Circuit / Components reduce motor's no load speed

[[email protected]]

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?

 

[Tony Williams]

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.

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.

 

[Andrew Wade]

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

 

[John Popelish]

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 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.

 

[Kitchen Man]

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

Big 300kW ones.

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

http://biz.yahoo.com/e/050311/psd8-k.html

 

[[email protected]]

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 Popelish]

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.

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 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.

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.

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

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.

 

[Terry Given]

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.

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.

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.

Cheers
Terry

 

[Rich Grise]

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~

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