Motors and variable frequency drives

The three-phase squirrel-cage induction motor drives almost everything in industry: conveyors, pumps, fans, compressors. It is tough and cheap, but you need to read its nameplate, connect it properly, start it the right way and, more and more often, run it from a variable frequency drive. This topic gathers what a technician needs day to day.

The nameplate

It is the first thing to look at, and it overrides any calculation:

• Power (kW or HP): the useful mechanical power at the shaft, not the power drawn from the supply.
• Voltages (for example 230/400 V Δ/Y): which supply voltage it runs on in delta and which in star (wye).
• Currents: the rated current for each connection. It is the value the overload relay or motor circuit breaker is set to.
• Speed (rpm), frequency (50 or 60 Hz), power factor (cos φ), efficiency or efficiency class (IE2, IE3…), degree of protection (IP55, for example) and duty type (S1 is continuous duty).

Speed: poles and slip

The speed of the rotating field (synchronous speed) depends on the frequency and the number of pole pairs:

n = 60 × f / p

At 50 Hz: with 1 pole pair (2-pole motor), 3000 rpm; with 2 pairs (4 poles), 1500 rpm; with 3 pairs (6 poles), 1000 rpm. At 60 Hz the figures are 3600, 1800 and 1200 rpm. The rotor turns a little slower than the field, for example at 1450 rpm in a 4-pole 50 Hz motor: that difference is the slip, and it is what lets the motor produce torque. That is why, to change the speed of an induction motor continuously, you have to change the frequency: that is what the drive does.

Star or delta in the terminal box

The practical rule: each motor winding withstands the lower voltage on its nameplate. If the plate says 230/400 V and the supply is 400 V phase to phase, the motor goes in star. If it says 400/690 V and the supply is 400 V, it goes in delta.

North American motors are often dual-voltage 230/460 V with nine leads numbered 1 to 9. The idea is the same, but the two connections are made by putting the winding halves in series (high voltage) or in parallel (low voltage): always follow the diagram on the motor.

Ways of starting

You can watch a star-delta start, with its current compared with a direct-on-line start, in the star-delta starter simulator, and wire start-stop and reversing circuits in the motor control circuit simulator.

Inside a variable frequency drive

A drive (VFD, inverter) has three parts: a rectifier that turns the AC supply into DC, a DC bus with capacitors that store energy, and an inverter with transistors (IGBTs) that builds a new AC supply of variable frequency and voltage by modulation (PWM). For the motor to give its torque at any speed, the drive keeps the ratio between voltage and frequency roughly constant.

Basic parameters

• Motor nameplate data: voltage, current, frequency, speed and power. Many drives also run a motor identification (auto-tune) to control it better.
• Acceleration and deceleration ramps: how long it takes to go from 0 to maximum speed and back.
• Minimum and maximum frequencies.
• Command source (terminals, fieldbus) and setpoint source (0–10 V or 4–20 mA analog input, potentiometer, fieldbus).

Typical drive faults

• DC bus overvoltage when braking: when decelerating, the motor acts as a generator and sends energy back to the bus. If the ramp is too short for the load's inertia, the voltage rises and the drive trips. Lengthen the ramp or add a braking resistor.
• Overcurrent: acceleration ramp too short, jammed load or a fault in the motor or cable.
• Earth (ground) fault: damaged insulation in the cable or the motor.
• Overtemperature: poor panel ventilation or a dirty drive fan.

Installing a drive

• Shielded motor cable, with the shield bonded at both ends over a good contact area, and kept apart from signal cables. Drives produce interference that can upset sensors and communications.
• The right residual current device: three-phase drives can produce smooth DC leakage currents, and manufacturers usually ask for a type B RCD where one is required.
• Do not open or close a contactor between the drive and the motor while the motor is running, unless the manufacturer allows it and the drive output is disabled: it can damage the output stage.
• Motors running slowly for long periods: the shaft-mounted fan cools less. Forced ventilation or protection with thermistors in the winding may be needed.

Motor protection

• Against overload: overload relay or motor circuit breaker set to the nameplate current, or the drive's own thermal protection configured with the motor data.
• Against short circuit: circuit breaker, motor circuit breaker or fuses.
• Against real overheating: PTC thermistors in the winding, which detect the motor temperature even when the current is normal (for example, with a blocked fan).
• Phase loss: a three-phase motor that loses a phase keeps turning with more current in the other two and overheats. Modern overload relays and motor circuit breakers detect it.

Summary

• The nameplate rules: useful power, Δ/Y voltages, currents, speed, power factor and efficiency.
• Synchronous speed: n = 60 × f / p. At 50 Hz, 3000, 1500 or 1000 rpm depending on the poles (3600, 1800 or 1200 at 60 Hz), minus the slip.
• Each winding withstands the lower nameplate voltage: a 230/400 V motor on a 400 V supply goes in star.
• Starters: direct on line, star-delta (a third of the current and torque), soft starter or drive.
• The drive rectifies, stores energy in the DC bus and inverts with PWM. Beware of its capacitors.
• Short ramps cause overcurrent when accelerating and overvoltage when braking; the fix is longer ramps or a braking resistor.