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Variable Speed Controller Selection For A Series-wound Universal Motor


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I would like to ask for your recommendation for adding a variable speed controller for this motor. It is a drill for steel plate, and so I'm treating it as a constant torque application.


Name Plate on Motor:

Rockwell Model 77750 "3/4-inch Heavy Duty Reversing Power Unit"

115 V AC-DC 12 Amps

Power Wiring:

The motor is a Series-Wound Brushed-Commutator motor.

5 wires (2 Field, 2 Armature, 1 Chassis Ground) are bought out to a Furnas J2 drum switch.

The drum switch reverses the motor by swapping the connections to the Armature.

Right now, the drill is plugged into 120 VAC, which is great for our purposes.


The motor is not equipped with any sort of tach feedback.


The motor is for a Bux magnetic drill. It is coupled to a gear box whose output shaft is rated:

No-load speed = 400 RPM,

Fully-loaded speed = 250 RPM.


The Bux drill has a cast aluminum electrical enclosure cavity available in it's main chassis that measures: 5" x 7" x 2-7/8".


These are my questions:


1. AC or DC? The triac-based AC units are the least expensive, but I've read that they lose most of their torque at low speeds. However, since there's no tach feedback, do I gain anything by using a DC controller?


2. In its current AC service, the motor is series-wound. With a DC controller, it seems that usually the field circuit is wired separately from the armature, that is, neither in series or shunt. Is there an advantage to this, and if so, how could I determine the voltages and max currents for the two windings?


3. Relating to Question 2 above, there are a lot of DC controllers available that specify an output of 90 VDC. This is probably ridiculous, but would these DC controllers be appropriate for this sort of motor?


4. Does the controller need to handle the reversing function internally, or can the output of the controller just be run through the drum switch as before, swapping the armature leads to reverse the motor direction?


5. Is a fuse needed both at the power input to the controller, and at the controller's output(s) to the motor?


Thank you very much for your help.



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  • 6 years later...



First of all frequency has no effect on motor speed.


The motor torque is proportional to square current.


At constant voltage supply, an small speed reduction means a big torque increment.


Triac converters is not a bad solution, not decreassea the torque at small speeds.


The speed is not proportional to voltage neither current neither frequency.


See rhe motor model


V=k x i2 x n + ixr

Torque= k x i2


Speed reversing is only possible interchanging stator-rotor coils.


There is not possible to plug an serial dc motor with AC (big iron looses), but is possible to plug an serial AC motor with DC current.


Stator coil has very small resistance, if you mount an serial motor as parallel one be carefull with the stator rotor (has to be very small).


This motors are used:


Drills: a small speed reductions means a big torque increment.


Trains: it is possible to mount some serial motors in serial mode. This trains has a motor in every wheel, mounting all this motors in serial mode makes differential effect

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