Magnets, Motors, Transformers

Question 1

• What type of permanent magnet is typically used in large offshore wind turbine generators?

Answer 1

-NdFeB magnets (rare earth material), use a nickel-plated layer to stop corrosion,

Question 2

-What is the main difference between a DC Induction Motor and a Brushless PM Motor?

Answer 2

• induction: generate magnetic field by passing current through a coil with a soft magnetic core
• soft material which directs the magnetic field
• onshore wind turbines use induction
• sometimes need brushes so more maintenance
• PM: have no gear box so less mass and maintenance
• no windings on rotor so no brushes
• greater efficiency
• higher power to weight ratio
• however, corrosion of magnets, which are expensive and technology not as well-known so harder to fix problems

Question 3

-List five of the principal mechanisms by which losses can occur in a transformer.

Answer 3

• winding resistance
• hysteresis losses: depends on maximum inductance of material
• eddy currents: reduces magnetic flux and produces heat
• magnetostriction: where material changes dimesons when magnetic field is applied (humming)
• mechanical losses
• stray losses

Question 4

What are the properties of soft magnetic materials

Answer 4

• permeability: indicates the magnetic induction generated by the material
• coercivity: should be as low as possible
• saturation magnetization: should be as high as possible
• Hysteresis loss: represents energy expanded during one cycle of hysteresis loop
• conductivity/resistivity: high electrical resistivity is required to reduce eddy current losses

Question 5

-What is “GOSS” material, and why is it often used in transformers?

Answer 5

• Grain orientated silicon steel
• Has grain orientation all in one direction so will increase magnetisation and reduces hysteresis and eddy current loss
• quite expensive and but useful as flux all in one direction which is good for transformer bad for motor