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4hv.org :: Forums :: General Science and Electronics
<< Previous thread | Next thread >>   

Does the wattage alone determine the power of a motor?

Author Post
ScottH
Tue Feb 28 2017, 10:21AM Print View
Registered Member #61373
Joined: Sat Dec 17 2016, 01:45PM
Location: San Antonio, TX
Posts: 81
I know that 1 horsepower=746 watts used. If an electric motor used 746 watts of power, it would put out 1hp.

I also know that the strength of an electromagnet is determined by the amperes flowing through the coil × the number of turns. I have a fan that consumes 0.1 amps using 120v. The fan is using 12w of power to blow air. That is 1.6% of a hp.

I hooked up that fan to 2 MOTS in anti phase. Check it out

For comparison, this is the same fan on 120v

Would that mean that the fan should be operating at 5.6hp since it has 4,100w going through it? The fan is a high speed fan on 120v, but since I am using 4,100v at 1 amp, shouldn't the fan put out way more power/speed than before? The fan only seemed to spin at 2 to 3× the rpm when hooked up to the MOTS.

Would the torque increase with minimal speed increase, or would the increase be all speed (horsepower is torque × rpm, so increasing either would increase power output)?
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Sulaiman
Tue Feb 28 2017, 10:47AM
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 2800
First you need to know which kind of motor is used;

I believe that your fan uses a 'universal' motor
if you read that article you may understand, if not, ask again.

other types; d.c. , induction , synchronous , brushless d.c. , shaded-pole , stepper
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ScottH
Tue Feb 28 2017, 03:26PM
Registered Member #61373
Joined: Sat Dec 17 2016, 01:45PM
Location: San Antonio, TX
Posts: 81
Sulaiman wrote ...

First you need to know which kind of motor is used;

I believe that your fan uses a 'universal' motor
if you read that article you may understand, if not, ask again.

other types; d.c. , induction , synchronous , brushless d.c. , shaded-pole , stepper


The fan motor has a coil of wire around part of a solid metal loop, which has the rotating part in the middle. It is a real basic and small motor. It is the Shaded Pole type.
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Sulaiman
Tue Feb 28 2017, 04:00PM
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 2800
so the MAXIMUM speed that the fan can rotate is 3000 (3600) rpm for 50 (60) Hz power,
increasing voltage will just increase heating.

The fan can rotate at less than 3000 (3600) rpm if a load is put on the motor, i.e. the fan.
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ScottH
Wed Mar 01 2017, 02:26PM
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Joined: Sat Dec 17 2016, 01:45PM
Location: San Antonio, TX
Posts: 81
Sulaiman wrote ...

so the MAXIMUM speed that the fan can rotate is 3000 (3600) rpm for 50 (60) Hz power,
increasing voltage will just increase heating.

The fan can rotate at less than 3000 (3600) rpm if a load is put on the motor, i.e. the fan.


So the torque is increased? Is the torque increase proportional to the amps (0.1 amps vs 1.0 amp = 10x torque) or the power (0.016 hp for 120v at 0.1 amps vs 5.6 hp for 4,200v at 1.0 amp = 350x the power)?
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Sulaiman
Wed Mar 01 2017, 04:59PM
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 2800
It is more complicated than that,

what you observed was influenced by the operation of your MOTs
unless you have knocked out the magnetic shunts,
the MOT output current is limited, by design.

IF you really had full MOT voltage across your motor winding
- you would have needed a fire extinguisher. !

.......................................................................................................
the motors are designed for economy, the steel operates near to magnetic saturation to minimise the ammount of steel required,
so increasing the voltage will increase the magnetic flux only a little before the steel completely saturates.
............................................................................................................
as a general rule,
experimenting with components operating at less than rated values is preferred to above rated values, where they die :)
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ScottH
Thu Mar 02 2017, 04:10PM
Registered Member #61373
Joined: Sat Dec 17 2016, 01:45PM
Location: San Antonio, TX
Posts: 81
Sulaiman wrote ...

It is more complicated than that,

what you observed was influenced by the operation of your MOTs
unless you have knocked out the magnetic shunts,
the MOT output current is limited, by design.

IF you really had full MOT voltage across your motor winding
- you would have needed a fire extinguisher. !

.......................................................................................................
the motors are designed for economy, the steel operates near to magnetic saturation to minimise the ammount of steel required,
so increasing the voltage will increase the magnetic flux only a little before the steel completely saturates.
............................................................................................................
as a general rule,
experimenting with components operating at less than rated values is preferred to above rated values, where they die :)


Cool. So my question/scenario would be accurate if it weren't for steel saturation and coil impedance being a factor; and the rest of the energy would just turn into heat?
For some reason the motor didn't get warm after the Mot. Thanks.
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KrowBar
Tue Mar 07 2017, 10:01PM
Registered Member #57401
Joined: Sat Sep 19 2015, 08:06PM
Location: Huntsville, AL
Posts: 10
Where did you get the impression that the fan was drawing 1amp through a 4100v potential difference? Please don't think of the fan having some number of Watts going through it. Instead think of the fan converting energy at some rate from electrical work to heat and blade motion. The rate at which the fan does this converting is the power it draws.
Also, be aware that your motor is not a resistive load and you aren't supplying a simple potential difference across a resistor - so you cannot calculate the power draw by multiplying a voltage and current. It is possible to have a very high rms current and high rms voltage but low actual power draw because the current and voltage are varying out of phase. Induction motors with little load exhibit this behavior and can easily consume less than 10% of the apparent power. This is referred to as low power factor and results in high losses in the supply wiring and components due to high currents, but low power actually being delivered to/consumed by the load.
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Enceladus
Wed Mar 29 2017, 02:05PM
Registered Member #61428
Joined: Sat Jan 14 2017, 12:39PM
Location:
Posts: 46
As somebody already pointed out, you're dealing with a shaded pole motor. These motors are the simplest and lowest power type of single phase induction motor. They pretty much produce all the mechanical power they ever will at their rated voltage at about 50-60% efficiency. They also draw about the same wattage regardless of load so even at their rated voltage they need constant airflow past the field winding or else they will overhead and fail. Increasing supply voltage only puts more pressure on a system already struggling to dissipate heat. It is only possible to run these motors at other voltages if you rewind the field coil appropriately.
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Sulaiman
Wed Mar 29 2017, 03:18PM
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 2800
ScottH,
time for you to do some more experimenting,
theory is very useful, but hands-on experience is essential - even if only to prove to yourself that your own theories are correct.
and you will learn quicker

I suggest permanent magnet direct current motors to start with,
they are in most battery-powered mechanical devices.
They can be a motor, or a dynamo.
Connect the shafts of two motors together to make a 'dynamotor' ... a dc/dc converter.
They make an excellent introduction to electric motors and generators, and are free.
Larger motors (cordless drills etc.) behave more like an 'ideal' motor than small ones.
Ideally you need to monitor motor and dynamo voltages and currents ... four meters :(

I learned so much from such a simple setup.
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