PWM, victors and motors

[SoD]

We need to get the Fisher-Price 9012 motor to work... but the problem is that we can't give him more than 40 Amps... which meens we need to control the PWM values in a way that the victor won't give more than 40 Amps... meening somewhere between 255 and 127. But how can we know the exact value?

[Mike Betts]

Originally Posted by SoD

We need to get the Fisher-Price 9012 motor to work... but the problem is that we can't give him more than 40 Amps... which meens we need to control the PWM values in a way that the victor won't give more than 40 Amps... meening somewhere between 255 and 127. But how can we know the exact value?

Sod,

You can give the motor more than 40A. The Victors (PWM control) regulate the voltage to the motor. The current is a factor of the voltage provided and the mechanical load on the FP.

I would start by reading some of the presentations on motors in the white papers section of this forum (anything written by Joe Johnson is great).

Next, I would search these forums on how a circuit breaker works and how you can deliver more than 40A to a motor.

Regards,

Mike

[MikeDubreuil]

I'm sure someone else can provide a detailed answer, but I'll give a quick one.

When you set a PWM output from 0 to 255 you are not setting voltage or current What you're actually setting is the duty cycle output of the Victor to the motor. Duty cycle is a percentage and the rough equation is (On Time)/(Off Time).

Examples:

• PWM Value: 127
  Duty Cycle: 0 %
• PWM Value: 255
  Duty Cycle: 100% (Normal polarity)
• PWM Value: 0
  Duty Cyle: 100% (Reverse polarity)

The current a motor draws is related to the amount of power required to turn the output shaft. The more friction in the mechanism the higher the current draw will be. This is a mechanical issue not a programming issue.

[Chaychay]

A couple years back FIRST provided current sensors in the kit. If you put one of these on your Fisher Price Motor circuit, you could just tone down your PWM output as the sensor output got closer to 40 amps. (Like a proportional controller)

EDIT: I just read the posts above mine, and they render my post incorrect. But just for your reference, you can stick a current sensor on their to monitor how much you are drawing.