How to Size a VFD in Pakistan: Current, Supply, Duty and Braking

The short answer

• Match output current to the motor nameplate, in amps. The kW or HP figure is a guide, and only for the duty printed next to it.
• Single-phase input is for small drives and usually means a 230 V three-phase motor, wired in delta if the motor is 230/400 V. Above that, use a three-phase 400 V drive.
• Heavy duty (constant torque) for conveyors, mixers, compressors and hoists. Normal duty (variable torque) for centrifugal pumps and fans. Buy the current for the duty you have.
• Derate for a hot panel. Drive manuals rate the full current up to a stated temperature and altitude, often 40 °C and 1,000 m. A closed panel in a Lahore summer can sit above that.
• A braking resistor is for a load that must stop quickly or is driven by the machine. Without it the drive trips on overvoltage while stopping. The ohms and the watts come from the drive manual.

From kilowatts and horsepower to current

The motor nameplate current is the number to match. A drive’s rated output current must be at least that, on the duty you are using. One horsepower is 0.746 kW, so a drive listed as 1 HP is not a 1 kW drive, and a 1.5 kW drive is not a 2 HP drive. Treat the two units as a conversion, then throw the result away if the nameplate disagrees.

If the nameplate is missing, the current of a three-phase motor is I = P ÷ (√3 × V × efficiency × power factor), with P the shaft output in watts and V the line voltage. At 400 V, with efficiency and power factor in the region of 0.85 and 0.8, that is a little over 2 A for each kilowatt of shaft power. Use that only as a check that a quoted current is in the right neighbourhood. The contactor sizing guide has typical full-load currents for 4-pole motors on 400 V. A different pole count or efficiency class changes the amps. When the nameplate exists, it wins.

The catalogue shows why the current, not the badge, is the match. The Delta VFD007L21A is listed as 1 HP, 240 V, 1-phase or 3-phase, 4.2 A, and marked CT. A motor whose nameplate is above 4.2 A does not belong on it, even if someone calls the motor “about a horsepower”. The ABB ACS355 listing says 1.5 kW. Read the output current on the drive rating plate and put it next to the motor nameplate before you fit it. The Delta MS300 listing is a 200–240 V unit. That voltage is the supply class, not a promise that every motor in the shop can be connected to it.

A contactor sized for the same motor is a different choice. The contactor is chosen on its AC-3 rating, as the contactor guide explains, and it does not set the speed. The drive output current has to cover the motor continuously. Size the motor cable from the motor current, the run and the installation, using the cable sizing guide. Where the drive maker asks for a screened motor cable, use that cable and land the screen as the manual shows. The screen is for electrical noise, not a substitute for the earth conductor.

Single-phase input or three-phase input

Pakistan’s low-voltage supply is 230 V single-phase and 400 V three-phase, at 50 Hz. Drives follow that split, with one trap: the input and the output are not the same thing.

A small drive often accepts a single-phase 230 V input and produces a three-phase output at about 230 V. The motor must be a three-phase motor wound for that voltage. A motor marked 230/400 V is connected in delta for 230 V and in star for 400 V. Leave it in star on a 230 V drive and it will not develop its torque. A motor that is 400 V delta only cannot be fed from a 230 V drive output and still deliver its rated power. The IG5A drives are listed as single-phase and three-phase, so read the particular unit. The LS SV008iG5A-2 is listed as 0.75 kW, 230 V, and the title says it is lotted refurbished stock. Treat a refurbished drive as used equipment: confirm the rating plate, and do not assume it still has every option a new unit of that family would have.

The current drawn from a single-phase supply is higher than the motor current, because the same power is arriving on two wires instead of three, with the drive’s own losses on top. Size the supply cable and the breaker or fuses from the input current in the drive manual, not from the motor nameplate. The circuit breaker guide is about the device type. The amp rating still comes from the drive maker’s table. A breaker that is only large enough for the motor will nuisance-trip, and a breaker that is larger than the drive allows will not protect the drive’s input.

From a few kilowatts upward, a three-phase 400 V input is the ordinary choice. The supply current is lower, the motor stays on its 400 V connection, and you are not asking a single-phase workshop feed to carry a large drive. Do not plan a large motor on a single-phase input just because a small drive in the same series can do it. If the site has no three-phase supply, that is a supply question, not a reason to oversize a single-phase drive past its listing.

A standard drive runs a three-phase induction motor. It does not run a single-phase motor with a start capacitor or a centrifugal switch. Those motors are not candidates for a VFD. Change the motor, or use a drive the maker rates for that motor type.

A solar pump inverter is a different product. The CNC YCB2000PV is listed under solar accessories as a solar pump inverter, not as a general 400 V industrial drive. Size it from the pump and the array, using that maker’s manual, not from the rules above for a mains-fed motor.

Heavy duty or normal duty

The same frame of drive is often sold with two ratings. Normal duty, also called variable torque, is for centrifugal pumps and fans, where torque falls as the speed falls and the overload is modest. Heavy duty, also called constant torque, is for conveyors, positive-displacement pumps, mixers, compressors, crushers and hoists, where the motor can be asked for full torque at low speed and for a higher short-term overload while starting or shock-loading.

Makers print the heavy-duty kilowatts lower than the normal-duty kilowatts on the same hardware. A drive that “is 15 kW” on the fan rating may be a smaller drive on the conveyor rating. Buy the output current of the duty you need, and make sure it covers the nameplate. Overload figures differ by maker. A common pair is about 110 percent for a minute on normal duty and about 150 percent for a minute on heavy duty. Those percentages are typical, not a promise. Read the datasheet of the unit you are ordering.

