Archive for the ‘Motors’ Category

How To Change The Speed Of A Motor

Posted on: June 23rd, 2014 by admin 17 Comments

It seems that everyone these days wants to change the speed of their motor. Whether it is because they want more air from their fan on a hot day or they want to reduce the cost of operating a motor, it has become nearly standard practice to change or vary the speed of electric motors.

From the time the three phase induction motor was invented (circa 1889) it was a foregone conclusion that it would run at a speed dictated by the frequency of the power supply. If a different speed was needed a device was attached to the output shaft of the motor to accomplish the task. Pulleys, gears, gear reducers – all were used to obtain the desired speed at the driven shaft. Some of us old timers remember devices like vari-drives and eddy current clutches that would allow the speed to vary with the demands of the application.

Sometime in the ‘70s the term Variable Frequency Drive (VFD) crept into our lexicon. The early units were very crude by today’s standards but they got the job done. It was then possible to operate a motor at virtually any speed from 0 to nameplate rpm with the twist of a dial. Next came the ability to operate above name plate rpm, follow an electrical signal for “automatic” speed adjustment to maintain system settings, and set up PID loops. Then vector drives, steppers and servos. Whoa… my head’s spinning.

Speed changes today can be as simple as adding an electronic drive or as invasive as rewinding the motor for a different speed. Like anything else, each method has its up side and down side.

Some (not all) small single phase motors can be operated at various speeds through the addition of a resistor(s). It should be pointed out that the resultant speed is dependent on the load placed on the motor. What is actually being done is the weakening of the motor by reducing the voltage across the winding (the resistor acts as a voltage divider). The “lost” voltage is converted into heat which, for all practical purposes, is wasted into the atmosphere.

Most large single phase motors can’t (or, more correctly, shouldn’t) be operated at reduced speeds. These motors often utilize a centrifugal mechanism to disconnect the start winding. If the speed of the motor is reduced too far the centrifugal switch recloses, allowing the high starting current to flow in the smaller start winding. The end result is a motor winding failure.

There are a number of different types of single phase motors: universal, shaded pole, permanent split capacitor, capacitor start, capacitor start/capacitor run, repulsion-induction. If you are not sure what type of motor you have, give the motor experts at Gem State Electric a call at 208-344-5461. We’ll help you figure it out.

Three phase induction motors are a different story. All three phase induction motors can be run at variable speeds with the help of a properly sized VFD. Some three phase motors are much better suited to variable speed operation than others but all can run on a VFD. A number of factors must be considered to determine a specific motor’s suitability however, so give us a call to discuss your specifics.

As stated above, for decades the three phase motor was designed for a specific rpm based on the frequency of the incoming line. Only rarely were devices employed to change the frequency of the incoming power; many of these rotary frequency changers are still in use however.

With the growing popularity of VFD electronic drives a whole new industry was opened. Companies now marketed the devices as a great way to save energy, particularly in variable torque applications such as pumps and fans.

By using a 60 Hz. input to create an easily variable frequency output the speed of the motor is now adjustable. By changing voltage and frequency with a constant ratio the output torque is consistent from nearly zero to full line voltage.

Various types of electronic drives, utilizing a dizzying assortment of bells and whistles, are now available. Your motor can do things now that its ancestors could only dream of!

Another way to (permanently) change the speed of a motor is to redesign the winding during the winding process. The speed choices are much more limited however. There are, for example, no speed choices between 1800 and 3600 rpm when redesigning the winding.

There are some practical limitations in doing so, not the least of which is the diameter of the rotor. The centrifugal forces on the rotor increase dramatically as the rpm goes up. In most cases a manufacturer will use a smaller diameter rotor in a 2 pole (3600 rpm) rated motor than they do in their 1800 rpm (and lower rpm). If the larger rotor is now spun at twice the rpm because of a winding redesign, the rotor is suddenly required to handle 4 times the centrifugal force it was designed to experience (force is increased by the square of the increase in speed).

