# Why does torque increase with rotor current in an AC induction motor?

In my understanding, the EMF induced on the rotor of an AC induction motor induces a current, and the larger this current, the greater the rotor's torque will be.

Could someone please explain: why does a larger current lead to a larger torque?

I'm looking for a qualitative explanation rather than a quantitative one, but any help would be very much appreciated!

• It's the other way around. Countertorque on the rotor will make the rotor spin slower than synchronous speed. That's what's creating AC in the rotor. – Janka Nov 7 '17 at 10:41
• Please see the torque over speed diagram of an induction motor first, around the synchronous speed. – Janka Nov 7 '17 at 10:46
• @Janka Ah yes thank you! I've been rather confused about this topic, but I've done more research and am starting to understand the relationship between speed and torque. But I've just got another question: is there a way to reduce the trade-off of speed for torque? For example, would a stronger magnetic field in the stator mean that the motor has a larger torque to begin with? – user-2147482591 Nov 8 '17 at 8:37
• Increasing the stator voltage scales the torque/speed diagram in the torque direction. However, you cannot increase the stator voltage over its design value because otherwise the motor goes into magnetic saturation and overheats greatly for very little effect. – Janka Nov 8 '17 at 10:47
• For increasing torque at standstill and low speeds, you want to use a en.wikipedia.org/wiki/Squirrel-cage_rotor – Janka Nov 8 '17 at 10:49

The increase in torque can be explained by the following physical relationships:

• The Torque that develops in a motor comes from the interaction between the stater's and rotor's magnetic fields.

• The strength of each magnetic field is proportional to the current flowing through the stater and rotor.

• The magnetic field of the rotor will attempt to align with the magnetic field of the stater(South to North and North to South), this is where the turning force exerted between the rotor and stater comes from.

Therefore if you push more current through the rotor a stronger magnetic field is developed and a greater turning force is exerted from the interaction of those magnetic fields. This concept is more simply illustrated by looking at a brushed DC motor. Picture below from: https://www.globalspec.com/reference/10791/179909/chapter-3-ac-and-dc-motors-ac-motors-ac-induction-motor

Hope this Helps!

Each electromechanical machine can be basically represented in two axes: direct (excitation) and quadrature (torque) which are perpendicular to each other. The torque is a product of both components, the Q is variable, while the D is constant in the nominal range of operation.

The easiest motor to understand is the permanent magnet DC motor. The D-axis is the permanent magnet - excitation, the rotor field is perpendicular to magnet and it is a Q-axis.

The induction motor follows the same rule, the current in the rotor cage produces a magnetic field that is summed to the rotating stator field, indeed these resultant field can be split into the Q and D components. As the torque is proportional to the product of D and Q components, it is obvious that larger current produces larger torque.