Effect of lower ac voltage on motor

[direction]

Hi

I am new to this forum and read somewhere that lower voltage (lower that nameplate) causes lower amps on a motor.


An assumption people often make is that since low voltage increases the amperage draw on motors, then high voltage must reduce the amperage draw and heating of the motor. Is this the case?

Does high voltage on a motor tend to push the magnetic portion of the motor into saturation. This causes the motor to draw excessive current in an effort to magnetize the iron beyond the point where magnetizing is practical.

 

[mc5w]

It all depends on how much load there is on the motor. If the load is light or none then a modest lowering of voltage will lower the iron losses by decreasing the magnetizing current. There are some variable frequency drives that can be programmed to do this if a motor has an operating cycle such that say 50% of the time the motor has no load.

However, for any significant load you need to stick to the nameplate values plus or minus 10%. A typical U.S. modern 3 phase motor will tolerate 190 to 250 volts when connected for 230 volts and 380 to 500 volts on a 60 cycle power system. What has been done with motor improvements since 1970 is to improve efficiency and to increase tolerance for a wide swing in voltage.

Prior to 1980, motors for use on 120Y208 volt systems had to be rated 200 volts on the nameplate - a 230x460 volt motor could not tolerate the lower voltage. In between 1980 and 2000 there were enough motor improvements ( higher temperature insulation, better steel ) that general purpose 200 volt motors were eliminated and replaced with 200-230x400-460 volt rated motors. The only 200 volt motors still being made are for submersible well pumps where the compactness requirements and so forth pretty much puts a straightjacket on hbow flexible motor design can be.

However, when these motors are operated on 120Y208 volts or 240Y416 volts 60 Hertz you generally have to take the service factor down from 1.15 to 1.0. The service factor is essentially the ability of the motor to tolerate abnormal conditions.

There has also been more recently motors that are rated 200-230x400-460 on 60 Hertz and 170-220x380-440 volts on 50 Hertz and full Horsepower for both ratings. Service factor ususally is 1.0 on anything other than 240 or 480 volts 60 Hertz.

Mike Cole, mc5w@earthlink.net

 

[direction]

I need direction with my question - lol

What I want to know:
What is the effect of higher than nameplate voltage on an induction motor(550hp). Does the amp's increase?

And:
How do I know how much unbalance current a motor can handle? Some websites mention 10% and other mention 5%?
SF=1.15

THX

 

[mc5w]

Significantly higher than nameplate voltage will increase the no load magnetizing current and increase the iron losses.

 

[Mstrvb19]

Let me break down what is going on here without getting into the physics too much. The 'rule' you are speaking of above is that induction motors can be approximated as constant kVA on the power system. However, this rule, as you are discovering, is just that-an approximation. The relationship between terminal voltage and current draw is really only linear over a short range- which can basically be considered to be the nameplate voltage +/- 10%, as mc5w mentioned. So the answer to your question is- yes, if the terminal voltage is high enough, the motor will draw more current. In reality, as mc5w was saying, the increased voltage does in fact increase the magnetizing current, but over the normal range of operation this effect is small and pretty much neglible. Most power systems do not operate so far above nominal voltage that this effect becomes noticeable. Additionally, this effect is dependant upon how heavily loaded the motor is. The closer to full load the motor is, the quicker it will reach saturation as the voltage goes up, so heavily loaded motors are more susceptible to this effect than others. If you are concerned that your motor is experiencing this (i.e. you are observing high currents at high voltage) check the running power factor. If the motor is saturating, the power factor will crash, so you would observe a power factor well below the usual or nameplate values. FYI- I have experienced motor saturation at high voltages as you mentioned- we had a NEMA class B motor in a typically class C application, and were operating it with 111% terminal voltage. The motor was saturating and the power factor was at about 21%. Also, some motors are designed to operate right to their limits and saturate quite often, including Motor Operated Valves (thus the reason for limitorque curves and the special requirements for MOV study) and some other types, such as strainer motors.
As far as voltage unbalance, I am pretty sure the manufacturers specify 5%. In the US, you can check NEMA MG-1, and as long as the motor was built to that standard, it will meet the requirements.

 

[buzzp]

NEMA recommendations for unbalanced voltage is no more than 1% unbalance without derating the motor(derate 3% on nameplate HP, I believe at 1% unbalance). Typical no derating occurs until the unbalance is at 3%. The maximum they recommend is 5% voltage unbalance with derating being 75% of nameplate HP. I know you stated current unbalance and this is generally kept below 5% (for derating purposes this is directly related to the current in any one phase not exceeding nameplate amps)(sorry I have no suggested derating based on current unbalance).
Assuming a motor is running at full load (nameplate) then as the voltage increases from 100% nameplate to 105% nameplate the current will generally drop (with an increase in starting current and drop in PF). As the voltage is increased above 105% of nameplate voltage the current will rise. Say the voltage was 110% of nameplate, you could expect to see around a 3-5% increase in current and reduction in PF of about 10%. At 115% voltage applied then the current is about 7-9% higher than nameplate amps.

 

[aolalde]

For a 550 HP similar to NEMA design B.

Voltage unbalance per NEMA MG1 is recommended not to exceed 1% . This 1% voltage unbalance will result in an average of 8% current unbalance.

If the Voltage unbalance reaches 5%, the current unbalance will be around 40% and to avoid overheating the maximum load power must be derated to 75%. 412.5 HP.

Unbalanced voltage is undesirable because it creates a negative sequence component acting like a burden break at double of the frequency (reverse flux rotation).

In regard to the voltage, when the motor has full load:
15% reduced voltage will increase the PF around 5%, reduce the EFF around 4% and the current will increase around 18%.

At 10% increased voltage, the PF reduces like 10%, the EFF drops 2% and the current increases around 3%.

More than 10 % over voltage is not recommended since the magnetic circuit will be saturated dropping the PF to dangerous levels of over current.