I had a customer back in the ‘70s that did not consider this when adjusting his brand new VFD. The drive was capable of outputting frequencies up to 300 Hz. His motor did not make it past 120 Hz before the rotor literally exploded!

Other considerations are the amount of torque required by the load at increased or decreased speeds, the bar/slot combination of the original motor, decreased cooling effect at lower speeds, core saturation at the new output levels, etc. Like many other things, just because you can change the speed of a winding design doesn’t mean you should do so. Before considering a change like this, consult a motor expert, like the experts at Gem State Electric, for example. (Now tell the truth – you were expecting that commercial weren’t you?)

If you do not have access to three phase power but need to change the speed of your motor you are still in the game. A customer recently came to us with a challenge. He needed more water to irrigate his field but the utility would only allow single phase power to be brought to his meter location. Single phase motors are, for all practical purposes, available up through 15 HP, but he needed a 20 HP motor/pump to move enough water for his crops. We were able to find a three phase motor/pump that we ran off of a VFD, and powered it from a single phase line – with the blessing of the utility!

With any such application it is always a good idea to consult the motor/pump/control experts at Gem State Electric (did you see that one coming?)

We are never further away than your phone, at 208-344-5461, to discuss your electric needs.

Motor Connections

Posted on: June 17th, 2014 by admin No Comments

One of the most important, yet most frequently misunderstood, parts of placing an electric motor in service is connecting the motor leads correctly. One would think it would be easy, but in fact it can be easy to get it wrong. Over the last 45 years I have seen literally scores of different connections used in electric motors. Let me say up front that, if you have any question about how to properly connect your motor, give the motor experts at Gem State Electric a call. We’ve gotten pretty good at figuring these things out over the phone. The major challenge in figuring out the proper connection for a given motor is the shear variety and the (sometimes) lack of standards. Color coding, NEMA numbering, IEC numbering and non-standard systems exist in both single phase and three phase motors. Single voltage, dual voltage, tri-voltage, wye-delta start, part winding start, two speed single winding, two speed two winding , constant torque, variable torque, constant horsepower, even two phase – we’ve seen just about everything. Then there is the occasional 18 lead motor with no markings on the leads! The point is – if you can’t figure it out don’t feel bad. Just give us a call at 208-344-5461. Some standards, of course, do exist. It’s just that some standards are more standard than other standards. So let’s look at some of the more common connections, keeping in mind that there can be variations on all of these diagrams.

SINGLE PHASE: It would be nice if all single phase motors could be reduced to two leads but that will never happen. We can however reduce the noise to a few groups that will handle most motors – single voltage, multi-voltage, set rotation and reversible. The single voltage and set rotation connections are typically a simplification of the multi-voltage reversible connections with some of the connections made internally, leaving fewer choices for the installer. Take a look at a few sample wiring diagrams, (right click on the link below to open in a new tab.)

NA Single-Phase1 connections

Some common modifications to this connection scheme: The use of colors instead of numbers (no standard between manufacturers is known to exist for colors) Some manufacturers interchange leads 2 and 3 GE made some larger single phase motors in the 1970s that used a 10 lead connection Some manufacturers use four leads for the start winding, adding lead numbers 6 and 7

