I need to control a 1/4 hp shunt wound DC motor. The former controller supplied full wave rectified 120 VAC to both the armature and field, no filtering. Needs to be reversible, either armature or field since it doesn't seem to have series fields. I know there are a lot of such controllers but have little experience with them.
Bill
Field reversing works with lower current, but there is latency unless over-driven.
5 HP and under, 3 HP and under especially, KB-Penta has been good for me, but I go there partly for their NEMA 4X housings.
At 1/4 HP you'd be served well-enough with a PWM drive instead of off-the-line 90/180 VDC SCR class.
The small PWM units can deliver the higher 120 VDC voltage cheaply - so long as at relatively low HP (2 HP or under is their best economy zone).
Field supply on all of these critters is ordinarily a naked FW bridge chip on one corner of the PCB, able to be strapped for 120 VAC or 240 VAC in.
No regulation, no alarm monitoring. No filtering. No Field economy circuit. Hot field all the time. Not really optimal unless Field input is switched-OFF with Armature input when motor is idle.
I've always abandoned those FWB bridge in-place in favour of DIY or use of some other, smaller DC drive's "Armature" output for my larger motor Field supply.
OTOH, that extra cost is justified largely because I want control over min/max Voltage and current, an alarm/shutdown signal for field too-low or lost outright when using Field Weakening. That replaces all those tuned and biased relays used in the 10EE DC panel I no longer use at all, for example.
A second DC Drive is not justifiable for a mere 1/4 HP load motor. One can do the protective part of that job with a Zener or two and a PUT or Crydom SSR more cheaply, if even you need Field power too low or Field power lost protection at all.
You probably DO NOT need it, actually. Many applications "squirrel caging" overspeed is just not a real-world risk.
I WOULD suggest use of a 4Q drive even if you don't need ANY of its regenerative features to operate the motor.
A 4Q unit manages potential switching and reversing rudeness inherently, and is less likely to be damaged by spikes it does not give rise to in the first place.
4Q also eliminates the need of reversing switches, relays, contactors, braking resistors, their logic, layout, mounting, wiring, and maintenance.
Not a lot of that to begin with on a mere 1/4 HP motor, but still.. sometimes, all it take is for one or more of those advantages to apply, and the modest extra cost over a 1Q drive is covered, all in one "wire and forget" package, Non Volatile RAM being a few Analog trimpots.
