Measuring Riddle No.4 - Measuring the winding length of a bldc motor
How we measure the winding length of a bldc motor from a given torque?

#1
Sat, January 22nd, 2011 - 19:22
It seems your means of measuring is computation, however Brushless direct-current motors (BLDCs) are so named because they have a straight-line speed-torque curve like their mechanically commutated counterparts, permanent-magnet direct-current (PMDC) motors. In PMDC motors, the magnets are stationary and the current-carrying coils rotate. Current direction is changed through the mechanical commutation process.
These motors are generally used where relatively high rotor inertia is beneficial to system performance. Common applications are computer disk drives and cooling fans. Construction is shown in Figures below.
The rotor assembly consists of a flexible magnet with four poles magnetized on it. It is enclosed by a magnetically soft steel cup or housing. A shaft which allows the rotor to turn with respect to the stator is attached to the center of the steel housing.
The stator assembly consists of a lamination stack with coils of wire wrapped around the pole pieces. This is supported by a mounting base which also contains a bearing support and a control circuit. This motor’s poles are alternately magnetized N-S-N-S.
As the rotor turns, the currents are turned off in one winding set and turned on in the other set by the Hall switch. This results in an S-N-S-N magnetization in which the S poles are induced at the interpoles and the poles where no current exists in the windings. This keeps the rotor turning. A close observation of this structure shows that it is in fact a single-phase motor and, as such, will have rotor positions where there is zero torque. This could result in failure to start. This is overcome by placing interpoles between the wound poles. The magnet provides some position bias, which always results in rotation when the pole is energized.

Two major motor performance parameters are required to be computed in order to establish overall motor performance. They are the torque constant Kt and the phase resistance Rphase. From these two parameters, all motor performance can be essentially derived.
The Kt value is derived for the case of RMS torque based on a trapezoidal torque versus position profile and near-perfect commutation. There is an adjustment of 10 percent to achieve a reasonable value for RMS torque. This value can be adjusted based on actual torque waveforms if required. The following equation computes the Kt value per phase as described.

HANDBOOK
OF SMALL
ELECTRIC MOTORS
William H. Yeadon, P.E. Editor in Chief
Alan W. Yeadon, P.E. Associate Editor