Operate DC Motor on Full-Wave Rectified 120 V? 2

[mwtucker]

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

 

[itsmoked]

Sounds like your motor is not continuous duty rated. With DC motors that is quite often the case. You will find the baby bottom smoothest voltage around will result in overheating too.

You should look for a motor that is designated "continuous" duty or service.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Motors usually do de-rate for DC with a form factor lower than unity. Pure DC has a unity form factor: anything with ripple is less than unity. Ripple causes losses in the iron core.

Form factor is defined as the ratio of the rms value of the current to the average current or Irms/Iav.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[potteryshard]

120 V AC rectified through a full-wave rectifier will RMS average to about 140 V DC. The motor voltage may simply be too high; this is consistent with the higher current.

 

[DickDV]

Both filtering the DC smooth and reducing the resulting voltage to 120 or less will help the motor run cooler.

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.

 

[waross]

The higher effective voltage of the rectified DC will cause the motor to run faster. Depending on the load this may be overloading the motor.
The shunt field may be a good filter in itself. Try a freewheeling diode across the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BrianE22]

This is more of a meter question, but for the above test (full wave rectifier, no smoothing capacitor) what is the proper selection for a typical (say Fluke) "True RMS" meter, DC amps or AC amps?

 

[starkopete]

Hi,
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

 

[Motorwinder]

The higher voltage shouldn't make it run hot. As the voltage goes up, so does the hp.

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.

 

[mwtucker]

Thanks for your replies and suggestions!

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

 

[OperaHouse]

have serious doubts the motor is overheating from ripple. adding a 200uf will likely make the problem worse from higher voltage. If you want to experiment further, add an inductor in series with the motor. This will lower the ripple and drop the voltage closer to the RMS of the line. Investigate choke input power supplies. A capacitor can be tried later between the inductor and the motor. For testing purposes, a power transformer would work for the inductor if the currents are appropriate.

 

[DickDV]

Add a series inductor ahead of that 200uf cap to form an L filter and you should be good to go, as long as you drop the voltage to motor nameplate rating.