The Delta listing above is marked CT, which on that family means the constant-torque, heavy-duty current. If your load is a centrifugal pump, a normal-duty rating may be enough and is sometimes the cheaper frame. If the load is a mixer, do not use the fan rating. A motor that must hold torque at low speed for long periods also needs its own cooling looked at: a shaft-mounted fan moves less air at low speed, and the motor can overheat even when the drive current is inside its rating. That is an inverter-duty motor or a separate blower, not a larger lug.

Derating for heat and altitude

Drive manuals state a maximum temperature and altitude for the full current. A very common pair is 40 °C and 1,000 m. Above those, the maker publishes a curve, and the curve is part of the selection. Do not apply a single percentage you found for a different brand.

Heat is the one that matters in Lahore. The city sits only a few hundred metres above sea level, so the altitude curve rarely changes the size. A closed panel in summer is another matter. The air around the drive, not the shade temperature on the news, is what the derating uses. Leave the clearances the manual draws, keep the fan inlet clear, and do not blank the vents with trunking. If the enclosure is outdoors in the sun, or indoors above a hot process, measure or estimate that internal temperature and apply the curve before you order. A drive that is one frame larger is sometimes the result. A drive crammed into a box with no airflow is how a correctly sized unit still trips on temperature.

Altitude starts to matter in the mountains, where the thinner air cools less and the clearance inside the drive is no longer what it was at sea level. Use the site height with the maker’s table. Do not add a flat extra “for Pakistan” on a Lahore job, and do not ignore the table on a site that is actually high.

Braking resistors

When a motor is told to slow down, or when the load pushes it (a lowering hoist, a large fan spinning down, a high-inertia drum), the motor becomes a generator. That energy returns to the drive’s DC bus. If the bus voltage rises too far, the drive trips. Coasting to a stop avoids the trip and does not stop the machine quickly. A braking resistor, switched by a brake chopper, turns the extra energy into heat.

Many small drives include the chopper and need only the resistor. Larger drives often need a separate brake unit. The manual says which, and it states a minimum resistance. A resistor below that minimum asks the chopper for more current than it can switch. The wattage is not the peak power printed in large type. It is the heat the resistor must survive for your duty: how often you stop, and for how many seconds. A resistor rated for a short stop will fail if the machine brakes every few seconds all day. Mount it where the heat can leave. It runs hot. Do not sit it on a plastic trunking lid or against the drive’s own air intake.

There is no universal ohm value to memorise. Two drives of the same kilowatts from different makers can want different resistors. Take the ohms and the watts from the manual of the drive you have bought, and buy a resistor that meets both, with a wattage that covers the duty cycle rather than a single stop.

A contactor is not a brake. Fit a supply contactor or a breaker if the manual or the lock-off procedure wants a way to remove power, and size that contactor from the contactor guide when it also starts a motor across the line. Do not use a contactor on the drive output to start and stop the motor while the drive is running, unless that manual allows it. Switching the motor leads under modulation is how output stages fail. Stop the drive, then isolate.

What else the datasheet is for

A soft starter is not a small VFD. It ramps the voltage and then runs the motor at full speed. Buy one only when you do not need to vary the speed. The VFD and drives category includes both, so read the title.

Set the motor parameters to the nameplate: voltage, current, frequency, power and base speed. The drive’s electronic overload only protects the motor if those values are the real ones. It does not replace the supply fuses or breaker the manual calls for, and it does not replace an emergency stop that has to remove power. Running above 50 Hz is constant-power territory on a normal motor: torque falls as speed rises. Do it only when the motor and the machine are allowed to, not because the keypad goes that high.

A drive, worked through

A 2.2 kW, 400 V, 4-pole motor on a mixer has a nameplate current. The typical current for that frame on the contactor chart is about 5 A, and the nameplate is what you use if it differs. The load is constant torque, so you take a 400 V three-phase drive whose heavy-duty output current is at least that nameplate, not the larger fan rating of a smaller frame. The supply breaker and the input cable come from the drive’s input current. The motor cable comes from the motor current and the run. In a closed Lahore panel, check the temperature derating before you decide the frame is big enough. If the mixer must stop in a second or two rather than coast, add the braking resistor the manual names. A pump of the same kilowatts could use the normal-duty rating and would usually coast to a stop with no resistor.

Selection checklist

1. Motor nameplate current, voltage, frequency and power. Three-phase induction motor, not a capacitor-start single-phase motor.
2. Constant torque or variable torque, and the drive output current for that duty at or above the nameplate.
3. Supply: 230 V single-phase in, with the motor on the drive’s output voltage, or 400 V three-phase in.
4. Input current from the drive manual, then the supply cable and breaker. Motor cable from the motor current.
5. Panel temperature and site altitude against the derating curve.
6. Braking resistor only if the load must stop quickly or can overhaul, with the manual’s minimum ohms and a wattage for the real duty.
7. Nameplate values entered in the drive, screened motor cable if the maker requires it, and a way to isolate the supply.

Browse VFDs and drives, contactors and motor starters and cables. Brands on the site with drives include Delta, ABB and LS. Prices are in PKR. Send a photo of the motor nameplate and, if you have it, the old drive’s rating plate on WhatsApp and our Lahore team will help you match the current and the duty.