I have a hypothetical question about running a DC motor on lower voltage than it is designed for. Lets say I have a 2hp 100VDC (yes, Direct Current) motor that I want run on 12 VDC. The motor is 2hp = ~1500 Watts, so I'd have to supply 125 Amps at 12 VDC. This would require some big wires, and result in some efficiency and heat losses. Would it burn out the wires in the windings of the motor? Would that be entirely dependent on the load? Would the motor turn at a lower speed? Lets say the rated speed of the motor is 4400rpm, what would the speed be with a 12VDC input?
Electronic – reducing input voltage to motor
dcmotorspeed
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As its only a 3V motor I'd limit the speed controller range by adding a resistor above and below the pot RV1. A 10k resistor either side will limit the output swing to +/- 5V and give you finer control over the speed. Increasing these values will reduce the voltage range even further if required.
By the way - I suspect your professor was looking for a closed loop type speed control rather than the open loop type you have. Hence the references to ADC, DAC and low pass filters. This would also require a tachometer (usually some form of pulse/frequency device the output of which would require conversion to an analogue signal - DAC) to measure the speed of the motor shaft.
It might be a good idea to clarify from your professor exactly what is required for this project (open loop or closed loop?) before committing to a final design.
(source: robotroom.com)
Here is a simple example of how to drive DC motor with Transistor and PWN. The diode is to protect the circuit while the motor is free spinning and to stop fail gating the transistor. The resistor is protection for the pulse generator i.e. micro controller or 555 chip or the many other ways to make PWN signal.
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Best Answer
If the rated speed at 100v is 4400rpm, then the nominal speed at 12v would be 4400*12/100 = 528rpm. In practice, it might be a little quicker or slower than that, depending on how the losses vary with speed.
The rated current at full power is 1500W/100v = 15Amps. It is this current that heats your windings.
With a motor that large, it's possible you could get 15A or more to flow through the windings with 12v applied. However, if the motor is self-cooled by a fan on the shaft, or simply by stirring air around in the casing, then the cooling air available would be much less than at rated speed, so you would have to reduce the current significantly below 15A to avoid the risk of it overheating.
If the motor has a cooling duct driven by a separate fan, and if you can run this cooling fan at full speed, you can cool it properly and it should be safe up to 15Amps.