Selection of Motor Power Rating Questions and Answers

Answer: A

Losses inside the machine turn into heat, and the insulation fails once the temperature exceeds what its class permits. Everything else about rating follows from that single constraint, which is why the subject is really applied thermodynamics.

Answer: B

Insulation ages chemically at a rate that roughly doubles for every ten degrees of excess temperature, so a modest overload can halve the machine's life. Failure is gradual rather than sudden, which is why the damage often goes unnoticed.

Answer: C

One lumped thermal capacity losing heat in proportion to its temperature rise gives a first-order equation that fits measured behaviour well enough for rating. The real machine has hot spots, which is why measured rises are compared against class limits with margin.

Answer: D

The lumped model gives \( \theta = \theta_f(1 - e^{-t/\tau}) \), so the rise is quick at first and then slows as the losses begin to escape as fast as they are generated. Around four time constants brings it within two per cent of the final value.

Answer: A

Correct answer: thermal capacity to its rate of heat dissipation

The heating time constant is

the thermal capacity in joules per kelvin over the dissipation coefficient in watts per kelvin, and it has the dimensions of time. It measures how quickly the machine responds thermally: a large machine, with much more mass than surface, has a long time constant and can absorb a brief overload that would overheat a small one within seconds.

Answer: B

With the shaft-mounted fan stationary the machine sheds heat far more slowly, so the dissipation term falls and the time constant lengthens. That is why an intermittently loaded machine may not cool as much during its rest period as a simple calculation suggests.

Answer: C

At equilibrium the heat generated equals the heat escaping, so the rise is fixed by the losses and by how effectively the machine sheds heat. Neither the thermal capacity nor the running time enters, since both concern the transient only.

Answer: D

Correct answer: 130 °C

The insulation classes give the total permitted temperature: Class A 105, Class E 120, Class B 130, Class F 155 and Class H 180 degrees. The figure includes the ambient, so a Class B machine rated in a 40 degree ambient may rise 80 degrees with a 10 degree hotspot allowance. Insulation life roughly halves for every ten degrees above the limit, which is why the figure is treated as absolute.

Answer: A

Correct answer: 155 °C

Class F permits 155 degrees in total. Most modern machines are wound with Class F material but rated for a Class B temperature rise, which leaves 25 degrees in hand - margin that can be spent on a hot ambient, on altitude derating, or simply on longer insulation life. It is one of the most useful facts in motor selection.

Answer: B

Correct answer: 40 °C

Standard ratings assume an ambient of 40 degrees and an altitude up to 1000 metres. A machine used in a hotter room has less temperature rise available before the insulation limit, so it must be derated or a larger frame chosen - and one used in a cooler room may carry more, though the manufacturer’s permission is needed to claim it.