I just recently purchased a broken Turbomolecular pump from eBay and repaired it, now that it works again I need to build a low-cost driver for it.

It has 4 Phases and normally relies on hall feedback which is still working.

Given Schematic

The pump is supposed to take frequencies up to 800Hz.

Now the actual question: What driving technique would you recommend for this motor? Simply relying on Hall feedback to regulate power and speed by adjusting driving current/voltage?

easiest driving method

Driving the motor like a unipolar stepper motor would the easiest way of running the motor.

What is the typical way of driving such a motor at those required speeds?

Update: This is currently the schematic i am most likely going to use. I am going to use a microcontroller or some other circuitry to monitor the RPM and thus adjust the maximum current delivered to the windings. I might add an option to use "pulse density modulation" to provide the speed control at a constant current. https://www.youtube.com/watch?v=0zlupI0UAGo enter image description here

  • \$\begingroup\$ I thought turbopumps used a BLDC motor and run at tens of thousands of RPM \$\endgroup\$
    – Voltage Spike
    Commented Jul 19, 2019 at 16:03
  • \$\begingroup\$ a stepper is basically a bldc, just with more poles. Also this one is really really old. \$\endgroup\$ Commented Jul 19, 2019 at 16:32

1 Answer 1


Not the easiest project you could find. From my point of viwew it should be as follows:

  • use an unipolar stepper motor driver with a chopper. Complementary motor phases can be grouped, so you need only one current sense resistor for each pair, total two current sense resistors.
  • The driver has to accept ABCD input sequence, not a step/dir interface
  • The hall sensors have to be translated with a truth table to ABCD sequence
  • The above combination shall give you the equivalent brush DC motor behaviour with current limiting: max. acceleration and spinning up until back EMF voltage does not reach the supply voltage, or until the load torque does not reach the available motor torque set by current limiting resistors.
  • To be able to change speed you should introduce logical AND gates in series with the ABCD inputs and with additional PWM signal.

enter image description here

This is almost what you need. You still need to parse the hall sensors signals to the sequence of IN1...IN4, for speed control you can use a PWM signal on Enalble pin.


I am not an expert in the dicrete circituit built driver. IMO also pulse density modulation is not what you need. If you use a MCU then, from your 1st schematics add 2 current sense resistors grouped on Q1/Q2 and Q3/4 then you measure the current in both complentary phases. While Q1 conducts, the Q2 doesn't, same for Q3 and Q4. Implement two current PI controllers (for each phase pair) with PWM output.

The hall sensors should provide the correct switching angle. Let's say combinations are as follow:

  • 00 / 0 degrees
  • 01 / 90 deg
  • 11 / 180 deg
  • 10 / 270 deg

You need to switch on phases in this pattern with respect to the rotor's flux:

  • 0 deg / the resulting stator flux vector shall be 135 deg (phase 180 + phase 90)
  • 90 deg / 225 deg (180 + 270)
  • 180 deg / 315 deg (270 + 0)
  • 270 deg / 45 deg (0 + 90)
  • 0 deg / repeat


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ so you think running the motor like an unipolar stepper motor will provide sufficent torque and speed? i am most likely going to go for a purely logic based circuit, no driver ic or similar. \$\endgroup\$ Commented Jul 22, 2019 at 15:34
  • \$\begingroup\$ @EricBAhapaid Edited the answer. \$\endgroup\$ Commented Jul 23, 2019 at 6:07
  • \$\begingroup\$ oh, the hall sensors actually put out all the required signals, one Phase and one leading by 45°. Why do i need to current chop Q1 and Q2? wouldnt i get more torque if you utilized the full 7A capabilities of each phase? PI loops also seem overkill for current control. \$\endgroup\$ Commented Jul 23, 2019 at 7:36
  • \$\begingroup\$ yeah honestly, the pump motor also supports watercooling, i will run it at 7A per phase even if i could be running the motor at twice the rated current. \$\endgroup\$ Commented Jul 23, 2019 at 18:01
  • \$\begingroup\$ oh, and wouldnt speed control over the enable pin be almost the same as lowering the current thresholds? As it would only decrease the ontime afterall which is what also happens when you lower the current threshold. \$\endgroup\$ Commented Jul 23, 2019 at 18:47

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