PWM motor drive and slow-down speed

If you run a DC motor off one of those dealextreme PWM motor controllers, and you turn the pot to "slow", does the motor "coast" down in RPM?

Application is for a variable-speed Leslie speaker.

Reply to
unk
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That's hard to say -- is that what you want?

If you want actual speed control, I suggest that you go shopping on eBay for a servo motor and driver combination that actually servos the motor speed.

There's a lot of stuff out there with a lot of different features, but a driver that takes encoder or tachometer feedback almost certainly will servo speed, as long as you mate it up with a compatible motor.

--
Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

** Yes, depending on how much load it is under.
** Forget it - *far* too noisy. DC motors whine and the PWM controllers add a whistle or a growl to the noise.

Leslie speakers use very quiet induction motors and belt drive.

.... Phil

Reply to
Phil Allison

IF the driver has both high and low outputs, source and sink, it should follow the PWM speed, meaning, the off duty of the cycle should shunt or force drag on the motor (braking), over all, you should see an effect of constant current.

Proper PWM drive should not allow the motor to free spin.

If all you have is a single output, then yes, it just may do that.

Jamie

Reply to
Maynard A. Philbrook Jr.

s,

** The ham radio dude is very confused, yet again.

PWM drive is generally at a frequency of a few thousand Hz or more - to mak e the current in the motor stay close to constant. When motor speed is stea dy and PWM drive is high, current rises in the armature, when PWM drive low a parallel diode instantly catches the back emf from the armature and the current falls until the drive goes high again. Far as the motor is concerne d, conditions are virtually the same as having a fixed DC voltage drive - t hat voltage being the DC supply times the duty cycle of the PWM drive.

However, if PWM drive stops, armature current falls to zero in a mS or so a nd the motor free spins under its own inertia - meanwhile generating a volt age across it's terminals that *reverse* biases the parallel diode.

If a short is placed on the motor, large current flows it will stop spinnin g very quickly. So, if some PWM controller dispenses with the parallel diode and uses a FET instead that is biased hard on whenever PWM low, it will perform the same function as the diode during normal operation - but when PWM drive stops al together, the motor will stop quickly instead of spinning on.

This is also true even when there is a fair amount of spinning inertia atta ched to the motor, as there would be in a Leslie speaker.

BTW:

Real Leslie cabs have NO motor breaking and take many seconds to spin up an d down in response to control settings - it's part of the famous Leslie sou nd.

.... Phil

Reply to
Phil Allison

ers,

ake the current in the motor stay close to constant. When motor speed is st eady and PWM drive is high, current rises in the armature, when PWM drive l ow a parallel diode instantly catches the back emf from the armature and th e current falls until the drive goes high again. Far as the motor is concer ned, conditions are virtually the same as having a fixed DC voltage drive - that voltage being the DC supply times the duty cycle of the PWM drive.

and the motor free spins under its own inertia - meanwhile generating a vo ltage across it's terminals that *reverse* biases the parallel diode.

ing very quickly.

ET instead that is biased hard on whenever PWM low, it will perform the sam e function as the diode during normal operation - but when PWM drive stops altogether, the motor will stop quickly instead of spinning on.

tached to the motor, as there would be in a Leslie speaker.

and down in response to control settings - it's part of the famous Leslie s ound.

** Anyone wanting more detail on PWM drive of DC motors could do worse tha n read this link - from a man with an honours degree from the university of hard knocks.

formatting link

.... Phil

Reply to
Phil Allison

DC supply times the duty cycle of the PWM drive.

Ha, you crack me up. ;)

Jamie

Reply to
Maynard A. Philbrook Jr.

Maynard is the epitome of the of the phrase "It is not so bad to be ignorant as it is to be unwilling to learn."

Reply to
John S

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