question about motor controller amp rating

have a sewing machine motor controler (wired in series) that uses a "carbon pile element" where pressure on the pile causes decrease resistance and more current to flow and at some pressure point a short by passes the carbon pile

controler is rated at 95-145 volts and 0.7 amps

now someone has claimed no troubles using the controller with 120v - 1 amp motor

is this true ? why or why not

ithought the motor would potentially try to draw more current than the controler could handle and of course overheat etc

any help greatly appreciated r

Reply to
robb
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(snip)

It might be. A 1 amp universal motor produces a torque roughly proportional to the square of the current. As long as that 1 amp motor is not loaded with a torque greater than about half of its rating, it will draw no more than about

70% of its rated current.
Reply to
John Popelish

i have a sewing machine motor controler (wired in series with motor) that uses a "carbon pile element" where pressure on the pile causes decrease resistance and more current to flow and at some pressure point a short by passes the carbon pile

controler is rated at 95-145 volts and 0.7 amps

now someone has claimed no troubles using the controller with 120v - 1 amp motor

is this true ? why or why not

ithought the motor would potentially try to draw more current than the controller was rated for and of course overheat etc

any help greatly appreciated r

Reply to
robb

great ! thanks John

will it matter that the original motor was 0.5 amp rated ? and the controler gets quite warm when controlling to slower speeds .

i am guessing that it should get no warmer than before with new 1 amp motor ?

thanks again for help robb

Reply to
robb

Let me think about that, a moment:

Producing a given speed and output torque implies a fixed mechanical power requirement. That sets a lower limit on how much electrical power is required to produce that mechanical power (the electrical power in must exceed the mechanical power out, because no conversion is 100% efficient).

It seems to me that the motor that requires the more voltage and the lesser current, to convert electrical power to that given mechanical power would produce the lesser heat in the series resistance controller. This because that motor would require the controller to drop the lesser voltage, while passing the lesser current, so the controller would produce the lesser volts*amps=watts of heat.

But knowing only the full load rated current for the motor does not, I think, give you enough information to calculate which motor is better in this situation, since we don't know what are the given speed and torque requirement, nor the rated speed and torque for each motor, where the rated current applies.

But, perhaps I am not seeing the simple generality that allows a solution. I would set up an experiment (say, a Variac, amp meter, volt meter and tachometer) and measure the motor voltage and current required to produce the given mechanical output.

Reply to
John Popelish

(trim)

i don't know what difference this makes but i think the most load the motor probably gets is at startup

because the motor has a ~1/2" diameter pully and the machine has a ~4" diameter pully (which has a heavy flywheel ). The pullies connected with a big rubber O-ring belt. Getting the machine going is a big tug on motor... then when it is going it rarely gets any load of significance as fly wheel helps drive needles through the work when resistance comes

Plus one rarely runs machine for extended periods it is usually stop and go where machine runs for 20-30 seconds stops and repeat

thanks for help john, rob

Reply to
robb

Increasing the pulley ratio so that the motor runs faster at lower torque would help lower the motor current and the voltage drop across the speed control. Effectively this is like shifting to a gear that uses the motor more efficiently.

Reply to
John Popelish

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