THREE PHASE: Most three phase connections are either a “wye” (sometimes referred to as a “Y” or “star” connection) or Delta. There are a number of combinations that utilize both a wye and a delta connection but these connections are best left for another discussion. Let’s take a look at some of the more common three phase connections. You should note when comparing the wye and delta dual voltage (nine lead) connections that the external high voltage connections are identical. This means that when connecting a nine lead three phase dual voltage motor for 460 volts it is not necessary to know the internal configuration of the winding. However, if it is 230 volts that you wish to connect to your three phase motor you must first determine what the internal connection was used. The easiest way to do this is to look at the low voltage wiring diagram. If there is no diagram, you must take continuity readings. In order to do this you must disconnect any leads that are already connected. Caution: It is always best, before disconnecting an existing connection, to notice two things: 1) Are the leads identified (alpha numeric or color coded) 2) What leads are currently connected to each other? If the leads are not identified in any manner, stop before it is too late and call the motor experts at Gem State Electric. We can help you figure out what steps to take before it is too late! If you don’t stop at this point you may be taking a minor challenge into a major project (also spelled m-a-j-o-r-e-x-p-e-n-s-e). If the leads are identified and currently connected together in some configuration right down which leads are connected before disconnecting anything. This way, if you get in over your head you can still go back to the start instead of launching that major project to which we were just referring. With all leads disconnected and not touching each other, check for continuity (a circuit) between leads 7, 8 and 9 with your ohm meter or continuity tester. If a circuit exists between all three leads, you have an internal wye connected motor. If no circuit exists, and you know the winding to be good, you have a delta connected motor. Using the appropriate connection diagram it is now an easy matter to reconnect your motor for the lower voltage.

Why your electrician may not be the best person to call with your motor problems

Posted on: December 18th, 2013 by admin No Comments

Why your electrician may not be the best person to call with your motor problems

If your lights won’t come on you’d call an electrician, right? You wouldn’t call a plumber, a roofer or, in all likelihood, a motor technician. So why, if you have a motor problem, would you call an electrician?

Only a few electricians understand the electric motor. If you need a piece of conduit bent, they’re your best bet. Don’t know what size and type of wire is needed for your 15 amp receptacle within 3 feet of a water faucet? Call your friendly neighborhood electrician!

But when the motor doesn’t start on your air compressor, or the return air fan is making noises that it has never made before you need to rely on the person who has the experience and knowledge dealing with problems similar to yours.

I’m sure that if you are a licensed electrician your blood pressure just went up. You may know the difference between a Wye/Delta connection and a PWS connection but, odds are, you don’t. I could not count the number of times an electrician has checked a three phase motor suspected of being single phased by measuring the voltage to ground at the load side of the contactor. (Hint – that won’t find the problem. If you don’t know why, you just proved my point). Does the average electrician, or even an above average one for that matter, understand the ramifications of using a Design C motor to replace a Design D motor? Let me give you an example to demonstrate.

A customer has a 60HP Lincoln motor on a Return Air Fan. One morning, after replacing filters and doing the usual preventative maintenance on the system, the staff maintenance personnel note that the fan did not start when they flipped the switch. So they call their electrician.

The electrician comes to the job site, opens the cover on the disconnect switch and sees a six pole knife switch. The leads across the top are color coded with tape, with three colors used, left to right and then repeated. The six leads coming off the bottom of the switch contacts are not color coded, but they are numbered. The electrician places the leads of his multi-meter across the different combination of motor leads and declares the motor winding to be bad – it is “open”. The motor should be pulled and sent to a motor shop.

The customer looks at the application. In order to enter the space where the motor is mounted he must boost himself about four feet into the air and crawl on his hands and knees into a space that is not tall enough to stand up. Then he is faced with a motor that is mounted vertically, shaft up, belt-driven to the shaft that supports the fan. There is no means of supporting the 500 pound motor while the mounting bolts are removed. In service man parlance – “It ain’t going to be pretty!”

The unanimous decision of the maintenance staff was “We want a second opinion”.

A second licensed electrician was called to the job site and asked to analyze the situation. It didn’t take long for the diagnosis – the motor has an open winding and has to be pulled. There is no continuity through any of the phases (it was a three phase motor).

Still hoping for a better result the customer called us; “We need you to come and check out a motor on our air handler” was their plea. The maintenance man told me “all I want to hear you say is that I don’t have to pull that motor out”.

We always try to get the background before we begin a diagnosis. We learned that the unit had been running until it was shut down for PM. The disconnect switch had been thrown to the “OFF” position, the work done inside the air handler, and the switch turned back on. The control for the system was remotely mounted so it was necessary to walk down to the control room and reset the control. Much to their chagrin, the motor would not restart.

