Motor and voltage drop
- Thread starter domnic
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rlundsrud
Senior Member
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- chicago, il, USA
I don't know that it would, depending on the application. When you reduce the voltage you also decrease the current and torque. If the motor requires more torque to start then is provided by the reduced current, that could be a problem, but reduced voltage starting is used to lower the starting current on motors.
If the line to line voltage is balanced and equally lower, the result of lower voltage is greatly reduced torque output. I would have to look up the formula but it may that torque drops proportional to V^2, the effect is quadratic.
If insufficient torque causes the motor to slow down, slip will increase by the difference for an AC induction type motor. Increased slip and lower efficiency outside the design range will result in conversion to heat.
Depends on the motor type. A universal type AC/DC motor will probably just see reduced torque and speed but no loss of efficiency or loss conversion to heating.
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- Massachusetts
Assuming the motor is fully loaded and assuming that load is contounuos reducing the volatge will increase the current causing the windings to overheat leading to insulation failure.
On the other hand if the motor is underloaded the reduction n voltage might reduce the currrent.
kwired
Electron manager
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- NE Nebraska
Also depends on the driven load type, slow down a variable torque type load (centrifugal fans and pumps) because of extra slip and the power demand from the load is reduced. Constant torque type loads will still demand same power from the motor, and current will go up to try to provide it. Unless you vary the frequency the motor is going to try to maintain base speed and will draw whatever current the supply will let it to try to do so until you reach a certain point - that point will be with a locked rotor condition though.
lauraj
Senior Member
- Location
- Portland, Oregon
Would it be correct in saying that slower speed would mean less CEMF, therefore increasing the current?
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- Placerville, CA, USA
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- Retired PV System Designer
It would. There are other ways of looking at it but that one works for me most of the time.
That is particularly good for universal motors, but is also useful for AC induction.
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Is the motor really a 120V single phase motor, or was this hypothetical? If real, the issue is not as simple as just reducing the torque. If you want to know more, you would have to tell us exactly which TYPE of single phase 120V motor it is, there are many different types, all with different effects.
Assuming the most common type, a Capacitor Start motor, starting/peak torque reduces at the square of the voltage change, but running torque is reduced at the same rate. So if already running when the voltage drops to 85VAC, the torque will drop to 85/120 which is 70%. But if trying to start from a stop, or if the load changes and you need to RE-accelerate the load, the peak torque is reduced to just 49% of normal. Since starting torque is usually 160% of Full torque, that means the starting torque us now roughly 80% of FLT instead of 160% FLT. That can mean, in a single phase motor under load, that the acceleration time might be too long for the starting capacitors or even the starting switch (if any) to handle. They over heat and fail.
Assuming the most common type, a Capacitor Start motor, starting/peak torque reduces at the square of the voltage change, but running torque is reduced at the same rate. So if already running when the voltage drops to 85VAC, the torque will drop to 85/120 which is 70%. But if trying to start from a stop, or if the load changes and you need to RE-accelerate the load, the peak torque is reduced to just 49% of normal. Since starting torque is usually 160% of Full torque, that means the starting torque us now roughly 80% of FLT instead of 160% FLT. That can mean, in a single phase motor under load, that the acceleration time might be too long for the starting capacitors or even the starting switch (if any) to handle. They over heat and fail.
nullsig
Member
- Location
- Louisville KY
Would the current not increase to a degree? P=IV. Voltage goes down, current goes up in order to maintain a certain power output.
kwired
Electron manager
- Location
- NE Nebraska
If power is maintained then current has to go up. You still can have reduced voltage, increased current but not at same overall power level, power factor sneaks in here as well and gives us true power and apparent power figures.
- Location
- Massachusetts
Correct if power output remains the same, however often it does not remain the same.
For example one power saving method is to reduce just voltage to motors that are running under loaded. Such as a conveyor that has to run constantly but only is heavily loaded a small percentage of the time. A controller can be added to reduce voltage during the lightly loaded periods and raise it back up when needed.
nullsig
Member
- Location
- Louisville KY
Agreed, it would be based on the type of load.
K8MHZ
Senior Member
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- Electrician
That's what I was taught.
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