To convince visitors to our website that we also make and test the odd prototype, as well as, on occasions, special test and measurement hardware, I thought I’d mention some of the test equipment we use. Some are off-the-shelf, and some are of our own invention. So it’s not all about magnetic simulation and finite element modelling sitting at a desk!
One of the most useful instruments we have is a flux meter. Ours was bought from Hirst in Cornwall. This type of instrument is an integrating voltmeter, but a high-performance one, which is calibrated in several ranges, is very sensitive and has low drift. The quantity measured is flux linkage, which in SI units is Weber-turns, or volt-seconds (1 Wb-turn = 1 volt-second).
This instrument can be used in conjunction with standard types of measurement coils, for example, a Helmholtz coil for measuring magnetic moment (a decent quality check for permanent magnets at goods in), or probe coils for measuring average flux density. But there are other ways it can be used to make measurements that are useful for confirming the performance of different kinds of motors or actuators.
To make a measurement of the back emf constant of a brushed D.C. motor (numerically equivalent to the torque constant in SI units), simply rotate the armature through a fixed, accurately known angle, and use a flux meter to measure the time integral of the generated voltage at the motor terminals. We often think of back emf constant in units of “volts per radian per second”, which immediately casts our minds into the idea of a motional emf. To understand this measurement technique it is more useful to think “volt-seconds per radian”, in other words, a time integrated voltage resulting from a change in flux linkage.
Magnetic Flux Measurements
Another potentially useful technique arose from a customer who wanted to de-bug his solenoid design. The force produced was not what he expected, and to establish the magnetic conditions inside the prototype we suggested winding a special coil with an additional small gauge “secondary” measurement winding of a known number of turns, and connecting this to a flux meter. Then ramping up the current in the main coil having first locked the armature and zeroed the flux meter. The resulting flux linkage measurement could be used in conjunction with the measurement winding turns count and the solenoid geometry to calculate the flux density inside the coil/armature. The test could be repeated for a range of locked armature positions and final current values to compare with design estimates of air gap flux density.