This information in itself would lead one to think it was not a motor issue as the motor was running fine when it was shut down. A quick continuity check confirmed that the winding appeared to be completely open. This also would usually be incongruous with a motor that was running fine upon de-energizing. So what’s up?

A simple explanation comes from an understanding of how an electric motor works and, in a related discussion, how a generator works.

When a voltage is present across a closed electrical circuit an electric current is passed through the circuit. When that current is flowing through the circuit it sets up a magnetic field. If a conductor (an element capable of conducting electricity) is moved through the magnetic field a voltage is induced across the conductor, the polarity of which is opposite in polarity to the polarity of the original applied voltage. The magnitude of the opposing voltage is dependent on the strength of the magnetic field and the direction and velocity of the motion of the conductor. What we have is a very basic generator.

An electrical meter, in order to measure resistance/continuity, must supply a voltage across a closed circuit. The magnitude of the current flow is monitored and interpreted by the meter in units of resistance (ohms). A known DC voltage across an unknown resistance will produce a known current flow, according to Ohm’s Law (E=IR). Most meters operate on a relatively low voltage (typically 9 volts DC). Even this low a voltage is sufficient to set up a small magnetic field in the motor’s stator (field winding). So the potential exists to generate electricity. Now all we need is relative motion between a conductor (like the cast aluminum rotor bars in the motor’s rotor) and our small magnetic field!

Because the fan is a Return Air fan it typically takes air from the space it services and “returns” it to the system. Because the Supply Fan was still running the air was being pushed into the room, building static pressure and creating air flow across the fan, creating rotation of the fan and therefore the motor’s rotor.

VOILÀ! We have a generator! Not a large generator. Not large enough to power your TV while watching Soap Operas all day, but a generator none the less; sufficient in output to overcome the small voltage of your meter and give a false reading. We had only to stop the rotation of the fan blade to confirm our suspicions. The motor winding was fine.

“You don’t have to pull that motor out” I told the customer.

Following a round of applause from the maintenance staff we were able to find a defective disconnect switch – one that had eluded the detection of two electricians. (Not to point fingers, but they had also missed the fact that the phasing tape was also misapplied, causing the power leads to be misidentified).

I am not asserting that we have never made a mistake. But the fact is that a license from the state does not, in and of itself, make someone qualified to troubleshoot. Go with the experience – Go with Gem State Electric!

Why repair an electric motor?

Posted on: January 24th, 2013 by admin No Comments

Why repair an electric motor?

The largest segment of the Electric Utilities market is the power consumed by electric motors. Electric motors drive everything from the electric screwdriver in your tool kit to the large refrigeration compressors that provide air conditioning for our largest buildings and plants.
Although the average electric motor will see a usable life of twenty years or more, many will see an early demise due to their operating conditions, lack of maintenance, improper application, improper repair or defects. When this happens the owner/operator must decide whether to replace or repair the motor.
Many factors have to be considered when deciding between repair and replacement. Certainly the cost to replace a motor is important. Often, however, the availability of a suitable replacement and the cost of repairing the “old” motor are just as important. With rising utility cost, the efficiency rating of the “old” motor and it’s replacement often out weigh repair/replacement cost. A large motor can cost an industrial user more to operate than he invested to purchase the motor in the first place. Looking closely at operational cost can sometimes yield tremendous savings when replacing a motor.
When a motor fails, a professional repair facility can help with the decision-making process. Often, a quick “bench test” (usually at no cost to the owner) can often help the motor repair shop determine whether or not a repair is economically feasible. If the decision is made to further examine the motor, a complete disassembly and assessment is typical. This in-depth examination often will disclose the root-cause of the failure. For example, if a motor winding has failed, an in-depth examination may disclose the reason for the failure. The professional repair shop can often determine the cause of a winding or bearing failure, thus avoiding a repeat failure.
Many in-house maintenance personnel are interested in one thing – getting the machine back “on-line” to avoid expensive down-time. They may not be capable of determining that a loss of voltage on one of three phases caused a winding failure. If a replacement motor is installed without determining and “curing” the root-cause of the failure the new motor may fail before anyone realizes that the motor failure was the “effect” and not the cause of the failure.
By the same token, a home-owner may know that the capacitor on his irrigation pump has failed. He may not be able to determine why it failed. If the owner replaces the capacitor it may not last more than a few seconds.
Repairing electric motors is more than replacing bearings. It requires the use of sometimes sophisticated diagnostic equipment as well as relying on years of knowledge to determine why a failure occurred, then repairing the motor to suit the owner and the application. Finally, testing of the motor under controlled in-shop conditions can assure the owner/operator that his motor is once again capable of doing the job for which it was designed.

