# How electric motor can have full torque at start? why dont IC engine can have this ?

1. May 9, 2010

### sr241

how electric motor can have full torque at start? is it because magnetic forces are in phase with turning moment curve of rotor of electric motor. i mean the peak of magnetic forces and peak of turning moment occurs at same time. if we impliment this in IC engine does it to can have full torque at start. I mean if we make presuure curve in phase with the turning moment curve of crank

2. May 9, 2010

### swraman

Re: how electric motor can have full torque at start? why dont IC engine can have thi

I was under the impression that an electric motor has its maximum torque when its angular velocity is 0 because if the angular belocity is 0, there is no change in flux, and no induced EMF (or Back EMF) opposing the direction of the EMF driving the motor.

3. May 9, 2010

### Phrak

Re: how electric motor can have full torque at start? why dont IC engine can have thi

It has nothing to do with the phasing. The torque in question is over several phase cycles.

Ignoring the finer and unenlightening details, for a DC motor, torque is proportional to current.

T = KI

The applied voltage minus the current through the series resistive loss (I times R) is proportional to angular velocity.

V - IR = Vmotive

Vmotive = Kω

This is as sr241 was saying. The angular velocity corresponds to a voltage of it's own that subtracts from the applied voltage with the name "back emf". With less voltage available, the current through the resistive loss is reduced; therefore less torque.

Last edited: May 9, 2010
4. May 9, 2010

### Lsos

Re: how electric motor can have full torque at start? why dont IC engine can have thi

An automobile ICE engine, by design, has to be spinning to make torque. It has to suck in air, compress it, and expel it, and it's not going to do that at 0 RPM. It's not going to do it at 10 RPM either, or even 100 unless it's some huge marine engine. Remember, a piston only makes power 25% of the time, and this power has to pay for the remaining strokes, including compression, which slows it WAY down. So in general, it has to spin at a decent speed to make enough power to be able to even pay for itself. Once it gets there, it has to spin faster to make extra power, and even faster to make the kind of power you want. Depending on design, you'll usually end up being in the thousands of RPM before you get there....

Electric motors, on the other hand...they're just some wires and magnets. You run a current through a wire and it becomes a magnet, which pushes against another magnet and vuoila! you've got force (torque).

The more current you supply, the more torque. I'm guessing the greatest current usually runs at 0rpm, since there's practically no resistance in the wires. Once the motor starts spinning, the moving magnetic field resists the current, and slows it down.

That's my layman's explanation of it, it's extremely simple and obviously will differ as there are many ways to make electric motors, and it may not even be 100% correct...in which case I invite someone to correct me.