90 VDC

[lilsparky]

Here's a dozey, I have 2-90 VDC gearmotors w/ brake that the instrumentation/mechanical folks want to feed but with a plug to a receptacle. Is that even possible? The receptacle would be on a mobile workstation if this is even possible.

All I can find are 90 VDC power supplies that I can either hard wire or install switches to turn each motor on one at a time.

Any ideas?

 

[gar]

120418-1745 EDT

You need to provide more information. Maximum required current? Is variable speed needed?What logic is required for control? Is dynamic or other braking required? Etc.

.

 

[petersonra]

Here's a dozey, I have 2-90 VDC gearmotors w/ brake that the instrumentation/mechanical folks want to feed but with a plug to a receptacle. Is that even possible? The receptacle would be on a mobile workstation if this is even possible.

All I can find are 90 VDC power supplies that I can either hard wire or install switches to turn each motor on one at a time.

Any ideas?

90 VDC drives are common. no reason you could not use a wall plug to feed the drive. most of these drives will come with a brake output (maybe an option) as well.

< $100.

something like this maybe http://www.dartcontrols.com/product-guide/analog-dc-speed-controls/125d/

 

[mike_kilroy]

in addition to a dart type small controller between the wall outlet and motor, if it is an SCR rated motor (most 90vdc models are), then a simple full wave rectifier (radio shack bridge) in series with your receptacle plug and fuse is all you need (if no variable speed required).

 

[petersonra]

in addition to a dart type small controller between the wall outlet and motor, if it is an SCR rated motor (most 90vdc models are), then a simple full wave rectifier (radio shack bridge) in series with your receptacle plug and fuse is all you need (if no variable speed required).

what about the brake?

 

[kwired]

Some of my reply has already been addressed.

Simple DC drive creates 90 V DC from 120 VAC.

If the speed control is not needed why did they use DC motors? An AC motor would cost less and no additional power supply or drive would be needed either.

The brake is not something only found on DC motors either.

 

[lilsparky]

.5 hp motor each. The 120 volt receptacle seems like the way to go with the rectifier bridge. The brake doesn't seem to be an issue with them.

Thanks!!!

 

[gar]

120419-1247 EDT

lilsparky:

If you simply use a bridge rectifier between the AC line and the DC motor you will get an average DC armature voltage of some value. This value will be dependent upon on the motor loading. Note: output speed of a permanent magnet DC motor is approximately proportion to internal armature voltage (motor terminal voltage minus the voltage drop from motor current times the armature internal resistance).

Theoretically if you had a resistive load on the output of a bridge rectifier and the input was a sine wave, then the average DC output would be approximately the AC line voltage times (0.636/0.707) or about 0.9 times the input RMS voltage minus diode drop.

A DC motor is not a resistive load. It is approximately a DC voltage (like a battery) in series with a resistance.

An experiment with a small Bodine PM (permanent magnet) DC motor with a worm gear load, but otherwise unloaded. With 123 V AC RMS input to a bridge rectifier the motor armature voltage was about 140 V DC average. The peak voltage of a 123 V sine wave is about 174 V.

Depending upon the load and what these motors do, you may want to consider a motor controller with speed and current control.

.

 

[mike_kilroy]

brake?

brake?

op did not spec the brake voltage so i assume it also is 90vdc? if so, it would just get put in parallel with the motor power from the output of the bridge recitifer. we have sold servo motors with 90vdc brakes for 50 years - we run from 120v thru bridge to brake.

 

[gar]

120421-1146 EDT

If you have a solenoid supplied from a sine wave excited full wave bridge rectifier, then the approximate current thru the coil is the average DC voltage across the coil divided by the coil resistance. For 120 V RMS input this is about 108 V DC. If the solenoid is rated for 90 V, then at 108 V the power dissipation is about (108/90)² = 1.44 times that at 90 V.

If this solenoid is paralled with and while running a DC motor from the bridge rectifier, then the solenoid power dissipation is much greater. Suppose the motor terminal average DC voltage is about 135 V, then the power ratio is about (135/90)² = 2.25 .

This type of brake would be spring loaded to apply braking on loss of power to the solenoid.

.

 

[Besoeker]

120419-1247 EDT

lilsparky:

If you simply use a bridge rectifier between the AC line and the DC motor you will get an average DC armature voltage of some value. This value will be dependent upon on the motor loading. Note: output speed of a permanent magnet DC motor is approximately proportion to internal armature voltage (motor terminal voltage minus the voltage drop from motor current times the armature internal resistance).

Theoretically if you had a resistive load on the output of a bridge rectifier and the input was a sine wave, then the average DC output would be approximately the AC line voltage times (0.636/0.707) or about 0.9 times the input RMS voltage minus diode drop.

A DC motor is not a resistive load. It is approximately a DC voltage (like a battery) in series with a resistance.

An experiment with a small Bodine PM (permanent magnet) DC motor with a worm gear load, but otherwise unloaded. With 123 V AC RMS input to a bridge rectifier the motor armature voltage was about 140 V DC average. The peak voltage of a 123 V sine wave is about 174 V.

Depending upon the load and what these motors do, you may want to consider a motor controller with speed and current control.

Gar

I agree with your assessment. Certainly, if the motor armature is fed by a plain rectifier bridge from the AC supply, the voltage (hence speed variation) with load could be a problem depending on the application.
There may be other technical issues to consider besides that and cooking the brake solenoid.

With potentially nearly twice rated voltage the motor could run at nearly twice rated speed. Any one for armature winding Spaghetti Bolognaise with a liberal sprinkling of comm segs?

How good (or bad) would the commutation be at elevated voltage and speed?

The 90V, 0.5 HP motor would have a running current of about 5A allowing for about 85% efficiency - and that may be optimistic. The DC motor, as you rightly say, runs with a back emf which is a little lower than the supply voltage by the armature resistance voltage drop, I_aR_a.
In my experience this usually isn't very much - maybe 5% voltage at full rated current.
The back emf is proportional to RPM. On initial starting there is no RPM and thus no back emf. So all the supply volts appear across R_a. So, 20 times the rated current would flow at that point.

I'm with you. I think a motor controller would be a good idea.

 

[mike_kilroy]

time for reality check

time for reality check

lilsparky, u now have had a bunch of replies. I would suggest it is now time to give the mfgr and part nos of the gearbox/motor/brake unit so no more guessing what ur particular motor/brake can or cannot tolerate. I suspect it is a Leeson or similar gearmotor meant to run off scr drive with same for the brake, and so a bridge rectifier to both motor and brake would be well within its design spec. But until you give the model details that cannot be guaranteed and u may have to go with much more expensive solutions for no good reason. If variable speed or exacting speed regulation is required, of course a drive is needed. If not, then you may well waste ur money on controls u do not need.

 

[Besoeker]

lilsparky, u now have had a bunch of replies. I would suggest it is now time to give the mfgr and part nos of the gearbox/motor/brake unit so no more guessing what ur particular motor/brake can or cannot tolerate. I suspect it is a Leeson or similar gearmotor meant to run off scr drive with same for the brake, and so a bridge rectifier to both motor and brake would be well within its design spec. But until you give the model details that cannot be guaranteed and u may have to go with much more expensive solutions for no good reason. If variable speed or exacting speed regulation is required, of course a drive is needed. If not, then you may well waste ur money on controls u do not need.

If it's meant to run off an SCR drive then it is meant to run with a controller.
Silicon Controlled Rectifier.