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I'm reading the procurement documents for the R211 railcar contract, which is for electric multiple unit subway cars designed for rapid transit applications in New York City. The railcar's propulsion is specified to use AC induction motors powered from PWM inverters with IGBTs.

The specification related to propulsion motor overcurrent is given as follows:

10.3.20.2 Over-temperature protection shall be provided as follows:

a) If the winding temperature rises 27°F above (15°C above) the operating class temperature limit, the control unit shall decrease the motor duty cycle by removing the dynamic braking and converting to frictional braking, until the temperature falls below the operating class temperature.

b) If the winding temperature rises above its design class operating temperature limit, the control unit shall remove power to the affected truck, until the temperature falls below the operating class temperature. The condition shall be reported and recorded by the MDS.

c) Motor temperature shall be calculated using a thermal model of the motor and the power that the motor is handling.

d) The measurement of winding temperatures using embedded sensors is not permitted.

Points c) and d) stand out to me as a little bit strange, since the motor is expected to be modeled with its loads, instead of directly measuring the temperature. The only reasoning I can determine for this is that it protects against the failure of the thermal sensor.

However, it's not clear to me why this failure more is harder to guard against, than (for example) the failure of current sensors needed to track the motor's power, or an obstruction of the normal cooling process by dust or debris. Is there a common technical reason why motor thermal protection is specified to use only modeled heating rather than measured heating or a combination of safeguards?

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Yes. Along with your reasoning of failure mode, reliability and cost, there is a disadvantage of slower response time of the sensors. The thermal model will give temperature values as fast as the speed of the digital processor you use. A Research by GE states:

An additional reason to reject such temperature sensors as the main basis for thermal protection, is the fact that the traditional Resistance Temperature Detector (RTD) has a relatively slow reaction time and can’t respond adequately to the high speed of the heating process during motor acceleration.

Also, this model ensures greater transparency between motor manufacturer and stakeholder (as manufacturer supplies a family of characteristic curves for the motor):

Another important part of thermal model implementation is “Expected values stored in Motor Protection Device”. This term implies that information is available from the motor designer and motor manufacturer, that is related to the thermal reserve, allowed performance and thermodynamics of the motor in question.

This paper goes on to further discuss the terminology like thermal capacity used and allowed performance and is a great read if you want to build a model.

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