Motor drives

Electric motors use roughly 40% to 45% of the world's electricity, mostly in pumps, fans, compressors and conveyors. Most of these motors are induction motors that, when connected directly to the supply, run at nearly constant speed (ELE-370). Flow and pressure are then controlled by throttling valves and dampers, wasting much of the energy. A variable-frequency drive (VFD), also called an adjustable-speed drive, combines a rectifier, a DC link and an inverter to supply the motor with variable voltage and frequency, so the motor runs at the speed the process needs. Drives save large amounts of energy, provide soft starting and precise control, and are essential in electric vehicles, elevators, robotics and data center cooling systems. This lesson ties together the previous lessons by examining drive structure, control methods, energy savings, braking, and the side effects of fast-switching inverters on motors and the electrical system.

Drive structure

A typical low-voltage drive contains:

• A front end: usually a six-pulse diode rectifier with a DC choke or line reactor, or an active front end for regeneration and low harmonics (Lesson 3).
• A DC link: film or electrolytic capacitors that store energy and smooth the bus, with a precharge circuit that limits inrush when power is applied.
• An inverter: a three-phase IGBT (or increasingly SiC) bridge using space vector PWM at 2 kHz to 16 kHz (Lesson 7).
• Control: a microprocessor that measures currents and voltage, runs the motor control algorithm and protection, and communicates with building or process systems.
• Optional components: a braking chopper and resistor, input and output filters, and safety functions.

Speed control of induction motors

An induction motor's synchronous speed is set by the supply frequency and the number of poles:

and the rotor runs slightly slower, at slip \(s = (n_s - n)/n_s\), typically 1% to 4% at full load. Changing the frequency changes the speed. To keep the magnetic flux, and therefore the torque capability, constant, the voltage must change in proportion to frequency, because the air-gap flux is proportional to \(V/f\). This is constant volts-per-hertz (V/f) control, the simplest drive control method:

At low speeds, the voltage drop across the stator resistance becomes significant, so drives add a voltage boost to maintain torque. Above rated frequency, the voltage cannot rise further (the DC bus limits it), so the flux weakens: the motor can run faster but with torque falling roughly in inverse proportion to speed, giving constant power operation.

Vector control

V/f control is adequate for fans and pumps but responds slowly and cannot give full torque at zero speed. Field-oriented control (FOC), or vector control, transforms the three motor currents into a rotating reference frame aligned with the rotor flux, splitting them into a flux-producing component \(i_d\) and a torque-producing component \(i_q\). Controlling these separately makes an AC motor behave like a separately excited DC motor, with fast, precise torque control:

Vector control can use a speed encoder (closed loop) or estimate speed from currents and voltages (sensorless). Direct torque control (DTC) is an alternative that selects inverter states directly from torque and flux errors. Permanent magnet synchronous motors, used in electric vehicles, high-efficiency pumps and compressors, and some data center fans, always require vector-type control; they offer higher efficiency and power density than induction motors because the rotor has no current-carrying conductors.

Energy savings with fans and pumps

The largest energy savings from drives come in centrifugal fans and pumps, which follow the affinity laws: flow is proportional to speed, pressure to speed squared, and power to speed cubed:

Reducing speed to 80% cuts the ideal power to about 51%. By contrast, throttling a fixed-speed fan with a damper or a pump with a valve reduces flow while power falls only a little. In practice, systems with significant static pressure (for example, pumps lifting water to a height, or systems with fixed pressure setpoints) save less than the cube law predicts, so savings should be estimated from the actual system curve.

P_rated = 75.0
hours = {100: 1500, 80: 3000, 60: 3000, 40: 1260}
price = 0.12
eff_drive = 0.97


def damper_kw(flow):
    return P_rated * (0.55 + 0.45 * flow / 100)


def vfd_kw(flow):
    return P_rated * (flow / 100) ** 3 / eff_drive


e_damper = sum(damper_kw(q) * h for q, h in hours.items())
e_vfd = sum(vfd_kw(q) * h for q, h in hours.items())
for q in hours:
    print(f"flow {q:>3}%: damper {damper_kw(q):5.1f} kW   VFD {vfd_kw(q):5.1f} kW   speed {q}%  ({hours[q]} h/yr)")
print(f"annual energy: damper {e_damper / 1000:.0f} MWh, VFD {e_vfd / 1000:.0f} MWh, saving {(e_damper - e_vfd) / 1000:.0f} MWh = ${(e_damper - e_vfd) * price:,.0f} per year at ${price}/kWh")

flow 100%: damper  75.0 kW   VFD  77.3 kW   speed 100%  (1500 h/yr)
flow  80%: damper  68.2 kW   VFD  39.6 kW   speed 80%  (3000 h/yr)
flow  60%: damper  61.5 kW   VFD  16.7 kW   speed 60%  (3000 h/yr)
flow  40%: damper  54.8 kW   VFD   4.9 kW   speed 40%  (1260 h/yr)
annual energy: damper 571 MWh, VFD 291 MWh, saving 280 MWh = $33,559 per year at $0.12/kWh

