Variable-speed drives
An induction motor turns at the speed imposed by the mains frequency. If the frequency is changed, the speed changes. The whole drive exists to generate that frequency at will, and to do so without destroying the motor.
01Why vary the frequency
In AC motors it was established that the synchronous speed is ns = 120 f/p. With 50 Hz and four poles, that is 1500 rpm, and there is no way to change it: the mains is in charge.
| Method | Range | Problem |
|---|---|---|
| Valve or damper | Whole flow range | The motor keeps drawing the same power: the outlet is throttled and the energy is thrown away. |
| Pulleys and gearboxes | Fixed | It can't be adjusted while running. |
| Pole changing | Two or three speeds | Special motors only, in steps. |
| Variable-frequency drive | Continuous, from 0 to over 50 Hz | Equipment cost and harmonics on the mains. |
In a centrifugal pump, the flow is proportional to the speed, but the power goes with the cube of the speed.
- At 80 % speed, the power consumed is 0.8³ = 51 %.
- At 50 %, it is 0.5³ = 12.5 %.
Throttling with a valve, on the other hand, barely lowers the consumption. That is why on pumps and fans the drive pays for itself within months, and it is the main reason they are everywhere.
02What is inside
| Stage | What it does | Practical detail |
|---|---|---|
| Rectifier | Converts the mains AC into DC | Three-phase diode bridge: on 380 V it gives about 537 V DC. |
| DC bus | Stores energy in large capacitors | They stay charged above 500 V for minutes after disconnection. It is the greatest hazard of the equipment. |
| Inverter | Six IGBTs that build the output AC by PWM | 2 to 16 kHz carrier (power supplies). |
| Control | Microcontroller or DSP that computes the modulation | Holds the parameters, the protections and the communications. |
| Braking | Resistor or return to the mains | When braking, the motor generates and raises the bus voltage: that energy has to be burned off or returned. |
03The volts-per-hertz law
It is the central concept of the topic. If the frequency is lowered without lowering the voltage, the magnetic flux of the motor grows until it saturates the iron: the current shoots up and the motor burns out.
V and f rise proportionally, the flux is maintained and the motor can deliver its rated torque at any speed. It is the usual working region.
At very low frequency a voltage boost is needed to compensate for the drop across the winding resistance, which at 2 or 3 Hz eats up almost all the voltage.
The voltage cannot exceed the mains voltage, so V/f drops, the flux drops and the torque falls in inverse proportion to the frequency. At 100 Hz the motor gives half the torque.
The mechanical limit must also be checked: bearings, balancing and the motor's own ventilation.
The fan of a self-ventilated motor is mounted on its own shaft: at half speed it ventilates much less, yet the motor can keep delivering the same torque and the same current. That is why prolonged operation at low speed with rated torque calls for independent forced ventilation or an oversized motor. It is one of the most common causes of burnt-out motors in installations with drives.
04Types of control
| Control | How it works | When it is used |
|---|---|---|
| Scalar V/f | Applies the V/f curve without measuring anything from the motor | Pumps, fans, conveyors: loads that don't demand torque at zero speed. It is the simplest and cheapest. |
| Sensorless vector | Estimates the flux position from the measured currents | When torque is needed at low speed and good dynamic response: elevators, extruders. |
| Closed-loop vector | Uses an encoder on the shaft | Positioning and exact torque at zero speed: machine tools, cranes. |
- Motor nameplate data: voltage, current, frequency, speed, cos φ. Without this the drive cannot protect it.
- Acceleration and deceleration ramps: if they are too short, the drive trips on overcurrent or on DC bus overvoltage.
- Minimum and maximum frequency.
- Current limit and thermal protection.
- Command source: keypad, terminal block, communication.
- Carrier frequency: the higher it is, the quieter the motor and the more heat in the drive.
05Installation: what ruins a drive
No switching device —contactor, switch, compensation capacitors— is placed between the drive output and the motor. Opening that circuit while the drive is delivering current destroys the output stage. If isolation is needed, it is done with the drive stopped and with an auxiliary contact that signals the drive.
The PWM output has nanosecond edges: it radiates a great deal. The cable to the motor must be shielded, with the shield grounded at both ends and over 360°, not with a “pigtail.” Without that, faults appear in sensors, in communications and even in neighboring equipment.
- Bearing currents: the switching induces voltages on the shaft that discharge through the bearing and pit it. With long cables or large motors, insulated bearings or shaft grounding brushes are used.
