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Motor Speed Calculation Formula

Motor speed refers to the number of revolutions completed by the motor shaft within a specified period, typically expressed in RPM (revolutions per minute). The rotational speed determines the motor's output speed, which is a critical factor in its performance. The motor speed calculation formula can be used for fault diagnosis. By comparing the theoretical speed with the actual speed, significant deviation may indicate issues such as winding short circuit and bearing failures. The speed formula can help identify and pinpoint the root causes of such faults. The formula for calculating motor speed varies depending on the type and operating principle of the motor, mainly including AC synchronous motors and DC motors. Understanding these formulars makes it easier to select a motor with the appropriate speed for a given application. Motor speed calculation formula

1. Calculation formula for motor speed of AC synchronous motor

 Speed formula: n= ( 60 * f ) / p Note: n is the actual rotor speed in revolutions per minute (rpm), f is the power frequency in Hertz (Hz), p is the number of magnetic pole pairs The key points of the motor speed calculation for synchronous motors are as follows: the speed of the synchronous motor is strictly equal to the synchronous speed ( n=ns ), and is independent of load size (as long as the load does not exceed the motor's pull-out torque); the speed depends only on the power supply frequency f and the number of poles pairs p; to change the speed of the synchronous motor, the power supply frequency f must be adjusted (a frequency converter is required).

2. DC motor speed calculation formula

 Motor speed calculation formula (separately excited/shunt-excited): n=( U - Ia * Ra ) / ( Ce * Φ ) Note: n is the rotor speed, U is the armature terminal voltage in volts (V), Ia is the armature current in amperes (A), Ra is the armature circuit resistance in ohms (Ω), Ce is the EMF constant determined by the motor structure, Φ is the magnetic flux per pole in Weber (Wb) According to the above formula, the speed of the DC motor can be adjusted through three main methods: voltage Control (U): reducing the armature voltage U will reduce the speed n, which is a commonly used smooth and continuous speed control; Field weakening (Φ)- for speeds above base speed: reducing the excitation current (and thus reducing the magnetic flux Φ) will increase the speed n; changing the armature circuit resistance (Ra): adding a series resistor in the armature circuit will increase the Ia * Ra voltage drop, thereby reducing the speed n. The ( U - Ia * Ra ) / ( Ce * Φ ) in the formula is equal to the back electromotive force E of the armature winding (E = Ce * Φ * n), so the formula can also be understood as n = E / (Ce * Φ). Summarizing the motor speed formulas discussed above, we can draw the following conclusions: when precise constant speed is required, use a synchronous motor: n= ( 60 * f ) / p; when wide range smooth speed regulation is required, use a DC motor: n = ( U - Ia * Ra ) / ( Ce * Φ ) .

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