[109jb]

I have a PWM speed control that i purchased off of e-bay. It is like the one in the picture. It has a pwm output to the motor. The input says up to 160VDC or up to 110VAC. I assume that the input is rectified since rectified 110VAC is about 155VDC. I also have a treadmill motor rated at 90VDC.

Here is what I am trying to determine. I'm thinking the PWM output of the speed control will have a peak voltage equal to the input. So, if I hook up 110VAC for the input to the speed control, the peak of the PWM output will be 155VDC. Am I OK if I keep the duty cycle of the PWM output such that the "average" output voltage is below the 90VDC rating of the motor?

[mike_Kilroy]

Typically 90vdc motors are ok upto nominal 160vdc bus but not more.

[109jb]

Typically 90vdc motors are ok upto nominal 160vdc bus but not more.

First off, I forgot to include the picture of the PWM speed control I have. It is attached to this one.

Attachment 271968

So can I infer from the comment above that I should be OK with 110VAC input? I would definitely keep the PWM duty cycle to about 55% which should place the "average" output voltage to about 85VDC (110VAC rectified to DC ==> 155VDC then 155VDC *0.55 duty cycle = 85VDC average

[mike_Kilroy]

Perhaps a description the limiting factor in armature voltage for brush motors will help understand the issue? when the commutator rotates, the supplied voltage is applied to the bars. there is a MAX BAR-to-BAR voltage the motor is designed for. Go over this and you get more and more sparking (lower brush life). go over this a lot and get ring of fire (sparking ionizes the air above the com surface, making its resistance zero ohms, hense big ring of fire in this short - it is called flash over.

Know your 'average' voltage equivalent has NOTHING to do with this; it is the PEAK of the PWMs - the max voltage - that is applied that is the limiting factor. So forget average discussion. Discuss dc bus voltage.

Notice I said 'typical' 90vdc motor; unless you can get manufacturer to bless YOUR particular motor @160v bus, you are own your own with 'typical' general replies. Cheaper motors will 'typically' be designed closer to the max bar-bar limit than better ones....

If you power up and see excessive sparking on THIS motor at 160vdc bus, then you either live with the shortened brush life, reduce the dc bus voltage, or add a choke (inductor) in series with the motor armature to reduce those pwm peaks.

[Al_The_Man]

The drive shown is very basic, no, or very few features such as current limit adjust etc.
Some other factors that limit the max applied to the motor is the maximum current (torque), in a PWM drive the current is exhibited by a average level slightly saw tooth wave form, the level of which is dependent on load.
It is not the same shape as the voltage which being PWM is square wave in shape with varying width, too high a voltage (pulse width) and you can overspeed the the motor, so the two things you need to monitor are RPM, and maximum current.
Power-Supply Considerations For Servo Amplifiers | Electromechanical content from Electronic Design
Al.

[109jb]

Thanks guys. I expected that the motor wouldn't care about "average" voltage and that the peak PWM voltage would be the decider. I plan to use 110VAC mains input, and I do have a choke that I can put inline with the motor. I'll give it a try and see how it does. It was a cheap motor, so I'm not too terribly concerned about burning it up, or the cheapo controller for that matter. I just wanted to give it the best chance of surviving. Obviously the best thing to do would be to use a DC power supply to get the input voltage to 90VDC, or a transformer to drop the AC voltage to about 60V to give a rectified DC of 90 or so. Just trying to do it on the cheap.

Al - your comments are appreciated. I will start out by using the pot that comes with the controller to adjust speed and will be careful to not overspeed it when using that. I'm planning to put it on my little lathe which has a spindle tachometer that I made a while ago. I will eventually set it up for pwm control from the CNC software and will set the max PWM signal for the max rpm that I want. As far as current, I could put an ammeter on it, but I think I will just chance it and keep the cuts on the light side as to not load the motor too much.

Thanks again guys.

[Al_The_Man]

Its fused so that will offer some protection.
Al.