Current and voltage relationship in an electric motor

In summary, a motor requires a current of 63 A under a voltage of 390 vac, and a current of 72 A under a voltage of 365 vac. Normally when the voltage increases the current increases with (ohm law).
  • #1
bachir1994
18
2
Hello,
I can not understand that a motor (pumping) requires a current of 63 A under a voltage of 390 vac, and requires a current of 72 A under a voltage of 365 vac. Normally when the voltage increases the current increases with (ohm law). Is that the power delivered by the engine which is 37 kw is maintained.
THANK YOU.
 
Engineering news on Phys.org
  • #2
bachir1994 said:
Hello,
I can not understand that a motor (pumping) requires a current of 63 A under a voltage of 390 vac, and requires a current of 72 A under a voltage of 365 vac. Normally when the voltage increases the current increases with (ohm law). Is that the power delivered by the engine which is 37 kw is maintained.
THANK YOU.

Power (3 phase) = 1.732 * V * I * PF
Power in watts, I=Amps, V=Volts, PF= power factor

Creating 37 kW of shaft power requires less current when the motor is supplied at a higher line voltage, and needs more current at lower line voltages.

An added consideration is shaft speed varies proportionally with line voltage, but pump power varies as the cube (see: pump affinity laws).

Induction Motor Characteristic - Voltage Effects.jpg

Is the motor voltage rating 380V? If so, then 390V is 2.6% above nominal, and 365V is 3.9% below nominal nameplate rating.
Inspect the generic curves above to gain a sense how voltage variation affects motor performance.
 
  • #4
Three phase motors operate at fixed RPM and draw whatever current they need to generate the torque they need to keep spinning or fail trying.
 
  • #5
thank you,
Therefore it is the velocity that imposes the torque therefore the current, it is a natural regulation of the phenomenon. Nothing is created, nothing is lost all changes.
 
  • #6
Induction motors have something called a "Speed-Torque" curve. It looks like this.
http://www.engineeringtoolbox.com/electrical-motors-torques-d_651.html
(the obvious annotations are mine)
speed-torque1.jpg

Note how small is the operating range , circled in purple.
When you lower voltage, the motor runs now on a lower torque curve.
So it has to 'walk back up' that curve by slowing down a little bit , in order to make desired torque.
Walking up the torque curve also walks up the current curve. It will require more than normal running current to make full torque at reduced voltage.

That's why real long extension cords can cause an induction machine like a fridge or pump to burn up its motor.

hope this helps

old jim
 
Last edited:
  • Like
Likes JBA
  • #7
thank you jim hardy
your explanation is very good, and it's ok for me. I begin to see things clearer
 
  • Like
Likes jim hardy
  • #8
bachir1994 said:
I begin to see things clearer

That's what counts. Keep up your good work !

old jim
 

1. What is the relationship between current and voltage in an electric motor?

The relationship between current and voltage in an electric motor is known as Ohm's Law. It states that the current flowing through a conductor is directly proportional to the voltage applied and inversely proportional to the resistance of the conductor. In an electric motor, as the voltage increases, the current flowing through the motor also increases, assuming the resistance remains constant.

2. How does the current and voltage affect the speed of an electric motor?

The current and voltage in an electric motor directly affect its speed. As the voltage increases, the speed of the motor increases, assuming the resistance remains constant. This is because a higher voltage means more electrical energy is available to power the motor, allowing it to rotate faster. Similarly, a higher current can also increase the speed of the motor, as it means more electrical energy is flowing through the motor.

3. Can the current and voltage in an electric motor be controlled?

Yes, the current and voltage in an electric motor can be controlled through various methods. For example, a variable resistor can be used to control the voltage, which in turn affects the current flowing through the motor. Additionally, the motor's speed can also be controlled by using a motor controller, which regulates the current and voltage going to the motor.

4. How does the current and voltage affect the torque of an electric motor?

The current and voltage have a direct effect on the torque (rotational force) of an electric motor. The torque produced by a motor is directly proportional to the current flowing through the motor and the strength of the magnetic field. As the current and voltage increase, so does the strength of the magnetic field, resulting in a higher torque output from the motor.

5. What happens if the current and voltage in an electric motor are too high?

If the current and voltage in an electric motor are too high, it can cause damage to the motor and its components. This is because excessive current can cause the motor to overheat, leading to insulation breakdown and potentially causing a short circuit. Similarly, a voltage that is too high can cause the motor to operate at a speed that exceeds its design limit, leading to mechanical failures. It is important to ensure that the current and voltage in an electric motor are within the recommended limits to prevent damage and ensure safe operation.

Similar threads

  • General Engineering
Replies
8
Views
3K
  • Mechanical Engineering
Replies
8
Views
1K
Replies
3
Views
6K
  • Electrical Engineering
Replies
2
Views
791
  • Classical Physics
Replies
9
Views
228
  • Engineering and Comp Sci Homework Help
Replies
2
Views
1K
  • Electromagnetism
Replies
16
Views
505
  • Electrical Engineering
Replies
5
Views
5K
Replies
3
Views
2K
  • Classical Physics
Replies
20
Views
1K
Back
Top