Which of the following is NOT an advantage of a PMMC instrument?

Permanent Magnet Moving Coil (PMMC)

Characteristics of the PMMC instrument

• PMMC is used to measure only DC or the average DC value of an electrical quantity. It cannot measure AC values.
• The PMMC instrument has a linear or uniform scale, hence, the deflecting torque produced is directly proportional to the current.
• The controlling torque in PMMC instruments can be produced by either spring control or gravity control.
• For spring control: Tc ∝ θ, and for gravity control: Tc ∝ sin θ 
• The damping torque in the PMMC instrument is provided by eddy current damping.
   

Advantages

• The PMMC has a high torque-to-weight ratio and, hence, has great accuracy.
• It produces no losses due to hysteresis.
• It has efficient damping characteristics and is not affected by a stray magnetic field.
   

Disadvantages

• Moving coil instruments are expensive.
• The moving coil instrument can only be used on the D.C. supply as the current reversal produces a torque on the coil.
• It may show an error due to the loss of magnetism of the permanent magnet.