Universal Motors

Universal motors have high starting torque, can run at high speed, and are lightweight and compact. They are low inductance motors.

Universal motors make good DC motors and can be used in servo systems with an h-bridge driver. The field is normally energized with the same motor voltage used to power the DC motor driver. The field and armature can be connect in parallel, but to reverse direction, one would have to be rectified so they can oppose either other.

Energizing the field separately seems more common and has some advantages; for example, you can "tune" the system by changing the field voltage (and therefore current) for the tradeoff of speed vs torque for any particular armature current. If we want a higher speed less torque motor we have less field current, and for lower speed higher torque we increase field current. Universal motors often come with the field pre-wired to the armature, so in that case, the wiring must be altered to separate the two.

The DC voltage required for full power is 0.707 times the rated AC voltage. e.g. for a 120 VAC motor, about 80 VDC is best. The power supply does not need to be well regulated, and can consist of a bridge rectifier and a few filtering caps.

Universal motors are commonly found in AC power tools, corded drills, vacuum cleaners.

As with any high power system, some form of fuse, circuit breaker, or active current limiting system is a good idea in case of a short or internal failure. For AC line powered systems, some tungsten light bulbs in series with one leg of the transformer on the 120v ac input side will just light up if there is a short and drop the power across the bulb.

Many thanks to George Dvorak for this imformation

Note: Many of the pictures found online showing the wiring of a Universal Motor only show the field winding, and comutator, ignoring the armature winding. This can be confusing. Remember that the armature has it's own windings.