In another thread, it was suggested that a 2.5 rated volt motor* could be run from 5v PWM'd to 50%.
My understanding of this whole PWM thing, is that for a dc motor, the voltage applied to the device is still the maximum (of say 5v). The average is say 2.5, I get that bit, so it runs at half speed, but the maximum voltage applied is still 5 surely, albeit for minute periods of time. Won't the motor still need to be a 5v motor even if it never goes over "2.5 PWM volts" so to speak?
In fact the post suggested this for a stepper, not a plain dc motor. Is PWM a concept applicable to steppers?
The reason why this works can be understood if you think the motor is broken by power it uses (it overheats) rather then a voltage. The power the motor uses is the same in both situations. If you were powering up a led, it would break because the current would exceed it's maximum value.
The inductance of the motor coil, combined with whatever resistance is in the circuit, limits the rise time of the coil current. You simply cut off the coil power source when the coil current is at its rated maximum. A circuit that does this is called an L/R chopper motor driver.
With this technique you can use a relatively high voltage power supply, combined with an appropriately chosen resistor in series with the windings, to get much higher torque out of a stepper than if you use a power supply that simply provides the rated coil voltage. Here is a randomly chosen copy of an application note that describes the waveforms: http://www.robotstorehk.com/AN-828.pdf
The newest chopper driver chips rely on the winding resistance for the "R" of the L/R circuit.
The voltage limit of a motor depends on at least two things. The heating issue, which is really caused by current, but more volts equals more current ; and the ability of the insulation of the windings or other critical parts to contain the voltage in the wires where it is supposed to be. Both of those constraints apply. So no, don't put 500 volts into your little toy motor, even on a 1% duty cycle. I guess you already knew that.
michinyon:
.So no, don't put 500 volts into your little toy motor, even on a 1% duty cycle. I guess you already knew that.
I don't have that many batteries anyway.....let's see, 400 odd penlights, each say 5cm long, 20m... difficult to keep the buggers nicely lined up, so no, I won't try that 8)
It's true that more volts equals more current, but time also comes into play.
A coil has inductance which slows the rate at which the current rises, as compared with a short straight piece of wire. Also, even with a large current it takes a little time for the wire to heat up. You need energy (watt-hours (or -seconds)) rather than just power (watts) to cause heating. The important thing is to prevent the wire from getting too hot. Switching the voltage on and off quickly (as PWM does) is one way to limit the amount of energy passing into the coil even though the voltage may be much higher (provided it is within the limits of the insulation resistance) than could be accepted without PWM.
But any old freq doesn't go with any old inductor, does it?
Reactance (XL) is relative to time, you need to know the motor's inductance.
Many windings results lots of L, so "no problem"?
If I have something "on" for 24 hours and then "off" for 24 hours, that's 50% duty (5.8 micro-Hz), but the DUT "somehow" failed the over-volts test [anyway]?