The Average Key Length Standard

Posted on: December 4th, 2012 by admin No Comments

The Average Key Length Standard

Manufacturers, end-users (and even many motor shops) seem to be unfamiliar with the Average Key Length standard as it relates to imbalance. There are a number of factors that can minimize the impact of not understanding this principal, but everyone who is responsible for the extended operation of rotating equipment should be familiar with it. Let’s take a quick look at a real-world example.

A couple of years ago a customer called up to have us check a large motor on a belt-driven air compressor. When I arrived at the job-site I found two air compressors sitting alongside each other. Each had a 150HPLincolnmotor belted to the large shaft of a high speed compressor.

One of the motors had experienced a winding failure and needed to be pulled. As I was about to find out, it was only the symptom of the failure.

As we discussed what would be needed to remove this 1500 pound motor from its base five feet above the floor, the other compressor suddenly turned on. The whole building seemed to shake as the compressor came to pressure then, just as suddenly, shut off. I asked the customer how long the compressor had been shaking that violently.

“Ever since it was installed. This one’s just as bad” he said as he pointed toward the unit we had been discussing. “We have to come in every so often and re-weld the supports ‘cause it breaks ‘em” he added.

While in the shop we set the rotor up in the balance stand to check the balance condition. The problem was soon apparent.

The motor shaft was 3.375” in diameter. The keyway was 7/8” wide x 8.375” long. The pulley that had been mounted on the shaft by the compressor manufacturer had a bore that was approximately 2.5” long.

The problem related to the “Average Key Length Standard”. More on that below, but first –

Let’s take a look at the manufacturing process to understand where this situation takes root.

When an electric motor is produced the piece of shaft material starts out very nearly perfectly balanced. As the material is machined, the various steps and shoulders appear as the result of material removed from the shaft. As these changes are virtually concentric (nothing is perfect) very little change is made to the balance condition of the shaft. However, two of the final steps introduce large changes to the concentricity of the rotor assembly.

When the rotor is pressed onto the shaft it brings with it a greater degree of eccentricity. The cast aluminum rotors used today throughout the industry can never be absolutely concentric. Air pockets and non-concentric shaping cause imbalance. The addition of a large non-concentric mass at a greater diameter than the shaft has a large influence on the overall balance of the rotating mass.

The final change is the cutting of a keyway on one side of the shaft. Material is removed to produce a large trough to accommodate the shaft key. As the material is removed, weight is removed and imbalance is introduced to the assembly.

To meet industry standards the final assembly is then dynamically balanced. To do so the machinist mounts the assembly in a balance stand and spins it. But the problem they face is as follows:

The motor manufacturer does not know what will be mounted on the shaft when the motor is placed into service. From a balance point of view the material that was removed when the keyway was machined must be compensated for.

By convention, one half (approximately) of the height of the key is considered to be material in the shaft, the other half is considered to be part of the attachment (the coupling or pulley that mounts on the shaft). When the rotor is balanced a weight equal to roughly 50% of the weight of the key that will be shipped with the motor is placed in the keyway. The assembly is spun and any necessary final adjustment to the balance is made by adding or subtracting weight at the proper position.

