HANDLING 50 HERTZ REQUIREMENTS
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INTRODUCTION

As American manufacturers increase exports to 50 hertz countries, there arises the problem of supplying motors for 50 hertz service at an array of unfamiliar voltages. Fortunately there are some possibilities available that make it feasible to handle many of these requirements without waiting for special designs.

The first choice should always be to utilize a stock 50 hertz motor from the wide variety offered in the current 501 catalog. If the basic motor exists but needs some type of modifications, they can frequently be handled through the Mod Express program to get exactly what is needed.

If a 50 Hz stock motor either doesnât exist or cannot be modified to match the requirement, then some other alternatives exist.

In order to provide a description of these alternatives, we must first break it into two major groups: Three Phase and Single Phase.

THREE PHASE MOTORS

When three phase motors are required, the situation can be quite simple. One ãRule of Thumbä that comes in very handy is as follows:

When the Ratio of Volts to Hertz Stays Constant, the Motor Can be Operated at the Reduced Frequency and Reduced Voltage.

Under this condition, the motor will provide the same operating torque that it would provide at its 60 hertz frequency. Please note that the stipulation ÷ the same torque should be remembered. An example may help illustrate the situation.

A standard induction motor rated at 1 HP, 3 phase, 230/460 volts, 60 hertz would be checked out as follows: 460 Ö 60 = 7.66 volts per hertz. In this case, the matching 50 hertz voltage would be 50 x 7.66 = 383 volts. Thus the standard 60 hertz motor could be used at 50 hertz on voltages of 190 or 380. Under this condition of reduced voltage and frequency, the motor could be expected to generate the same amount of torque as it would on the normal 60 hertz application. In this case, it would be 3 lb. ft. of torque.

The speed of the motor would of course be lower then it would be on 60 hertz. Normally, you would expect to get a speed that is roughly five-sixths of the 60 hertz speed. In the case of a 1725 RPM motor, you would normally be 1425 RPM when the motor is operated on a 50 hertz power system.

WHAT ABOUT HORSEPOWER?

Since horsepower is the product of speed and torque, you would expect that the horsepower output would be five-sixths or slightly over 80 percent of the 60 hertz rating. In order to overcome this problem, there are two approaches: The first would be to select the next larger HP rating. Thus, in the example cited above, a 1-1/2 HP motor could be used to handle very nicely the 1 HP requirement at 50 hertz. In most cases, the incremental cost of selecting the next higher horsepower is substantially less than the cost and time involved in ordering a special unit. This derating approach is a sound and conservative one that can be used on virtually all applications involving open drip-proof and totally enclosed motors and brake motors. A motor selected in this manner can be renameplated to the new voltage, HP, speed and frequency combination.

Due to the inherent conservative designs used in Baldor motors and the normal voltage tolerances, many stock motors can be operated on 200 volts, 3 phase, 50 hertz or 400 volts, 3 phase, 50 hertz. Some can also be operated on 415 volt, 50 hertz systems. These combinations of 200, 380, 400 and 415 are the most frequently occurring 50 hertz voltages.

A second approach allows you to handle many of the 50 hertz requirements without derating. This is a little more involved and might normally be considered where special motors exist or where there are specific frame size restrictions that do not allow for an increase to the next larger HP rating.

The approach in this case involves asking a few specific questions and having a reasonable understanding of the type of load that is being driven. The basic question is this: ãIs the machine going to be identical in all respects to its 60 hertz counterpart?ä If the answer to that question is ãyesä, a second question should be asked as follows: ãAre you going to allow your machine to run at five-sixths of the 60 hertz speed or are you going to change transmission components such as gearing, belts, pulleys, etc. to increase the output speed up to the normal rate that you would get if the motor were to be running on 60 hertz?ä In this case, if the customer is going to change components in the machine to maintain the performance of the machine up to the 60 hertz capability, then the approach of oversizing, as discussed previously, should be used.

If, on the other hand, the machine is identical and the customer is going to operate it at reduced capability, then the torque required to drive the machine would normally be the same torque or in some cases less than the 60 hertz torque requirement. If the torque requirement is the same or less, then the motor need not be derated since the machineâs requirements have been decreased and the motor would still be a perfect match for the machine. There are also many Baldor motors that can be operated at the rated horsepower on 50 hertz requirements without exceeding their rated temperature rise. Thus, a third option also exists but it involves a good deal more searching to determine if a motor can be utilized to handle specific requirements.

OTHER VOLTAGES

Aside from the three commonly occurring 50 hertz voltages that have been described previously, there also arises from time to time requirements for others such as 440 volts, 50 hertz. When the rule of thumb is applied to standardly available motors, it turns out that this voltage is not one that can be handled by normal derating processes. In this instance, a special motor would have to be wound or an existing motor could be rewound by a service shop to match this requirement. In some instances, 575 volt, 60 hertz motors can be utilized to handle voltages of 480, 50 hertz or as high as 500 volts, 50 hertz. When this occurs, the normal procedures for derating as listed previously can be applied.

SINGLE PHASE MOTORS

Single phase motors present a unique problem since there are two items involved:

  1. The winding must match the 50 hertz frequency and voltage.
  2. The centrifugal starting switch must be set to operate at the right point as the motor accelerates during its starting period.

110 The simultaneous requirement for both of these items usually makes it impossible to utilize normal 60 hertz motors for 50 hertz, single phase requirements. In most instances it may be possible to rewind an existing 60 hertz motor and change the centrifugal starting switch to one that is appropriate for 50 hertz operation. This procedure is fairly costly and time consuming.

A second option exists with Baldor motors since we now offer a good selection of single phase, 50 hertz motors in the range of horsepowers of from 1/3 to 5. These motors are specifically designed for 50 hertz operation on either 110 volts or 220 volts (5 HP, 220 volts only). They are rigid base motors in both open drip proof and totally enclosed. When C Flanges are required, footless C Face 1425 and 2850 RPM motors are offered in a range of sizes from 1/3 to 2 HP. C Flange kits are available to convert stock motors from the standard mounting to a C Flange mounting. Since the bases are welded on, it is not feasible to remove the base in order to get a footless motor but most customers will not object to having both the C Flange and rigid base if they can get availability of the basic unit.

EXPLOSION PROOF MOTORS

Explosion proof motors present some unique problems. Basically, they conform to the same rules that have been discussed previously. However, due to the UL (Underwriters Laboratory), many of these motors cannot be renameplated to alternate voltages or frequency. The reason for this hinges on the safety aspects of the explosion proof designs as well as the thermal overload coordination situation. Thus, explosion proof, 50 hertz motors, that are not available from stock, both single and three phase, have to be ordered as special units.

SUMMARY

By using the techniques described, it is possible to handle a very high percentage of the normally occurring 50 hertz voltage requirements. If you should have questions, please contact us and we will try to be of assistance.




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