AC Drives – – AC Variable Frequency Drives (VFD)

  • Do you have ac motors in your facility?

  • Do these motors need a variable output?

  • Do you use mechanical means to vary the output?

In the United States 50% of the total electrical energy generated is consumed by rotatingVariable Frequency Drive equipment. 65% of this total is consumed by centrifugal or flow related applications such as fans, blowers, compressors, and pumps according to current estimates. Variable speed drive technology offers a cost-effective method to match driver speed to load demands and represents a state-of-the-art opportunity to reduce operating costs and improve overall productivity.

What is a Variable Frequency Drive?

You can divide the world of electronic motor drives into two categories: AC and DC, a motor drive controls the speed, torque, direction and resulting horsepower of a motor. AC drives control AC induction motors. DC drives typically controls a shunt wound DC motor, which has separate armature and field circuits.

Motor drives are also known as DC Drives, AC Drives, SCR Drives (Silicon Controlled Rectifier Drive), Variable Frequency Drives, Variable Speed Drives, Adjustable Speed Drives, Electric Motor Drives, Electronic Motor Drives, AC Motor Controllers, AC Inverters, Pulse Width Modulation (PWM) Drives and a few other names.

What is a VFD?

You can divide the world of electronic motor drives into two categories: AC and DC. A motor drive controls the speed, torque, direction and resulting horsepower of a motor. A DC drive typically controls a shunt wound DC motor, which has separate armature and field circuits. AC drives control AC induction motors, and-like their DC counterparts-control speed, torque, and horsepower.

Application As An Example

Let’s take a brief look at a drive application. In Fig. 1, you can see a simple application with a fixed speed fan using a motor starter. You could replace the 3-phase motor starter with Variable Frequency Drive (VFD) to operate the fan at variable speed. Since you can operate the fan at any speed below its maximum, you can vary airflow by controlling the motor speed instead of the air outlet damper.

Figure 1, Fixed Speed Fan Application

Figure 1, Fixed Speed Fan Application

A drive can control two main elements of a 3-phase induction motor: speed and torque. To understand how a drive controls these two elements, we will take a short review of AC induction motors. Fig. 2 shows the construction of an induction motor. The two basic parts of the motor, the rotor and stator, work through magnetic interaction. A motor contains pole pairs. These are iron pieces in the stator, wound in a specific pattern to provide a north to south magnetic field.

Figure 2, Basic Induction Motor Construction

Figure 2, Basic Induction Motor Construction

Figure 3, Operating Principles of Induction Motor

Figure 3, Operating Principles of Induction Motor

With one pole pair isolated in a motor, the rotor (shaft) rotates at a specific speed: the base speed. The number of poles and the frequency applied determine this speed (Fig. 4). This formula includes an effect called “slip.” Slip is the difference between the rotor speed and the rotating magnetic field in the stator. When a magnetic field passes through the conductors of the rotor, the rotor takes on magnetic fields of its own. These rotor magnetic fields will try to catch up to the rotating fields of the stator. However, it never does — this difference is slip. Think of slip as the distance between the greyhounds and the hare they are chasing around the track. As long as they don’t catch up to the hare, they will continue to revolve around the track. Slip is what allows a motor to turn.

Motor Slip:

Shaft Speed =

120 X F
P

– Slip
Slip for NEMA B Motor = 3 to 5% of Base Speed which is 1800 RPM at Full Load
F = Frequency applied to the motor
P = Number of motor poles
Example:

Shaft Speed =

120 X 60 Hz
4

– Slip

Figure 4, Induction Motor Slip Calculation

We can conveniently adjust the speed of a motor by changing the frequency applied to the motor. You could adjust motor speed by adjusting the number of poles, but this is a physical change to the motor. It would require rewinding, and result in a step change to the speed. So, for convenience, cost-efficiency, and precision, we change the frequency. Fig. 5 shows the torque-developing characteristic of every motor: the Volts per Hertz ratio (V/Hz). We change this ratio to change motor torque. An induction motor connected to a 460V, 60 Hz source has a ratio of 7.67. As long as this ratio stays in proportion, the motor will develop rated torque. A drive provides many different frequency outputs. At any given frequency output of the drive, you get a new torque curve.

Figure 5, Volts/Hertz Ratio

Which Drive is Right for your Application?

The AC Variable Frequency Drive market is huge, due to the great advances in highly reliable AC power electronics and high power microprocessors. The Flux Vector AC Drive for example, can compete on an application basis that DC Drives would in earlier years be required.

