What is the current drawn by the motor at one-third normal speed?

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In summary, a motor designed to operate on 130 V draws a current of 13.8 A at start-up and 4.40 A at normal operating speed. The resistance of the armature coil is 9.42 ohms and the back emf developed at normal speed is 88.6 V. At one-third normal speed, the current drawn by the motor is 10.7 A. This can be calculated using the equation I=(V-E)/R, where E is proportional to the speed and is equal to 88.6/3 = 29.5 V in this case.
  • #1
candyq27
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Homework Statement


A motor is designed to operate on 130 V and draws a current of 13.8 A when it first starts up. At its normal operating speed, the motor draws a current of 4.40 A.
(a) What is the resistance of the armature coil?

(b) What is the back emf developed at normal speed?

(c) What is the current drawn by the motor at one-third normal speed?

Homework Equations



V=IR
I=(V-E)/R (E=induced emf)

The Attempt at a Solution



I figured out parts (a) and (b) but I'm not sure how to do part (c).
(a) V=IR 130=(13.8)R R= 130/13.8 = 9.42 ohms
(b) I=(V-E)/R so 4.4=(130-x)/9.42 x=88.6V
(c) I'm not sure how to figure this out. I know at normal speed the I is 4.4A, but I can't just divide that by 3 to figure it out. Can someone help? Thanks.
 
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  • #2
In (c), the back emf is proportional to the speed.
 
  • #3
Ok so the back emf would be 88.6/3 = 29.5
so then I can use the equation I= (V-E)/R = (130-29.5)/9.42 = 10.7 A
Thank you!
 
  • #4
That looks OK to me.
 
1.

What is motor back emf/current?

Motor back emf/current is a phenomenon that occurs in electric motors when the motor is turned off or slowed down. It is the voltage or current that is generated in the opposite direction of the applied voltage or current, due to the motor's magnetic field collapsing. This back emf/current can have a negative impact on the motor's performance and efficiency.

2.

What causes motor back emf/current?

The main cause of motor back emf/current is the motor's magnetic field. When the motor is turned off or slowed down, the magnetic field collapses and generates a voltage or current in the opposite direction. This is due to the principle of electromagnetic induction.

3.

How does motor back emf/current affect motor performance?

Motor back emf/current can have a negative impact on motor performance in several ways. It can cause the motor to run at a slower speed, reducing its torque and power output. It can also cause the motor to draw more current, leading to overheating and potentially damaging the motor.

4.

How can motor back emf/current be controlled?

Motor back emf/current can be controlled by using a device called a snubber circuit. This circuit absorbs the back emf/current and dissipates it, preventing it from causing damage to the motor. Another way to control motor back emf/current is by using a motor controller that can adjust the voltage and current supplied to the motor.

5.

Is motor back emf/current always a negative thing?

No, motor back emf/current is not always a negative thing. In some cases, it can be beneficial as it can act as a self-regulating mechanism to prevent the motor from drawing too much current and overheating. However, in most cases, it is important to control and minimize motor back emf/current to ensure optimal motor performance and longevity.

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