volts per hertz ratio and ac motor torque

Discussion in 'General Electronics Chat' started by recca02, Oct 15, 2007.

1. recca02 Thread Starter Senior Member

Apr 2, 2007
1,211
0
i need to know why exactly does torque remains constant (or is proportional)
to V/Hz ratio?

2. Mike M. Active Member

Oct 9, 2007
104
0
More voltage = more current = stronger field = more torque for a given unchanged loading = faster frequency.

3. recca02 Thread Starter Senior Member

Apr 2, 2007
1,211
0
i m consfused
for more voltage the current shud decrease for same voltage (ac motors are like transformers)
isnt this why undervoltages are said to be dangerous since current draw increases IIRC.
but may be the load isnt constant in my case only torque is constant.

i was thinking along the lines that
power=torque*speed
torque=power/speed
speed prop to frequency
power prop to voltage?????????????
but my books tell me
power prop to sq (V)
and torque pro to sq(V) / speed.

4. Mike M. Active Member

Oct 9, 2007
104
0
Power dissipated is directly releated by the loading. If there is more load, the motor will turn slower and the back EMF will be reduced and therefore the applied voltage doesn't have as much reverse voltage holding it back. This will cause a current increase on the input until the system stabilizes back near the original frequency but at a higher power dissipation. It also depends on whether you are talking about a synchronous motor or and inductance motor which is torque regulated through it's slip speed.

Seriously, I could go on and on for about 5 pages on this subject. There is a lot of information that depends on the type/configuration of motor because they all act different but share similarities.

Just remember that back EMF depends of the RELATIVE speed of the rotor wires with respect to the magnetic field. This always generates a voltage in opposition to the applied voltage via the right-hand-rule for generators. Every motor has an effective generator as the rotor that is cancelling out a portion of the applied voltage. The slower the rotor turns, whether it be increasing the load or decreasing the applied voltage, the back EMF will be reduced. If it is due to an increasing load, the applied voltage minus the back EMF will be greater allowing for a greater current flow and thus power dissipation. If it is due to lowering the applied voltage, the applied voltage minus the back EMF narrows and the current will decrease and lower the power dissipation and thus the torque so the rotor will slow down with a constant load.

5. recca02 Thread Starter Senior Member

Apr 2, 2007
1,211
0
while i have already learned those facts what i can not seem to figure out is
in both asynch and synch m/c the torque is power/Ns and power is prop sq(V)
so how come torque is proportional to V/Hz instead of Sq(V/Hz).
if possible i find relations easier to understand when given with equations.

another question is what happens to life of a motor which runs on VFD's or any other controls like PWM (in cases where the waveform is departing from sinusoidal waveforms). Does this not cause reduction in life of motor for unsmoothness in performance?