Variable Speed DC Drive.Even though AC Variable Frequency Drives are readily replacing DC Drives and motors, there is still a large base of DC equipment that has to be supported by experienced technicians. Joliet Technologies believes that by not abandoning the DC market we can better serve our customer base demands. For example, older machines with multiple exciting DC Analog drives and motors may be a great candidate for retrofitting newer digital DC drive equipment. Saving cost and down time.

If your needs are much simpler and speed control is not part of the work scope. Joliet Technologies can provide Custom AC Drive Controllers consisting of contactors, starters, relays, pushbuttons and PLCs. Our engineers can provide all PLC and Peripheral Programming needs. Panels can be designed by another OEM or we can help with engineering to specifications provided.

DC Variable Speed Drive
PROSCONS
Torque control and limitingBrush life commutation maintenance.
Wide speed range (under 5000 RPM)Motor high speed limit (high speeds use expensive gear boxes)
Trimming capabilitiesExpensive large HP motors
Constant HP capabilitiesPower system harmonics
True 4-quadrant regenerative energy conversionNA
Few distance limitationsNA
Cost (especially single phase sizes)NA
AC Variable Frequency Drive
PROSCONS
Improved efficiencyDistance limitations
Low motor maintenanceLarge system designs (high HP drive foot prints)
Tight controlCost (encoder feedback)
Digital controlExpensive regenerative braking
High speed capabilitiesPower system harmonics
Reversing capability without extra power elementsNA

How to Maintain a VFD

Do you know how to maintain a VFD? Doing so is easier than you might think. By integrating some simple, logical steps into your preventative maintenance program, you can ensure your drives provide many years of trouble-free service.

How To Maintain a VFD

Do you know how to maintain Variable Frequency Drives (VFDs)? Doing so is easier than you might think. By integrating some simple, logical steps into your preventative maintenance program, you can ensure your drives provide many years of trouble-free service. Before looking at those steps, let’s quickly review what a VFD is and how it works.

A Quick Overview

A VFD controls the speed, torque and direction of an AC Induction motor. It takes fixed voltage and frequency AC input and converts it to a variable voltage and frequency AC output. See Training Note “What is a VFD?” for a more detailed description of VFD concepts and operating principles. In very small VFDs, a single power pack unit may contain the converter and inverter.

Fairly involved control circuitry coordinates the switching of power devices, typically through a control board that dictates the firing of power components in the proper sequence. A microprocessor or Digital Signal Processor (DSP) meets all the internal logic and decision requirements.

From this description, you can see a VFD is basically a computer and power supply. And the same safety and equipment precautions you’d apply to a computer and to a power supply apply here. VFD maintenance requirements fall into three basic categories:

  • keep it clean;
  • keep it dry; and
  • keep the connections tight.

Let’s look at each of these.

Keep it Clean

Most VFDs fall into the NEMA 1 category (side vents for cooling airflow) or NEMA 12 category (sealed, dust-tight enclosure). Drives that fall in the NEMA 1 category are susceptible to dust contamination. Dust on VFD hardware can cause a lack of airflow, resulting in diminished performance from heat sinks and circulating fans (Photo 1).

Photo 1, Fan Injecting Dust into Drive Enclosure

Dust on an electronic device can cause malfunction or even failure. Dust absorbs moisture, which also contributes to failure. Periodically spraying air through the heat sink fan is a good PM measure. Discharging compressed air into a VFD is a viable option in some environments, but typical plant air contains oil and water. To use compressed air for cooling, you must use air  that is oil-free and dry or you are likely to do more harm than good. That requires a specialized, dedicated, and expensive air supply. And you still run the risk of generating electrostatic charges (ESD).

A non-static generating spray or a reverse-operated ESD vacuum will reduce static build-up. Common plastics are prime generators of static electricity. The material in ESD vacuum cases  and fans is a special, non-static generating plastic. These vacuums, and cans of non-static  generating compressed air, are available through companies that specialize in static control equipment.

Keep it Dry

In Photo 2 you can see what happened to a control board periodically subjected to a moist environment. Initially, this VFD was wall-mounted in a clean, dry area of a mechanical room and moisture was not a problem. However, as is often the case, a well-meaning modification led to problems.

In this example, an area of the building required a dehumidifier close to the mechanical room.  Since wall space was available above the VFD, this is where the dehumidifier went. Unfortunately, the VFD was a NEMA 1 enclosure style (side vents and no seal around the cover). The obvious result was water dripping from the dehumidifier into the drive. In six months, the VFD accumulated enough water to produce circuit board corrosion.

