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.