Circuit / Components reduce motor's no load speed

I have an universal motor operating at certain loading with a 5000RPM. However, when it is not in loading condition, motor is in free-load and speed is very fast (let say 20000RPM). I would like to make a circuit or add some components into the motor in order to reduce the motor's free load speed to 10000RPM or lower. Actually, it is no need to make it absolutely in 10000RPM, but just want to drop down to certain speed level. It can be within 10000RPM +/- 3000RPM is fine.

Any good idea on the design? I am thinking that can I use some passive components at the terminal of the motor to achieve the tasks?

You could consider adding a fan load to the shaft.

The torque required by a fan is roughly proportional to the square of the speed, whereas the torque from the motor is proportional to the reciprocal of the square of the speed.

If you add a fan whose required torque is 1/15th of the load torque at 5000rpm, then the fan torque load matches the delivered motor torque at 10000rpm.

I did some work a few years ago on wind generators.

Big 300kW ones.

THey were 660V and direct onto a local grid. Problem was when the wind dropped they went from generator to motor. (where is this leading u ask) .

So what i did was:- Monitor the current and voltage in a phase. calculate the power factor (phase difference between I and V).

As the wind dropped the power factor moved from about 0.7 (on load and generating) towards 1.0 (off load) and it would have gone on to -0.7 (on load motoring).

I detcted this condition and then using a modified soft starter (like a three phase dimmer!!). Delayed firing of TRIACs then cut the current / power loss by only running on 10% of the cycle.

So you can do the same. Detect the off load (unity?? PF condition) and then use a TRIAC or SCR to chop the waveform and thus reduce the speed. Reverse for going on load detect power factor 0.7? and switch to full volts again.

Andrew

You might add something to the external terminals that reduces the line voltage, but it will reduce the loaded speed at least as much as it reduces the unloaded speed. The property of series wound (universal) motors is that the field excitation drops strength as the motor load is reduced, making the armature go faster and faster to generate approximately line voltage back emf as it spins in the weakening field. If you can get to the brush holders, you might try connecting a big, power resistor across them, so that even if the armature current falls to a very low value, the field will get a minimum current through the resistor. If the motor runs on 120 volts, you might try something like a 2k 10 watt resistor. This will provide something like 60 mA minimum field current. Of course, you will have to pay for the energy wasted by the resistor, and make sure the resistor has the air flow to get rid of that heat.

You're showing your age a bit. Big ones these days are on the order of 1.8MW. =8^O

In my idea, I want to keep the motor performance when it is on loading. As most of the universal motor has the following characteristic: very high speed at the free load stage, speed will be increased and increased if no load applied.

So, I want to make a circuit for detect the motor's speed and if it is over speed. Then I will chop off a little voltage and make it not over speed at free load stage. If we add a power resistor at the terminal of motor, it maybe not a good way as it will affect the operation performance of the motor in on load stage and drop down the motor power.

Any good and simply suggestion for that application? I really want to find some ideas which is very cheap and simple device. Thanks~

John P> > I have an universal motor operating at certain loading with a 5000RPM.

You are welcome to go this route. But the total package may get quite complicated (speed sensor, speed control circuit and voltage control device).

If you are willing to accept a somewhat reduced unloaded speed (say, reduce it to 1/4th of what it is without the resistor), not try to hold the speed constant, regardless of load, the added resistor will have almost no effect on the full load speed.

You probably won't find anything cheaper or simpler than the armature shunt resistor. I think you should try it and see if it meets your requirements before you discard this idea.

Sure - PWM. :-)

Pulse-width modulate the motor's drive. If it's running on AC, use a triac and phase control. Almost nothing to it!

Good Luck! Rich

the voltage-reducing approach wont be cheap. Plus if you havent much experience with that sort of thing, it'll blow up quite a few times before you get it to work. Johns idea is probably the best one.

Cheers Terry