question about motor ratings

WayneW

WayneW

Senior Member
can somebody explain this math?

a pool pump motor is rated at 115v@19amps (or [email protected]). W=V*A, so that works out to approx 2200 watts. I measured 240 volts and 9.5A with a clamp on ammeter, so I am seeing about 2200 watts.

but the motor plate also says 1.1 KW and 1.5HP.

How is the motor rated at 1100 watts? Is this due to the power factor? Isn't a power factor of .5 pretty bad? I assume I am paying for 2200 watts, but only getting 1100 watts of work for my money? Since the rating match the measurements, I assume the motor is not bad, but is there a more economical way?
 
Is the pump working properly? Has a good impeller? A lightly loaded motor can have poor power factor.

I don't think you would be paying for the power because the voltage and amperage would be out of phase due to poor power factor and your meter probably wouldn't register it properly.

If the current and voltage were in phase, and you were not getting appropriate torque and RPM, I think the motor should be getting very hot, because it's gotta be goin' someplace. Or you may be getting the RPM and the torque indicated by your measurements, and I have no explaination for the nameplate, and this post and possibly myself have no redeeming value. :)
 
The output power of the motor is 1.5 HP, which is about 1100 Watts.

The rating of the motor is 2200 VA ( these are not the real input watts ). There are two components for the discrepancy:
  • Power Factor - This energy is not really consumed. It is returned back to the grid, but the wiring must be capable of carrying it. I believe that most utility companies do not charge to the residential customers for this pf consumption.
  • Efficiency - This is the energy transformed into heat. How big is this component depend on the motor efficiency
I dont remember the typical efficiencies of the electrical motors. If you get a number you can make an assumption for real input watts (1100w/efficiency)- the rest is explained by the pf.
 
The back emf is probably what accounts for the difference. Motors are never rated in watts, but in volt/amps (VA) to indicate the power that must be supplied (but not necessarily consumed). If the nameplate has a rating in watts, it is probably referring to the heat that the motor will normally dissipate at rated load. A horsepower is nominally equal to 746 watts, so 746 x 1.5 gives you about 1119 watts, hence the rating.

A single phase motor is usually about 80% effecient, although this will vary with the type of winding (squirrel cage, etc) and construction.

Power factor is a calculation that only takes into account the leading or lagging current vs voltage due to the inductive or capacitive load. It does not take into consideration back emf, which plays a major role in motors.
 
Guy Lavoie said:
Power factor is a calculation that only takes into account the leading or lagging current vs voltage due to the inductive or capacitive load. It does not take into consideration back emf, which plays a major role in motors.
Click to expand...
Guy - But my understanding is that a motor with pf of 1.0 does not have any back emf. In other words, the shift in phase between the voltage and current is the cause for the back emf. I has been a long time since I studied this so I might be wrong. Would you please confirm/correct me?

WayneW - Assuming the effiency of .8 and assuming the the motor yields 1.5HP (or 1119w), the real input power (consumed) is 1400 Watts. The difference (800VA) are not really consumed as Guy clearly explained above.
 
I looked at Baldor's

Seems your measurements and conclusions are pretty typical.

.80 % efficiency and .62 power factor.

I don't think you're paying for power factor directly though, because your meter doesn't record it, but you are heating up your wiring and what not.

They did have high efficiency motors .94 PF and 82% eff.
 
elcano said:
Guy Lavoie said:
Power factor is a calculation that only takes into account the leading or lagging current vs voltage due to the inductive or capacitive load. It does not take into consideration back emf, which plays a major role in motors.
Click to expand...
Guy - But my understanding is that a motor with pf of 1.0 does not have any back emf. In other words, the shift in phase between the voltage and current is the cause for the back emf. I has been a long time since I studied this so I might be wrong. Would you please confirm/correct me?

WayneW - Assuming the effiency of .8 and assuming the the motor yields 1.5HP (or 1119w), the real input power (consumed) is 1400 Watts. The difference (800VA) are not really consumed as Guy clearly explained above.
Click to expand...
No, the back emf is produced by the "slipping" of the magnetic field induced in the rotor in relation to the magnetic field in the windings. An AC induction motor has no brushes to feed a winding to create a magnetic filed in the rotor (it "induces" it, hence the name), so this slippage is necessary to its operation. That is why a motor is usually rated as spinning at 1725 (or 3450) RPM when the theoretical speed would be 1800 (or 3600) with 60Hz power, depending on the number of poles. Even with no load, it still slips. With no load, the slip is very little and much of the magnetic field produced is induced back into the windings and produces a lot of back emf. That is why a lightly loaded motor draws little current. As the load is increased, the magnetic field slips more and more so it is converted to mechanical energy and there is less back emf.

In a purely inductive or capacitive load (like a coil or a capacitor) the power factor simply indicates the relation (in phase angle) between the voltage and the current rise and fall. With motors you do have a power factor due to the inductive nature of the windings and this value will be relatively constant. Back emf is not considered to be part of the power factor because it is the back emf intensity that varies, not the phase angle. It has also been a whilke since I studied this so I might have a few things a bit off myself :unsure:

Motors are complex impedances!
 
WayneW said:
can somebody explain this math?

a pool pump motor is rated at 115v@19amps (or [email protected]). W=V*A, so that works out to approx 2200 watts. I measured 240 volts and 9.5A with a clamp on ammeter, so I am seeing about 2200 watts.

but the motor plate also says 1.1 KW and 1.5HP.

How is the motor rated at 1100 watts? Is this due to the power factor? Isn't a power factor of .5 pretty bad? I assume I am paying for 2200 watts, but only getting 1100 watts of work for my money? Since the rating match the measurements, I assume the motor is not bad, but is there a more economical way?
Click to expand...
Hi

Its been quite a while since i did motor theory but Guys comments are pretty spot on from memory.......

Your power meter (assuming they are the same as in Oz) reads in watts which in not true power...to read true power you need to install VA meters....So you are using more power than what your paying for.....

Large users of power would have VA meters installed so they would have Power Factor correction Capacitors installed to get their power factor as close to unity as possible...

Frank
 
I have messed w. motors for 25+ years and never appreciated what you have pointed out in this thread. I was always interested in HP, torque, RPM, current for overloads and overcurrent devices, and resistance of windings to ground in troubleshooting. I have even installed power factor correction capacitors.


This is the first I realized how bad a typical motor is re. power factor

The KW vs. the HP never really soaked in before.

Thanks for the education.
 
You must log in or register to reply here.
Back
Top