Start delta starter of an extraction fan is cutting power when it suppose to switch to delta after a certain time. I think my timer relay has a problem, so I want to replace it. I understand that the switching over supposed to be done when a motor reached 80% of its rpm, but I don't know how to set timer relay to achieve that. If are there any calculations involved, please assist. How can I set the time?

  • \$\begingroup\$ you should possibly use a current relay the slower an induction motor is the higher the current. \$\endgroup\$ Mar 25, 2018 at 7:36

1 Answer 1


The switch-over time could be calculated by calculating the time to accelerate the load. To do that, you need to know the load inertia and torque vs. speed profile. You also need to no the motor torque vs. speed capability with the delta connection. Even a crude estimate would provide a starting point.

Without calculating, you could use trial and error. Set the timer to something less than the time it takes for the overload relay to shut the motor off. Initiate start while measuring current and observing the speed. Use a stopwatch to measure the time it takes for the current to drop significantly and the speed to reach 80%.

You could use a current sensing relay, but a fan is not likely to change characteristics over time, so all you really need to do is determine the time once. The starter should work fine with the proper time setting without needing to change the design to a current sensing system.

With full motor nameplate data, you could make a good estimate of the time required using the assumption that the motor capability is matched to the fan requirement.

  • \$\begingroup\$ +1, but fan characteristics indeed can change if there are obstacles in the air way. E.g. dust and mold. \$\endgroup\$
    – Janka
    Mar 25, 2018 at 12:14
  • \$\begingroup\$ ... or just throw out all of that ancient technology and replace it with a solid state soft starter. This is the major down side of Y-Delta starting. \$\endgroup\$
    – JRaef
    Mar 26, 2018 at 22:18
  • \$\begingroup\$ I think star-delta starters are sufficiently crude that most changes in load characteristics can be ignored until the equipment is seriously in need of maintenance. They are ancient technology, but that doesn't automatically mean it is better to replace than repair them. Everyone that deals with motor controls should understand the advantages, disadvantages and costs associated with every alternative as related to their individual conditions. \$\endgroup\$
    – user80875
    Mar 26, 2018 at 23:16
  • 1
    \$\begingroup\$ In my experience, the majority of people that use wye-delta don't truly understand the down sides of it, especially the transition issues that impose spikes on the line and damage the loads mechanically. People use it because it's the cheapest way to satisfy a requirement for reduced voltage starting and discover the negative aspects only after the damage is done. But that's just an opinion... \$\endgroup\$
    – JRaef
    Mar 27, 2018 at 0:25
  • \$\begingroup\$ If the OP has a Y-D starter that has been in service long enough tor the timing relay to go bad, those down sides may not have been an issue. Specific shortcomings are better information than "ancient technology." \$\endgroup\$
    – user80875
    Mar 27, 2018 at 3:41

Your Answer

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

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