At full flow the drive costs a little energy (its own losses), but over a year of part-load operation it roughly halves consumption. Data center cooling systems rely heavily on drives: computer room air handlers, chilled water pumps, chiller compressors and cooling tower fans all modulate speed to match the IT load, which is a major reason for improvements in power usage effectiveness (DCE-330).

Braking and regeneration

When a drive slows a motor quickly, or when the load drives the motor (a lowering hoist, a downhill vehicle, an overhauling conveyor), the motor acts as a generator and returns energy to the DC link. With a diode front end, this energy cannot flow back to the supply and raises the DC bus voltage, so the drive must limit deceleration or dissipate the energy:

• A braking chopper switches a resistor across the DC bus when the voltage exceeds a threshold, converting the energy to heat (dynamic braking).
• An active front end or regenerative unit returns the energy to the grid, which pays off in elevators, cranes, test stands and centrifuges.
• A common DC bus lets one drive's braking energy power others.

Side effects of PWM drives on motors

The fast voltage edges of PWM inverters, with rise times of 50 ns to a few hundred nanoseconds, create problems that do not occur with sinusoidal supplies:

• Reflected wave overvoltage: the motor cable behaves as a transmission line. Because the motor's surge impedance is much higher than the cable's, a voltage edge reflects at the motor terminals and can nearly double the voltage if the cable is longer than a critical length related to the rise time. Peaks of 1,300 V to 1,600 V on a 480 V or 600 V system can break down motor winding insulation over time.
• Uneven voltage distribution: the first turns of a winding see most of a fast edge, stressing turn-to-turn insulation.
• Common-mode voltage and bearing currents: the inverter's output phases do not sum to zero, producing a common-mode voltage that couples through stray capacitances to the rotor and discharges through the bearings, causing electrical discharge machining that pits races (fluting) and leads to bearing failure.
• Electromagnetic interference from high-frequency currents in cables and ground paths.

where \(l_c\) is the critical cable length, \(v\) the propagation velocity in the cable (roughly 150 to 200 m/µs), and \(t_r\) the voltage rise time.

Motors intended for drive operation should meet inverter-duty insulation requirements, commonly specified using NEMA MG 1 Part 31 (for example, withstanding peaks of 3.1 times rated voltage for motors up to 600 V), and drives are listed to UL 61800-5-1. Installation follows NEC Article 430 Part X for adjustable-speed drive systems, which sizes conductors at 125% of the drive's rated input current (430.122).

Drives and the electrical system

Drives affect the supply as well as the motor. Their rectifiers draw harmonic currents (Lesson 3); the precharge and DC link make their input power factor near unity in displacement but distorted; and they can trip on voltage sags because the DC bus discharges quickly. Ride-through features keep drives running through short sags by using the load's kinetic energy to hold up the DC bus, which is important in data centers where cooling must restart quickly after a transfer to generators. Drives also provide soft starting, eliminating the large inrush of direct-on-line starting, which reduces the generator capacity needed (ELEC-220).

Modern drives include safety functions such as Safe Torque Off (STO), which removes the gate signals through a certified safety channel so the motor cannot produce torque, allowing safe maintenance of machinery without removing all power, defined in IEC 61800-5-2. Drives also report energy, faults and condition data through industrial and building networks.

Applications

Beyond fans and pumps, drives power electric and hybrid vehicles (with SiC traction inverters at 400 V to 800 V), electric trains, ship propulsion, wind turbines (full or partial converters), compressors in chillers and heat pumps, conveyors, cranes, elevators, machine tools and robots. Medium-voltage drives (2.3 kV to 13.8 kV) using multilevel inverters control large pumps, compressors and mills of several megawatts. In each, the same building blocks appear: a front end, a DC link, an inverter, and a control algorithm.

Lesson 8 explains the electromagnetic interference and layout principles behind several of the side effects described here, and Lesson 12 offers a drive design as one of the course project options.