Page 18 - Industrial Technology EXTRA - 14th April

process, calling for a design that minimises losses to copper winding, can be easily modified: by changing
achieve the best solution possible. This is even more both the wire diameter and the number of turns, the
critical for battery operated tools, where only limited torque constant (Kt) and resistance (R) can be fine-
power is available yet where the design engineer tuned.
wants to be able to achieve the maximum number of In the speed phase of the industrial power tool’s
operations from a single charge. operation profile, the motor needs to run fast with
little resistance. Here, a lower Kt value gives
Conflicting requirements increased speed. During the torque phase, however,
Given these conflicting requirements, it might seem the motor is expected to deliver peak torque at low
that there is no ideal solution. The electrical speeds. A higher Kt value gives a higher output torque
performance of a motor is defined by the magnetic at a given current. Here we have conflict.
circuit, so every option requires a compromise. The We might think to select a low Kt value to increase
first component within the circuit, the magnet, has a speed, and compensate the low Kt with more current
fixed value. However, the second component, the to reach higher output torque. However, a higher
current would increase the copper losses, generating
Typical working cycles for industrial power tools repeated heat and thus limiting the maximum torque available.
continually Excess heat impacts on the comfort of the user, while
the increased current impacts on battery life.
There are also iron losses to consider, which are
related to speed. Eddy current losses increase with
the square of speed, heating up the motors simply
when rotating – even in a no-load condition.

Optimised for power tools
There is no unique solution, and in the past design
engineers would have had to accept the performance
compromise in the different phases of operation and
in the broader utilisation requirements for the tool.

April 2020 • INDUSTRIAL TECHNOLOGY EXTRA! • p18