Optimizing Overcurrent Protection for a 4200 kW Slip Ring Induction Motor

Without a doubt, with a motor that valuable, the overcurrent sensing should be on the motor terminals. That way the overcurrent relay sees the actual motor conditions the motor designer anticipated when designing the machine. To measure the current before the capacitors, with no confirmation of each capacitor’s serviceability, is a recipe for poor manufacturing reliability.

Thank you for your feedback. Your point about the importance of overcurrent sensing directly on the motor terminals is well taken. This ensures that the overcurrent relay accurately reflects the motor conditions anticipated by the designer, rather than potentially inaccurate readings influenced by capacitor serviceability.

To validate our concerns, we conducted a sample test on the Mill main drive, recording data over a 15-second interval. The results are summarized below.

Sample Data (Energy Meter Readings):

Load Projection (Estimated) if overload setting reference is 471 Amp:

We observe that the motor operates up to 111% of its rated capacity for an indefinite amount of time, unnoticed by the relay since the threshold for overload is 471 Amps according to the nameplate of the motor. This is a point of significant concern. Additionally, current sensing is performed before the capacitor, potentially leading to inaccuracies in protection.

Your expertise and any further insights on this matter would be greatly appreciated.

To meet your goal of tripping your motor off in 10 seconds at 100% load will require you to significantly lower your trip current setting. For example, a standard Class 10 Overload Relay will trip in less than 10 seconds at 2500 amperes, when set at 421A motor rated current.

At this point, I assume you may be questioning the setting and design of the overload sensing relay? No relay is designed to protect a typical induction motor, wound rotor or squirrel cage, so that 10 seconds of operation at rated amperes results in a trip. Relays are designed to replicate the effects of amperes flowing in the stator windings, to predict the temperature in the windings. That is all. 10 seconds will add some heat, but the temperature will not rise a measurable amount. The motor designer recognizes this, and designs the machine to achieve a normal service life under those conditions.

A machine this size no doubt has at least 6 embedded temperature detectors, likely RTDs. If it was my motor, I would purchase a motor protection relay that monitors all 6, and set the motor trip setting according to the relay instructions. MultiLin is a respected name in motor protection, I think GE owns them now, I’m sure Siemens and ABB also have similar devices.

You are right taking 414A for setting the relay.

Please bear in mind that if you have to remove the PFCC at the motor terminals, the relay setting needs to be increased to prevent motor trip < 100% load.

Further, how do you prevent this protection tripping the motor during starting as well as during short time / permissible overloads??