# Calculating Current in DC Motor at 11V, 2000 RPM

• farry1024
In summary, we have a 12V permanent magnet DC motor with a stall current of 2 amperes and a no load speed of 3000 r.p.m. When supplied with 11V, the motor is moving the legs at a rated speed of 2000 r.p.m. and we need to calculate the current it would draw. Using the equation for back emf, we can find the resistance to be 6 ohms and then calculate the current using the given values.
farry1024
Question:
Assume that a 12V permanent magnet DC motor is to be used for moving the robot legs.
It has a stall current of 2 ampere and a no load speed of 3000 r.p.m.
Assumer that the friction at no load spped was so little that it can be neglected .
When the suply is at 11V, the motor is moving the legs at its rated speed of 2000 rom due to firction, what current would it draw?

Vmax = 12V
Imax = 12A
ωmax = 3000 rpm <<<ω=omoega, i.e. rotational speed

V1 = 11V
I1 = unknown
ω1 = 2000 rpm

back e.m.f proportional to ω

ω1/ ωmax = back e.m.f 1 / back e.m.f max
and back e.m.f = V- Ir

ω1/ ωmax = [ V1 - I1 ( r ) ] / [ Vmax - I max (r)]

However, I find that I do not have the resistance, how can I calculate? thanks.

You know that the stall current is 2 amperes, and when the motor is stalled there is no back emf. So the resistance is 12 V/2 A = 6 ohms.

To calculate the current in the DC motor at 11V and 2000 rpm, we can use the formula P = VI, where P is the power, V is the voltage, and I is the current. Since the motor is operating at its rated speed of 2000 rpm, we can assume that the friction can be neglected and the back emf is equal to the supply voltage of 11V. Therefore, the power output of the motor would be 11V x I1.

To find the current, we can use the formula P = VI, where P is the power, V is the voltage, and I is the current. Rearranging the formula, we get I = P/V. Plugging in the values, we get I = (11V x I1)/11V = I1.

Therefore, the current drawn by the motor at 11V and 2000 rpm would be equal to 1 ampere. This is because the motor is operating at its rated speed and the back emf is equal to the supply voltage, meaning that there is no resistance in the circuit.

However, if the motor was operating at a different speed or if there was friction present, the calculation would be different. To calculate the current in those scenarios, we would need to know the resistance of the motor, which can be obtained from the motor's datasheet or by measuring it with a multimeter.

## 1. How do you calculate the current in a DC motor at 11V and 2000 RPM?

The formula for calculating current in a DC motor is I = (V - E)/R, where V is the supply voltage, E is the back EMF, and R is the total resistance in the circuit. To calculate E, use the formula E = (N * P * 2 * pi * f)/60, where N is the number of windings, P is the number of poles, and f is the frequency. Once you have calculated both V and E, plug them into the first formula to find the current.

## 2. What is the difference between supply voltage and back EMF in a DC motor?

Supply voltage is the voltage that is being supplied to the motor from an external source, while back EMF is the voltage that is generated by the motor as it rotates. Back EMF is caused by the motor's rotation and opposes the supply voltage, which is why it is subtracted in the formula for calculating current.

## 3. How does the speed of a DC motor affect the current?

The speed of a DC motor is directly proportional to the back EMF, which means that as the speed increases, the back EMF also increases. This results in a decrease in current, as seen in the formula I = (V - E)/R. Therefore, at higher speeds, the current in a DC motor will be lower.

## 4. What is the significance of 2000 RPM in the calculation of current in a DC motor?

2000 RPM is the speed at which the motor is rotating and is used to calculate the back EMF in the formula E = (N * P * 2 * pi * f)/60. This value is important because it directly affects the back EMF and, in turn, the current in the motor.

## 5. Is it possible to determine the current in a DC motor without knowing the back EMF?

No, it is not possible to accurately determine the current in a DC motor without knowing the back EMF. The back EMF is an essential component in the formula for calculating current, and without it, the calculation will not be accurate. Therefore, it is necessary to know both the supply voltage and the back EMF to determine the current in a DC motor.

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