Simple enough – right?

But what about the “attachment” manufacturer? When the keyway is cut in the bore of his assembly it must be compensated for in the balance procedure. But they do not know what shaft their piece will be mounted on. The pulley and coupling makers must also supply a key, half of which is considered the material of their assembly, the other half is the compensation for the keyway to which it mounts.

Do you see the problem yet?

In the case of the air compressor motor mentioned above the pulley manufacturer supplied a key that matched the length of the bore of the pulley (approximately 2.5”). Their key weighed approximately 250 grams, half of which was to keep the balance of the pulley within specifications.

But the poor motor manufacturer had balanced the rotor assuming that their key would be used (7/8”x7/8”x8.375”). Their key weighed a whopping 837.5 grams. So when the pulley was mounted on the shaft an imbalance of 293.75 grams was immediately introduced into the rotating system at a distance of approximately 1.68” from the center of the shaft. The influence on balance increases with both the weight of the key and the distance from the center of the shaft. The imbalance is magnified by speed.

So how do you work around this situation?

 The Average Key Length Standard

 

Measure the full length of both keyways, add the two dimensions together, and divide by two. In the discussion above we have 8.375” + 2.5”/2 = 5.43”

Cut a piece of key stock as close as possible to the calculated length and you will have corrected the majority of your imbalance in many cases. It is important to know that the motor manufacturer supplies a full length key with every motor. It is the customer’s responsibility to use the correct portion of that key, whether the customer is an O.E.M. or an end user.

In this specific case, the imbalance dropped form 6.0 mils to 1.0 mils displacement. Even if you have never had to balance a rotating piece of equipment, you can probably deduce that this is quite an improvement.

 Epilogue

 After returning to the job site with the freshly rewound and rebuilt motor I examined the pulley/shaft and key size on the driven shaft. The same condition existed there with one additional factor thrown in. The pulley ratios were such that the compressor shaft was rotating even faster and the keyway in the shaft was even longer, magnifying the effect on the driven end.

Instead of the manufacturer’s representative having to re-balance the compressor and motor, a change of keys was all that was needed to stop breaking steel supports and their welds. The customer was happy and is now fully aware that when replacing a motor or the shaft attachment, care should be taken to cut the new key to the appropriate length, instead of simply using the full length of the supplied key.

Torque vs Horsepower

Posted on: October 22nd, 2012 by admin No Comments

    The relationship between torque and horsepower is often misunderstood but is very important. To understand these two terms and how they are affected by other variables we have to start with definitions.

Torque is a twisting or turning ‘force such as the turning of a shaft in an electric motor. Horsepower is the same twisting or turning effect as related to time. Torque is usually measured in pound-feet (or foot-pounds) or some derivative such as ounce-inches or inch-pounds. In Europe the standard unit of measure for torque is Newton-Meters. Horsepower is pound feet per second (or minute). In terms of mechanics: one foot-pound of torque is the twisting effect created by applying a force of one pound against a lever, at a distance of one foot from the fulcrum or pivot point. Horsepower was established as the amount of effort that it takes to move a 3300 pound weight one foot in one minute. Note that torque has no time component, horsepower does.

The formula that relates these two to each other is as follows: T = HP X 5250 I rpm where T is torque, HP is horsepower, RPM is shaft speed expressed in minutes and 5250 is a mathematical constant. By looking at the formula we can see that:

1) For a given speed, if horsepower increases so does torque.

2) If rpm increases torque DECREASES for a given horsepower.

 

You may have noticed that for a given horsepower rating an 1800 rpm motor develops twice the torque that a 3600 rpm motor does at full load (3 ft-Lbs/hp vs. 1.5 ft-Lbs/hp). Likewise for a given rpm the way to get more torque is to increase horsepower.

