Variable frequency drives (VFDs) and electric motors are strange companions: The VFD is a static device, delicate, intolerant of wide variations in environmental conditions; extremely adjustable and controllable by microprocessors; capable of being monitored and controlled from remote locations; and a product of modern electronic engineering and precision—the beauty.
The motor is a rotating machine, robust and capable of wide variations in environmental conditions; not easily controlled unless assisted by external devices; and a product of nearly two centuries of electrical and mechanical engineering—the beast.
Yet, the two work together successfully to provide a means of speed control of rotating machinery. But before it can happen, there are many issues of compatibility that engineers must carefully evaluate and resolve.
Matching nameplate ratings
The first issue is matching the nameplate ratings. The author has encountered cases where incorrect matching led to incompatibility. Manufacturers nominally rate the VFD in horsepower, but this is only an approximate guide to VFD selection. The actual ratings to be determined are input and output voltage, current, power factor, and frequency.
In one such case at a manufacturing plant, the engineer specified a 25-hp, 480-V, 900-rpm, 3-phase, 60-Hz electric motor to drive a pump at variable speeds of 90 to 900 rpm. The specified VFD had the following ratings:
• Input: 480 V, 3-phase, 60 Hz, 32 amp
• Output: 0 to 460 V, 3-phase, 0 to 66 Hz, 0 to 32 amp.
Facility engineers procured the correct VFD. However, a 25-hp, 480-V, 3-phase, 60-Hz,1,800-rpm motor was available in the warehouse. Believing that the VFD could adjust the motor speed from 90 to 900 rpm, the engineers coupled this motor to the pump. When they started the system, the pump could not be brought up to its full speed. The VFD tripped out at approximately 400 rpm on overload.
At first it was thought that debris in the pump piping caused the overload. The real reason for the tripping was the mismatch in motor and VFD ratings. For an 1,800-rpm, 60-Hz motor to run at 900 rpm, the output frequency of the VFD must be 30 Hz. Because all VFDs are programmed to maintain a constant voltage-to-frequency ratio—the V/f ratio typically is 7.66 for 460-V motors—the output voltage of the VFD must be 230 V. The pump demands a brake horsepower of 25 from the motor at 900 rpm. To produce 25 hp output at 230 V, the motor input current should be 64 amps instead of the rated 32 amps. The VFD input current also should be about 64 amps. The VFD would trip out on overload.
Therefore, a rule to follow is to make sure that at the rated speed of the driven equipment, the nominal ratings of the VFD should be the same as those of the motor.
Harmonic heating
Harmonics in motor currents produce additional heating due to induced high-frequency currents in the rotor bars. Does the motor need to be de-rated when driven by a VFD? In other words, can the motor supply its rated horsepower without overheating? In older current source inverter (CSI) VFDs, which have a large inductor in the dc link, the motor current waveform was a “double-hump” (see Figure 1), rich in the 5th, 7th, and other harmonics. It was common practice to de-rate the motor by 5% to 10%. However, modern pulse-width-modulated (PWM) drives produce almost a sinusoidal motor current except for a small high-frequency component, typically 5 to 10 kHz, because of the PWM carrier switching frequency (see Figure 2). No de-rating is required. To be safe, the VFD manufacturers recommend motors with 1.15 service factor.
Heating due to harmonic currents is not the only consideration when selecting motors to be driven by VFDs. Standard motors that are not inverter duty motors generally are unsuitable because of considerations such as insulation, cooling at low speeds, bearing currents, and voltage spikes.
Variable or constant torque
Manufacturers offer two types of VFDs: variable torque and constant torque. The two are basically the same except that theconstant-torque has a slightly larger kVA rating. The selection depends upon the load driven.
Centrifugal pumps and blowers are variable-torque loads: The torque demanded by the load is reduced as the speed is reduced. In fact, the torque demanded is proportional to the square of the speed. The following proportional relations hold:
Horsepower (hp) a (rpm)3
Torque (ft-lb) a (rpm)2
Flow (gpm) a (rpm)
Therefore, if the pump speed is reduced to 50% of the rated speed, the required torque is reduced to 25% of the rated torque. The torque produced by the motor that drives the pump or fan is proportional to the product of the airgap flux and the stator winding current. The VFD that powers the motor keeps the airgap flux constant at all speeds (constant V/f ratio). Therefore, the motor current also reduces to 25% of the rated current. The motor I2R losses reduce to 6.25% of the losses at the rated current. Motor and VFD would have no problem dissipating the heat.
It’s a different story with constant-torque loads, such as rotary and screw compressors, elevator drives, hoists and cranes, and reciprocating pumps. The motor torque, which must be the same as the load torque during steady-state operation, is the same at all speeds, and the motor current remains the same at all speeds. Motor efficiency is lower and heat dissipation is a problem because of the reduced airflow from the shaft-mounted fan.
The problem is more severe in totally enclosed, fan-cooled motors used in Class 1, Division 2 hazardous locations, particularly when the motor operates at reduced speed for extended periods of time. In such cases, an independently driven blower would be necessary to provide adequate airflow.
Flux-vector drive
Many applications demand unusual operating conditions of the electric motor. For example, an elevator or a crane drive requires operation in both directions. In addition, the motor is required to produce a positive torque while rotating in the negative direction and a negative torque while rotating in the positive direction, required for braking when the load is being raised or lowered. Such drives are said to operate in all four quadrants of the speed-torque plane (see Figure 3).
In some other applications the motor is required to produce the rated torque at zero speed, such as a marine winch motor. An ac induction motor cannot meet these requirements without the help of VFDs, but dc motors can. But dc motors are expensive and require constant maintenance because of the commutator and brushes. A flux-vector drive is used in such applications.
From a control point of view, conventional VFDs for pumps and fans provide open-loop control; the VFD supplies a certain voltage at a certain frequency, and the motor rotates at a speed determined by its characteristics. Speed adjustment is either manual or by a remote signal. In a flux-vector drive, there is a continuous feedback of the motor speed and torque to the VFD. The VFD adjusts the voltage and frequency to produce the required operation. The flux-vector drive gives the ac motor the same capabilities as the dc motor. The direction of rotation can be reversed without actually switching the phases. Earlier flux-vector drives needed feedback from a shaft position sensor. Modern drives are sensor-less drives relying on the current, voltage, and speed measurement to compute the torque.
Voltage doubling, critical cable length, and dv/dt
PWM drives produce an output voltage in the form of high-frequency rectangular pulses of the same magnitude. The frequency of the pulses (known as the PWM carrier frequency) is in the range of 4 to 16 kHz. The width of each pulse is modulated such that the instantaneous average value is a sine wave. Figure 4 shows a line-to-line output voltage of a PWM VFD. For illustration, only seven pulses in one half-cycle are shown. Actually, for a typical carrier frequency of 4 kHz, there would be 33 pulses in each half-cycle. The magnitude of each pulse is equal to the dc link voltage of the VFD (typically 648 V in 480-V 6-pulse drives). The pulses are rectangular with a rate of rise of approximately 5,000 V/microsec. Higher frequency in the range of 4 to 16 kHz makes the motor current more sinusoidal, reduces the noise in the motor, and reduces the switching losses in the VFD transistors, thus increasing the efficiency of the VFD. However, the high-frequency pulses create greater stress in the motor insulation. Read More
