Motor performance

If I was to run an electric motor rated at 24V 20A at 15V 10A and at 10V 15A what would the difference in performance be? Assuming everything is rated to safely handle more than that power.

• That all depends on the type of motor. Is this a brushed motor? Wound field, or permanent magnet? BLDC? – Phil G May 23 at 19:33
• 'V' for volt, 'A' for ampere with a space between the number and the symbol as per SI standard. Welcome to EE.SE. – Transistor May 23 at 19:36
• Can you give me details for the different types of motors? – user222622 May 23 at 20:31
• Comments are for clarifying the question, not expanding it. Asking for all of the details for even one type of motor would be a question that is too broad for this forum. – Charles Cowie May 23 at 22:03
• – Sunnyskyguy EE75 May 23 at 23:35

You cannot "run" a motor at a certain amperage, because of the two fundamentals:

• voltage determines speed

• torque determines amperage

Okay, but why I can't select the torque then?

You can, but because both the motor and the part driven have a torque over speed characteristic, they automatically run at the crossing point of their characteristics, the working point. At that point, the torque of the drive and the countertorque of the part driven are balanced so the speed does not change any more.

That means the torque determines the speed. You can play with the voltage to ramp up the speed, but get another torque balance then, which most times also means the amperage goes up as well.

• does that mean using a dimmer switch to run the motor I would always draw the maximum amperes and therefore at two different wattages would still have the same pulling force but not speed? – user222622 May 23 at 20:28
• The countertorque applied to the motor determines the current. If your motor runs free, the current is minimal, as the countertorque is only from the bearings and the fan on the axle, if any. The countertorque depends on the load, as well as its characteristic. For example, a crane has a fixed torque over speed depending only on the mass attached, a wheel drive has a roughly linear torque over speed, a pump or fan has a quadratic torque over speed. – Janka May 23 at 21:08
• A typical dimmer switch is itself a complicated beast, as it doesn't control the voltage, but the voltage integral over time. This creates a lot of undesireable side effects especially at higher speed. – Janka May 23 at 21:11

All motors have a characteristic curve that shows the motor's torque capability vs. the motor's operating speed. Driven loads have a similar curve that show the torque required to drive the load, the torque demand, at a given speed. The actual operating torque and speed is the intersection of those two curves.

To a great extent, reducing the voltage applied to a motor creates a new torque vs. speed curve for the motor with a lower speed at every given torque point. For some types of motors, there are other factors that have an effect on the curve. For a permanent-magnet DC (PMDC) motor with a commutator the other factors are relatively insignificant. A group of torque vs. speed curves for a PMDC motor is shown below. The curves correspond to various voltages increasing from left to right. For this type of motor, the speed increase is quite linearly proportional to torque.

The maximum torque is relatively constant regardless of speed, but the safe operating time at lower speeds is limited by the cooling air flow produced by the motor itself. The maximum speed is limited by the insulation voltage, the commutator construction and mechanical considerations.