# How to calculate HP for motors (alternators)

• rizwan25
In summary, the conversation discusses the purchase of an alternator with specific RPM and power requirements. It is mentioned that a motor with 40.25 HP is needed to spin the alternator, and there are better alternators available for faster motors. The possibility of running the alternator with a low HP motor or solar panels is also explored, but it is determined that this would not be feasible due to the conservation of energy and the concept of perpetual motion. The conversation concludes with a request for clarification on the purpose of the alternator.
rizwan25
Hi,
I want to purchase a alternators that have 300 rpm and 20kw three phase can any body guide to me that which motor is best to spin this alternator means in HP.
Thanks

Welcome to PF.

20 kW = 26.82 HP
What type of motor?
The 300 RPM will be a problem as you will probably need a gear reduction.
Can you please give us a link to the 300 RPM, 3PH alternator.

russ_watters
Its from AliExpress

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Sorry its 30kw

rizwan25 said:
Sorry its 30kw
1 HP = 745.7 watt
30 / 0.7457 = 40.25 HP

There are better 3PH alternators for faster motors.
500 RPM is for wind turbines.
What type of motor, diesel, gasoline?

Baluncore said:
1 HP = 745.7 watt
30 / 0.7457 = 40.25 HP

There are better 3PH alternators for faster motors.
500 RPM is for wind turbines.
What type of motor, diesel, gasoline?
Actually i want to run with a single phase motor.
Or with solar panels that runs the motor then solar motor runs the alternator.

Is it possible to run alternator with low hp motor to get 30kw laod.

rizwan25 said:
Is it possible to run alternator with low hp motor to get 30kw laod.
No.
Conservation of energy says you cannot get more energy out than you put in.
Power is the rate of flow of energy.

The electric energy you can take from the alternator, will be less than the energy the electric motor is consuming while it turns the alternator. The missing energy is lost as heat.

There is no such thing as a free lunch.
We do not discuss mythical free energy, or perpetual motion, on this forum.

russ_watters
Thank you for your guide.but in this scenario source of motor is different.mean i just take energy for 3 or 4 HP motor from an electric company that runs alternator.the production of alternator consumes the energy separately it's doesn't provide energy to single phase motor.

rizwan25 said:
Thank you for your guide.but in this scenario source of motor is different.mean i just take energy for 3 or 4 HP motor from an electric company that runs alternator.the production of alternator consumes the energy separately it's doesn't provide energy to single phase motor.
"Perpetual motion machine" doesn't literally just mean self powered, it is an overall category of devices that violates conservation of energy. Label the power inputs and outputs (both mechanical and electrical) on your diagram and you will see they don't match. They have to match.

Lnewqban
@rizwan25 Can you please just tell us what are you trying to do?

Do you wish to convert solar panel output to 3 phase AC?
Or do you want to convert 1 phase AC to 3 phase AC?

russ_watters

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