Operate DC Motor on Full-Wave Rectified 120 V?
Operate DC Motor on Full-Wave Rectified 120 V?
(OP)
Hi,
I have DC "brushed" (i.e. it has 2 brushes) motor that is rated at 120 VDC. The full-load current is 3 amps. My questions relate to running this motor with a "simple" power supply.
My first shot was to use a single rectifier (half-wave) to rectify the 120 VAC line. The motor runs, but it overheats quickly and starts to smell.
I then went to a full-wave bridge connected to the 120 VAC line. The motor runs, but still gets very hot after only 1 minute of operation. There is no filtering capacitor, so the DC applied to the motor is the pulsating DC (120 Hz) from the bridge. I believe that this would be a full-wave rectified voltage, varying from 0 to +170 volts peak at 120 Hz. Although the motor is rated 3 amps at full load, I am measuring about 3.5 amps with no load.
I'm not sure why the motor gets so hot? If I had a filtered (i.e. constant 120 VDC) supply, would the heating diminish?
Since there is no requirement for speed control, what is the best (i.e. simplest, lowest cost) way to drive the 120 VDC motor?
Thanks in advance for your input!
Regards,
Mike
I have DC "brushed" (i.e. it has 2 brushes) motor that is rated at 120 VDC. The full-load current is 3 amps. My questions relate to running this motor with a "simple" power supply.
My first shot was to use a single rectifier (half-wave) to rectify the 120 VAC line. The motor runs, but it overheats quickly and starts to smell.
I then went to a full-wave bridge connected to the 120 VAC line. The motor runs, but still gets very hot after only 1 minute of operation. There is no filtering capacitor, so the DC applied to the motor is the pulsating DC (120 Hz) from the bridge. I believe that this would be a full-wave rectified voltage, varying from 0 to +170 volts peak at 120 Hz. Although the motor is rated 3 amps at full load, I am measuring about 3.5 amps with no load.
I'm not sure why the motor gets so hot? If I had a filtered (i.e. constant 120 VDC) supply, would the heating diminish?
Since there is no requirement for speed control, what is the best (i.e. simplest, lowest cost) way to drive the 120 VDC motor?
Thanks in advance for your input!
Regards,
Mike





RE: Operate DC Motor on Full-Wave Rectified 120 V?
You should look for a motor that is designated "continuous" duty or service.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Operate DC Motor on Full-Wave Rectified 120 V?
Form factor is defined as the ratio of the rms value of the current to the average current or Irms/Iav.
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If we learn from our mistakes I'm getting a great education!
RE: Operate DC Motor on Full-Wave Rectified 120 V?
RE: Operate DC Motor on Full-Wave Rectified 120 V?
After doing that, the motor heating should be a function of the shaft loading. You may find that, given your source of DC power, you cannot load the motor to full nameplate rating due to some remaining ripple on the DC especially at higher loads.
RE: Operate DC Motor on Full-Wave Rectified 120 V?
The shunt field may be a good filter in itself. Try a freewheeling diode across the motor.
Bill
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"Why not the best?"
Jimmy Carter
RE: Operate DC Motor on Full-Wave Rectified 120 V?
RE: Operate DC Motor on Full-Wave Rectified 120 V?
Section 12.66 of NEMA MG 1 deals with power supplies for DC motors. Check the motor nameplate for a Power Supply letter (A, C, D, E, or K). If it lists one - what is it?
Thanks
RE: Operate DC Motor on Full-Wave Rectified 120 V?
High amps could indicate weak fields.
I'm assuming this is a PM motor. It could have weak or cracked magnets. If it's a shunt field, it could have shorted fields or low voltage to the shunts.
RE: Operate DC Motor on Full-Wave Rectified 120 V?
A few comments after experimenting:
ITSMOKED: The motor is rated for continuous duty by the manufacturer;
STARKOPETE: There is no data plate on teh motor that indicates "A,C,D,E, or K). I have asked the manufacturer if that is specified, but they are in China and I haven't heard back.
WAROSS: Freewheeling diode produced no difference; I also tried 1uF 400V capacitor--no difference;
MOTORWINDER: It is a PM motor.
I have determined that the overheating appears to be related to the amount of ripple in the power supply (agrees with DICKDV's post). If I add a 200uF 200V capacitor across the 120 VDC (and then use my Variac on the input line to reduce the filtered voltage to 120 VDC) the heating is much less. If I use an industrial regulated power supply ($$$) with the output set to 120 VDC, the motor barely gets warm.
So here's a new question: Is there a relatively simple circuit that can output a "reasonably filtered" 120 VDC from the 120 VAC line?
I know that I can buy a $200 open-frame unit that will do the trick, but that is about $175 more than I budgeted for this!!
SIDE NOTE: I have another project that uses a different 120 VDC motor(also PM brushed motor) and I am using only a full-wave bridge off the 120VAC line to power it. It does not overheat. It does run at high RPM (i.e. about 20,000 RPM) whereas the motor on this project runs at about 4800 RPM.
Thanks very much for your help!!
Mike
RE: Operate DC Motor on Full-Wave Rectified 120 V?
RE: Operate DC Motor on Full-Wave Rectified 120 V?