Diode bridge

[Rick Scheiba]

I am want to operate a 90 volt DC motor 1.25 hp (Treadmill) for a project but need it to run on AC power. I read that a diode bridge can do that for me. I have no clue but looks pretty simple. Is this possible and are there anything special I would need for this motor? Thanks

 

[davenn]

I am want to operate a 90 volt DC motor 1.25 hp (Treadmill) for a project but need it to run on AC power. I read that a diode bridge can do that for me.

Hi welcome to EP

nope, not sure where you heard that

a diode bridge DOESNT change DC to AC, It changes AC to DC ( a rough DC that needs smoothened before use)


Dave

 

[Rick Scheiba]

Thanks Dave but isn't that what I' m trying to doing, changing AC to DC to operate the motor

 

[Bluejets]

Read it again Dave....

Rick,
That would need a fairly hefty diode bridge.
1.25hp = approx 932watts = load current somewhere around 10 amp so possibly stud type diodes and allowance would have to be made for starting current which can be up to 6 times more, depends on many things.
Voltage of bridge diodes would need to be around 150v to be on the safe side.
One problem would be , where are you going to get 90 v dc?
There would be others I'd imagine.

 

[Rick Scheiba]

Maybe this will help. The motor I'm using came from a treadmill. It is 1.25 hp DC90V 10amps. Does that help?

 

[davenn]

Sorry Rick

Read it again Dave....

............

yeah realised I misread LOL was too early in the morning ... before my first coca cola ( I don't drink coffee )


Dave

 

[Minder]

Generally the majority of T.M. controllers are either SCR bridge, which is just a way of controlling the mean level output of a diode bridge set up, the other is PWM control which is a bit superior to the SCR bridge method.
If you only want it to run flat out, then a straight bridge set up off of 120vac would do the trick.
M.

 

[Rick Scheiba]

Let me see if I'm getting this right. Get 4 150v diodes and make a diode bridge and use a pwm control to adjust the rpms of the motor. Can you tell how confused I am ?

 

[Minder]

That is the PWM version, you need a DC supply and a PWM circuit to control RPM.
With the SCR bridge circuit, two of which are rectifiers, two are SCR's, no other power supply needed.
OR, a full bridge off of AC power, no cap needed. but no rpm control.
M.

 

[Rick Scheiba]

Is this what I need ?

 

[Bluejets]

As Minder said, that will give you full speed.

Then again, if the motor is a series type with wound field windings, you could just run it off AC.
Still the query about where you will get your supply.

 

[Rick Scheiba]

As Minder said, that will give you full speed.

Then again, if the motor is a series type with wound field windings, you could just run it off AC.
Still the query about where you will get your supply.

Then would I add a controller for speed adjustment ? I'm not sure what you mean by " where will I get my supply"

 

[Bern1937]

Hi Rick, so far you have received some good input. I would like to back up and get more info. from you. Do you want to run this at full power? Do you need to have it speed controlled? Do you have any transformers with current ranges in the 5 to 10 amp area? How about voltage from maybe 24 to 80 volt, or somewhere in these rough areas? One feedback mentioned that it maybe a series wound motor. The ones I have seen are PM motors, so if this is true and you put 90 volts DC on it, without a load, I would expect it to over speed and possibly come apart. Get back with some info. and I think you will get some more good ideas.

 

[Minder]

Then would I add a controller for speed adjustment ? I'm not sure what you mean by " where will I get my supply"

There are KB controllers on ebay that will do the trick, they run directly off the mains supply, similar to the original T.M. controller, you can confirm any model as to being SCR or PWM by looking up the model No. details on the KB site.
I have not come across any wound field DC motors for some time now, the only ones you might see is in the Very large HP types.
M.

 

[IamJatinah]

Hi There....whoa......slow the horses....you cannot drive any motor with a simple bridge rectifier for longer than say.....a few seconds for the diodes to fry. As someone pointed out....POWER is huge in the motor world.
Your treadmill motor and controller are from what brand? I can maybe help you but you will need to throw away what "someone told you" and read a bit on motors.....

Here is a simple way to understand your treadmill or any motor controlled device.....

