Selecting VSDs and AC motor converters--part 4

Control: speed, torque, accuracy

In most VSD applications, simple open-loop speed control without a high degree of accuracy is quite adequate. In these cases, the speed can be set manually or from a PLC and adjusted when required.

In most VSD applications, closed-loop control of the speed is achieved on the basis of a feedback loop from a process variable (PV), such as fluid flow rate in pumping systems.

In these systems, the speed does not need to be controlled very accurately because the control system continuously adjusts the speed to meet the process requirements.

But there are some applications where the speed needs to be accurately controlled. In these cases, the following should be considered:

• Does the motor, which has been selected according to the thermal considerations, provide sufficient speed accuracy? i.e. Is normal motor slip acceptable?

• It may be advisable to select a larger motor and converter in order to reduce the slip and improve the speed accuracy.

• It may also be necessary to use speed feed-back from the motor, E.g. a tachometer or digital speed encoder, to obtain accurate speed control. This is called closed-loop speed control?

Select correct size of motor, converter

Manufacturers of electric motors and frequency converters have evolved various methods for quickly selecting the size of motors and frequency converters for a particular machine load. The same basic procedure is used by most applications engineers. These days, applications selections are usually done on the basis of PC based software. However, it’s important for engineers to clearly understand the selection procedure.

One of the best procedures uses a simple Nomogram based on the load limit curves to make the basic selection of motor size. This procedure is described below.

The other factors are then checked to ensure that the optimum combination of motor and converter is selected.

The following selection principles are recommended:

1. First, the type and size of motor should be selected. The number of poles (basic speed) should be chosen so that the motor runs as much as possible at a speed slightly above the base speed of 50 Hz. This is desirable because:

• The thermal capacity of the motor improves when f = 50 Hz because of more efficient cooling at higher speeds.

• The converter commutation losses are at minimum when it’s operating in the field weakening range above 50 Hz.

• For a constant torque load, a larger speed range is obtained when the motor operates well in the field weakening range at the maximum speed. This means that the most efficient use is made of the torque/speed capability of the variable speed drive. This could mean cost savings in the form of a smaller motor and converter.

• Although many manufacturers claim that their converters can produce output frequencies of up to 400 Hz, these high frequencies are of little practical use except for very special (and unusual) applications. The construction of standard cage motors and the reduction of the peak torque capability in the field weakening zone, restrict their use at frequencies above 100 Hz. The maximum speed at which a standard squirrel cage motor can be run should always be checked with the manufacturer, particularly for larger 2-pole (3000 rev/m) motors of more than 200 kW. The fan noise produced by the motor also increases substantially as the speed of the motor increases.

• A comparison of the torque produced by a 4 pole and a 6 pole motor. This illustrates the higher torque capability of the 6 pole machine.

++++ Comparison of the thermal capacity limit curves for two 90 kW TEFC squirrel cage motors:

a) 90 kW 4 pole motor (1475 rev/min)

b) 90 kW 6 pole motor (985 rev/min)

2. The selection of an oversized motor just to be 'safe' is not usually advisable because it means that an oversized frequency converter must also be selected. Frequency converters, particularly the PWM-type, are designed for the highest peak current value, which is the sum of the fundamental and harmonic currents in the motor. The larger the motor, the larger the peak currents. To avoid this peak current exceeding the design limit, a converter should never be used with a size of motor larger than for which it’s specified.

Even when the larger motor is lightly loaded, its harmonic current peaks are high.

3. Once the motor has been selected, it’s reasonably easy to select the correct converter size from the manufacturer's catalogue. They are usually rated in terms of current (not kW) based on a specific voltage. This should be used as a guide only, because converters should always be selected on the basis of the maximum continuous motor current.

Although most catalogues are based on the standard IEC motor power ratings (kW), motors from different manufacturers have slightly different current ratings.

4. Although it seems obvious, the motor and converter should be specified for the power supply voltage and frequency to which the variable speed drive is to be connected. In most countries using IEC standards, the standard supply voltage is 380 volts ±6%, 50 Hz.

In U.S., this is 415 V ±6%, 50 Hz. In some applications where the size of the drive is very large, it’s often economical to use higher voltages to reduce the cost of cables. Other commonly used voltages are 500 V and 660 V. In recent years, AC converters are manufactured for use at 3.3 kV and 6.6 kV. Frequency converters are designed to produce the same output voltage as that of the supply, so both the motor and the converter should be specified for the same base voltage. Although the output frequency of the converter is variable, the input frequency (50 Hz or 60 Hz) should be clearly specified because this can have an effect on the design of inductive components.

Selection Procedures--Summary

To select the correct motor/converter combination, the following information must be available:

• • Voltage and frequency of the power supply (volts)
• • The breakaway or starting torque (newton meters)
• • The load torque (newton meters) and its dependence on speed.
• • Speed range of the variable speed drive (rev/min)
• • Acceleration requirements or 'ramp times'
• • The moments of inertia of the motor and load (kg-m^2)

1. Initial data for drive apps: The number of poles determines the synchronous speed of the motor and this is usually selected according to the maximum speed required by the application. Modern VVVF converters are available with output frequencies of up to 400 Hz, although, as pointed out above, there are few practical applications above 100 Hz.

