Three parameters determine how a VFD handles its motor: the acceleration ramp time, the deceleration ramp time, and the V/f ratio. Getting these wrong causes nuisance trips, mechanical damage, or motor overheating.
This guide explains how each parameter works, how to calculate the right values for your application, and how to identify the fault codes that wrong settings produce.
A VFD that trips on overcurrent at startup almost always has an acceleration time that is too short for the load.
A VFD that trips on overvoltage during braking has a deceleration time that is too short.
Both faults are caused by misconfigured ramp parameters and are corrected without any hardware change.
The 3 Parameters That Control Every VFD Start and Stop
The time for the VFD output frequency to ramp from 0 Hz to maximum. A longer acceleration time reduces the starting current and the mechanical shock on couplings and gearboxes.
Too short: overcurrent trip (OC) at startup. Too long: motor stays in low-speed high-slip region too long, causing overheating in fixed-cooling motors.
The time for the VFD output frequency to ramp from maximum down to 0 Hz. A longer deceleration time reduces regenerative energy fed back into the DC bus and prevents overvoltage trips.
Too short: overvoltage trip (OV) during braking. Too long: load coasts and overruns in gravity or inertia-driven applications such as downhill conveyors.
The ratio of the VFD output voltage to output frequency. In V/f mode, voltage and frequency increase together in a fixed ratio set by the motor nameplate base frequency and rated voltage.
Too high a ratio: magnetic saturation, high no-load current, motor overheating. Too low: insufficient torque at low speed, the motor stalls under load.
Acceleration Time: How to Calculate the Right Setting
The minimum acceleration time depends on the motor and load inertia and the available accelerating torque. The formula below gives the theoretical minimum.
In practice, always set the acceleration time 20 to 50 percent longer than the theoretical minimum to allow for load variations and starting current peaks.
The standard ramp time formula used by most VFD manufacturers is:
For pumps and fans: start at 10 to 30 seconds. For conveyors: 5 to 15 seconds. For compressors: 20 to 60 seconds. For light loads with no gearbox: 2 to 5 seconds.
The VFD working principle guide explains how the VFD generates the ramp by incrementing its output frequency reference every few milliseconds, not by physically measuring motor speed.
Deceleration Time: Managing Regenerative Energy
When a VFD decelerates a motor, the motor acts as a generator and pushes energy back into the DC bus.
If this raises the DC bus voltage above the overvoltage threshold, the VFD trips.
A braking resistor connects across the DC bus through a braking IGBT (also called a chopper transistor). When the DC bus voltage rises above the threshold during regeneration, the chopper switches the resistor in to absorb the excess energy as heat. This allows very short deceleration times on high-inertia loads. Size the resistor for the peak regenerative power: P = (0.5 x J x (omega_max squared)) / t_decel, where omega_max is the maximum angular velocity in radians per second.
V/f Ratio: Setting the Correct Volts per Hertz
The V/f ratio is set directly from the motor nameplate: V/f = Rated Voltage / Base Frequency. This keeps motor flux constant across the full speed range.
For a 400 V, 50 Hz motor, V/f = 400 / 50 = 8 V/Hz.
The VFD maintains this ratio so the motor flux stays constant and torque is maintained at all speeds below base frequency.
| Frequency (Hz) | Correct Vout at 8 V/Hz (V) | Effect if Vout too high | Effect if Vout too low |
|---|---|---|---|
| 10 Hz | 80 V | Magnetic saturation, high no-load current, core heating | Insufficient flux, low torque, motor stalls under load |
| 25 Hz | 200 V | Magnetic saturation, motor draws excess current at no-load | Motor speed drops under load, poor speed regulation |
| 50 Hz (base) | 400 V | This is the rated point. V/f ratio is correct at base frequency. | Operating below rated voltage at rated frequency, reduced power output |
| 60 Hz (above base) | 400 V (capped) | Above base frequency, voltage is capped at rated value. Motor enters field weakening: torque falls as frequency rises. | Same voltage is already at maximum. |
At very low frequencies (0 to 5 Hz), the stator resistance of the motor becomes significant compared to the inductive impedance, and the standard V/f ratio delivers insufficient flux to develop the required starting torque. Most VFDs provide a manual voltage boost setting (sometimes called IR compensation or startup boost) that adds a fixed voltage offset at low frequencies. A typical starting boost of 5 to 10 percent of rated voltage corrects this. Setting boost too high causes motor overheating at low speed due to excess stator current.