So where does all of this good stuff come into play in the real world? Most commonly, when frequency changes are made in induction motors. The speed of an induction motor is determined by the number of electrical (not mechanical) poles and the frequency of the applied power. A common method of changing the speed of a three phase induction motor nowadays is by using a frequency inverter. The inverter typically starts with an A.C. input, rectifies it to a D.C. voltage and then, through a variety of methods, converts it back to A.C. through the use of triggering devices. The speed of the trigger determines the apparent frequency output of the inverter. So by turning an adjustment knob or reprogramming the output the frequency can be adjusted over a range, typically 0-90 Hz, although some inverters can go to 360 Hz. or beyond.

What happens to the voltage as the frequency changes?

Remember that the ratio of the voltage and frequency needs to remain constant. The volts per hertz ratio is adjustable in most inverters, but once it is set the ratio remains the same throughout the output range. This means that with an input voltage of 480 volts, 60 Hz. a properly adjusted inverter will have a volts per hertz ratio of 480 / 60 = 8. At 20 Hz. the voltage output will be 20 X 8 = 160 volts. At 57.2 Hz. the output voltage will be 57.2 X 8 = 457.6 volts. Because the strength of the magnetizing force remains the same, the torque exerted on the rotor (and therefore the output torque of the motor) remains constant throughout the speed range. That is until the inverter reaches 480 volts at 60 Hz. When the output is increased above the input rating the frequency still increases, the speed still increases, but the voltage can not increase above 480 volts. So at an output of 90 Hz. for example, the volts per hertz ratio is now 480 / 90, or 5.3 instead of 8. The strength of the magnetic attraction between the stator field and the rotor is now reduced and less torque is produced.

    Jumping back to the torque/horsepower formula for a moment, we can see that for a 480VAC drive, from 0-60 Hz. the volts per hertz ratio remains constant, so the torque remains constant. At a setting of 50% the frequency is 30 Hz. the voltage is 240 volts, the speed is 50% of the nameplate value. The torque is unchanged, so the horsepower output is 50% of the nameplate rating. This means that as the speed increases from 0 to nameplate rated speed the motor is operating as a constant torque motor, and the horsepower goes from 0-100% of nameplate value.
After we pass the input voltage rating, the horsepower remains the same because the voltage remains the same, speed increases because the frequency increases, therefore the torque is reduced. At 90 Hz. the torque is only 66% of what it was at 60 hz (60/90). So the motor, above nameplate rpm, acts as a constant horsepower, variable torque motor.

Practical application time: The customer no longer needs to operate his machine at various speeds but instead would like to operate at a speed equal to 50% of nameplate. The motor was rated at 25 HP, 3600 rpm, 480 volts, 3 phase, 60 Hz. What motor does he need to accomplish 50% speed without over loading?

His constant frequency input voltage at 60 Hz is still 480 volts 3 phase. The speed at 50% would require a 4 pole motor (i.e. 1800 rpm), but his horsepower requirement is now only 50% or 12.5 HP. He would need a 15 HP motor, assuming a constant torque type of load. If he is pumping liquids with a centrifugal pump or moving air with a fan his load would be a variable torque load, requiring only a 7.5 HP motor.

Shop question: A customer calls and wants to know if you can rewind his 380 volt 50 Hz. 1500 rpm motor to operate on 480 volt 60 hz. What do you tell him?

You cautiously tell him that, depending on his load, he can probably use the motor as is. The volts per hertz ratio of 380/50 and 460/60 is still virtually the same. The one precaution would be to keep in mind that there will be an increase in speed from 1500 to 1800. The motor will need to produce 20% more horsepower for constant torque loads. If the motor was not fully loaded to start with, it will probably be okay. If the load is a variable torque load (i.e. pump or fan) it will probably not be sufficient to pull the load at the higher speed. Yes, the motor can be rewound to the new frequency and the customer will be very happy he doesn’t have to import a motor.