Variable Frequency Control of Industrial Motors

GOALS of this DISCUSSION:

• Explain how the speed of an induction motor can be changed with a change of frequency.
• Discuss different methods of controlling frequency.
• Discuss precautions that must be taken when the frequency is lowered.
• Define the terms ramping and volts per hertz.

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The speed of a three-phase induction motor can be con trolled by either changing the number of stator poles per phase, as is the case with consequent pole motors, or by changing the frequency of the applied voltage.

Both methods will produce a change in the synchronous speed of the rotating magnetic field. The chart shown in Ill. 1 indicates that when the frequency is changed, a corresponding change in synchronous speed results.

Ill. 1 Synchronous speed is determined by the number of stator poles per phase and the frequency.

Ill. 2 An alternator controls the speed of several induction motors.

Changing frequency, however, causes a corresponding change in the inductive reactance of the windings (XL = 2pfL). Since a decrease in frequency produces a decrease in inductive reactance, the amount of voltage applied to the motor must be reduced in proportion to the decrease of frequency in order to prevent overheating the windings due to excessive current. Any type of variable frequency control must also adjust the output voltage with a change in frequency. There are two basic methods of achieving variable frequency control: alternator and solid state.

Alternator Control

Alternators are often used to control the speed of several induction motors that require the same change in speed, such as motors on a conveyer line ( Ill. 2).

The alternator is turned by a direct current motor or an AC motor coupled to an eddy current clutch. The output frequency of the alternator is determined by the speed of the rotor. The output voltage of the alternator is determined by the amount of DC excitation current applied to the rotor. Since the output voltage must change with a change of frequency, a variable voltage DC supply is used to provide excitation current. Most controls of this type employ some method of sensing alternator speed and make automatic adjustments to the excitation current.

Solid-State Control

Most variable frequency drives operate by first changing the AC voltage into DC and then changing it back to AC at the desired frequency. A couple of variable frequency drives are shown in Ill. 3A and Ill. 3B.

There are several methods used to change the DC volt age back into AC. The method employed is generally determined by the manufacturer, age of the equipment, and the size motor the drive must control. Variable frequency drives intended to control the speed of motors up to 500 horsepower generally use transistors. In the circuit shown in Ill. 4, a three-phase bridge rectifier changes the alternating current into direct current. The bridge rectifier uses six SCRs (Silicon Controlled Rectifiers). The SCRs permit the output voltage of the rectifier to be controlled. As the frequency decreases, the SCRs fire later in the cycle and lower the output volt age to the transistors. A choke coil and capacitor bank are used to filter the output voltage before transistorsQ1 through Q6 change the DC voltage back into AC. An electronic control unit's connected to the bases of transistors Q1 throughQ6. The control unit converts the DC voltage back into three-phase alternating current by turning transistors on or off at the proper time and in the proper sequence. Assume, for example, that transistors Q1 and Q4 are switched on at the same time. This permits statorwindingT1 to be connected to a positive volt age and T2 to be connected to a negative voltage. Cur rent can flow throughQ4 to T2, through the motor stator winding and through T1 to Q1.

Now assume that transistors Q1 andQ4 are switched off and transistors Q3 and Q6 are switched on. Current will now flow through Q6 to stator winding T3, through the motor to T2, and through Q3 to the positive of the power supply.

Since the transistors are turned completely on or completely off, the waveform produced is a square wave instead of a sine wave ( Ill. 5). Induction motors will operate on a square wave without a great deal of problem. Some manufacturers design units that will produce a stepped waveform as shown in Ill. 6. The stepped waveform is used because it more closely approximates a sine wave.

Some Related Problems

The circuit illustrated in Ill. 4 employs the use of SCRs in the power supply and junction transistors in the output stage. SCR power supplies control the output voltage by chopping the incoming waveform.

This can cause harmonics on the line that cause over heating of transformers and motors, and can cause fuses to blow and circuit breakers to trip. When bipolar junction transistors are employed as switches, they are generally driven into saturation by supplying them with an excessive amount of base-emitter current. Saturating the transistor causes the collector-emitter voltage to drop to between 0.04 and 0.03 volts. This small voltage drop allows the transistor to control large amounts of current without being destroyed. When a junction transistor is driven into saturation, however, it cannot recover or turn off as quickly as normal. This greatly limits the frequency response of the transistor.

Ill. 3A Inside of a variable frequency AC motor drive.

Ill. 3B 2 Hp. variable frequency drive

IGBTs

Many transistor controlled variable frequency drives now employ a special type of transistor called an Insulated Gate Bipolar Transistor (IGBT). IGBTs have an insulated gate very similar to some types of field effect transistors (FETs). Since the gate is insulated, it has very high impedance. The IGBT is a voltage controlled device, not a current controlled device. This gives it the ability to turn off very quickly. IGBTs can be driven into saturation to provide a very low voltage drop between emitter and collector, but they don't suffer from the slow recovery time of common junction transistors. The schematic symbol for an IGBT is shown in Ill. 7.

