How does Timing effect the power of an electric motor?

In general terms this describes motor action:-) But (bet you knew that was coming) more specifically you need to first visualize that the permanent magnets create a fixed field in magnitude and position (for this discussion). This field has a shape with a maximum intensity exactly between the magnets (in a 2 pole motor - most DC motors - directly opposite each other). To generate the maximum torque from the motor, the electric field generated by applying current to the windings must be located 90 ELECTRICAL degrees from the center of the magnet field (90 physical deg. in a 2 pole motor). This is defined as the max. Kt (torque constant - oz.in./A)


What happens in a real motor under load is that the electric field lags the magnet field due to inductance effects in the windings and possibly some saturation effects in the steel parts (magnetic iron). The amount of lag varies with load (and rpm), so is a complex variable.


We try to preset the motor timing (brush position) so that at a selected load current, the fields are at the 90 E deg point for max. Kt., and max. magnetic power conversion efficiency. At no-load (without a prop) this will be an advanced position and since it is ahead of the 90 E Deg. point, will have a reduced Kt (take the SIN of the angle x KT90, eg SIN108 x 1 = .951 x Kt), which requires more current to turn the shaft (at no-load. The current will pull into phase at the selected load current. This will actually look like the current decreasing as the load increases, up to a point.


In a brushless motor the same thing is happening, but we advance the hall sensors for this effect. The big difference is that we have to visualize the analogy while sitting on the rotor (stationary frame of reference) and look at the stator field.


This is explained in most texts of ac and dc electric machines. It's a lot more complex than this, so I hope I've given you some insight as to the goings on in that little bundle of wire and magnets.

www.rcecho.com