number is the one that goes into the drive.

Sources and method

Three kinds of statement appear on this page, and each is labelled where it occurs:

Standards and documents

Further reading