Comparison of Motor Speed Control Methods

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Why motor speed control?

It is often desirable to control the motor speed, usually for reasons process control for such variables as flow or pressure. Such applications as fans and pumps often have varying output requirements, and control of the motor speed is more efficient than mechanically limiting the process output with such devices as throttling valves or dampers.

The reason for this is due to the fact that for centrifugally-based processes (such as fans and centrifugally-based pumps), the following relationships exist:

Torque = RPM2

Power = RPM3

So, for these types of processes the torque required to turn them is proportional to the square of the speed.

But, the power required to turn them is proportional to the cube of the speed, and this is what makes motor speed control economically attractive. To further this argument, consider the energy wasted when mechanical means such as the throttling valves or dampers are used to control a process which is being driven from a motor running at full speed.

It is clear that motor speed control can be used to save energy by reducing wasted energy used to mechanically control the process.

Adjustable-speed drives (ASDs)

By far the most commonly-used AC motor control method is the use adjustable-speed drives. In most commercial and industrial environments these have supplanted virtually every other motor speed control method.

An adjustable-speed drive works on the principle of varying the frequency to vary the speed of the motor. Recall that from equations above the synchronous speed of a motor is a function of both the system frequency and the number of poles of the motor. By varying the frequency, the motor speed may be varied so long as the motor is equipped to dissipate the heat at reduced speeds.

Unlike soft-starting, specialized definite-purpose inverter-rated motor designs are preferred since reduced-speed operation can cause thermal issues and overspeed operation can result in safety issues.

Further, pulse-width modulated (PWM) drive outputs can cause repetitive voltage overshoots referred to as ringing, which can reduce the life expectancy of a general-purpose motor. The motor manufacturer should be consulted before applying a general-purpose motor in an adjustable-speed drive application.

Table 1- Motor starting methods summary

Across-the -LineSimple, Cost-EffectiveHigh Current Inrush
High Starting Torque
Abrupt Start
Reduced-voltage autotransformerHigh output torque vs. starting current.
Some Flexibility in starting characteristics due adjustable taps on autotransformers
Limited duty cycle
Large equipment size due to autotransformers
Reduced-Voltage Resistor or ReactorHigh output torque vs. starting currentLimited duty cycle
Limited flexibility in starting characteristics
Higher inrush current than with reduced-voltage autotransformer
Large equipment size due to resistors/reactors
Wye-DeltaRelatively low inrush current
Relatively simple starter construction
Good for long acceleration times
Relatively low output torque vs. starting current
Limited flexibility in starting characteristics
Requires special motor construction
Part-WindingRelatively Simple starter constructionRelatively low output torque vs. starting current
Not suitable for frequent starts
Requires special motor construction
Solid-state soft starterSmooth Acceleration
Low inrush current
High flexibility in starting characteristics
Typically offers deceleration control also
Typically integrates with industrial automation
Relatively Expensive
Sensitive to power quality
Heat dissipation and ambient temperature are a concern
Rotor ResistanceSmooth acceleration available
Good flexibility in starting characteristics
Can be used for speed control also
Complicated controller design
Requires expensive wound-rotor motor construction
Speed Drive
Smooth Acceleration
Low inrush current
High flexibility in starting characteristics
Offers deceleration and speed control also
Typically integrates with industrial automation
Cost-prohibitive unless speed control is required also
Sensitive to power quality
Heat dissipation and ambient temperature are a concern
Continuous harmonic currents can create power quality issues

Various designs exist for adjustable-speed drives, however for low voltage drives the most prevalent is the voltage-source pulse-width modulated design. As its name implies, the output is pulse-width modulated to reduce the output harmonic and noise content.

The AC input to the drive is typically a diode rectifier. A simplified circuit topology for al voltage-source PWM drive is given in Figure 1 below.

Voltage-source PWM adjustable-speed drive: simplified circuit topology for low voltage implementation
Figure 1 – Voltage-source PWM adjustable-speed drive: simplified circuit topology for low voltage implementation

The output stage for the circuit in Figure 1 consists of Insulated-Gate Bipolar Transistors (IGBT’s), which are commonly used in low voltage PWM adjustable-speed drives instead of SCR’s due to their superior switching rate capability.

Adjustable speed drives offer superior speed control for motors through 10,000 hp, depending upon the system voltage. They usually incorporate protection for the motor as well, allowing the omission of separate motor protective relays if desired.

Due to the high switching frequencies involved and their interaction with the cable capacitance, the length of the cable runs between the output of the drive and the motor are limited, and, as mentioned above for soft-starters, power factor correction capacitors and surge capacitors should not be used at the output of an adjustable speed drive.

Also due to the high switching frequencies, common-mode noise on the grounding conductors can be an issue when these drives are employed.

AC Variable Speed Drive and IE2 Motor Kit - 1.5kW (2.0HP) 230V Single Phase
On the incoming line, adjustable speed drives produce harmonics which must be taken into account in the over-all system design. This topic is addressed in a later section of this guide. Adjustable speed drives, like soft-starters, are microprocessor-based devices. Therefore, they can interface with the automation infrastructure of a facility.

With the exception of a few isolated cases, for most industrial and commercial facilities adjustable speed drives are the speed control of choice for AC motors.

Older methods 

Various other methods exist for AC motor speed control. A few of these are:

  • Rotor-resistance speed control – similar to rotor-resistance starting, this method consists of varying the effective resistance in the rotor of a wound-rotor induction motor to vary the speed. Variants of this method include rotor power recovery systems using a second machine or an auxiliary solid-state rectifier and converter.
  • Multi-speed motor – This type of motor is typically a squirrel-cage motor which has up to four fixed speeds.
  • Primary voltage adjustment using saturable reactors – This method is only applicable to NEMA Design D motors and offers a very narrow range of speed control.

Because of the limitations of these methods and the fact that they do not fit a wide range of motors, the adjustable speed drive is typically the solution of choice for most commercial and industrial facilities.


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