DC motor blows rectifier

[91bronc300]

I've been trying to teach myself about electricity and motors and transformers and such in my spare time. I picked up a treadmill for free off of craigslist a while ago, the guy said it worked fine but the main board went out and it was too expensive to replace. The motor in the treadmill is a 2.5 HP continuous duty permanent magnet DC motor with these specs:

3200 rpm
21 amp
DC90V
Insulation F
Ambient 40 degrees C

Well I bought these rectifiers off of eBay http://www.ebay.com/itm/251274906610?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

I figured that being rated 50 amp and 1000 volts would make them more than adequate. I connected my variac to a wall outlet and the two outputs from the variac to the AC terminals on one of the rectifiers (even had the rectifier screwed to a heat sink) and the POS and NEG terminals on the rectifier to the red and black wires of the motor. This setup keeps burning up rectifiers, it's already torched 3. I hooked up a DC current clamp around one of the motor wires and the motor never draws more than ten amps. Can someone help me figure out what I am doing wrong? Thanks.

 

[Alchymist]

I've been trying to teach myself about electricity and motors and transformers and such in my spare time. I picked up a treadmill for free off of craigslist a while ago, the guy said it worked fine but the main board went out and it was too expensive to replace. The motor in the treadmill is a 2.5 HP continuous duty permanent magnet DC motor with these specs:

3200 rpm
21 amp
DC90V
Insulation F
Ambient 40 degrees C

Well I bought these rectifiers off of eBay http://www.ebay.com/itm/251274906610?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

I figured that being rated 50 amp and 1000 volts would make them more than adequate. I connected my variac to a wall outlet and the two outputs from the variac to the AC terminals on one of the rectifiers (even had the rectifier screwed to a heat sink) and the POS and NEG terminals on the rectifier to the red and black wires of the motor. This setup keeps burning up rectifiers, it's already torched 3. I hooked up a DC current clamp around one of the motor wires and the motor never draws more than ten amps. Can someone help me figure out what I am doing wrong? Thanks.

Sound like you are killing the diodes with inductive spikes from the motor. Dis you put any capacitors across the output of the diodes? Any snubber circuit?

 

[91bronc300]

No, no capacitors, and I don't know what a snubber circuit is. What specs of capacitor would be needed? And the capacitors just get wired across the NEG and POS from the rectifier? Thanks for your input.

 

[Alchymist]

Google is your friend:

https://www.google.com/#bav=on.2,or.r_qf.&fp=39aa1df25dc11213&q=dc+motor+snubber+circuit

 

[theoldwizard1]

Why the variac ? I am assuming it is to get the 90VDC.

You are missing lots of details.

• Instantaneous starting current for all motors can several times the motors rated current.
• The output of a rectifier is not "pure" DC, but it is probably okay for what you are doing (assuming no load).
• How are you checking voltages ? Multimeters measure AC voltage in "RMS". The peak voltage is actually much higher. Because the output of the rectifier is not "pure" DC, the multimeter is also probably giving you an incorrect DC voltage.

The reason there is a controller board instead of your setup is because of some of the issues I have brought up.

The controller board likely "pulse width modulates" the DC voltage to the motor. PWM means that the AC is first turn into "pure" DC (rectified and filtered). Then it is "chopped" in to little slices, probably between 400 and 4,000 slices per second. When no slices are fed to the motor, the motor does not spin. If 100 slices per second out of 400 are feed to the motor you will get approximately 25% speed (actually it will probably be less for a lot of other reasons). 200 slices per second should give you about 50% speed.

Now you understand why that controller board cost so much.

 

[91bronc300]

Why the variac ? I am assuming it is to get the 90VDC.

You are missing lots of details.

• Instantaneous starting current for all motors can several times the motors rated current.
• The output of a rectifier is not "pure" DC, but it is probably okay for what you are doing (assuming no load).
• How are you checking voltages ? Multimeters measure AC voltage in "RMS". The peak voltage is actually much higher. Because the output of the rectifier is not "pure" DC, the multimeter is also probably giving you an incorrect DC voltage.

The reason there is a controller board instead of your setup is because of some of the issues I have brought up.

The controller board likely "pulse width modulates" the DC voltage to the motor. PWM means that the AC is first turn into "pure" DC (rectified and filtered). Then it is "chopped" in to little slices, probably between 400 and 4,000 slices per second. When no slices are fed to the motor, the motor does not spin. If 100 slices per second out of 400 are feed to the motor you will get approximately 25% speed (actually it will probably be less for a lot of other reasons). 200 slices per second should give you about 50% speed.

Now you understand why that controller board cost so much.

Yeah, the variac is to vary the speed of the motor because I still want to use it as a treadmill.

I'm not sure why the motor controller would need to slice the DC current into little bits and then feed only a certain percentage of bits to the motor to control motor speed. I figured just simply changing the voltage would do that.

So the motor is creating inductive spikes that are exceeding the rating of the diodes and blowing them out and adding some capacitors between POS and NEG will create the buffer needed to protect the rectifier?