Photo 2, Corrosion on Board Traces Caused by Moisture

What about condensation? Some VFD manufacturers included a type of “condensation protection” on earlier product versions. When the mercury dipped below 32 degrees Fahrenheit, the software logic would not allow the drive to start. VFDs seldom offer this protection today. If you operate the VFD all day every day, the normal radiant heat from the heatsink should prevent condensation. Unless the unit is in continuous operation, use a NEMA 12 enclosure and thermostatically controlled space heater if you locate it where condensation is likely.

Keep Connections Tight

While this sounds basic, checking connections is a step many people miss or do incorrectly – and the requirement applies even in clean rooms. Heat cycles and mechanical vibration can lead to sub-standard connections, as can standard PM practices. Retorquing screws is not a good idea, and further tightening an already tight connection can ruin the connection (see Sidebar).

Bad connections eventually lead to arcing. Arcing at the VFD input could result in nuisance over voltage faults, clearing of input fuses, or damage to protective components. Arcing at the VFD output could result in over-current faults, or even damage to the power components. Photos 3 and 4 show what can happen.

Loose control wiring connections can cause erratic operation. For example, a loose START/STOP signal wire can cause uncontrollable VFD stops. A loose speed reference wire can cause the drive speed to fluctuate, resulting in scrap, machine damage, or personnel injury.

Photo 3, Arcing Caused by Loose Input Contacts

Photo 4, Arcing Caused by Loose Output Contacts

Re-torquing – A Screwy Practice
Although “re-torquing” as a way of checking tightness is common in many PM procedures, it violates basic mechanical principles and does more harm than good. A screw has maximum clamping power at a torque value specific to its size, shape, and composition. Exceeding that torque value permanently reduces the clamping power of that screw by reducing its elasticity and deforming it. Loosening and then re-torquing still reduces elasticity, which still means a loss of clamping power. Doing this to a lock washer results in a permanent 50% loss. What should you do? Use an infrared thermometer to note hot connections. Check their torque. If they have merely worked loose, you can try retightening them. Note which screws were loose, and be sure to give them an IR check at the next PM cycle. If they are loose again, replace them. Finally, don’t forget the “tug test.” This checks crimps, as well as screw connections. Don’t do this with the drive online with the process, though, or you may cause some very expensive process disturbances.

Additional Steps

  1. As part of a mechanical inspection procedure, don’t overlook internal VFD components. Check circulating fans for signs of bearing failure or foreign objects – usually indicated by unusual noise or shafts that appear wobbly.
  2. Inspect DC bus capacitors for bulging and leakage. Either could be a sign of component stress or electrical misuse. Photos 5 and 6 show fan and capacitor stress problems.

    Photo 5, Foreign Object in Fan

  3. Take voltage measurements while the VFD is in operation. Fluctuations in DC bus voltage measurements can indicate degradation of DC bus capacitors. One function of the capacitor bank is to act as a filter section (smoothing out any AC ripple voltage on the Bus). Abnormal AC voltage on the DC bus indicates the capacitors are headed for trouble.Most VFD manufacturers have a special terminal block for this type of measurement and also for connection of the dynamic braking resistors. Measurements more than 4VAC may indicate a capacitor filtering problem or a possible problem with the diode bridge converter section (ahead of the bus). If you have such voltage levels, consult the VFD manufacturer before taking further action.With the VFD in START and at zero speed, you should read output voltage of 40VAC phase-to-phase or less. If you read more than this, you may have transistor leakage. At zero speed, the power components should not be operating. If your readings are 60VAC or more, you can expect power component failure.
  4. What about spare VFDs? Store them in a clean, dry environment, with no condensation allowed. Place this unit in your PM system so you know to power it up every 6 months to keep the DC bus capacitors at their peak performance capability. Otherwise, their charging ability will significantly diminish. A capacitor is much like a battery-it needs to go into service soon after purchase or suffer a loss of usable life.

    Photo 6, Capacitor Failure

  5. Regularly monitor heat sink temperatures. Most VFD manufacturers make this task easy by including a direct temperature readout on the Keypad or display. Verify where this readout is, and make checking it part of a weekly or monthly review of VFD operation. You wouldn’t place your laptop computer outside, on the roof of a building or in direct sunlight, where temperatures could reach 115 degrees Fahrenheit or as low as -10 degrees Fahrenheit. A VFD, which is basically a computer with a power supply, needs the same consideration. Some VFD manufacturers advertise 200,000 hours-almost 23 years-of Mean Time Between Failures (MTBF). Such impressive performance is easy to obtain, if you follow these simple procedures.