Which of the following statements are INCORRECT about PMMC instruments?

I. The torque-to-weight ratio is high, which gives a high accuracy.

II. A single instrument can be used for several, different current voltage ranges by using the instrument transformer.

III. The scale is uniformly divided.

IV. The cost of PMMC instruments is lower than that of moving iron instruments.

The correct answer is option 4.

Permanent Magnet Moving Coil (PMMC)

Characteristics of PMMC instrument

• PMMC is used to measure only DC or the average DC value of electrical quantity. It cannot measure different voltage or current values by using the instrument transformer.
• 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.