Ill. 4 Solid-state variable frequency control using junction transistors.

Ill. 5 Square wave.

Ill. 6 Stepped wave.

Ill. 7 Schematic symbol for an Insulated Gate Bipolar Transistor.

Drives using IGBTs generally use diodes, not SCRs, to rectify the AC voltage into DC ( Ill. 8). The three-phase rectifier supplies a constant DC voltage to the transistors. The output voltage to the motor is con trolled by pulse width modulation (PWM). PWM is accomplished by turning the transistor on and off several times during each half cycle ( Ill. 9). The output voltage is an average of the peak or maximum voltage and the amount of time the transistor is turned on or off.

Assume that 480 volts three-phase AC is rectified to DC and filtered. The DC voltage applied to the IGBTs is approximately 630 volts. The output voltage to the motor is controlled by the switching rate of the transistors.

Assume that the transistor is on for 10 microseconds and off for 20 microseconds. In this example, the transistor is on for one-third of the time and off for two thirds of the time. The voltage applied to the motor would be 210 volts (630/3). The speed at which IGBTs can operate permits pulse width modulation to produce a stepped wave that's very similar to a standard sine wave ( Ill. 10).

Ill. 8 Variable frequency drives using IGBTs generally use diodes in the rectifier instead of SCRs.

Ill. 9 Pulse width modulation is accomplished by turning the voltage on and off several times during each half cycle.

Ill. 10 The speed of the IGBTs can produce a stepped wave that's similar to a sine wave.

Advantages and Disadvantages of IGBT Drives

A great advantage of drives using IGBTs is the fact that SCRs are generally not used in the power supply and this greatly reduces problems with line harmonics.

The greatest disadvantage is that the fast switching rate of the transistors can cause voltage spikes in the range of 1600 volts to be applied to the motor. These voltage spikes can destroy some motors. Line length from the drive to the motor is of great concern with drives using IGBTs. Short line lengths are preferred.

Inverter Rated Motors

Due to the problem of excessive voltage spikes caused by IGBT drives, some manufacturers produce a motor that's inverter rated. These motors are specifically designed to be operated by variable frequency drives.

They differ from standard motors in several ways:

1. Many inverter rated motors contain a separate blower to provide continuous cooling for the motor regard less of the speed. Many motors use a fan connected to the motor shaft to help draw air though the motor. When the motor speed is reduced, the fan cannot maintain sufficient air flow to cool the motor.

2. Inverter rated motors generally have insulating paper between the windings and the stator core ( Ill. 11). The high voltage spikes produce high currents that produce a strong magnetic field. This increased magnetic field causes the motor windings to move, because like magnetic fields repel each other. This movement can eventually cause the insulation to wear off the wire and produce a grounded motor winding.

3. Inverter-rated motors generally have phase paper added to the terminal leads. Phase paper is insulating paper added to the terminal leads that exit the motor. The high voltage spikes affect the beginning lead of a coil much more than the wire inside the coil. The coil is an inductor that naturally opposes a change of current. Most of the insulation stress caused by high voltage spikes occurs at the beginning of a winding.

4. The magnet wire used in the construction of the motor windings has a higher rated insulation than other motors.

5. The case size is larger than most three-phase motors. The case size is larger because of the added insulating paper between the windings and the stator core. Also, a larger case size helps cool the motor by providing a larger surface area for the dissipation of heat.

Ill. 11 Insulating paper is between the windings and the stator frame.

Variable Frequency Drives Using SCRs and GTOs

Ill. 12 Changing DC into AC using SCRs.

Variable frequency drives intended to control motors over 500 horsepower generally use SCRs or GTOs (gate turn off device). GTOs are similar to SCRs except that conduction through the GTO can be stopped by applying a negative voltage-negative with respect to the cathode-to the gate. SCRs and GTOs are thyristors and have the ability to handle a greater amount of cur rent than transistors. Thyristors are solid-state devices that exhibit only two states of operation: completely turned on or completely turned off. An example of a single-phase circuit used to convert DC voltage to AC voltage with SCRs is shown in Ill. 12. In this circuit, the SCRs are connected to a phase shift unit that controls the sequence and rate at which the SCRs are gated on. The circuit's constructed so that SCRs A and A’ are gated on at the same time and SCRs B and B’ are gated on at the same time. Inductors L1 and L2 are used for filtering and wave shaping. Diodes D1 through D4 are clamping diodes and are used to prevent the output voltage from becoming excessive. Capacitor C1 is used to turn one set of SCRs off when the other set is gated on. This capacitor must be a true AC capacitor because it will be charged to the alternate polarity each half cycle. In a converter intended to handle large amounts of power, capacitor C1 will be a bank of capacitors. To understand the operation of the circuit, assume that SCRs A and A’ are gated on at the same time. Current will flow through the circuit as shown in Ill. 13. Notice the direction of current flow through the load, and that capacitor C1 has been charged to the polarity shown.

