# What is the relationship between field strength and efficiency in a DC motor?

This refers to a DC motor with permanent magnets.

What is the relationship between the efficiency of the motor and the magnet field strength?

• Field strength mainly causes torque and back-EMF, I know of no link to efficiency per se, but machines are not my forte. Commented Jan 30, 2018 at 10:40
• Any properly designed PM motor will have the core sitting near saturation with the original magnets. If you increase the magnet strength you'll just saturate the core and it is unlikely you'll observe any improvement in efficiency. It will just run rougher due to the asymmetric armature reaction. If you increased the airgap to reduce saturation, then you'll be able to get a higher peak torque from the motor, but the efficiency still won't change.
– Jon
Commented Jan 30, 2018 at 11:48
• @Jon funnily enough, with a cheap motor where the can is too thin, you can increase field strength by wrapping mild steel round it, to keep the can out of saturation.
– user16324
Commented Jan 30, 2018 at 11:52
• Don't you feel your question lacks research? There isn't even an educated guess in it, which I suppose you have done mentally. Commented Jan 30, 2018 at 11:56
• @DmitryGrigoryev My educated guess was there is no effect. However, physics is not engineering reality Commented Jan 30, 2018 at 12:07

To a first order approximation, none.

Modifying field strength is used as a speed control on some very large DC motors, and they don't dissipate huge powers and catch fire at either end of the useful range.

What it does do is modify the generator EMF at a given speed. Reduce the field and the generator EMF reduces. In a motor, that's called back EMF, so the supply voltage isn't cancelled by back EMF, current increases, torque increases, motor speeds up, back EMF increases, current decreases to the correct level to maintain the new speed.

To a second order approximation, there will be effects on efficiency.

Too much field will saturate the magnetic circuit. I'm not sure how that affects efficiency in a PM motor, but it certainly wastes power driving the field winding in a wound field motor.

Too little field attempts to run the motor very fast, but with little torque. Unless the motor is correctly (very lightly) loaded, it will tend towards stall, which definitely reduces efficiency. And ultimately, bearing/brush/windage friction will consume all the mechanical power leaving none at the shaft.

So, loosely, high field is generally more efficient.

But if the motor is correctly rated and not overloaded, there should be a reasonably wide range of field strengths at close to peak efficiency.

(If you have a fixed field, you can achieve a similar variation in results by winding the motor with more or fewer turns : more turns being roughly equivalent to higher field. If you don't feel like rewinding motors to experiment, radio control model car suppliers sell the same motor wound with different turns for performance tuning)