(DC/AC) PMMC, MI, Electrodynamometer type - Electrical Engineering (EE) PDF Download

Permanent Magnet Moving Coil (PMMC) instrument

One of the most accurate type of instrument used for D.C. measurements is PMMC instrument. Construction: A permanent magnet is used in this type instrument. Aluminum former is provided in the cylindrical in between two poles of the permanent magnet (Fig. 1.7). Coils are wound on the aluminum former which is connected with the spindle. This spindle is supported with jeweled bearing. Two springs are attached on either end of the spindle. The terminals of the moving coils are connected to the spring. Therefore the current flows through spring 1, moving coil and spring 2.

Damping: Eddy current damping is used. This is produced by aluminum former. Control: Spring control is used. 

Principle of operation

When D.C. supply is given to the moving coil, D.C. current flows through it. When the current carrying coil is kept in the magnetic field, it experiences a force. This force produces a torque and the former rotates. The pointer is attached with the spindle. When the former rotates, the pointer moves over the calibrated scale. When the polarity is reversed a torque is produced in the opposite direction. The mechanical stopper does not allow the deflection in the opposite direction. Therefore the polarity should be maintained with PMMC instrument. If A.C. is supplied, a reversing torque is produced. This cannot produce a continuous deflection. Therefore this instrument cannot be used in A.C. 

Torque developed by PMMC

Let  

Td =deflecting torque

TC = controlling torque

θ = angle of deflection  

K=spring constant

b=width of the coil 

l=height of the coil or length of coil  

N=No. of turns

I=current  

B=Flux density        

A=area of the coil 

The force produced in the coil is given by 

F = BIL sin θ   

When θ = 90°

For N turns, F = NBIL

Torque produced Td = F × ⊥r distance 

T_d = NBIL × b = BINA

T_d = BANI

T_d ∝ I

Advantages

• Torque/weight is high
• Power consumption is less
• Scale is uniform
• Damping is very effective
• Since operating field is very strong, the effect of stray field is negligible
• Range of instrument can be extended

Disadvantages 

• Use only for D.C.
• Cost is high
• Error is produced due to ageing effect of PMMC
• Friction and temperature error are present

Extension of range of PMMC instrument

Case-I: Shunt 

A low shunt resistance connected in parrel with the ammeter to extent the range of current. Large current can be measured using low current rated ammeter by using a shunt. 

Let

R_m =Resistance of meter

R_sh=Resistance of shunt

I _m = Current through meter

I _sh =current through shunt

I= current to be measure  

∴V_m = V_sh                    

I _m R_m = I _sh R_sh

Apply KCL at ‘P’ I = I _m + I _sh

Eqⁿ (1.12) ÷ by I_m

Shunt resistance is made of manganin. This has least thermoelectric emf. The change is resistance, due to change in temperature is negligible.

Case (II): Multiplier 

A large resistance is connected in series with voltmeter is called multiplier (Fig. 1.9). A large voltage can be measured using a voltmeter of small rating with a multiplier. 

Let  

Rm =resistance of meter

Rse =resistance of multiplier

Vm =Voltage across meter

Vse = Voltage across series resistance  

V= voltage to be measured

Apply KVL, V = V_m + V_se

Eqn (1.19) ÷V_m

Moving Iron (MI) instruments

One of the most accurate instrument used for both AC and DC measurement is moving iron instrument. There are two types of moving iron instrument.

• Attraction type
• Repulsion type

1.8.1 Attraction type M.I. instrument

Construction: The moving iron fixed to the spindle is kept near the hollow fixed coil (Fig. 1.10). The pointer and balance weight are attached to the spindle, which is supported with jeweled bearing. Here air friction damping is used. 

Principle of operation

The current to be measured is passed through the fixed coil. As the current is flow through the fixed coil, a magnetic field is produced. By magnetic induction the moving iron gets magnetized. The north pole of moving coil is attracted by the south pole of fixed coil. Thus the deflecting force is produced due to force of attraction. Since the moving iron is attached with the spindle, the spindle rotates and the pointer moves over the calibrated scale. But the force of attraction depends on the current flowing through the coil. 

Torque developed by M.I

Let ‘θ ’ be the deflection corresponding to a current of ‘i’ amp

Let the current increases by di, the corresponding deflection is ‘θ + dθ ’