How to Calculate a Current for a Given Torque

v You could get them from products specification or relative reference handbook. different type of ac motor has different cm curve. you can also use this formula M=aCI(fei)*cos(thita)

can you illustrate the formula for me:

what are the abbreviations for the consequent letters;M,a,C

thanx

From the manufacturers.

Torque and current are linearly related.

I do not know where you can find such an equation or graph. There are several factors to consider. Current will be different for different voltages. For instance, if you have a 5 hp 3 phase motor running on 460 volts, full load current will be about 6.5 amps. The same motor connected for 230 volts would draw about 13 amps. In addition to this, different types and efficiencys will develop their full load torque at different speeds (slip). Beyond that, there is a magnetization current that the motor needs before it can begin to produce any torque for the load. These factors vary with motor size and design.

The short answer is:

Rated torque of a motor = kW x 9550/rpm (at rated full speed)

rpm = 120 x f / P

kW is the motor kilowatts. 1kW = 1.3405 x Hp

where:

Hp is horsepower

f is electrical frequency (usually 60Hz in North America)

P is number of motor poles

Torque at any speed:

Not a simple answer - will depend on many variables. The following equation generally holds true for induction motors:

Torque = (1.5 x P x wm x Rr' x V1^2) / (w x slip x ((Rs + (Rr' / slip))^2 + XL^2)

(^2 means "squared")

where:

Torque: Motor torque at motor shaft, varying with slip. Unit: Nm

P: Number of poles of the motor

Rr': Total equivalent resistance of the rotor. Unit: ohms

V1: Applied line voltage at the stator terminals. Unit: Volts

w: Electrical frequency of applied current at the stator terminals. Unit: rad/s

slip: Motor slip

Rs: Internal total stator resistance. Unit: ohms

XL: Total equivalent leakage reactance referred to the stator. Uni: ohms

The long equation will give you the complex motor curve unique to each motor. I did a thesis on how to change the motor characteristic to accomplish various load-speed dynamic controls.

From: Synergy motor gal

NICE!!!!!!!!!!!!!!!

ELECTRICAL AND MECHANICAL MEET AT LAST lol thanks

Our guest is correct. You can easily calculate the rated torque of a given motor. I misunderstood your question. I thought you wanted to calculate what the current would be for a motor when the load required less than the rated torque.

Mech.Power(KW)= Torque (Nm)xSpeed(RPM)/9550

Elec.Power= Mech.Power/Efficiency

Current=Elec. Power/Voltage for DC Motors

Current=Elec. Power/Voltage/Power Factor/1.7328

Just curious...

I was doing a motor calc... (been a long time) searched here and found this thread.

Where does the 1.7328 factor come from when calculating current?

It only applies when calculating for 3 phase systems. That is the square root of 3.