Why does wattmeter's reading go negative?

Raman Jain Profile Answers by Raman Jain Questions by Raman Jain

• Sep 30th, 2008

When the power flow goes in opposite direction then the wattmeter shows negative reading.
It means, let initially power flow from P1 to P2 direction, wattmeter shows positive reading and when direction of power flows change to P2 to P1 wattmeter shows negative reading.

SURAJTHEGREAT Profile Answers by SURAJTHEGREAT Questions by SURAJTHEGREAT

• Mar 3rd, 2009

It all depends on sense of winding.

For example if wattmeter shows negative reading when connected to motor or generator, then there are two obvious mistakes. One you must have connected wrongly and the other the sense of winding is wrong.
The first one can be easily verified. Even after verification of the first mistake the problem continues then connect negative of winding to common of wattmeter and -ve 2 other end of wattmeter. If the problem is solved, then the sense of winding has to be changed.

SURAJTHEGREAT Profile Answers by SURAJTHEGREAT Questions by SURAJTHEGREAT

• Mar 3rd, 2009

If you have seen the wattmeter ratings, it will be (300V,10A,UPF) (Voltage,
Current, Power Factor)
That is wattmeter needs voltage and current to measure power.  So a
wattmeter consists of two coils pressure coil, and current coil.
Pressure coil measure quantity proportional to voltage and current coil measures
quantity proportional to current.

bittu123 Profile Answers by bittu123 Questions by bittu123

• Nov 9th, 2011

Suppose, phase difference between V and I is "A"(angle). and wattmeter reads +ve. Now if phase difference became (A+180), wattmeter will read -ve of same magnitude.

It simply means, direction of power flow gets changed just because of changing in direction of either current or voltage

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