Starting a 30kW Motor by Soft Starter

92 pages is a lot of waiting, unless you're Evelyn Woods.

Well just shooting from the hip, I'd say the amperage looks a little high, I would start by experimenting with raising the voltage levels, and at some point I would want to meg out the windings, you might need a redip...the insulation does wear out....

The longer it takes for your motor to get up to speed, the more heat you are putting into the rotor circuit. The 7 starts per hour is likely D.O.L. or across the line starting.

If you can give it more voltage and shorten the time, your motor will last longer, and you can get more starts per hour, up to 7, if the motor was designed to NEMA standards. This link points to the method US NEMA gives as design guides for motor starting, and does not factor in the additional heat added when the starting time is extended, however much that is (?).

www.landbelectric.com/download-document/78-maximum-nema-inertia-starts.html

Common misconception.

Starting time is extended, but only because the starting TORQUE and thus CURRENT going into the motor is lower. If you were to plot the energy that goes into starting a motor from a dead stop to full speed as a curve with a soft starter or Across-the-Line / DOL, the area of that curve is exactly the same; instead of steep, high and and short duration, it is shallower, lower and longer duration. There is a net zero change in actual motor heating effect.

the only thing that changes that situation is a VFD, because the VFD can limit the starting current to 100% of the motor FLC, regardless of how long that may take to accelerate; 1 minute, 1 hour, 1 day, 1 week, etc.

That is true, the real work (kw) is the same over any time, but the heating of the rotor bars is a function of slip frequency and time. Many soft start manufacturers warn against extending the time to get to motor design slip speed, a common mistake when setting the starting voltage too low. Since most (induction squirrel cage) motors we see are rotor limited on start, stator winding protection will not catch rotor overheating until too late.

One rule of thumb, if the motor voltage is more than 10 times the horsepower rating, then the motor may not be rotor limited on start, like a 500hp 6900V motor.

Hi rwilliams;

I could not understand your words

"If you can give it more voltage and shorten the time, your motor will last longer, and you can get more starts per hour, up to 7, if the motor was designed to NEMA standards".

Here if initial voltage is 235, the current rises up to 255A (FLA 55A) when i increase the initial voltage 250V, current rises up to 292A. More the current drawn, more the heat. Is it?

The time running at less than rated slip speed will shorten significantly, that reduces the total heat.

These motors are designed to be started in a very short time, at a very high current. If you deliberately make the motor operate at slow speed by limiting the accelerating torque, the rotor bars will take higher current over time than expected, and you shorten the life of the motor, unless you take steps to limit the temperature rise.

Depending on the features of the soft starter, it may be possible to save some energy, but only a bit. And you might save a bit on power factor, but again not much, and that's not the same as saving energy anyway.

If there are periods of time where less air flow is sufficient, you could save energy with a VFD.

Speed-up drives are ordinarily pretty silly. Just get a motor with the right number of poles.

Are you really saving energy? Most big motors take a lot of energy to start up & levels out once it is running, A case in point is I used to work with four 1000cuft compressors and it was found to be more economical to leave them running all day & to "unload" them when the pressure was reached rather than stop and start them.

Bazzer

Hi;

As I discussed earlier that I have installed soft starter Schneider brand "ATS01N272Q" which dont have built-in over load, so if we install regular thermal overload Schneider "GV3P 65 48-65A", it gets trip during starting time. The starting time of motor is almost 8-10sec and peak current goes up to 265A for 4-5sec then drops gradually. Kindly guide which type of thermal overload will be suitable for this application.

you need to increase the voltage/current adjustment of the soft start to get the motor to start in a shorter time.

If you cannot do this, then a higher class of Overload Relay is required. However, the possible damage to the motor running it for 8 to 10 seconds close to locked rotor conditions is what you risk.

The motor designer can tell you the actual thermal margins built into the motor. This motor is likely rotor limited, so the damage curve for the rotor is what needs to be calculated, along with your duty cycle.