Max motor rating appear on all motors, voltage, current, power, spin-rate(RPM)
These factors now need a drive circuit to drive the motor which is a DC motor using "Brushes"

First we take a Large ugly unruly "Bulk Voltage" of say 165vdc.....rectified and filtered
NEXT ... we add a switch that allows superfast pulses of 165vdc to pass to the motor, enabling it to spin
NEXT ... we add regulation to keep that "spin" at a constant rate(by varying that pulse-width)
NEXT ... we add an "isolated human interface"(switches and buttons) board for users settings
NEXT .... we supply feedbacks for spin rate (RPM), motor voltage & current back to interface
NEXT .... we MUST be sure to design the switching FET and kick-back diode circuit critically or we fail~!
please see inductive kickback for an explanation of this electrical condition and how it relates to inductive loads

Now, that superfast pulse of 165v shooting thru the motor brushes, then windings, builds a magnetic FLUX which "pulls" magnets inside the motor to one direction or another, with unbelievable force(3-5x internal combustion power)

If we stretch that "superfast" pulse to the motor a bit longer, the motor spins faster, eats more power......if we reduce that superfast pulse we slow that motor....the voltage applied to the motor leads is always 165vdc but your meter will not read that.....you're meter will read an RMS value of that pulse at a certain frequency(root-mean-square = RMS), so a servicer may see 55vdc on the motor leads with the unit running.....and 75vdc when it's running faster....

but in reality....the applied voltage is always a sliced section of an available "bulk drive voltage" simply selected for how much power, spin, speed, or pressure we need or want..... have a super day!!
;o)

 

[Minder]

Hi There....whoa......slow the horses....you cannot drive any motor with a simple bridge rectifier for longer than say.....a few seconds for the diodes to fry. As someone pointed out....POWER is huge in the motor world.

If we stretch that "superfast" pulse to the motor a bit longer, the motor spins faster, eats more power......if we reduce that superfast pulse we slow that motor....the voltage applied to the motor leads is always 165vdc but your meter will not read that.....you're meter will read an RMS value of that pulse at a certain frequency(root-mean-square = RMS), so a servicer may see 55vdc on the motor leads with the unit running.....and 75vdc when it's running faster....

but in reality....the applied voltage is always a sliced section of an available "bulk drive voltage" simply selected for how much power, spin, speed, or pressure we need or want..... have a super day!!
;o)

The inrush current will be very momentary, as soon as the motor turns the BEMF voltage generated opposes the supply voltage until the motor reaches its operating speed where the current will then be minimum and eventually the opposing voltage will almost equal the supply, at which point the any increase in current at that point will be dependant on load.
M.

 

[IamJatinah]

The inrush current will be very momentary, as soon as the motor turns the BEMF voltage generated opposes the supply voltage until the motor reaches its operating speed where the current will then be minimum and eventually the opposing voltage will almost equal the supply, at which point the any increase in current at that point will be dependant on load.
M.

Hmmm...first of all....where would you find a bridge rectifier capable of 20-30A constant current at the loaded supply voltage once the motor reaches it's constant-horsepower region? There are two stages, constant-torque and constant-horsepower.

This device, your bridge you're talking with a simple BEMF theory, would need to supply a constant 300Watts of rectified power to said motor, keeping in mind that most motors can consume "up to" 600x it's rated current to overcome "inertia" and dependent on design and loading, and you're BEMF at power-off, would require the bridge to be rated for input motor voltage rating(rail voltage max) x 3, or in the case of a treadmill motor, a PRV rating of near 600vdc.

Also, BEMF only would come into play with a "switched" motor supply rail or power down of the motor rail, yes? If there is no drive "off time" there can be no to minimal BEMF even generated, so I don't follow the theory well here. BEMF is generated by a spinning motor during it's "Off-Cycles" when windings are in decay and tossing BEMF back onto the drive line, which is why you need a kickback diode, for the "inductive kickback" or BEMF of the motor.....in it's off state. These folks are talking about large motors here, not a disc drive motor allowed to whiz at a resonant frequency dependent on pole-design. Maybe I need to do a recheck of my motor drive theory ;o)

 

[Minder]

I am not talking about the BEMF of a device such as a relay or solenoid where when switched off the BEMF is of the opposite polarity, this is a DC motor (other motors also) where when supplied with a Steady DC voltage, as the motor rpm increases it will reach an rpm where the (self) generated voltage of the armature reaches the applied voltage, or close to it, and opposes it, thereby limiting the current..
This is the current limiting factor of the motor.
Otherwise the actual resistance of the armature, which can be exceeding low, would result in a very high current regardless of rpm and load.
If any load is applied to the motor, it results in a reduction in rpm, which in turn lowers the generated voltage (BEMF) and thereby the current increases.
The load of any DC motor cannot be any higher than the current due to actual armature resistance.
Any motor, DC included has a Continuous torque level, which is maximum at zero rpm, this is a safe torque value that can be applied in a continuous situation, the other is the peak torque level which can only be entered briefly, otherwise can result in destruction of the motor.
This is why 'intelligent' drives are generally used to control a motor, the drive will have a settable point for both the above torque values in order to prevent damage to the motor.
Incidentally I use stud mount individual rectifiers for anything above 40amps and 1Kv.
M.