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Special Reports

2010 Quarter 4 Issue 5

Impact Analysis on the Application of 60Hz Three Phase Squirrel Cage Asynchronous Motor to 50Hz Power Supply

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Overview

The analysis is designed to probe into the inconsistence problem, which refers to whether the exported three-phase Squirrel Cage Asynchronous Motor with 60Hz, power supply 575VAC, 480VAC, 460VAC and 440VAC can be applied to 50HZ power system.

 

Analysis on the condition that 60Hz motor is applied to 50Hz motor power system with the same voltage.
Based on Electromechanics, Ф flux of motor each pole can be calculated through formula below:


 

In the formula, ke stands for reduction voltage coefficient, U stands for additional power voltage (phase voltage) (V), ƒ stands for power frequency (Hz), W stands for number of turns for series coil in stator winding, kdp1 stands for stator winding coefficient.

For the finished product of motor, kdp1, W, and ke are constant value. When U value is fixed, ƒ is in proportion to Ф:

Based on formula above, the analysis on performance parameters when 60Hz motor is applied to 50Hz motor power system can be calculated as follows:

(1) No-load Current

When power frequency drops down form 60Hz to 50Hz, Ф2 equals to 1.2Ф1. Flux value of each pole increases 20 percents accordingly while flux density of motor parts increases 20 percents.

Under normal circumstances, flux of the motor magnetic circuit keeps constant; the reasons are listed as follows:

Electromagnetic torque TM is the result of interaction between I2 and Ф.

In the formula, KT stands for torque constant, I2 stands for rotor winding current and cosφ2 stands for rotor side power factor.

Formula above indicates that if Ф value decreases, Electromagnetic torque value will reduce and belt load capacity of motor will lower; if Ф value increases, magnetic circuit will get into saturation, which leads to serious distortion of field current wave pattern and the peak wave will come out. See Picture 2-1:

Picture: Relationship Between Flux and No-load Current

Relationship between Flux and No-load Current follows the regulation of magnetization curve, as it is shown (a) of Picture 2-1 and curve ① of Picture (b).

Under normal circumstances, the motor magnetic circuit stays in the unsaturated zone, as shown AA section of picture (a). When flux value varies between -Ф1~+Ф1, exciting current (no-load current) changes as it is shown with the curve of Picture (a) with a small amplitude value.

If the magnetic flux peak increases to Ф 2, compared with a very close increasing value of Ф 1, magnetization curve can not reach BB section because magnetic circuit reach saturation. When flux value varies between -Ф2~+Ф2, exciting current (no-load current) changes as it is shown with the curve ③ of Picture (b) with a serious distortion and peak leap, causing a high leap for motor current value.

Motor should be forbidden to use if no-load current value approaches or exceeds the rated current value.

(2) Rotate speed

Synchronous speed is calculated through formula below:

In the formula, pole logarithm remains constant, but value of synchronous speed decreases due to reduction of power frequency from 60Hz to 50Hz.

Thus rotate speed of motor fall by 17 percents.
Rated revolution of motor is calculated as follows:

In the formula, s stands for difference of synchronous speed and rated revolution, na stands for actual motor rotate speed value.

Suppose a motor with rated frequency of 50Hz, rated revolution of 1440rpm, 4 motor pole values and synchronous speed 1500rpm.

Thus, difference of synchronous speed and rated revolution s equals to 60rpm with 1500 subtracting 1440.

So actual motor rotated speed would reduce.

(3) Starting current

Because motor is inductive load type machine, main reactance of each phase can be calculated through formula below:

In the formula, Xm stands for motor main reactance of each phase, μ0 stands for magnetic conductivity, N stands for iron core number of turns, stands for stator core effective length of alternating-current motor, δ stands for air gap and K3 stands for ratio of motor magnetic circuit total potential drop and magnetic potential drop of air gap.

The result is that, Xm value is direct proportion to ƒ value. If ƒ value is lowered, Xm value drops accordingly. While starting current is inverse proportion to Xm value. Thus starting current value will be more than 20 percents than that of original one.

(4) Torque

Motor torque value is calculated through formula below:

In the formula, TM stands for motor rated torque (Nm), Pe stands for motor output rating (kW), ne stands for motor rated revolution (r/min).

Motor in the frequency of the rated operating process, generally speaking, if the constant flux, the constant torque motor can output. Below is the rated operating frequency, rated output torque.

Because rotate speed value is decided by frequency, it will decrease as frequency value drops. And motor electromagnetic torque is inverse proportion to the square of supply frequency, that is the formula of . Torque value will increase when supply frequency drops from 60Hz to 50Hz.

