DC motor with half wave rectified source
DC motor with half wave rectified source
(OP)
I am evaluating a shunt dc motor to determine if it will produce the min. torque required for my valves. It's ratings are .361 hp, 95 arm.volts, 4.7 arm.amps. It's being supplied by 104 Vac source (120V under min. volt conditions) that is half-wave rectified (through SCRs) by a Modutronic 10A controller. This gives me an average(DC) voltage about 35-40 V. Does it make any sense to supply a 95V motor with half its rated voltage? How will a dc motor react in this situation?





RE: DC motor with half wave rectified source
Are you going to have a current regulator to fire the SCR's? A Speed regulator to supply the current regulator with a reference?
With this type of supply, since it is hardly DC, you will have more heating than with a full wave supply. Can the motor take this?
So I guess the answer to your question will depend on the questions I asked. What is the application?
RE: DC motor with half wave rectified source
Therefore, it would be much better to have a full-wave rectification and reasonably good ripple filter. This would raise the voltage close to the motor terminal rated voltage. Consequently, the better motor operation will result.
RE: DC motor with half wave rectified source
RE: DC motor with half wave rectified source
The circuit sounds incredibly crude, it isn't fit to call it a servo as that would imply some finesse and precision.
The motor will produce somewhat less than 50% rated torque, and will get hot because of the huge AC component. A DC motor isn't normally designed to minimise eddy losses in the core, because a DC supply doesn't generate eddy currents in the iron in the way that AC does. As your controller is feeding the armature a mixture of AC and DC, the motor will heat up due to eddy losses, but not produce a power output commensurate with that heating effect. There will probably be significant torque ripple, although in this application you might not be too bothered.
The 47R / 100n network is a snubber designed to protect the SCR's from high dv/dt which would damage them.
I think you could make a useful modification by converting your circuit to a half-controlled bridge rectifier, which would use your existing SCR's and drive circuitry and add two power diodes of similar rating to your SCR's. This would give you a full-wave rectified supply which causes less harmonic pollution of your supply and give a far better DC supply to your motor. A win-win situation.
You need to do some more investigation into your firing circuit to determine what range of delay angle control it has, but the principle will work.
RE: DC motor with half wave rectified source