Also, I know running AC through a bridge rectifier will give you DC voltage that still drops to 0 at twice the AC frequency you are rectifying and when you say 'rectified and filtered' by filtered do you mean then running that rough DC through capacitors to smooth it out?

Thanks for all your input fellas.

 

[dave*99]

Yeah, the variac is to vary the speed of the motor because I still want to use it as a treadmill.

I'm not sure why the motor controller would need to slice the DC current into little bits and then feed only a certain percentage of bits to the motor to control motor speed. I figured just simply changing the voltage would do that.

So the motor is creating inductive spikes that are exceeding the rating of the diodes and blowing them out and adding some capacitors between POS and NEG will create the buffer needed to protect the rectifier?

Also, I know running AC through a bridge rectifier will give you DC voltage that still drops to 0 at twice the AC frequency you are rectifying and when you say 'rectified and filtered' by filtered do you mean then running that rough DC through capacitors to smooth it out?

Thanks for all your input fellas.

The motor controller is a pulse width modulator circuit. This allows the motor to vary speed while providing the needed high torque for the load. Simply varying the voltage will vary the speed and torque. Not good for a treadmill application where you want a constant speed.

 

[91bronc300]

The motor controller is a pulse width modulator circuit. This allows the motor to vary speed while providing the needed high torque for the load. Simply varying the voltage will vary the speed and torque. Not good for a treadmill application where you want a constant speed.

I see. So I'm jiggered.

 

[MTW]

Just a thought, but have you disconnected the motor from the load for testing?
Seems to me that from what you stated the same thing is happening to your rectifier as happened to the original control. From the specs you gave sounds like a standard DC motor to me.

I would remove the motor and inspect it for signs of internal damage or mechanical faults first. If none were found then try running it with no load connected. If it ran under no load, then I would look to the treadmill mechanism itself for mechanical binding. Standard troubleshooting procedures. Isolate whether it's the motor or the driven mechanism. You never stated if the machine ran at all before the parts burned up!

The other thing is your heat sink, what are you using for a sink? Did you use thermal paste under the bridge rectifier? Power semiconductors will burn up in short order if they cant get rid of the heat fast enough. Those 50A rectifiers are probably only good for 5A with a poor or nonexistent heat sink.

Take a look here for some technical specifications on your rectifier, this listing shows a 50A unit is only rated for 1HP
http://www.newark.com/multicomp/gbpc5002/bridge-rectifier-1hp-50a-200v-gbpc/dp/24M2287

How about your Variac, what kind of rating does it have? Is it large enough to supply 20A and still keep the voltage from sinking? That would be a larger than normal Variac for most folks. 10Amps may be all that your variac can muster at that votage.

 

[nehog]

I see. So I'm jiggered.

No, you just headed down the wrong path!

90V motor controllers are common and available, I use one to control my small lathe which has a 90 volt motor. My controller (a board about 4x4) has feedback to (try to) stabilize the motor speed (as load increases it increases the power to the motor) and soft start so the motor speed ramps up (great for a treadmill, and of course any equipment that you don't want to accelerate too fast.) All these devices take AC line in, and provide DC voltage for the motor. The one I use has a 'calibration' resistor to allow the feedback to work correctly. A simple rheostat controls speed.

There are several examples of these on eBay and I'm sure Amazon offers them as well. Perhaps even Craig's list would have some?

 

[91bronc300]

Thank you for all the excellent answers. It looks like I need to know a lot more about motor controllers so I'll be ordering 'Industrial Motor Control' from Amazon. I have to admit that this kind of stuff gets pretty complicated real quick but I'm having fun toying around.

By the way MTW, my variac is a heavy ******, 120V input and rated 7.0 KVA so I think it should handle that size motor fine. I'm no expert though, obviously.

 

[2ManyProjects]

I've been trying to teach myself about electricity and motors and transformers and such in my spare time. I picked up a treadmill for free off of craigslist a while ago, the guy said it worked fine but the main board went out and it was too expensive to replace. The motor in the treadmill is a 2.5 HP continuous duty permanent magnet DC motor with these specs:

3200 rpm
21 amp
DC90V
Insulation F
Ambient 40 degrees C

Well I bought these rectifiers off of eBay http://www.ebay.com/itm/251274906610?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

I figured that being rated 50 amp and 1000 volts would make them more than adequate. I connected my variac to a wall outlet and the two outputs from the variac to the AC terminals on one of the rectifiers (even had the rectifier screwed to a heat sink) and the POS and NEG terminals on the rectifier to the red and black wires of the motor. This setup keeps burning up rectifiers, it's already torched 3.

Thank you for all the excellent answers. It looks like I need to know a lot more about motor controllers so I'll be ordering 'Industrial Motor Control' from Amazon. I have to admit that this kind of stuff gets pretty complicated real quick but I'm having fun toying around.

Good luck with that. Seriously. But be aware that you're sort'a starting in the middle. Normally, before one attempts to grapple with AC circuits, motors, inductance, capacitance, and all the myriad of things which go with that, one FIRST needs to have a solid grasp of DC concepts, and electricity in general. No insult intended, but it seems strongly evident that you aren't there yet.

By the way MTW, my variac is a heavy ******, 120V input and rated 7.0 KVA so I think it should handle that size motor fine. I'm no expert though, obviously.