The value increases by 44 percents. As the same way, the peak torque and least torque value will increase accordingly. Meanwhile, due to the effect of excitation, current value of bringing onto load will increase greatly. If the current value is limited less than rated load value, bringing onto load capacity will be weakened enormously.

(5) Efficiency

There are several major kinds of dissipation as follows:
1) Iron loss PFe
Values of Hysteresis and eddy-current loss PFe caused by the alternation of main flux in motor iron core is calculated through formula below:

In the formula, K stands for correction factors of increasing iron loss such as the process of iron core and uneven flux density distribution, P1/50 stands for unit loss (W/kg) of 50Hz, 1T flux density in iron core material (silicon steel sheet). B stands for flux density of iron core (T); ƒ stands for flux alternation frequency (Hz), GFe stands for iron core quality (kg).

2) Stator copper loss PCu1

If the load current value is the same, value of Stator copper loss PCu1 will remain unchanged.

3) Rotor copper loss PCu2

Due to the 20 percents of density increase, the value of rotor current will reduce by 17 percents to maintain the same torque speed, and the value of rotor copper loss PCu2 will also reduce.

4) Added losses: the blower loss value P ƒ reduces to about 60 percents of the original as rotate speed lowers. The value of added losses reduces a lot because motor output power drop greatly.

(6) Power factor

No-load current increases the value. Although the value of motor reactive resistance drops, the changed value could not compensate the losses. So power factor also drops its value somewhat.

(7) Temperature rise

Flux density enlarges itself by 20 percents of the original, causing the saturation of iron core flux density. Besides, ventilation quality declines as rotate speed drops. Due to two factors above, motor temperature rise would be higher than the original by a great volume.

 

Situation analysis of applying 60Hz motor to 50Hz power system through reduction voltage

(1) Calculation of value of reduction voltage

To avoid current overheat when applying 60Hz motor to 50Hz power system needs to maintain a constant value flux. According to the formula , the only changeable value is supply voltage. Take a motor with 3Ф, 400VAC, 60Hz for instance, suppose Ф1 value equals to Ф2 value, the voltage in the 50Hz power is:

(2) Alteration of motor rotate speed and power after reducing voltage.

1) Rotate speed: due to the rule that the rotate speed of non-synchronous motor is only direct proportion to frequency ƒ with unchanged pole logarithm P value, thus rotate speed drop by 17 percents based on previous calculations.

 2) Power: positive setting Ф remain constant so current value I after reducing voltage also maintain unchanged. According to the calculation of output power:

Substitute the formula of  to above pattern, motor power with reduction voltage would be 83 percents of the original. If the motor stator winding adopts connection type, the phase voltage would drop to  of the original by changing stator winding connection type as Y. Flux value drops meanwhile, current I is inverse proportion to Ф. So according to , current value is 0.692 times of the original. Output power would be calculated as 40 percents of the original one:

(3) Methods and possibilities of realizing reduction voltage

1) As for the area with lower supply voltage, to adjust gears unit of supply transformer tap switch is advisable way. The pressure regulating range is ±10 percents.

2) Add a voltage regulator. The cost of replacing a voltage regulator is 30~60 percents of that replacing a motor. That is relatively economical.

To adjust voltage through replacing voltage regulator is almost the only resort because extra losses would be brought about no matter motor operates with excitation or non-excitation.

Passing excitation will greatly increase the motor exciting current, plus extra heating, which causes an unstable motor operation. Thus this situation should be absolutely avoided to reduce possibility of burnout.

Underexcitation will reduce output torque greatly and make a high possibility of launching load. Power factor will also change. But if adding a rated load to motor, rotor current will sharply increase to cause a burnout.

Using voltage regulator of adjusting a proper value can ensure motor normal operation and output rated torque. Motor power will drop as rotated speed drops.

Generally speaking, machinery and equipment on time with motor has 20~30 percents of the residual, even more, just after the lower voltage motor produce stall.

 

Conclusions:

With a same voltage value, the effects from application of 60Hz motor to 50Hz power system on the motor performance can be drawn as follows:

 No-load current will exceed by 20 percents.
 Rotate speed will drop by 17 percents.
 Starting current will increase by 20 percents of the original.
 Torque will increase by 44 percents.
 Power factor will drop by a certain amount.
 Motor temperature rise will increase a lot.

By the same analyzing means, performance parameters when applying 50Hz motor to the 60Hz power system can be calculated as follows:

 No-load current reduce by 17 percents.
 Rotate speed will increase by 17 percents.
 Torque will reduce by 30 percents.
 Starting current will reduce by 20 percent of the original.
 Power factor and temperature sire will get an improvement.

To sum up, application of 60Hz to the motor with 50Hz power system is not under recommendation except for the particular circumstances.