# reducing input voltage to motor

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?

• The motors would run slowly, if at all. Furthermore, they may indeed burn out. Apr 11, 2016 at 17:11

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.

• Just in case it is not obvious, at 12 volt, the rated power will be something less (depending on cooling) than 2 Hp X 12/100 = 0.24 Hp.
– user80875
Apr 11, 2016 at 18:23
• OK, that's helpful. Some followup questions:So the motor and its windings are designed to handle 15 amps and, regardless of the voltage input, if I run it at higher amperage I am heating the components more 6than they are designed for, correct? Apr 11, 2016 at 18:44
• more questions: I know that the amps that the motor will draw will increase as i add more load to the motor output shaft, but if I run it with no load (other than the friction of the bearings and whatever fan it has) at 100VDC, is it pulling the full 15A while it is sitting there spinning at 4400rpm? If I run it on 12VDC, and its spinning at 528rpm, how many amps is it pulling (again, with no significant load). Apr 11, 2016 at 18:47
• I am trying to understand the relationships between voltage, load, current, and angular velocity. Apr 11, 2016 at 18:48
• In the simplest terms, Voltage = Speed, Current = Torque. If you have little or no load, there is a very low torque requirement to maintain a speed (at any voltage). DC Motors operate in a constant torque mode (they can deliver full rated torque, but only if loaded) from 0RPM up to their rated base speed. Going above base speed, say with field weakening, they transition to a constant horsepower operation. Apr 12, 2016 at 13:56