The capacity of the Variac is not the problem. The problems is, as "nehog" and others have pointed out, you really DO need a proper motor controller. Your home-brew Variac/rectifier setup is simply too far from the mark. In and of itself, it doesn't even make "real" DC. Take a look at the following illustration:

That first (red) line represents the AC input to the rectifier. Of the three horizontal dotted lines running through that part of the image, the center one represents 0 Volts. The magnitude of that waveform (i.e., the distance from that "zero-crossing" line on either the positive- or negative-going sides) is analogous to the voltage.

The output from the rectifier will resemble either the second (green) or third (blue) line, depending in whether it is set up as a "Half-Wave Bridge" or a "Full-Wave Bridge". In both cases, the diodes do their job by blocking the negative-going current (and therefore, the corresponding negative voltage swings); but this is effectively still basically an AC signal, now carrying a positive voltage offset. To get "real DC" out of this would require filtering (in the form of large capacitors and/or inductors) later in the circuit.

 

[ddawg16]

I can tell you right now that motor control rates up there with FM (Fuc&ing Magic). The technology that goes into variable speed motors and the motor controllers (VFD's) is bleeding edge.

Expect to see some significant advances in motor control as the electric car spurs research....I think they are only taping the surface.

 

[cnc-me]

I would not hook that motor up to your new control, because it might burn it out.
There must be something wrong with the motor or the unit itself.
Have run plenty of DC motors with setups like you describe and I know other people doing
the same with no problems. Used to run an Enco mill like that for years, only recently did
I change it over to a DC drive.

Don't think much if any filtering is required for DC motors, although they might operate better
with it. I don't have a scope, but would think the output from a DC drive is far from the
perfect straight line of true DC. As matter of fact the motors sound better on a variac
than they do on a drive. Just my $1.02 (2 cents adjusted for inflation)

 

[cnc-me]

I can tell you right now that motor control rates up there with FM (Fuc&ing Magic). The technology that goes into variable speed motors and the motor controllers (VFD's) is bleeding edge.

Expect to see some significant advances in motor control as the electric car spurs research....I think they are only taping the surface.

I agree, AC drives are great and now, they are getting cheap enough
you can actually afford to buy them.

DC on the other hand is more of a "cut & dried" technology as electronic
drives have been around a lot longer, although I'm sure it steadily advances too.

As complicated as AC drives are they seem to be quite reliable, have some
Eaton Dynamatics that were built in 1988, still going strong.

 

[91bronc300]

I think what I'll do when I get the time again this week is try adding some capacitors between the rectifier and motor and see if buffering these induction spikes that way will keep from blowing out my last rectifier. If that doesn't work then CNC and MTW may be right and there is a fault in the motor itself. Either way I'll be reading up on motor controllers just for giggles.

CNC, so you are saying that your run a DC motor (or did) with a variable speed and variable loads at both high and low speeds with nothing other than a variac and a rectifier?

 

[nehog]

Also perhaps add a small resistor/capacitor pair across each diode in the rectifier assembly.

See http://en.wikipedia.org/wiki/Snubber for some information on this problem and techniques to resolve it.

However, realize that any DC motor run at lower voltages (such as provided by a variac) will exhibit very poor low speed torque response.

 

[nehog]

BTW, an SCR speed controller, supplied with about 90 VDC, may run the motor better than a variac. However finding a SCR (most are triac today) controller is usually very difficult.

 

[RPH]

Scr's are not hard to find. We use them daily on the job. Some are huge. What you want him to build is known as a gated rectifier. Lots of induction systems employ this on the dc side of things.

 

[cnc-me]

I think what I'll do when I get the time again this week is try adding some capacitors between the rectifier and motor and see if buffering these induction spikes that way will keep from blowing out my last rectifier. If that doesn't work then CNC and MTW may be right and there is a fault in the motor itself. Either way I'll be reading up on motor controllers just for giggles.

CNC, so you are saying that your run a DC motor (or did) with a variable speed and variable loads at both high and low speeds with nothing other than a variac and a rectifier?

Did so for years with my Enco mill, had a real DC motor on it though, not a treadmill motor. Should of had a snubber circuit on it, but I never lost a bridge
rectifier, even if I did, what are they three bucks to replace.
It didn't have quite as much torque down low as the drive does, but it worked
just fine. I ran an Allen-Bradley DC drive for a while, it did not work very good at all, would not hold a speed, also speed seem to change as the drive warmed up. A-B makes great products that drive is not one of them though.
In fact it didn't work as well as the Variac. I'm now running one of those little Dart drives, seems to work real good, and inexpensive to buy compared to some.

 

[cnc-me]

Also perhaps add a small resistor/capacitor pair across each diode in the rectifier assembly.

See http://en.wikipedia.org/wiki/Snubber for some information on this problem and techniques to resolve it.

Have you ever seen a neon bulb used for this?
Just got done gutting a 13 volt Progressive Dynamics RV power supply,
for travel trailers, and that's what they used. I think the neon bulb fires at around 80 volts IIRC. They also had a couple of small caps across the diodes.