Frequently Asked Questions For Lenze Drives

Lenze 3/15/2018
Frequently Asked Questions For Lenze DrivesHow do I select the Proper Size VFD?
For most applications, select a Lenze VFD that has an output current rating that equals or exceeds the nameplate current rating of the motor. Do not just select the VFD based on the motor Hp rating, as this can result in a VFD that is too small. Some severe applications, such as punch presses, vibratory conveyors, mixers, etc, may require oversizing the VFD in order to handle the peak current demands of the load. Please contact AC Tech's Application Engineering Department for more information.

What type of AC motor should be used with the VFD?
In general, the AC Tech VFD will operate any standard 3-phase NEMA design B induction motor that has a full load current rating that is less than or equal to the VFD's output current rating.

Can a VFD be used on a smaller horsepower rated AC motor?
Yes. All AC Tech VFD's come standard with an electronic thermal overload that allows the VFD to deliver 150% of the rated output current for one minute, and higher current levels for shorter periods of time. This electronic thermal overload can be adjusted to protect smaller motors. Consult the appropriate AC Tech Installation and Operation Maual for details on programming the electronic thermal overload.

Can more than one AC motor be operated from a single VFD?
Yes. But be aware of the following points:
• All motors will run at the same time.
• All motors will run at the same speed.
• The motors cannot be controlled individually.
• The full load current ratings of all motors combined must be less than or equal to the VFD output current rating.
• Individual overload protection must be provided for each motor per NEC.
NOTE: With The TCF Series Drive, it is not recommended to operate multiple motors in Vector mode. V/Hz or Enhanced V/Hz mode should be used instead.

Can single-phase motors be used with VFDs?
No. VFDs are designed to operate 3-phase motors only, and will not work with single-phase motors. Drives that are referred to as "single-phase" will operate on single-phase input power, but the output is still 3-phase, thus requiring a 3-phase motor.

Do the VFDs have a power supply for customer use?
The MC1000 & MC3000 series drives do not have a power supply for customer use. The MCH series drive, when equipped with a Bypass, is available with a 24 VDC, 100 mA power supply. The SC and TC drives have a 12 VDC, 50 mA power supply for customer use.

Can your 3-phase VFDs be operated on single-phase input?
The 3-phase versions of the MC, SC, and TCF Series VFDs can be operated on single-phase input, but the output current rating must be derated by 40%. For example, a 10 Hp, 230 Vac drive has an output current rating of 28 amps. If operated on single-phase input, this would have to be derated to 16.8 amps (28 x 0.6 = 16.8). If the load requires more than 16.8 amps, a larger VFD would be required. In most cases, this derating results in the VFD needing to be twice the HP of the motor, but always check the current to be sure. Be aware that AC Tech does offer some models for single-phase input only (model numbers with "S" at or near the end), and others that are fully rated for both single-phase or 3-phase input (model numbers ending in "Y"). These models do not need to be derated for single-phase input.

How can I determine what "Series" my VFD is?
All current AC Tech models contain a "C" in the Series name (such as SCF Series). However, the "C" is dropped in the actual model number (such as SF2100). This sometimes leads to confusion when talking about the products. For example, one person may say MC1000, while someone else may say M1000. They are both referring to the same product. Here is a listing of AC Tech drive Families and Series:

QC Family contains the following:
• QC1000/QC2000 Series (model numbers begin with digits "Q1"/"Q2")
• QC3000 Series (model numbers begin with digits "Q3")
MC Family contain the following:
MC1000 Series (model numbers begin with digits "M1")
• MC3000 Series (model numbers begin with digits "M3")
MCH Series (model numbers begin with digits "MH")
SC Family contains the following:
SCF Series (model numbers begin with digits "SF")
SCM Series (model numbers begin with digits "SM")
SCL Series (model numbers begin with digits "SL")
• SCD series (model numbers begin with digits "SD")
TC Family contains the following:
• TCF Series (model numbers begin with digits "TF")

Can VFD's be mounted above one another?
It is not recommended to mount VFD's above other VFD's or other heat producing equipment that will impede the cooling of the VFD, unless sufficient space and/or additional cooling (fans, blowers, air conditioners, etc.) is provided.

Can a motor contactor or disconnect switch be installed between the VFD and the AC motor?
Operating a motor contactor or disconnect between the VFD and the AC motor, while the VFD is running, can cause severe damage to the VFD. Such devices should only be operated when the VFD is in a STOP mode. In cases where such a device is required, an "early-break" auxiliary set of contacts on the device should be interlocked with the VFD's External Fault input or Stop input so that if the device is opened while the VFD is running, it will shut off the output of the VFD. If wired to the VFD's Stop input, the stop method must be set to Coast.

Does the VFD require any type of circuit breaker or input fuse protection?
Yes. Branch circuit protection via circuit breaker or disconnect switch and fuses must be provided to comply with the National Electric Code (NEC) and all local codes. Consult Article 430, Section H, of the NEC handbook for more information. The MCH Series is available from the factory with optional circuit breaker or disconnect switch and fuses. Please consult AC Tech for more information.
An external brake resistor is needed to decelerate larger moments of inertia or in the event of longer operations in generator mode. It converts braking energy into heat. The brake resistors recommended by Lenze are designed for around 1.5 times the regenerative power for a cycle time of 15/135 s (brake/pause). The brake resistors are fitted with a thermostat (potential-free NC contact).

When should input and output reactors be used?
Input and output reactors serve very different purposes. An input (or line) reactor helps protect the VFD from input power disturbances that could cause nuisance tripping or damage, and also reduces harmonics that the VFD generates back onto the line. Therefore, input reactors should be used in the following cases:

1. The supply power is subject to disturbances such as surges, spikes, dips, transients, etc.
2. The supply power is very stiff (greater than 10 times the kVA rating of the connected VFDs)
3. Harmonics are a concern (VFDs with input reactors almost always meet the IEEE-519 standard)

An output (or load) reactor, on the other hand, is used to protect the motor if the wiring distance between the VFD and motor is very long. The general rule of thumb is that an output reactor should be used if the motor wiring is over 100 feet, but this value can vary greatly, depending on the motor.

NOTE: If the motor leads are extremely long (500 feet or more), a dV/dt filter should be used instead of a reactor, as they provide better protection at extreme distances.

Will the VFD retain customer parameter settings when there is a loss of input power?
Yes. The VFDs are equipped with non-volatile memory that is not affected by loss or removal of input power.

How can I clear a fault on an AC Tech VFD?
On all QC Series and MC Series VFDs , press the STOP key on the keypad or open the Stop circuit between terminals 1 and 2 on the control terminal strip. On SCF, SCM, SCL, SCD and TCF Series VFDs, open the Stop circuit on the control terminal strip or, if the variable torque inverter is equipped with the optional remote operator interface, press the STOP key on the operator interface. Faults can only be cleared if the condition that caused the fault has cleared.