Above-synchronous speeds are of particular advantage for constant torque loads, where the maximum speed should, ideally, be in the range of 50-100 Hz.

2. Select no. of poles for motor: This is not the case for pump and fan drives, where the load torque increases as the square of the speed. The optimum use of the motor's torque characteristics occurs when the motor speed is chosen so that the maximum speed of the drive occurs at 50 Hz.

3. Select motor pwr rating: Using the load torque requirements, the power rating of the motor can be selected from a motor manufacturer's catalogue using the formula:

kW 9550 (rev/m) (Nm)

Speed Torque = Power ×

However, the de-rating of the motor for harmonic heating, reduced cooling at lower speeds and reduced torque at higher speeds must be taken into account. It’s quick and convenient to use a motor selection nomogram, an example of which will be discussed at the training course. This nomogram makes allowance for the harmonic heating and reduced cooling of the motor when used with a VVVF converter.

The procedure for using the nomogram is as follows:

• In quadrant 1, first select the column corresponding to the number of poles (synchronous speed) of the selected motor.

• Then select the maximum speed in rev/min of the required speed range. The corresponding frequency can be read off the scale on the right side of quadrant if required.

• Trace horizontally into quadrant 2 up to the load limit curve. The per unit value of torque limit can be read off the scale at the top of the quadrant if required.

• From the intersection of the horizontal trace and the load limit curve, trace vertically downwards into quadrant 3 up to the line corresponding to the calculated load torque. The slope of this curve corresponds to the formula in 3 above.

• From the intersection of the vertical trace and torque line, trace horizontally left to the motor power scale corresponding to the chosen number of poles.

The intersection of the horizontal trace and the power scale gives the required motor power in kW.

• For a square-law torque load (pump or fan drive), select the standard motor corresponding to the motor power rating.

• For a constant torque load, repeat the above steps to determine the motor power for the minimum speed. Select the standard motor corresponding to the larger of the two power ratings.

4. Select best frequency converter

A converter with a rating suitable for the motor selected should then be selected from the manufacturer's catalogue. Converters are usually manufactured for power ratings that match the standard sizes of squirrel cage motors. Catalogues usually give the current rating as well as a check to ensure that the motor current is below that of the converter.

The following factors must be considered:

• • Supply voltage and frequency
• • Rated current of the motor
• • Duty type (variable torque or constant torque)

A converter is selected so that the rated current of the converter is higher than the rated current of the motor. Also, the type of converter should be suitable for the duty required.

Some manufacturers have different converters for the two duty types.

5. Final checks

The following final checks should be made:

• Is the continuous power rating of the motor (de-rated for altitude, temperature, harmonics, etc) greater than the continuous power requirements of the load?

• Is the starting torque capability of the variable speed drive high enough to exceed the breakaway torque of the load?

• If the VSD is operating in the over-synchronous speed area, is the motor torque capability at maximum speed adequate for the load torque?

• Is the speed accuracy adequate for the application?

Example selection calculations:

A variable speed drive application has been proposed for a crusher feed conveyer on a mineral processing plant. The required speed range is 600 rev/m to 1400 rev/min. The calculated power requirement of the load, reduced to the motor shaft, is 66 kW at 1400 rev/m. The breakaway torque is expected to be 110% of rated torque. The supply voltage is 415 volts, 50 Hz. Select the optimum size and rating of squirrel cage motor and converter for the most cost effective solution.

The load is a typical constant torque load type. From the previous equations, the constant load torque requirement across the speed range is:

Breakaway torque required:

Nm 495 = 450 1.1 B × = T

There are two alternative solutions for this application

• Consider a 4 pole motor with a rated speed of 1480 rev/min Using the motor selection nomogram and plotting the torque requirements for the minimum and maximum speeds, a 110 kW, 415 V, 4-pole motor should be selected based on the motor load capacity at the minimum speed of 600 r/min.

(A 75 kW motor would have been chosen on the basis of 1400 rev/m.) This motor frame size is 280 M and current rating 188 amps.

From the motor catalogue, the rated torque of this motor is 710 Nm. The VSD can deliver current equivalent to 150% torque at starting, so the ability to overcome the breakaway torque is generous.

Therefore, the recommended converter is a 110 kW, 415 volt, 220 amp unit.

The rating of the motor and the converter could be decreased if the motor was forced cooled by a separately powered fan.

• Investigate a 6 pole motor with a rated speed of 985 rev/min Using the motor selection nomogram and plotting the torque requirements for the minimum and maximum speeds, a 75 kW, 415 volt, 6-pole motor should be selected based on the motor load capacity at both the minimum speed of 600 rev/m and maximum speed of 1400 rev/m. The motor frame size is 280 M and current rating 135 amps.

From the motor catalogue, the rated torque of this motor is 727 Nm. The VSD can deliver current equivalent to 150% torque, so the ability to cover the breakaway torque at starting is adequate.

Therefore, the recommended converter is a 75 kW, 415 volt, 140 amp unit.

The second alternative is the most economical solution because, although the cost of both motor alternatives will be roughly the same (same frame sizes = approx same cost), the converter required for the second alternative will have a lower initial cost, because it has a lower current rating.