Voltage Output to Motors

[steveski]

I have a 7A RoboClaw unit and am using 6VDC motors. I had the unit working well on my desk with a 12VDC supply to the RoboClaw. When I went to put the unit on a 24VDC supply it started locking up the motors. I measured the voltage output to the motors and found that when I supply 12VDC to the RoboClaw it is sending 6VDC to the motors. When I supply 24VDC to the RoboClaw it sends 12VDC to the motors. The max voltage for these motors is 9VDC, so this is probably why they don't run well when I use the 24VDC supply. My question is what is the spec for motor voltage output? Is it always half of the power supplied to the RoboClaw main battery terminals?

Anyone have ideas on this?

[Basicmicro Support]

THe voltage output by the controller is a PWM signal at near the input voltage(eg 12v power input means about 11.8v PWM pulses). The motor acts like a low pass filter(its a big inductor/resistor circuit). So at 50% duty the PWM will be averaged(inside the motor) to about half the input voltage. However the motor brushes still "see" the high voltage(the filtering happens inside the coil of the motor after the brushes). If the motor is a 9v rated motor you should not use more than about 13.5v(eg 50% over the rated voltage of the motor).

I'll also note that most motors that are not rated for 12v or more(in my experience) tend to have very cheap brushes and should not be run at higher voltage than their actual voltage rating in general(eg I would not run a 9v rated motor over 9v(input voltage)).

Second note. If you are reading the voltage of the motor outputs with a multimeter it is averaging the voltage it sees over a period of time(much longer time than the PWM period of the Roboclaw). You cant see the high voltage PWM without using an oscilloscope.