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This isn't exactly related to the Prius but I kinda need the answer for a project. Maybe there are electrical guys here that can help.

Is there any way to reduce the power output of a motor?

If a 5 HP motor is used to drive a load but when the load drops to a point where only 2 HP is needed. Is there a way to reduce the motor output to 2 HP? Or does the motor still runs at 5HP regardless of load and extra energy simply get wasted?(in resistor or other ways)

I have no knowledge of electric motors but common sense dictates that it is probable that if only 2hp is used then only slightly more than 2hp of input is consumed. Maybe I have got this wrong and will soon be told so if that is the case.

Someone said "alternating current motors generally will draw only sufficient current to match the load. An AC motor under no load draws very little current
regardless of its rated horsepower. It will increase its draw as the load increases. There is such a thing as over-motoring a piece of equipment with an AC motor. I'm not sure how it works, but there is an inefficiency
in say, attaching a ten HP motor where a 1 hp would be adequate. Direct current motors will draw a fixed amount of current regardless of the load."

Just want to get things straight once and for all.

Thanks.
 

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There are many different kinds of motors. A brush DC motor probably has little control of the power output, except by limiting the current to the motor.

The motor in the Prius is a sychronous 3 phase AC motor. Current is pulsed into the motor with the required width to provide the correct average power input for the load and power output requirement. Sensors in the motor tell the computer what position the rotor is in, so that the right leg of the motor is sent power at the correct time.

Motors work by creating opposing magnetic forces to cause motion. The opposing magnetic forces can be two sets of electromagnets, or one electromagnet set and one fixed magnet set. Either the electromagnet set or the fixed magnet set can be on the rotor. It is often easier to put the fixed magnet set on the rotor, so that there is no need to provide a means to deliver electric power to a moving rotor. When the rotor is electric, slip rings are often used, along with brushes. This can cause power loss due to arcing, and wear on the brushes. The Prius uses a fixed coil set on the stator, and a permenant magnet set on the rotor.
 

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Pretty much all electric motors generate what is called "Back emf". The spinning armature within the electromagnetic field coils (or permanent magnets in some motors) actually generates a voltage which causes the current flow into the motor to drop. As the current flow drops the armature speed drops which reduces the back emf that is generated allowing the current to increase again. This description fits best for a DC motor but similar thing happens in AC motors. The bottom line is the motor draws only as much current as it needs to overcome the friction of the armature bearings and the load it connected too. Theres some ohmic loss in the coils as well. If you have a 1 hp AC motor with no external load running at 3600 rpm on 120 volts the current flow will be fairly low (1 amp or so). As soon as you put a load on it, the current will climb to 10 - 12 amps until the motor is producing 1 hp. As you put more load on it, the current will continue to increase but no your heading for trouble since the windings will start heating up a lot and burn-out is not far away.
 

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Also bear in mind that the larger the HP the higher the no-load current will be.
 
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