- Harmonics toward the mains: the input rectifier draws current in peaks. It is corrected with a line or DC-bus reactor, and in large installations with filters (AC power).
- Enclosure ventilation: the drive dissipates 2 to 4 % of the power it handles. A 15 kW unit can be giving off 500 W inside the cabinet.
- Distance to the motor: long cables produce reflections that double the voltage at the motor terminals. Beyond a few tens of meters an output filter is needed.
Cut the power, wait the time the manufacturer indicates —typically 5 to 15 minutes— and verify with an instrument that the DC bus voltage has dropped below 50 V. The bus capacitors store enough energy to kill long after the equipment has been disconnected, and a dark front panel means nothing.
06In the lab
With a training drive and a small motor, enter the nameplate data and the ramps, and start from the keypad. Use a tachometer to measure the speed at 10, 25, 50 and 60 Hz and compare it with n = 120 f/p. Note the slip at each point.
Use a True RMS multimeter to measure the output voltage at different frequencies and build the table of V versus f. Verify that the ratio stays constant up to the rated frequency and that after that the voltage flattens out. Plot it.
Shorten the acceleration ramp until the drive trips on overcurrent, and the braking ramp until it trips on DC bus overvoltage. Note the limit values and explain each trip. It is the best way to understand what each parameter protects.
Change the carrier frequency between 2 and 12 kHz and listen to the motor: the characteristic whine changes pitch until it becomes inaudible. Measure the drive's temperature in each case: silence is paid for with heat.
07Common mistakes
| Symptom | Usual cause |
|---|---|
| Trips on overcurrent at start-up | Ramp too short for the inertia, or current limit set wrong. |
| Trips on overvoltage when braking | The motor returns energy and the bus voltage rises. Lengthen the ramp or add a braking resistor. |
| The motor heats up at low speed | Insufficient self-ventilation. Forced ventilation is needed. |
| No strength at low frequency | Voltage boost missing, or the application requires vector control. |
| Faults in sensors and communications when the motor starts | Output cable unshielded or shield badly connected. |
| Bearings pitted after a few months | Shaft currents induced by the switching. |
| The drive was destroyed when an output contactor opened | Never disconnect between drive and motor while the equipment is running. |
| The motor vibrates and makes odd noise | Low carrier, or mechanical resonance at a certain frequency: solved with “skip frequencies” in the parameter setup. |
08Self-assessment
Why does the voltage have to be lowered when the frequency is lowered?
Because the magnetic flux is proportional to V/f: if f drops and V doesn't, the flux grows, the iron saturates and the magnetizing current shoots up, burning out the motor.
A 380 V, 50 Hz motor runs at 30 Hz. What voltage should the drive apply?
The ratio is 380/50 = 7.6 V/Hz, so 7.6 × 30 = 228 V —plus the boost that compensates for the resistive drop—.
What happens above the rated frequency?
The voltage cannot rise any further, so V/f falls: the flux drops and the available torque decreases. The motor enters the constant-power region and its mechanical limit must also be checked.
Why does a pump with a drive save so much energy?
Because the power of a centrifugal pump varies with the cube of the speed: at 80 % speed it consumes 51 %, and at 50 %, barely 12.5 %. Throttling with a valve does not lower the consumption.
What are the three stages of a drive?
Rectifier (mains to DC), DC bus with its capacitors, and inverter of six IGBTs that builds the output AC by PWM.
Why isn't a contactor placed between the drive and the motor?
Because opening it while the drive is delivering current destroys the output stage: an inductive current is abruptly interrupted. If isolation is needed, it is done with the drive stopped.
What is the difference between V/f control and vector control?
V/f applies a curve without measuring the state of the motor: it is simple and sufficient for pumps and fans. Vector control estimates or measures the flux position and controls the torque directly, giving strength at low speed and better response.
Why must the cable to the motor be shielded?
Because the PWM output has nanosecond edges that radiate a lot of interference. The shield, grounded at both ends and over 360°, contains it and prevents faults in sensors, communications and neighboring equipment.
What precaution must be taken before opening a drive?
Cut the power, wait the indicated time —5 to 15 minutes— and verify with an instrument that the DC bus is below 50 V. The capacitors keep more than 500 V long after the power is cut.
A motor runs at 30 % speed with rated torque and burns out. What was missing?
Forced ventilation. The motor's own fan turns with the shaft: at low speed it barely cools, while the current is still the rated one.