The wattmeter in general use for power frequency work is of the
Answer: A
Two coils are used, a fixed current coil and a moving pressure coil, and the torque is proportional to the product of their currents. Being a product it gives true power on any waveform and at any power factor, which no single-coil movement can do.
Please log in or register to report this question.
In an electrodynamometer wattmeter the fixed coil carries
Answer: B
The fixed coil is the current coil, wound with few turns of thick wire and connected in series with the load. The moving coil is the pressure coil, of many turns of fine wire with a series resistance, and is connected across the load.
Please log in or register to report this question.
The deflecting torque of a dynamometer wattmeter is proportional to
Answer: C
Averaged over a cycle the torque is proportional to \( V I \cos\phi \), which is exactly the active power. The cosine term is what makes the instrument read power rather than volt-amperes, and why it reads correctly at any power factor.
Please log in or register to report this question.
The scale of a dynamometer wattmeter is
Answer: D
The rate of change of mutual inductance with angle is made nearly constant by the shaping of the coils, so the deflection is proportional to the power and the divisions are equal. This is in contrast with the moving iron and hot wire movements, whose scales are square law.
Please log in or register to report this question.
The pressure coil circuit of a wattmeter is made of many turns of fine wire in series with a high resistance in order to
Answer: A
The current in the pressure coil must be proportional to the voltage and in phase with it, which needs a circuit whose resistance far outweighs its reactance. Keeping the current small also limits the error it introduces into the current coil reading.
Please log in or register to report this question.
Damping in a dynamometer wattmeter is provided by
Answer: B
Eddy current damping is ruled out, because a permanent magnet near the coils would distort the very field on which the reading depends. A light vane moving in a closed sector supplies the damping without introducing any magnetic disturbance.
Please log in or register to report this question.
With the pressure coil connected on the load side of the current coil, the wattmeter reads the power in the load plus
Answer: C
The current coil then carries the load current only, and the pressure coil measures the voltage across the load plus the drop in the current coil, so the current coil loss is included. This connection suits a heavy load current at low voltage.
Please log in or register to report this question.
With the pressure coil connected on the supply side of the current coil, the wattmeter includes in its reading
Answer: D
Here the pressure coil is across the supply and its current passes through the current coil, so the power it consumes is added. This connection is the better one when the load current is small and the voltage high, where the pressure coil loss is relatively less.
Please log in or register to report this question.
A compensated wattmeter carries an extra winding on the current coil former whose purpose is to
Answer: A
The compensating winding has the same number of turns as the current coil and carries the pressure coil current in the opposite sense, so the flux due to that current is annulled. The instrument then reads the load power alone, whichever way it is connected.
Please log in or register to report this question.
The error caused by the inductance of the pressure coil circuit of a wattmeter is
Answer: B
The pressure coil current lags the voltage a little because of the coil's inductance, so the angle between the two coil currents is reduced and the cosine is increased. On a lagging load the instrument therefore reads high, and the error grows as the power factor falls.
Please log in or register to report this question.
