0
\$\begingroup\$

I have a project in mind where rather than using weights for resistance in exercise training, I would use a BLDC for resistance. I want to have a pulley system hooked up to a BLDC, similar to the pulleys at the gym but with a BLDC rather than physical weights.

After some research the general idea I have is to use the BLDC as a generator and then short it with resistors or hook it up to a battery to collect the electricity rather than dumping it into a resistor.

I was wondering how would I vary the counter-torque of the BLDC. For example if I were to short the three terminals of the BLDC with resistors would a higher resistor result in the pulley system being harder to pull?

\$\endgroup\$
5
  • 2
    \$\begingroup\$ A higher resistor would be easier to pull; a direct short circuit would be hardest. \$\endgroup\$
    – user16324
    Commented Jul 23, 2022 at 20:02
  • 1
    \$\begingroup\$ you would need active counter-torque to simulate weights \$\endgroup\$
    – jsotola
    Commented Jul 23, 2022 at 20:14
  • \$\begingroup\$ Some bridges driven with "PWM" would be useful for this task ... \$\endgroup\$
    – Antonio51
    Commented Jul 23, 2022 at 20:56
  • \$\begingroup\$ Higher resistance = more like an open circuit. Lower resistance = more like a short-circuit. The one that pulls the most current works the generator the hardest. \$\endgroup\$
    – DKNguyen
    Commented Jul 23, 2022 at 22:50
  • \$\begingroup\$ This might be able to work. But you will probably end up needing some form of variable electronic control rather than just simple resistors. I think you will want the torque to be constant, and you can't get constant torque with a simple resistor. The torque will be dependent on speed and it won't feel at all like lifting a weight. \$\endgroup\$
    – user57037
    Commented Jul 24, 2022 at 5:42

1 Answer 1

Reset to default
4
\$\begingroup\$

Connecting a resistor across the terminals of a motor used as a generator will give you a braking torque proportional to speed, so-called dynamic braking.

Control the constant of proportionality by changing the resistors. Open circuit / high value resistor -> no or little braking torque, short circuit / low value resistor -> maximum or high braking torque, mid-range resistor -> somewhere between these limits.

The mechanical input power to the motor/generator will be dissipated in the total resistance of the motor + load resistor circuit. As an exercise machine could produce 1 kW peak, and half of that continuously, you need the load resistor power-rated accordingly. If you go for the maximum torque of no external resistance, that means all the power will be dissipated in the motor, which needs to be cooled appropriately.

Many exercise machines use an automotive alternator as the load generator. It has a number of significant advantages.

  • It is mass-produced, so new ones are cheap. It's thrown away in large numbers, so secondhand/rescued ones are free.
  • At several kW, the power rating is more than adequate.
  • The load torque can be controlled by using a fixed resistive load on the output and varying the field excitation current, this tends to be cheaper and easier than switching or controlling resistors.
  • If you do want to recover the electrical output, then the output voltage can be controlled by the field excitation current, just as it is in a car, without further power electronics.
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.