# Dc motor back emf and speed relationship

### AB : Measuring RPM from Back EMF - Precision Microdrives

Rotating Speed Of DC Motor And Counter-electromotive Force When you measure this relationship on an actual motor, it will form a clean straight line. Find DC motor calculations including torque in MICROMO's motion control library . The proportionality constant is the back EMF constant of the motor. The proportionality constant for the relationship between motor speed and motor torque. The constants KM (motor size constant) and Kv are values used to describe characteristics of electrical motors. Contents. 1 Motor constant; 2 Motor velocity constant, back EMF constant; 3 Motor . is such that the back-EMF is equal to the battery voltage (also called DC line voltage), the motor reaches its limit speed.

The current induces on the motor because of the main supply. The back emf opposes the supply voltage.

• Back EMF in DC Motor
• What Are Motors?
• Motor constants

The supply voltage induces the current in the coil which rotates the armature. The electrical work required by the motor for causing the current against the back emf is converted into the mechanical energy.

And that energy is induced in the armature of the motor. Thus, we can say that energy conversion in DC motor is possible only because of the back emf. The mechanical energy induced in the motor is the product of the back emf and the armature current, i. The back emf makes the DC motor self-regulating machine, i. Consider the motor is running at no load condition. At no load, the DC motor requires small torque for controlling the friction and windage loss.

The motor withdraws less current. As the back emf depends on the current their value also decreases. The magnitude of the back EMF is nearly equal to the supply voltage.

If the sudden load is applied to the motor, the motor becomes slow down. As the speed of the motor decreases, the magnitude of their back emf also falls down. The small back emf withdraw heavy current from the supply. Simply speaking, it means that a motor is a bidirectional energy converter between electricity and machine. We can interpret that Fleming's left-hand rule views the energy conversion in the direction of "electricity to machine" whose conversion coefficient is KT.

### What is Back EMF in a DC Motor? - Circuit Globe

Meanwhile, Fleming's right-hand rule views it in the direction of "machine to electricity" whose conversion coefficient will be KE. Thus, KT and KE are the same thing. In this book, however, we will continue to use KT and KE separately to clearly indicate the direction of conversion.

Studying Power Generating Function of the Motor The power generating function also appears while a motor is energized and rotating. To observe such a condition, we will rotate the motor we talked about earlier using step-down alternating current half-wave rectified by a diode for power supply as shown in Fig. Input should have been one half of a sine wave, but we can observe a voltage waveform as shown in Fig.

This refers to the voltage counter-electromotive force of equation 2. The rotating speed increases in this sequence until the motor reaches a new steady state. Next, let's see what will happen if the external load is increased while the voltage remains constant.

This time, the motor revolution becomes stable at slower speeds than before. The DC motor has the following characteristics: In other words, effects explained by the left-hand and right-hand rules that we thought were different from each other do exist simultaneously in the motor, and the rotating speed of the motor has been determined when the effects are balanced with each other. Relationship between Rotating Speed and Torque The relationship between the rotating speed and torque of a DC motor can be shown by the diagram shown in Fig.

You can read the no-load angular speed at the upper left-hand end of the line and the starting torque at the lower right-hand end.

## Application Notes

In addition, if the supply voltage rises, this line moves in parallel toward the upper right-hand corner; and, conversely, toward the lower left-hand corner if the voltage drops. You already learned that a DC motor has a feature of increasing torque in proportion to current.

back emf explained

This time, we have learnt that it has another feature in which rotating speed increases if voltage is increased. This is quite a convenient characteristic in terms of controlling rotation. This means that the stop position can be maintained by controlling the current even if a large external force is applied during position control. This is a characteristic of DC motors and brushless DC motors only and is not found on induction or stepping motor.

DC motors and brushless DC motors are used for servo control due to this feature of being able to maintain the stop position. Actual Example of Coreless Motor Fig. As its name suggests, the coreless motor does not use a core for the rotor. Instead, it uses a coil hardened with resin for the rotor. Another type of coreless motor has a flat coil. Because this motor is manufactured using the same method as that for printed-circuit boards, it is called a printed motor.