When an SCR is gated on, it can only be turned off by permitting the current flow through the anode-cathode section to drop below a certain level, called the holding current level. As long as the current continues to flow through the anode-cathode, the SCR won't turn off.

Now assume that SCRs B and B’ are turned on. Because SCRs A and A’ are still turned on, two current paths now exist through the circuit. The positive charge on capacitor C1, however, causes the negative electrons to see an easier path. The current will rush to charge the capacitor to the opposite polarity, stopping the current flowing through SCRs A and A’, permitting them to turn off. The current now flows through SCRs B and B’ and charges the capacitor to the opposite polarity ( Ill. 14). Notice that the current now flows through the load in the opposite direction, which produces alternating current across the load.

To produce the next half cycle of AC current, SCRs A and A’ are gated on again. The positively charged side of the capacitor will now cause the current to stop flowing through SCRs B and B’, permitting them to turn off. The current again flows through the load in the direction indicated in Ill. 13. The frequency of the circuit's determined by the rate at which the SCRs are gated on. A variable frequency drive rated at 125 horsepower is shown in Ill. 15.

Ill. 13 Current flows through SCRs A and A’.

Ill. 14 Current flows through SCRs B and B’

Ill. 15 A 125 Hp. variable frequency AC motor Controller.

Features of Variable Frequency Control

Ill. 16 Most variable frequency drives provide current limit and speed regulation.

Although the primary purpose of a variable frequency drive is to provide speed control for an AC motor, most drives provide functions that other types of controls don't. Many variable frequency drives can provide the low speed torque characteristic that's so desirable in DC motors. It is this feature that permits AC squirrel cage motors to replace DC motors for many applications.

Many variable frequency drives also provide cur rent limit and automatic speed regulation for the motor.

Current limit's generally accomplished by connecting current transformers to the input of the drive and sensing the increase in current as load is added. Speed regulation is accomplished by sensing the speed of the motor and feeding this information back to the drive ( Ill. 16).

Another feature of variable frequency drives is acceleration and deceleration control, sometimes called ramping. Ramping is used to accelerate or decelerate a motor over some period of time. Ramping permits the motor to bring the load up to speed slowly as opposed to simply connecting the motor directly to the line.

Even if the speed control is set in the maximum position when the start button is pressed, ramping forces the motor to accelerate the load from zero to its maxi mum RPM over several seconds. This feature can be a real advantage for some types of loads, especially gear drive loads. In some controllers, the amount of acceleration and deceleration time can be adjusted by setting potentiometers on the main control board ( Ill. 17). Other controllers are completely digitally controlled and the acceleration and deceleration times are programmed into the computer memory.

Some other adjustments that can usually be set by changing potentiometers or programming the unit are as follows:

Current Limit: This control sets the maximum amount of current the drive is permitted to deliver to the motor.

Volts per Hertz: This sets the ratio by which the volt age increases as frequency increases or decreases as frequency decreases.

Maximum Hertz: This control sets the maximum speed of the motor. Most motors are intended to operate between 0 and 60 hertz, but some drives permit the output frequency to be set above 60 hertz, which would permit the motor to operate at higher than normal speed. The maximum hertz control can also be set to limit the output frequency to a value less than 60 hertz, which would limit the motor speed to a value less than normal.

Minimum Hertz: This sets the minimum speed the motor is permitted to run.

Some variable frequency drives permit adjustment of current limit, maximum and minimum speed, ramping time, and so on, by adjustment of trim resistors located on the main control board. Other drives employ a microprocessor as the controller. The values of current limit, speed, ramping time, and so on, for these drives are programmed into the unit and are much easier to make and are generally more accurate than adjusting trim resistors. A programmable variable frequency drive is shown in Ill. 18.

Ill. 17 Some variable frequency drives permit setting to be made by making adjustments on a main control board.

QUIZ:

1. What is the synchronous speed of a six-pole motor operated with an applied voltage of 20 hertz?

2. Why is it necessary to reduce the voltage to a motor when the frequency is reduced?

3. If an alternator is used to provide variable frequency, how is the output voltage of the alternator controlled?

4. What solid-state device is generally used to produce variable frequency in drives designed to control motors up to 500 horsepower?

5. Why are SCRs used to construct a bridge rectifier in many solid-state variable frequency drives?

6. What is the main disadvantage of using SCRs in a variable frequency drive?

7. How are junction transistors driven into saturation, and what is the advantage of driving a transistor into saturation?

8. What is the disadvantage of driving a junction transistor into saturation?

9. What is the advantage of an IGBT over a junction transistor?

10. In variable frequency drives that employ IGBTs, how is the output voltage to the motor controlled?

11. What type of motor is generally used with IGBT drives?

12. What is the primary difference between a GTO and an SCR?

13. What is a thyristor?

14. After an SCR has been turned on, what must be done to permit it to turn off again?

15. What is meant by "ramping" and why is it used?

Ill. 18 Programmable variable frequency drives permit setting such as current limit, volts per Hz., max. and min. Hz., acceleration and deceleration to be programmed into the unit.