VFD Ramp Time and V/f Calculator
Worked Example: 7.5 kW Pump, 400 V, 50 Hz
Vr = 400 V, Fb = 50 Hz, Nr = 1450 rpm, Pr = 7.5 kW
Total inertia J = 0.08 kg.m² (motor + impeller estimate)
Step 1: V/f ratio
V/f = Vr / Fb = 400 / 50 = 8.0 V/Hz
At 25 Hz: Vout = 8.0 x 25 = 200 V
At 10 Hz: Vout = 8.0 x 10 = 80 V
Step 2: Motor rated torque
omega = 1450 x 2 x pi / 60 = 151.8 rad/s
Tr = Pr / omega = 7500 / 151.8 = 49.4 Nm
Step 3: Minimum acceleration time
Assume VFD delivers 150% torque = 1.5 x 49.4 = 74.1 Nm accelerating torque
t_min = J x omega / T_acc = 0.08 x 151.8 / 74.1 = 0.16 s (theoretical)
Apply safety margin x 1.5, then round up for pump application: set 10 s
Step 4: Recommended deceleration time
Pump has low inertia. Regeneration risk is low. Set 8 s
Step 5: Verify settings under load
Monitor output current during acceleration ramp.
If current exceeds 150% FLC: increase acceleration time by 5 s and retest.
If overvoltage fault on stop: increase deceleration time by 5 s and retest.
4 Common VFD Faults Caused by Wrong Parameter Settings
Cause: Acceleration time too short. The VFD demands more speed than the motor can deliver, causing output current to spike above the trip threshold (typically 150 to 200% FLC).
Fix: Increase acceleration time. Start at double the current setting. Also check that V/f ratio is not set too high, which increases no-load current and reduces the margin before the OC threshold.
Cause: Motor running below 20 Hz for extended periods. Self-cooling fans are speed-dependent. At low speed, cooling airflow reduces significantly, causing overheating even at rated current.
Fix: Install a separate forced-ventilation fan on the motor for continuous low-speed operation, or use a TENV motor. Also reduce the V/f boost at low frequency.
Cause: Deceleration time too short for the load inertia. Regenerative energy raises the DC bus voltage above the overvoltage trip threshold. High-inertia loads (fans, flywheels, compressors) are most prone.
Fix: Increase deceleration time. If overvoltage persists, add a braking resistor and enable the braking chopper. See the VFD overload trip causes guide.
Cause: V/f ratio too low, or a supply voltage dip limits VFD output at startup. The motor cannot develop enough torque, slip increases, and the VFD reports a stall or undervoltage condition.
Fix: Check the V/f ratio against the motor nameplate. Verify supply voltage is within specification during starting. If using manual boost, increase in 1 to 2 percent steps and monitor no-load current.
Watch: VFD Parameter Setup and Tuning Guide
VFD Parameter Configuration Questions Engineers Ask
Related Articles on This Site
- VFD Overload Trip: Causes and Solutions
- Motor Starting Methods Compared
- How AC Induction Motor Works
- Induction Motor Slip Explained
- Motor Full Load Current (FLC) Explained
External References
- VFD Parameter Configuration Guide | ABB ACH550 User Manual
- VFD Programming and Parameter Setup | Yaskawa Technical Manual
What We Learn Today
- Acceleration time controls how fast the VFD ramps frequency from 0 to maximum. Too short causes an overcurrent trip. Calculate using t = J x omega / T_acc and add a 50 percent safety margin. For pumps start at 10 to 30 seconds. For fans and compressors start at 30 to 120 seconds.
- Deceleration time controls how fast the VFD ramps frequency down to zero. Too short causes an overvoltage trip as regenerative energy raises the DC bus. High-inertia loads (fans, flywheels, compressors) need long deceleration times or a braking resistor with a chopper transistor.
- The V/f ratio is set from the motor nameplate: V/f = Rated Voltage / Base Frequency. For a 400 V, 50 Hz motor this is 8 V/Hz. Maintaining this ratio across the speed range keeps motor flux constant and delivers rated torque at all speeds below the base frequency.