Since there is more resistance from it locking up and from it heating up, why doesn't this resistance limit the current flow enough to prevent it from burning up? I would think as this happens the extra resistance would limit the current flow thus regulating the temperature to a safe level preventing damage.
2007-06-02 12:09:25 · 9 answers · asked by Matt F 1 in Science & Mathematics ➔ Engineering
you have it backwards....the windings in a motor is just a coil of copper wire.... depending on the motor, the resistance of the coil is low, like 1 to 10 ohms.... when the motor spins it produces a counter electromotive force which raises the Resistance.... when it locked up and with such low Resistance it pulls enough amps to burn up the wire.... the slower a motor turns from its designed rpm's the less Resistance there is and hence, more amps....that's why the lights dim when a large motor starts.... because with 120v and 3 ohms of Resistance it pulls 40 amps.. (volts= Resistance x amps) when it speeds up it almost acts like a generator and tries to send voltage backwards which raises the Resistance
2007-06-02 12:27:27 · answer #1 · answered by Mr. Ree 5 · 3⤊ 0⤋
Actually, there is LESS resistance, i.e. less ELECTRICAL resistance.
Consider : When an electric motor is just switched on, a large current flows. As it speeds up, a back-emf develops, so that if the motor is spinning freely at high rpm, very little current is used. If a load is applied, some current is used but since it is still spinning, there is still a large back-emf. If the load is much higher, it does not spin as fast, so the back-emf decreases, and more current flows. Take that to its natural conclusion :if it does not spin at all, there is no back-emf, less electrical resistance, more current, more heat. Besides, even the cooling fans (which are mounted on spinning part) would not function since the spinning part is not spinning !!
Note: You may not need this since it is a basic electromagnetic principle. But just to make sure you get the point, here goes : If current is supplied to a coil of wire in a magnetic field, the coil spins. This is the motor effect. If a coil of wire is made to spin in a magnetic field, an emf is generated. This is the generator effect. In the case of the electric motor, when current is initially applied, the coil begins to spin. But this is now a spinning coil in a magnetic field. So, there is an emf in the OPPOSITE direction to the one supplied to the motor.
2007-06-02 19:24:15 · answer #2 · answered by flandargo 5 · 3⤊ 0⤋
When a motor heats up the resistance in the coils decreases with increasing temperature. As the resistance decreases more current flows increasing temperature even further. Circuit breakers are used to prevent motor burnout.
2007-06-02 19:26:42 · answer #3 · answered by Kainoa 5 · 1⤊ 1⤋
The other poster got it right. A spinning motor will produce a counter EMF force that opposes current flow. This is why a motor with a greater load and slower speed will have a greater current draw. If you stop the motor, there will be now counter currents developed and it is smoke time.
2007-06-02 19:36:25 · answer #4 · answered by mr.answerman 6 · 2⤊ 0⤋
When a motor first starts, the "starting inrush" of a motor is generally more than five times the running current. This inrush is also referred to as a "locked rotor" current. In a lab you can actually measure the inrush by clamping the shaft and then energizing the motor. -- similar to your description of "locked up".
In some cases this inrush can be 10 to 20 times the running load. See the link below. (page 15 of the PDF, but labeled page 9 of the actual document)
2007-06-02 21:29:34 · answer #5 · answered by Thomas C 6 · 3⤊ 0⤋
One more piece -
When the motor is spinning, there's usually some kind of fan that's part of the motor which provides cooling. When the motor locks up, no more cooling.
But mostly, it's what everyone else said - higher current, lower resistance.
2007-06-03 20:55:21 · answer #6 · answered by Doug B 3 · 1⤊ 1⤋
Locking of a motor makes the motor infinitely loaded,therefore yhere will be a high flow of current through the armature winding.This high flow of current causes a heavy amount of heat produced,the heat is sufficient to blow up a motor.
2007-06-03 08:41:19 · answer #7 · answered by SHUBHO 1 · 0⤊ 1⤋
when the motor is locked, it acts exactly as a transformer.
with a short circuited secondary.
would it burn?
i think you got the answer. if you know how transformers work.
2007-06-03 01:36:10 · answer #8 · answered by lord 3 · 1⤊ 1⤋
Because the current goes through the roof.
2007-06-02 21:08:56 · answer #9 · answered by Scott S 4 · 0⤊ 1⤋