Motor speeds and voltage 3

[koel246]

HI, i was wondering if anybody could give me a simple relationship between the speed at which at motor turns and the voltage supplied. Also when a motor is started and has a load attached, there is obv. a period of acceleration before constant velocity is reached. Would they remain constant during this period or would they change - how??

Thank you

Koel

 

[sanborn]

The simplest relationship between voltage and speed is back EMF. This is usually given in the motor's data sheet as V/kRPM or V/Rad/s. If the back EMF is say 20 V/kRPM and you apply 20 Volts to the motor it will turn about 1000 RPM with no load.

Volts = Speed * Back EMF

To make things a little more complicated if you apply load you have to obey Ohms law. As you apply load the motor will start to draw current. This will require more voltage to push the current through the motor. To calculate this additional voltage requirement you will need to know the resistance of the motor as well as the torque constant of the motor (or use an ammeter). The torque constant has units of torque/current. You will also need to know the torque you are applying to the motor. Remember every motor has a friction torque of some sort.

I = (Tload + Tfriction)/Torque Const

V = I * R + BackEMF * Speed

This is fairly accurate in a cold motor with few poles. Since resistance of copper rises with temperature you would have to correct the resistance value according to the running temperature. Usually a factor of 1.5 by 155C. Additionally voltage is required to overcome the motor's viscous damping. How much this affects the calculation depends on the motor. It is typically more of a problem for high pole count per second applications like large frameless motors or very high speed servos.

 

[acmotorengineer]

is this an AC motor or a DC motor?

 

[jwerthman]

Sanborn's explanation applies to DC motors and universal motors, but not to induction motors or synchronous motors. Induction motors and synchronous motors are both essentially constant speed devices, with the speed determined by the frequency of the applied AC voltage. Within the window of normal operating voltage, the speed is affected little or none by the magnitude of the applied voltage. The speed of a synchronous motor will not change at all with variations of the voltage. The speed of an induction motor will change slightly as the voltage is changed because the motor slip will increase (slower speed) with lower voltage or decrease (higher speed) with higher voltage.

 

[koel246]

Well the motor in question was DC so there was no problem. i just didnt want to overload it when testing - its instructions werent very detailed. Thanx for all ur help!!

 

[19gabi50]

For an induction motor, one can write the formula:

s2=s1*(u1/u2)^2, where:
s1 is the slip when the motor is supplied at the voltage U1 and the load torque has a certain value.
s2 is the slip when the motor is supplied at the voltage U2 and the load torque has the same value.
That means the load torque is assumed constant (not speed dependent). This hypothesis is acceptable if the speed/slip changes a few percent.
Besides, the formula is correct in case the sleep is higher than the critical value (related to break down torque).
The above formula is useful if one wants to know the effect of a voltage dip of 20-30 % on an induction loaded motor.