Wednesday, 23 September 2015

Controlling treadmill motors by Pulse Width Modulation.

Permanent magnet DC motors used in fitness industry

Home use treadmills are equipped with DC (Direct Current) motors.  The Permanent Magnet DC Motor (PMDC) is the most commonly used type of direct current motor available producing a continuous rotational speed that can be easily controlled.

The rotational speed of a DC motor is proportional to the Voltage applied to the motor.

PWM Regulation as standard regulation for home use treadmills motors

PWM regulation on a treadmill DC motor

Pulse width modulation (PWM) speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant.

The voltage applied to the motor can be controlled by varying the width of these applied pulses and thereby varying the average DC voltage applied to the motors terminals, ie, the longer the pulse is “ON”, the faster the motor will rotate and likewise, the shorter the pulse is “ON” the slower the motor will rotate.

Advantages and disadvantages of treadmill motor regulation by PWM

The use of pulse width modulation has the advantage in that the power loss in the switching transistor is small because the transistor is either fully “ON” or fully “OFF”. As a result:
  • the switching transistor has a reduced power dissipation 
  • linear type of control which results in better speed stability
  • the motor can be rotated very slow without it stalling
The main disadvantage is that the motor is driven with high voltage pulses (about 300VDC for a network current of 230VAC, 150VDC for 110 VAC). When a reduced rotational speed is required by the application (such as walking at slow speed on the treadmill) associated with the heavy load (user weight, treadmill maintenance conditions, running belt friction factor) the motor is subjected to heavy stress and overheating. This could bring easily to electric leaks inside the motor and consequent damage to the motor itself and/or controller.

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