Newbie needs help with formula for DC Series motor.

Thread Starter


Joined Aug 5, 2018
Hi Everyone :)
Just started my coursework online and Im starting to get my head around the basic formulas but have had a brain malfunction.

Im trying to work out the formula for the following question.

The output of a series dc motor connected to a 210v supply running at 1230rev/min is 3.2kw
The supply current for this load is 20.5A
Calculate the Torque developed & Operating efficiency.

Torque = E Ia/ 2πn (but I need the Emf for this calculation?)
Emf = V - Ia (RaRf) (But I dont know the field winding or armature resistance?)

Can anyone point me in the correct direction as to what calculation I need to do first to find the torque and efficiency??

Thanks :)


Joined Mar 31, 2012
You have the EMF (more commonly known as "voltage"). It's given as part of the problem.

If you know the basic relationship between power output and torque, then you can determine the torque without knowing the voltage at all.

Similarly, if you know the voltage and current you can determine the input power. If you have the input and output power, how can you determine operating efficiency?

Your equation for Emf is not dimensionally consistent. Your first term is a voltage, but you second term is the product of a current and two resistances, that yields units of V·Ω and you can't add two terms unless they have identical units.