Don't know if this would be more appropriate on any other stackexchange site but my question is :

How do you evaluate the sizing of a softstarter for no-load testing of an induction motor ?? For example, a 75kW starter would be used for on load starting of a 75kW motor but on no-load the starter required should be much less. I'm guessing around 25 ~ 35 kW but have no idea on how to best evaluate this.

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    \$\begingroup\$ It all depends on the starter ratings; you need to make sure the starter can handle the startup current draw. What is your motor NEMA design class? What's your application? Will you be running the motor at rated load or will the typical load run above nameplate? Most starters will state something like "600% for 30s, 125% continuous" or similar; this gives you an idea of whether the starter is capable of a) starting the load and b) running the application. \$\endgroup\$ – akohlsmith Apr 19 '12 at 14:04
  • \$\begingroup\$ Agreed... but the issue here is not really the load characteristics but the no-load characteristics. All the data and design parameters you specified are for starting the motor on load. I'm looking for a rule of thumb (with appropriate safety margin) or guidelines that I can apply for selecting the starter based on the load characteristics to calculate the no-load characteristics. \$\endgroup\$ – Saad Farooq Apr 19 '12 at 15:53
  • \$\begingroup\$ I should also bring to your knowledge that the reason I require this is that I want to test motors in my workshop once they are repaired or rewound. I need to know the maximum rating of motors that I can test in my workshop through a soft starter. \$\endgroup\$ – Saad Farooq Apr 19 '12 at 15:56
  • \$\begingroup\$ What is a motor "soft starter"? I'm surprised because I've never heard of one and I work with brushless motors for a living. Do you mean some kind of an SCR/triac combination, or a relay, or what? \$\endgroup\$ – Jason S Apr 19 '12 at 23:05
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    \$\begingroup\$ en.wikipedia.org/wiki/Motor_soft_starter -- go figure. Wikipedia has nearly everything. \$\endgroup\$ – Jason S Apr 19 '12 at 23:27

It sounds like you want to find the smallest soft starter you need to just do basic unloaded spin-up testing of repaired motors in your shop. In that case all you will care for is voltage requirements and overload current ratings of the starter.

AC motors will draw their locked-rotor (typically 6x for NEMA B motors) current when starting up under load. Even unloaded, there will be a brief (several seconds) of higher-than-nameplate current. This will not really be visible for small (<25-50HP) motors, but for larger motors with large rotors you can see 2x or maybe even 3x nameplate as the big, heavy rotor slowly spins up.

Having said that, however, any starter worth it's salt will have a decent (3x to 6x) overload rating, and should also feature a current ramp start. I'd size the starter so that its overload rating is twice your biggest motor's nameplate (e.g. a starter with an overload current rating of 400A should be fine for a 200A or even 300A motor) -- just make sure the current limit is set up and it'll keep the current within the safe operating area of the starter, and it's up to you to make sure that the motor can start up unloaded with the current limit in place. The motor will be a slow start, but your starter and wiring should be safe, which is the whole point. The starter should also have short-circuit protection to protect everything from wiring mishaps or a badly rewound motor.

  • \$\begingroup\$ So I basically decide based on the motor's nameplate current under the assumption that the the no-load starting current will not be a whole lot more than that and then rely on the starters securities and my control over the starting ramp ? \$\endgroup\$ – Saad Farooq Apr 19 '12 at 20:01
  • \$\begingroup\$ Also, would you mind adding tags for induction-motors and soft-starting.... I'm new on this site so don't have enough reputation to add tags (I'll probably have more questions on Programmable Logic Controllers later :-) ) \$\endgroup\$ – Saad Farooq Apr 19 '12 at 20:05
  • \$\begingroup\$ I'd select a soft starter that has an overload current rating (usually overload ratings are for 30s) that is 2x nameplate of the biggest motor you're going to test. e.g. for a 200A nameplate motor, select a 300-400A overload rating on the soft starter (which will probably be a 50-75A soft starter if it's not a really cheap brand.) \$\endgroup\$ – akohlsmith Apr 19 '12 at 23:05

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