kHz to RPM Converter for Turbo Pumps and Spindles

Turning a Turbo Pump's Drive Frequency Into Rotor Speed

A turbomolecular pump controller almost never shows revolutions per minute. It shows a drive frequency in hertz or kilohertz, because that is what the inverter commands, while the datasheet rates the pump at 60 000 or 90 000 RPM. Ultra-high-speed spindles have the same split: the drive talks in kilohertz, the tooling catalogue in RPM.

Conversion factor: 1 kHz = 1 000 Hz, and one cycle per second is 60 revolutions per minute, so 1 kHz = 60 000 RPM — provided one cycle of the quoted frequency equals one full turn of the shaft. On that basis 1.5 kHz is 90 000 RPM and 0.4 kHz is 24 000 RPM.

Check what the cycle counts. Hertz is a true frequency and RPM a rotational rate; the ×60 000 factor bridges them only when the cycle and the revolution are the same event. A brushless drive with p pole pairs produces p electrical cycles per turn, so shaft speed is 60·f / p: at 1 kHz a two-pole-pair motor turns at 30 000 RPM, not 60 000. A tachometer emitting several pulses per revolution behaves the same way.

Why These Machines Live at Six Figures of RPM

Blade Speed Against Molecular Speed

A turbomolecular stage strikes gas molecules with a surface moving at a speed comparable to their own. Nitrogen averages about 470 m/s at room temperature, and a 60 mm rotor at 90 000 RPM has a tip speed near 283 m/s. Hydrogen averages roughly 1 755 m/s — hence the far weaker compression for light gases.

Bearings That Never Touch

Above roughly 40 000 RPM a contacting bearing becomes a consumable. Active magnetic bearings suspend the shaft in a field; air-bearing spindles float it on a pressurised film. Both keep lubricant out of the vacuum.

A Revolution in 667 Microseconds

At 90 000 RPM the rotor completes 1 500 turns a second, so one revolution lasts 667 μs — the timescale a once-per-turn trigger works on.

From a Controller Readout to a Rated Speed

Enter the frequency the drive reports

Type the kilohertz figure from the front panel or the serial log. If the display is in plain hertz, pick Hz on the left-hand dropdown. Commas and dots both work.

Divide by the pole pairs if the value is electrical

Read the RPM field, then divide by the motor's pole pairs or the tachometer's pulses per revolution. Only when that count is one does the raw ×60 000 figure equal shaft speed.

Compare it against the rated speed

Hold the result next to the nameplate figure. A pump idling at 82 per cent of rating is still running up, throttled by gas load, or overheating.

Go the other way for a commissioning target

Press the swap arrow, or type into the RPM field, to find the frequency a rated speed corresponds to before setting a trip point.

Rated Speeds and Drive Frequencies Across the High-Speed Range

The third column is the shaft's own rotational frequency, one cycle per revolution. The fourth is what an inverter outputs for a motor with two pole pairs — a common arrangement on pump drives — and is exactly double. Reading them together tells you whether a controller value has already been divided down.

Machine	Rated speed	Shaft frequency, 1 cycle/rev	Electrical frequency, 2 pole pairs
Large turbomolecular pump, ~1 000 L/s	24 000 RPM	0.4 kHz	0.8 kHz
Mid-size turbo pump, ~300 L/s	36 000 RPM	0.6 kHz	1.2 kHz
Compact turbo pump, ~80 L/s	60 000 RPM	1.0 kHz	2.0 kHz
Small split-flow turbo pump	90 000 RPM	1.5 kHz	3.0 kHz
Micro-milling spindle	100 000 RPM	1.667 kHz	3.333 kHz
PCB drilling air-bearing spindle	200 000 RPM	3.333 kHz	6.667 kHz

Tach Pulses and Shaft Turns Side by Side

Enter the pulse frequency, read the rate in RPM, then divide by pulses per revolution — seeing both at once stops a pole-pair factor being dropped.

Revolutions Per Second for Trigger Timing

Switching the output dropdown to rev/s gives the figure a balancing pickup works in, with no second calculation.

Bare Figures for a Commissioning Log

The copy button on each field yields digits alone, no unit and no grouping spaces, which is what a controller trip-point field expects.

Vacuum and High-Speed Spindle Questions

Why does a turbo pump controller display drive frequency rather than RPM?

Frequency is what the inverter commands directly, so it is the honest reading: the controller ramps its output and the rotor, held by a permanent-magnet field, follows with no slippage to account for. It also makes run-up logic easy to express — "reach 80 per cent of set frequency within 300 seconds" compares two numbers the drive already owns.

How do pole pairs on the motor or tachometer change the arithmetic?

A brushless motor's field sweeps once past every pole pair per mechanical turn, so electrical frequency equals pole pairs times mechanical frequency and shaft speed is 60·f / p. With one pole pair, 1 kHz gives 60 000 RPM; with two, 30 000; with three, 20 000. The same trap sits in the speed sensor: a pickup seeing one target per revolution maps straight to RPM, while one reading a toothed wheel must be divided by the tooth count.

What are critical speeds, and why does the rotor pass through them quickly?

A critical speed is a rate at which the once-per-turn excitation from residual unbalance coincides with a natural frequency of the rotor-bearing assembly, so amplitude grows sharply while the shaft sits there. The drive sweeps through the band rather than dwelling in it, and a magnetic-bearing controller may change its stiffness to shift the resonance aside. It is also why the operating point lies well above the last rigid-body critical.

What sets the maximum speed a rotor can be rated for?

Material stress at the blade root, which grows with the square of rim velocity. That is why the smallest pumps spin fastest: a 60 mm rotor at 90 000 RPM reaches about 283 m/s while a 150 mm rotor at 24 000 RPM sits near 189 m/s. Alloy strength also falls as the rotor warms, which makes cooling part of the rating.

How long do spin-up and braking take at these speeds?

Run-up takes two to five minutes, and failing to reach the set frequency inside that window is treated as a fault. Coasting down is far slower, because in a good vacuum almost nothing slows the rotor — an unbraked pump can keep turning for a quarter of an hour. Controllers therefore offer active braking, and a vent valve admits gas at a controlled rate.