# how do i work out rpm of a universal motor

#### daveyjones97

Joined Oct 21, 2011
46
hi im struggling to find how you work out the rpm of an universal motor. ac will do for now as thats 50hz in the uk. i have an universal motor with 2 field windings and 20ish commutator segments on the rotor.
i'd like to know what rpm it would run at 240v 50hz ac and on dc. my guess is 3000rpm ac and 10000+ maybe explode with cenrifugal force or just arc excessively if given no load dc and a high enough voltage. can any one correct me or is that about right?
the motor is 2000w from a karcher jetwash.
thanks

#### crutschow

Joined Mar 14, 2008
25,991
Universal motor speed is basically determined by the voltage and the load. With no load the speed will get very high. It is generally independent of the AC frequency (if designed for AC/DC operation) and will have a similar or slightly higher speed with DC.

#### strantor

Joined Oct 3, 2010
5,382
I just tested a 120V 8A universal fan motor with the impeller removed with a 20V power supply, limited to 1A and the thing almost self destructed before I could disconnect the leads.
The current in the armature is the same as the current in the field, so after initial startup current, the current trails off lower and lower creating everincreasing field weakening, which speeds the motor up. With no load, it's only mechanically limited, and will go as fast as required to spin itself into nonexistence. That goes for both AC and DC. The universal care nothing about number of poles or synchronous speed on AC.

#### daveyjones97

Joined Oct 21, 2011
46
thanks guys, didnt realise it would run away on ac too. il make sure it always has a load.

#### thatoneguy

Joined Feb 19, 2009
6,359
When loaded, they can put out a lot of torque as well.

They are limited with torque at high speed, though.

#### BillB3857

Joined Feb 28, 2009
2,541
When I had my technical interview for my job, one question was, "What controls the speed on a series wound motor?" Correct answer is, "The load" That is one reason nearly all series wound motors , like in a food blender, have an internal fan. It provides a minimum load.